Building Envelope Design: Completing the Design pt. 2

Wall performance categories: Barrier, drainage, and rain screen

We will classify walls into the three different wall performance categories and then provide some examples of variations within each. Depending on the climate, the program, and initial cost versus long-term maintenance considerations, one wall type may fit best with your planned project. Ill. 25 shows wall sections constructed of wood frame, concrete block, metal stud, stone veneer, and other systems in each of the three basic categories. The three wall performance categories based on performance are (1) barrier, (2) drainage plane, and (3) rain screen or cavity wall.

As you have seen, water, vapor, and air intrusion can lead to leaks, condensation, and increased potential for mold growth. Designers are faced with count less conflicting criteria. Your clients may have limited knowledge of building envelope performance, yet many of them try to provide you with directions as to what they want. More important, some try to tell you what they don't want: "Oh, we don't need any of that membrane stuff in the walls; that's a waste of money!" We have faced the same difficult situations with informed clients, who tell us: "We would like to have cavity walls, but we just can't afford them.

Just do it in tilt concrete and forget about it." Are these the same clients who will come after us if the new offices leak in five years or if the condensation we warned them about begins to create strange smells in the boardroom? What is a designer to do? The first thing is to build a good relationship, and the second is to try and teach your clients the scientific and mathematical reasons on which you are basing your recommendations. If all this fails to bring them around to your point of view, bring out the big guns. The best ally we have is the building code. Recent changes have been made in many codes that require better wall system design and performance. "We would love to leave out the air and vapor barrier as you wanted, but the building code won't allow it" is an excellent reason to support your preference. It maintains your camaraderie with the client. If the codes don't support your recommendations, quote the best experts in the field.

Ill. 24 Pearl River Tower Terrace

Ill. 25 Three types of walls; barrier, drainage and rain screen. Concrete Block Metal Stud Tilt or Precast Stone on CMU Wood Frame Table 3 Rainfall with systems. Rainfall amounts Extreme (>60'') High (40''-60'') Moderate (20''-40'') Low (<20'') Mixed humid A respected expert in building design and construction, Dr. List, with the Building Science Corporation, has authored several articles and books on the subject. Table 3 was derived from his recommendations and was expanded based upon our experience.

Barrier walls can be acceptable in areas with less than 20” of annual rainfall. Barrier walls, as a general rule, have lower initial cost than rain screen walls of the same system. Barrier walls rely on the paint, stain, vinyl coating, aluminum, or surface treatment to stop all water at the face. If it doesn't rain much, or if you don't mind the wall materials behind the surface getting wet on occasion, this may be all you need. Some barrier walls rely on paint or other coatings to stop water. These products typically need maintenance and reapplication over time. If, for example, you have a west-facing barrier wall built using painted plywood to stop water, this plywood may crack and split over time as the sun's heat bakes it at more than 125ºF.

Cracks and splits often exceed the bridging ability of paint. The void could allow water in, so it needs to be filled and paint applied. This is what we are referring to as maintenance. Barrier walls rely on the skin to stop wind and water. In hot, humid climates with a lot of rain, we would try to get the most flexible coating with the lowest perm rating. In a colder climate or an arid area, we like a breathable exterior coating such as a latex paint or perhaps a penetrating stain.

Drainage walls are different from both barrier and rain screen walls in that you are allowing water into the wall and hoping it drains out before it causes problems. The old standard for many years was what we call a mass wall, constructed out of thick layers of stone, brick, block, or a combination of them. Mass walls can absorb a lot of water before it reaches the inside face of the wall. As long as the rain stops before the wall becomes saturated all the way through to the inside face, and as long as vapor drive is working in your favor, this type of wall should perform adequately. The table indicates that drainage walls are acceptable for use in areas where average annual rainfall is between 20 and 40”; however, it's really the intensity and duration of the rain event that matters. Since temperature and pressure differentials will drive vapor through a wall, you should consider the rates of transmission for the sum of the selected materials and thicknesses before relying on it. Because these walls need to dry to the out side, permeable coatings are recommended. Plaster cement coatings are commonly applied to the interior and exterior faces of mass walls. These provide permeable exterior wall coatings that reduce air transmission by sealing most of the irregularities in the stone, brick, or block. At the same time, plaster cement coatings are quite durable. Mass walls may be built without insulation if temperature differences don't exceed the thermal lag caused by the mass of the composite whole. If it doesn't stay too cold for too long, the temperature in the wall may never reach the dew point.

Drainage walls can be built out of the most porous construction materials.

Many of our example sections show stucco as the hygroscopic material that permits moisture to be absorbed, but it could be almost anything. Face brick, concrete, and wood are all porous. For a modern wall built in the drainage category to perform well in hot, humid climates, it will need a moisture barrier.

The lower the permeability of the membrane, the less water gets into the interior of the wall. The less water that gets into the wall, the fewer chances there are for mold growth. The barrier system needs to restrain both air and vapor in most walls. This point in the wall is the only low-permeability vapor barrier product that we use in the entire wall section. A combination of insulation and HVAC system modulation should minimize the potential for the dew point being reached inside the wall. If well detailed and well constructed, the wall should keep liquid water out.

Rain screen walls or, as we like to call them, cavity walls are the most expensive of the three (as a general rule) and also perform the best in areas with a lot of rain. The choice of wall type depends on many factors, but climate and contents should be considered first. Rain screen walls are not required for most industrial uses, for storage of durable materials, for unconditioned spaces, for seasonal-use buildings, etc. Depending on what the building is used for and who the occupants will be, a barrier system may suffice. If aver age rainfall exceeds 40” per year and /or the units are to be sold as condominiums, then rain screen walls are the only wall type that should be used.

This is what we have been recommending for years and is what many of the world's leading experts recommend.

Rain screen walls require more three-dimensional planning because of their increased thickness. Rain screen walls also perform far better in high-wind and rain events than any other wall types (in the same material category). The cavity in rain screen wall construction has three key advantages in hot, humid climates: (1) It separates the outer wythe from the inner wythe, preventing moisture from being transmitted through materials hygroscopically, (2) it serves as a thermal break between planes, and (3) it reduces the pressure and force of any water in liquid or vapor states that reaches the membrane. Before we go into the more detailed review of their differences, we need to let you all know that the graphical examples provided in the book are intended to illustrate differences in systems and wall types. They are not intended to be used for construction as they are depicted herein. We might have left out some important sub-component in order to more clearly make our point with what is shown and discussed. Please don't copy them and paste them into your plan sets. We would be glad to talk with you about a possible review of your details, or develop some especially for your projects. (Author)

Wood frame construction

Ill. 26 Wood frame barrier wall.

Pros: Low Thermal Mass, Cheap, Fast, Breathable

Cons: Does not Stop Wind or Water Well, Requires More Maintenance We start off with a wood frame construction example and then go on to look at concrete block, metal stud, concrete, metal panel, stone veneer, and glass wall systems. Regardless of the type system, you have to stop water from causing problems in your built environment. In the barrier system, you try to stop it at the face, for example, with paint on wood siding (see ill. 26). Its performance depends totally on fit. In order to stop high winds, it needs to be installed so that no space exists between the many pieces and parts. This is highly unlikely.

The better drainage system version relies on the combination of sheathing as a vapor retarder and an applied air barrier to prevent any water that defeats the paint and lapped siding from getting into the wall behind the moisture-reducing membrane. The continuous membrane extends down past the floor level toward grade. The best system uses a multitude of components, each playing a role in stopping air and moisture, whether liquid or vapor. Rain screen or cavity wall system performance is far better than that of the drainage wall because of the addition of the air cavity between the sheathing and siding. This cavity provides many benefits. It provides a drainage pathway for any moisture that defeats the siding to drain down, and perhaps more important, it reduces wind pressures acting on the air moisture barriers, reducing the amount of movement through any voids or a permeable membrane component. This particular example uses a self-adhesive membrane (SAM) as an air and moisture barrier applied to the exterior face of the sheathing, (see ill. 27).

Ill. 27 Wood frame rain screen wall.

Pros: Keeps Out Moisture Best, not as Reliant Upon Maintenance Cons: Higher Initial Cost, Thicker

Since typical wall sheathings allow moisture transmission through them, we feel that it's beneficial to stop moisture from reaching the sheathing. This is what makes this the best performing of the wood frame systems. If you wanted to take it a step further, you could add a separate air infiltration barrier (AIB) as well. In our opinion, it's not necessary if the SAM is well sealed, except in warm climates with more than 80” (2.5 meters) of annual rainfall. In cold, humid climates, we would not locate the SAM on the outside of the wall framing. We would want to stop moisture from moving through the wall from the inside face toward the insulation (not shown) and toward the outside in the winter. This would mean that the MRB would be on the interior face of the wall insulation. Depending on the design conditions, the MRB might be vapor permeable, such as a coat of paint on drywall, or vapor impermeable, such as foil-backed batt insulation or closed-cell styrene products.

CMU walls

Ill. 28 CMU barrier wall system

Pros: Low Initial Cost, Stucco Fills Voids in CMU

Cons: Paint may not Bridge Cracks in Stucco, Relies Upon Paint for Moisture Barrier

The general conditions described previously are similar for each of the sample walls. What is important is matching the wall type to the requirements. In CMU barrier type wall section (see ill. 28), you will see that the example indicates paint over stucco on concrete block. The paint serves as the air barrier and water-reducing membrane, stopping wind and rain to the best of the paint's ability. This is where we frequently see an elastomeric coating specified because of many beneficial properties. Among them is its elastic qualities, to span and bridge small voids or cracks over time. It is relied upon to stop as much air and water penetration as possible. If it rains less than 15” (0.5 meter) per year, and the temperatures are mild, this might suffice. Moreover, if there are no interior finishes and some moisture moving through the wall is acceptable, a barrier system with a breathable paint is fine. There are pros and cons of using non-breathable paints such as epoxy, oil-based enamels, and elastomeric coatings. The selection and application should be based on a careful evaluation of all factors, most important of which are vapor drive and temperature/pressure differentials. We have seen misapplied elastomers cause large blisters on expose exterior wall surfaces as a result of condensation taking place on the backside of the paint. These blisters or bubbles are unsightly and point out the bigger problem-moisture migration and trapped condensation in the wall. We have never seen this condition on a rain screen system, mostly in barrier walls.

If there are more than 20” but less than 40” (0.5 to 1.2 meters) of rain per year, then perhaps the drainage system may suffice. This is well-suited to a cool, dry climate and works adequately if moisture is permitted to drain down and out slowly through the stucco. In humid climates, it may be necessary to place an MRB behind the stucco on the outside face of the CMU. This can be liquid or sheet products (if self-sealing) and typically would have expanded metal lath fastened to the block. The lath reduces cracking, thereby improving the wall's performance over time.

The rain screen version of the CMU wall can work well in either warm or cold, wet climates. The rain screen wall is recommended where rainfall exceeds 40” (1.2 meters) per year or where tropical rains are likely to last longer than it takes to saturate the block. The cavity permits drainage and effectively reduces the wind-driven rain pressure on the concrete block portion of the exterior wall system. If you want to limit moisture transmission, we recommend a sheet or fluid-applied MRB applied to the block. In ill. 29, the exterior finish again is stucco, this time applied on lath over fiber-reinforced sheathing. We typically see this done with an air barrier on the sheathing.

Paper-backed lath does not function as a sealed air barrier unless the laps are sealed, and we have never seen this done successfully.

Paper-backed lath functions as a moisture-reduction barrier and is part of a bond breaker system. More important, it interrupts capillary action through the stucco. It creates a slight but effective drainage plane between sheet materials.

Studies have shown that when not in contact with adjacent materials, stucco or any hygroscopic material does not transmit moisture. Thirty-pound building felts are suitable for use in a wall as an MRB because they are more self-sealing than the lighter 15-pound versions. The void between the lapped felts permits air to aid in drying after rain events.

Ill. 29 CMU rain screen wall system.

Pros: Excellent Performance, Long Term Reduced Wind Pressure on MRB Cavity Acts as Drainage Pathway Cons: More Labor and Material Intensive Higher Initial Cost

Metal stud

The third type of a wall system (see ill. 30) is the metal stud exterior wall system. In the barrier version, you see a section example constructed with stucco on sheathing. As in most barrier systems, the paint is supposed to stop the water. In the drainage version, the stucco is intended to absorb water, and gravity is supposed to convey it down and away. MRB is required between the stucco and the sheathing. The J-bead at the lower limits of each stucco plane is required to let water escape from the stucco. We refer to these specific types of specialized J-beads as weep screeds. They have a series of small holes in them to promote drainage. When applying sealant to a weep screed, always install the sealant bead behind the most remote of the drain holes. If you don't , you will effectively trap moisture behind the sealant (see ill. 31).

Ill. 30 Metal stud barrier wall system.

Pros: Goes up Fast, Thin Walls

Cons: Does not Stop Wind or Water Well Requires More Maintenance

Ill. 31 Apply sealant behind weep holes. Backer Rod Weep Screed Sealant Flashing Paint At the base of the wall in this example you can see the introduction of a piece of closure metal to close off the potential pathway between the bottom of the wall sheathing and the slab edge. In ill. 32, you can see a blow-up of the closure metal taken to a new level. This was designed to combat the pressure-washer situation described earlier. Many public buildings and entertainment or resort facilities have as a part of their routine maintenance procedures weekly cleaning of exposed surfaces with pressure washers. The closure metal indicated in this section has been bent 180 degrees and installed in such a manner that if any water is forced past the bottom of the wall (stucco on J-bead, weep screed), that water will be deflected back down again and not be permitted up into the building or even the wall cavity.

We recommend 304 or 316 series stainless steel, preferably welded at the joints, installed before the J-bead and membrane. If stainless is too expensive, galvanized metal, foil-backed self-adhesive membrane (SAM), or even thick plastic is better than nothing. Notice also that there is no sealant at this joint! We want any water that gets to this point in the wall to be able to drain out.

In the next example, the metal stud wall uses counterflashing behind the sheathing to convey water out and beyond the face of the next-lower sheet. In the barrier system, you are counting on the paint and sealant to stop water at the face. The drainage system uses membrane behind the stucco to stop water from reaching the sheathing. It is designed to drain through capillary action in the cementitious stucco itself. The membrane laps over the counter-flashing. As with all wall types, the cavity wall system has the most reliable performance period and is recommended wherever average annual rainfall exceeds 40” (1.2 meters). In the cavity wall, we build the interior wall and apply sheathing, appropriate membranes, and insulation depending on the climate. In the cold climate, we apply vapor retarder on the inside face to prevent moisture from migrating toward the insulation from the occupied side in the winter. In hot, humid climates, we apply moisture barriers to the outside of the insulation. In the graphic example, we install fiberglass sheathing products to the metal studs and apply vapor barrier membrane to the outer face. Then we create the drainage cavity by applying furring members and an air barrier and the stucco on lath. We prefer nonpaper products as the air barrier. On the outside of traditional three coat stucco, we like elastomeric paint products. An excellent alternative might be white stucco because stopping air and water at the face is not important to the overall performance of the composite wall system.

Ill. 32 Closure metal at wall edge. AIB SAM on Sheathing Metal Flashing Bend Metal Closure Over on Top to Stop Water from Pressure Washer No Sealant here, to Permit Drainage from Membrane, Above Closure Metal Fully Adhered

Ill. 33 Concrete barrier wall.

Pros: High Thermal Mass Simple, Low Labor Cost, Fast Cons: Concrete is Hygroscopic Concrete is a Poor Insulator Relies Upon Paint to Keep Out Water

High Thermal Mass Ill. 34 Concrete drainage wall.

Pros: Retarder Reduces Moisture Reaching Concrete Painted Texture Coat Bridges Small Cracks Cons: Wind Pressure Still Upon Membrane can Lead to Condensation Problems Ill. 35 Concrete rain screen wall.

Pros: Exterior System is Good AIB Cavity Reduces Pressure on Membrane Water has a Pathway to Drain Cons: Thick Wall System, More Expensive

Concrete

The next example is for concrete exterior wall systems, such as formed and poured in place, tilt, or precast. Ills 33 through 35 show three levels of performance, with the best once again employing the drainage cavity. As tilt walls have become more prevalent, resulting from increased labor costs across the board, more and more client groups are choosing them. Many of these clients have no previous experience with this type of construction but make the choice based on initial cost considerations. Unfortunately, the cost information they are using may not be a fair comparison with previous systems. Many clients are choosing the tilt wall based on the price of a barrier system in tilt (see ill. 33) and comparing that with drainage or rain screen material and labor costs in frame or block walls. One can only hope that these decisions are not regretted years from now when the exterior protective coatings need replacing and a big storm event hits the area. In the barrier system in tilt concrete, we often rely on a coating to prevent water intrusion. Many of these are based on elastomers with a binder. Water that may penetrate the coating gets right into the concrete wall.

As long as the intensity and duration of the rain event don't exceed the wall's capacity to hold and store water molecules and , more important still, the molecules don't penetrate all the way through the wall to the inside face (see perm ratings for concrete in section 3) where it's more likely to fuel bacterial growth, this incidental moisture should cause little concern. If, on the other hand, the moisture exceeds the wall's ability to store water and the rain continues, then water will exit the interior face of the wall. These scenarios can change with temperature and pressure differentials as well.

The other important consideration that we feel is often overlooked on all wall systems is the issue of condensation from dew point conditions being reached in the wall. Most clients want the tilt wall to be exposed on the outside face.

They expect durable coatings to be applied to the outside. This means that any MRB needs to be applied under or with the coating. This basically prevents the use of sheet-applied membranes with low permeability. This usually results in the insulation (if any) being placed on the inside of the wall system, the conditioned side. Any moisture that works through the wall must be removed by the building HVAC system, if it's operating.

Tilt concrete in barrier systems is probably much more suitable for warehouses (non-conditioned but well-ventilated or unoccupied spaces) in the hot, humid climates than for classrooms or offices. In the cold climates, tilt concrete panels often get insulated where it's cost-effective and energy-efficient. This is on the cold side or interior face of the wall. In mixed climates, you must consider condensation from vapor drive wherever you place the insulation and MRB. Some concrete panels have been constructed with integral rigid insulation foam between inner and outer concrete layers. This is seen as an improvement in thermal resistance and dew point and could be considered in hot and humid climates. Tilt wall construction can be built in the drainage wall (see ill. 34), and rain screen (also called cavity wall) form (see ill. 35). In the drainage version, a vapor retarder is applied to the outer face of the concrete wall surface. The lower the perm rating, the more moisture is prevented from entering the concrete. The stucco acts as the drainage medium. This might make sense in hot, humid climates if the drainage cavity were located on the outside of concrete panels. In the rain screen scenario, the vapor barrier and insulation would typically be located on the inside face of the cavity in hot humid climates. The rain screen version has the advantage of having wind pressures reduced outside the wall, and keeping the elements off the membrane. It will outperform other versions of concrete walls, and is recommended if average rainfall exceeds 40” per year, and when interior spaces are conditioned and occupied. One way to achieve this would be with rigid insulation such as expanded polystyrene adhered to the outside face of the concrete panel. This would be installed using furring members to space the insulation from the concrete to make the cavity. Air and vapor barrier material would be applied to the outer faces of the EPS board.

Another method would be to use extruded polystyrene and a liquid or sheet membrane. In the drainage wall, an AIB would be installed with lath and stucco.

In the cavity wall, furring would create the airspace depth desired. Stucco could be installed over paper-backed lath or on fiber-impregnated exterior sheathing (preferably with lath). Insulation could also be placed inside the wall sandwich with concrete on both sides. This section works reasonably well in heating or cooling mode and has several other advantages. It protects the soft insulation from impact and allows easy fastening to both inner and outer faces, (see ill. 36).

Obviously, concrete wall systems come in many other shapes and composites than these. We have seen the popularity of foam and concrete walls increase in the past 20 years as energy costs continued to rise. They use hollow foam shapes as forms for site-placed concrete and reinforcing that results in a well-insulated and durable wall system. They perform well in missile impact tests owing to the inherent strength of concrete. Thus it's easy to see that concrete can be used as an exterior structural system with no added insulation, as an internal core in foam composite sandwich construction, or with interior or external insulation depending on the climate. It is truly a versatile product.

Ill. 36 Internal insulated concrete wall.

Pros: Places Insulation Where It Works Well Low Labor Cost Cons: Reduces Structural Integrity, Can Cause Condensation in the Wall

Metal wall systems

Metal panels come in several shapes and forms. These military-, industrial-, or agricultural-based engineered metal buildings have made their way into main stream society. For over a decade in the late twentieth century, metal buildings were one of the most popular commercial building types based on cost and time considerations. The system shown as an example in ill. 37 uses metal-clad composites that are a little more expensive and used more commonly today in commercial construction than the purlin and girt-based ribbed wall panels.

There are many recent examples of metal-clad wall systems winning design awards throughout the world. One of the reasons for this is the clean lines and concealed fasteners available, but the systems go up fast as well. Saved time means saved money.

Ill. 37 Metal panel system, rain screen version.

Metal Skin Membrane Sheathing Metal Framing Interior Finish Weep Hole Sealant Pros: Pre-Finished Durable and Non-Porous Two-Step Application, No Curing Cons: Works Best in Flat Planes, Needs Substrate for Screw Fastening

There are three common types of metal wall panel systems, too. We have indicated three common expressions basically consisting of the dry joint, wet joint, and rain screen systems. The main difference between the dry and wet joint products is the addition of backer rod and sealant used in the wet system.

The dry system lets water get behind the face plane to the bent-metal rout and return fasteners beyond. The wet system is really a barrier system. The sealant makes up a part of the barrier. The rain screen system employs an MRB on top of the sheathing and under the metal composites. This has the best performance against air and water intrusion.

Masonry veneer

The stone or brick on CMU example (see ill. 38 ) is probably the highest-cost example illustrated to this point. It takes more labor and time to build but typically lasts longer and requires less maintenance. It possesses a much higher thermal mass than most other systems. This can be a positive or a negative feature depending on your climate. It requires you to look at thermal (time) lag and moisture transmission as they affect the dew point and condensation. The barrier system depends on the stone veneer face to stop water. This system is suitable for areas with low rainfall, less than 15” (0.5 centimeter). Perhaps if you specified nonpermeable grout and nonporous stone, you could stop rain at the face, but this system is better suited to low rainfall and low humidity.

Ill. 38 Stone veneer on CMU, drainage wall type.

Stone Veneer Membrane CMU Wall Interior Finish Furring, Insul.

Pros: Membrane Helps Reduce Transmission Veneer can be Applied Directly to CMU not as Thick Cons: Membrane in Contact with Masonry, Wind Pressure on Membrane

The drainage system assumes that voids exist in the materials and that moisture can move through mortar that gets saturated and conducts water down and away.

We would recommend an MRB on the face of the backup wall if rainfall exceeds 15” (0.5 meter) per year. More the rainfall, the better the MRB needs to be.

If you get more than 25” (0.6 meter) of rain per year, we would anticipate possibly using a maximum 1.0 perm rating liquid or sheet product and lath behind the scratch coat of stucco. Then mortar the stones to the scratch coat. Again, the rain screen system (see ill. 39) performs better than the others and is recommended in areas where rainfall exceeds 40” (1 meter) per year. For hot, humid climates, we would recommend closed-cell foam insulation on the outside of the block and an MRB outside the insulation. In cold climates, the insulation might be inside the CMU wall. As always, the climatic data for the specific site should dictate what insulation to use and where it needs to go.

Structural brick

Ill. 39 Stone veneer on CMU, rain screen wall type.

Pros: Exterior System is Good AIB Cavity Reduces Pressure on Membrane Water has a Pathway to Drain Cons: Thick Wall System, Labor Intensive, More Expensive

A final opaque wall construction method is the structural brick type of wall system. This system is in a niche of its own and does not fit into the same categories very easily because of the material properties. You may put such a system in between barrier and drainage cavity systems. In order to present this system in an "apples to apples" comparison, we will discuss the brick wall system as fitting into the three wall performance categories, whereas their physical properties pre vent them from performing the same. A single-wythe wall made of structural brick could be thought of as a barrier system because the face of the brick does a fairly good job of stopping liquid water penetration. It is composed of vitreous clay material like face brick and is fired at 2,000ºF for several days in a kiln.

Structural brick are slightly hygroscopic and , as such, could be thought of as a drainage system. Water that gets through the face can work its way into the cores, where it can be drained vertically down and away from the wall. A double-wythe wall made of brick could be considered a cavity wall or rain screen system. All these structural brick examples would have the potential to use the brick as facing material for both interior and exterior surfaces. This precludes the need for paint, sheathing, membranes, furring, and drywall finishes. These systems are available for use with internal insulation to achieve R values around 10 to 11. Coupled with its inherently high thermal lag, structural brick is well suited for use where temperatures fluctuate greatly from day to night and where maintainability is important.

Glazed wall systems

Ill. 40 Show shapes and fasteners.

There are more window system manufacturers today than ever before. This pro motes competition, which leads to advances in products. Many of these advancements are related to the performance of the glass, but a lot has to do with frames, extrusion shapes, and fastening methodologies. Regardless of the manufacturer or shape, there are certain constants that we will discuss that hold true for most systems. Let us take this opportunity to recommend that you draw your window and door details carefully. You should show the extrusion at large scale on the basis of the design. Show all fasteners, where they are likely to be installed, what material the screws go into, the shim space, where the backer rod and sealant go, and where water drains out of the sills and /or covers. Take special effort to show two-piece systems where one piece fastens to the jamb or sill framing, block, concrete, etc.

Storefront and curtain wall systems are not the same. Curtain wall systems can go to greater unsupported lengths and heights and , as such, usually cost more. Curtain wall systems come in a variety of shapes and sizes, some with internal stiffeners (concealed within the extrusion, where they are not visible) and others with visible bracing such as Kawneer's truss-style system. Curtain wall systems range in depth from 4” (10 centimeters) to more than 12” (30 centimeters).

They can easily span more than 30 feet (9 meters) in height. New advancements in technology have allowed manufacturers to offer photovoltaic curtain wall systems that generate electricity and reduce heat gain. Glass is available in many colors and in variety of thicknesses. Storefront systems start at about 2 1/2” (6 centimeters) in depth and span more modest openings. Owing to their differences in heights and resulting loads, their profiles will be correspondingly different.

There is also a difference in their fastening. Most transfer wind loads at the head and sill only. The jamb deflection can exceed 1 1/2” (4 centimeters), which requires special detail considerations.

At exterior perimeter conditions at floor level, both should be detailed with the recessed slab-edge detail discussed earlier. This enables any water that may pass through the sill fasteners or along side the sill or jamb to drain harmlessly out and prevent it from reaching finish floor elevations. If possible, coordinate the pocket depth and height so that the sill flashing extends just past the out ward edge of the concrete recess so that workers can apply sealants from the inside to fully seal the window systems. Metal sill pans can be difficult to install with most curtain walls because the fasteners in sills are around 1/2” in diameter, which can result in more than eight big holes in the sill pan. This is why we recommend the slab-edge recess for curtain wall systems. No sill pan is required at a recessed slab edge, but it's still a great secondary protection.

Curtain wall sills that are not at floor level are more difficult to seal. Most occur at the top of internal red iron structural steel, metal frame box headers, or per haps a concrete bond beam. You must get any water that penetrates the face sealant or that drains down the glass into the coverplate area out past the wall in order to prevent it from getting into the wall. In a rain screen wall, you just have to get that water out past the vapor barrier on the inside of the cavity. This can be done much more easily than getting it to the face of the finish wall surface.

Since these systems typically get shimmed at or near fasteners with short pieces of shim material, they rely on sealant to fill the voids between and around shims. Require the sills to be set in a bed of sealant, and make certain that the drain pathway in the extrusion or sill is not impeded by sealant placement.

Among the many manufacturers' products, there are some important differences. Some of them rely on concealed fasteners (F-clips and T-clips) for their mounting supports (see ill. 41) and are difficult to seal. Grommets, sealant, or SAM patches can be used, but it's difficult to be sure that they are working right, and they are even more difficult to maintain over time. We have found several ways and means for improving the seals at window jambs, sills, and heads. One of the most successful ways we have found to provide long-lasting closure at the jambs is to install bent metal closure strips under the pressure plates. These pieces of metal close off the void between substrates and window systems. If the closure metal is adequately fastened and set in a bed of adhesive sealant, then we have formed a long-lasting seal well behind the finish system (brick in this example, see ill. 42).

Ill. 41 Image of sill pan with holes for fasteners through Clips.

Ill. 42 Curtain wall mockup with metal air and moisture barrier.

Ill. 43 Window jamb and sill image with sealant

One of the companies we frequently see in competitive bidding situations depends on installers to apply sealant to fasteners that protrude through the sill extrusion that can be inspected visually and resealed over time (see ill. 43).

While we are still relying on sealant, we have a better opportunity for quality assurance of completed work than if nothing were done at all. The sealant effectively reduces opportunities for wind-driven, or gravity induced water to move through the extrusion and enter the wall cavities.

Windows in walls

Punched openings and strip windows are treated differently from storefront and curtain wall systems. Concrete-formed sill pans can be cast into tilt or pre cast wall panels to help limit water intrusion. These should be constructed at no or little cost to the owner and provide long-lasting benefits in terms of reducing water intrusion. If well designed and well constructed, these shapes (see ill. 44) function like three-dimensional dams to keep wind-driven water out of the walls when (not if) sealants fail. It would be nice if there were no fasteners through the sill into structure below. Most manufacturers offer fixed and operable windows for punched openings. Many of them can be fastened at the jamb. This allows a sill pan to be used without fastener penetration. Depending on spans, wind loads, and extrusion strengths, there are limits to the width of a window system that does not require fastening at the sill.

If all wind loads were positive, we might be able to avoid holes in our sill pans or precast sills, but we usually design for positive and negative forces on our exterior surfaces.

Ill. 44 Sill pan formed of concrete in tilt wall.

Sill flashings

Ill. 45 Simple sill flashing.

A sill flashing is nothing more than a means to reduce water penetration behind or under a wall, door, or window sill. Wall sill conditions are similar.

In traditional wood frame construction, we see termite shields or similar metal flashings. These served two important functions. First, they acted as a capillary break between floor or foundation and wall systems. Second, they serve to make it more difficult for tunneling insects to penetrate the sole plate for the wall. In concrete floors, especially at grade, we see the use of recessed slab edge pockets discussed previously. This is another form of sill flashing. The depressed concrete forms a sill of sorts for the wall.

Sill flashings take on many shapes and are made of many materials, but they play the same important role. Their job is to make it more difficult for water to get into the occupied spaces, wall, floors, etc. Many window manufacturers provide sill flashings that are designed to be integral to their structural sill components. The simple isometric drawing provided in ill. 45 illustrates the geometry of a window sill flashing. There is a continuous piece of sheet material that extends from under the window to a point where it turns down the face of the exterior wall. There is no vertical face on the rear or sides of sill flashing. The front (exterior) face of the flashing should extend vertically down to provide a way to cover the void beneath the window sill and also to keep water out past the face of the finishes below. Ideally, the horizontal portion of the flashing is sloped slightly toward the front and is not penetrated by fasteners. Only if the window systems transfer all wind loads to the jambs can you avoid fasteners through the sill flashings.

Sill pans

Ill. 46 Sill pan isometric.

A sill pan is an improved version of sill flashing. Sill pans have vertical components on the jamb sides, as well as the rear. A good way to show these in construction plans is an isometric drawing with dimensions for fabrication (see ill. 46). These three-sided vertical flanges keep water from dripping off the sides of the sill flashing if it accumulates. Most sill pans are constructed out of bent metal, and the preferred way of sealing the joints is by welding. Lower-cost alternatives include sealants in the seams or pieces of SAM applied to close them off. There are inherent challenges to sill pans, in that they must be installed behind and under subsequent finishes (see ill. 47).

They must be installed before the windows or doors and perhaps before air or moisture barriers are complete (depending on the wall system). In the ideal world, the rough opening is framed in and sheathed, and then MRB is installed and sealed. This is followed by the AIB installation, and then the welded sill pans are installed. Coordinated with J-beads and other trim pieces, the opening is then stripped in from the bottom up with SAM, making sure that the jamb legs of the sill pan are behind the SAM, (see ill. 48). In this way, any water that gets to the SAM is carried down into the sill pan and causes no harm.

Ill. 47 Sill pan for door coordinated with other materials.

Ill. 48 Sill pan installation in progress.

Sill pans play a very important role in moisture control in areas with heavy rain and snow fall. Sill pans have three different levels of performance over time, and they are closely related to initial cost. The lowest-cost sill pan is formed out of a flexible membrane material that's bent into shape. A piece of plastic sheathing or SAM would work better than nothing. Shower pan material would perform well over time. The second level of performance is achieved by the use of cut metal that's bent to shape. The joints or seams would be sealed with SAM stripping or perhaps a good sealant. These are not as good in the long term as bent metal with welded seams. We recommend minimum 0.05-inch aluminum or stainless steel sill pans. The minimum vertical height of the legs depends on the wind speed to be experienced. We have found that a minimum of 2-inch vertical leg stops most wind-driven water that a sill pan might see in a well-designed and well-constructed building with less than 50 mile per hour winds.

Ill. 49 Sill pan at CMU and stone rain screen wall.

Another key element to the overall performance of a sill pan is proper fit with the window and rough opening. Given the thickness of folded SAM strips and the flashing itself, you have to take those dimensions into account in framing the rough opening. After the sill is installed, it should be checked for positive drainage to the front. A well-designed sill pan slopes down at 1/8” per foot from the front face of the window sill extrusion to the face of the wall and beyond by 1/2” or more. The drip leg bends on kick-out flashing will then minimize staining on the wall below, as well as reducing incidental water behind the wall membrane. Sill pan flashing must be coordinated with brick vents and other relief details at window sills in brick or stone cavity wall systems. See Figs. 49 and 50 for detailed examples of sill conditions for several of the wall types.

For sliding-glass door systems and folding-glass wall systems, the details must be done differently. The point at which water is discharged from the Nanawall system, for example, is about 2” (~5 centimeters) below finish floor. In the example in ill. 51, you can see that the drain point is coordinated with the installation of a hot liquid applied membrane system well below finish floor elevation. In this condominium example, the finished walking surface is brick pavers set about 3/8” (1 centimeter) below the interior floor elevation to get as much of a barrier to water as possible and still be less than the accessibility codes permit.

Ill. 50 Sill pan at wood frame wall.

Since we are still very concerned about wind-driven rain in this detail, we sloped the pavers at about 1/4” per foot (2 percent) away from the opening. In addition, the pavers were set in very porous screenings from a local concrete block plant to maximize the porosity and drainage. Bi-level drains were installed, one at the surface of the pavers and the other at the top of the post-tensioned structural slab. As an added precaution, we placed overflow scuppers through adjacent exterior walls and located them so that they were always at least 2” below finish floor elevations. This required coordination with the drain elevations of the paver-level drains but was seen as the minimum level of protection by the lead designer on the architect's design team. The plumbing engineer was not certain the scuppers were necessary, and the owners tried (in vain) to talk us out of spending the money for eight scuppers, but we felt the obligation to future owners. Sometimes we believe that we have to protect owners or developers in this case, from themselves. They can get so caught up in trying to cut costs that they lose sight of the cost-benefit ratio of critical components. All they had to do was omit a few amenity deck-level plants in pots to pay for the incremental added cost for the scuppers (about $6,000).

Ill. 51 Sill pan detail for sliding wall at pavers.

Threshold over Paver Concrete Pavers Coarse Sand or Screenings Slope Lightweight Conc. Fill Liquid Applied Membrane Tensioning Member Post-Tensioned Slab Rigid Insulation Interior Finish Floor Rear Lip of Sill Pan Door Sill

Jambs

We have talked about sill flashings and pans, so let us discuss the second important location for window and wall moisture intrusion considerations- the jamb. Whereas you often have a barrier to wind-driven moisture in sills, it's not as common in the jamb. An exception to this is the use of fin-type windows. Fin windows (and doors such as sliding-glass doors) are supplied with integral flanges that effectively reduce the path for wind-driven rain along side jamb openings. Fin windows were used commonly in wood frame construction.

Some manufacturers provided them with full corners, whereas others offered the corner covers as an option. These were common in vinyl and aluminum.

They typically would be installed after the AIB and MRB. Many window systems don't offer fin product lines. With non-fin windows, the jambs are more difficult to close off from wind and rain. Jambs come in one- and two-piece systems. In the one-piece systems, exposed fasteners get installed through the extrusion into wood, metal, or concrete alongside the jambs. The rough openings are oversized to accommodate construction tolerances, and shims are placed strategically to tighten the jambs to the structure in proper alignment.

The voids between the window system jambs and the wall openings are never filled completely with shim material. Shims usually take up less than 20 percent of the jamb length. This leaves a large void to be filled in some manner or left open (see ill. 52). In this example, bricks were to be returned back toward the frames, and sealant was expected to fill the voids. Window installers in the 1960s began to use an expanding foam product to better seal the voids. Most of these canned foam products are open-cell foams that are temporary air barriers at best. We don't recommend reliance on foam fillers or sealants for long-term solutions. They have been shown to deteriorate over time. Wood windows may be constructed using wooden jamb and applied trim pieces to create confining pathways for double- or single-hung wood windows. The fasteners typically are exposed, unless they are installed in such a place as to be concealed by another piece of wood, such as casings, moldings, metal tracks, or stops. In wood window detailing, there are often removable closure pieces such as brick mold that effectively seal off the opening between jamb and wall (see ill. 53). Ill. 52 Shim space at window jamb.

Ill. 53 Removable trim pieces.

Removal of the casing can be accomplished by setting the finish nails and prying off the old piece of trim. Whenever you are designing or building opening protectives, you must consider future repair, maintenance, and replacement. A future owner should be able to replace a broken window without removing interior finishes such as wall sheathing or exterior finishes such as brick, block, or stucco. Ideally, screw-type fasteners are used in such situations. Metal or wood trim pieces can be installed so as to allow future access to repair damage.

Sealants should be installed similarly so that it does not have to be removed in order to re-glaze a window. In condominium designs, take extra precautions to seal window against high winds. The detail illustrated in ill. 54 shows a high-rise project's solution to close off the shim spaces at the jamb. It uses redundant sealant applications and a strip of SAM. The sealant selected must be compatible with the SAM, or the sealant will react with the SAM, causing it to degrade.

Ill. 54 Redundant sealant in condominium jambs.

In two-piece aluminum jambs, the outer piece gets fastened into the structure first, and the second piece gets snapped on afterward. In this system, the fasteners are concealed. Different manufacturers have totally different extrusions and dimensions, so it's important to have a basis of design that's well detailed as to dimensions and methods of closure. We recommend drawing the window and door details at a sufficiently large scale that the fastener sizes and depths, as well as what they are to be fastened into, are readily visible. The details also should extend beyond the window to illustrate how the window ties into adjacent wall materials and systems. Show shim spaces, and show membranes and sealants as you want them placed. In this way, you make sure that the structural and waterproofing considerations can be solved in concert.

Ill. 55 Misapplied sealant in rain screen brick wall.

Condensation and Liquid Water would Cause Mold Here Open Pathway Fasteners Backer Rod and Sealant here would Trap Vapors Behind Sealant Without Seal here We would have a Totally Open Path for Vapors

When shop drawings are provided, take care to review attachment materials and methods proposed, and see that they don't deviate substantially from that which was shown on design documents and specifications. Make sure to avoid potential galvanic action between dissimilar materials, such as steel and aluminum.

Look at the particular makeup of the materials submitted, and make sure that you're getting 304 or 316 stainless steel if it's to be in contact with other materials such as aluminum or copper and not a 400 series stainless, which has more reactive characteristics. Another important consideration in jambs is where in the wall section they occur, how far back from the face of wall, etc.

Several wall sections are provided, employing the different types of walls and performance levels. Make sure that there are no breaks in the air barrier, thermal insulation, or moisture-resistant barriers. For example, in ill. 55, you don't want to seal the jamb to the face of the brick in a rain screen wall system if the MRB is on the face of CMU backup masonry wall. The sealant at the face of the brick could be a secondary barrier. The sealant in a cavity or rain screen wall should seal the face of the backup wall to the window extrusion.

This is where the MRB is applied, that's , where (in hot climates) the moisture will be.

Ill. 56 Rain Screen stucco on metal framed wall. Stucco Paper Backed Lath Cavity Vapor Barrier Sheathing Stucco Soffit Drip Screed Sealant Aluminized SAM Window System Wall, Beyond

Heads

In wood frame construction with lap or bevel siding on a barrier wall, we used to install a simple water table at window heads. The function was the same in masonry, where masons placed protruding stones or bricks out past the face of the wall to get water from above to drip out further than the face of the window head. This also helps to minimize the amount of rain striking the head joint and possibly getting into the wall. Head flashings in drainage and cavity walls do the same job. Head flashings should be installed last because the systems are all designed to keep water from getting behind layers below. In a rain screen stucco-on-metal-frame wall (see ill. 56), you can see how wall membranes should be lapped over the head flashing to avoid backwater laps and problems associated with them. Since we have heard of numerous projects where stains were observed long after sealants were applied directly to the bituminous face material on SAM products, we began using aluminized SAM products where in contact with sealants. The cut ends of the aluminized sealants should be behind the point where sealants will be in contact, and we believe you can avoid future stains in this manner. We have been using them in heads, jambs, and sills where in contact with sealants. Since leaking head conditions are not as common as jamb and sill leaks, we will not spend as much time focusing on them. The same issues discussed in terms of jambs apply. We have illustrated other types of head conditions in other common materials (see ill. 57).

Ill. 57 Head detail, stucco on CMU wall.