Building Envelope Design: Schematic Design

Going back to the traditional delivery system, the designer now begins the schematic design (SD) phase, wherein the overall building areas and volumes are established. Different client groups have varying requirements for the SD phase, but the intent of SDs is to give the client an idea of where the architect is heading with the building. Sometimes designers will provide several different SDs for review and discussion before narrowing it down to one. In many projects, mechanical and electrical systems, as well as structural systems, are then introduced over the architectural framework. Throughout SD phase, decisions are made as to the type of systems being planned. This permits coordination to begin taking place to make sure that all the systems have adequate clearances, etc. Another benefit is the opportunity for material takeoffs in a gross sense. SDs are typically the first opportunity for anyone to compare the estimated cost of the planned project with the target budget established earlier. These pricing checks are typically rough order of magnitude (ROM) checks and price systems based on previous experience with similar systems, etc. They are not generally used as absolute facts but rather as general guides. Depending on how complete the SDs are and the expertise of those estimating, the SD price check usually can be accurate to about 2-10 percent of the cost when complete. The most useful estimates are projected to project start dates so that inflation is factored in in such a way that the owner can clearly see the contributing factors, the rates used, etc. SD estimates can let designers know if they are moving in the right direction, if some decisions resulted in cost increases, and so forth. Budgetary control is important to prevent envelope failure that can result from cost-cutting. All too often the performance of the building's skin is compromised because of a budget shortfall. Architects can play an important role in preventing this from happening and should do so to limit their liability.

Avoiding future problems

Architects and engineers typically make decisions in the SD phase that affect performance of the completed building. These decisions include systems-level selections that affect the cost as well. The best way to cut costs if an initial estimate comes in high is by reducing square footage. If this is not acceptable, there are several other ways that don't result in a poor envelope. The worst thing is to confuse cost-cutting with value engineering. In value engineering (VE), decisions are made that result in nearly equal performance at a lower cost. We have seen VE result in millions of dollars in savings and actually improve performance over the initial design. An example is the change from driven piles to a mat foundation system that was designed for the National High Magnetic Field Laboratory. The estimated cost for the 140 driven piles was more than $840,000, more than the cost of using a 3-foot-thick concrete mat foundation. The mat also served to reduce vibration transmission, which resulted in better test results in the laboratory experiments. We did not have to use up the owner's contingency budget or reduce the building envelope performance to solve the crisis.

Another building envelope VE consideration could include overflow scuppers in lieu of piped internal overflow drains. There is a resulting lower plumbing cost, as well as a more reliable drainage system in using scuppers if the building height and style allow them. We have seen VE effectively improve creature comfort and dehumidification in office complexes while returning savings to the owners. We also have seen occasions when cost-cutting was employed, thinly disguised and referred to as VE, resulting in terrible lawsuits and poor performance. Cost-cutting should be honest. It can be perfectly all right to accept a lower-cost substitute if the performance is still acceptable to the designer, builder, and owner. Other common costs that are cut are square foot costs of material and labor, accepting a lower-cost carpet or tile, for example.

This isn't true VE because the value is not comparable.

What hasn't been stated is the obvious issue of getting a lower price for the same thing. Perhaps there are other potential suppliers with better buying power or somebody who is willing to accept a lower profit margin. If the con tractor can save time, he or she might be willing to accept a higher-cost material if he or she can get it faster or install it quicker. Before proceeding to design development (DD), the designer and client should insist on a detailed and accurate estimate. If it exceeds the target budget, scope cuts should be made to get it back to budget, or the owner may have to commit some of his or her contingency funds. It is not recommended that the design moves forward without balancing the budget. An alternative means for balancing the budget might be to identify certain scope items that can be deducted at SD phase in the hope of being able to add them back in as design and pricing are more refined. What one must try to prevent is getting forced into cost-cutting on the building envelope, lower-cost windows, cheaper air-conditioning systems, reduced insulation thickness or average R value, omitted membranes, sill pan omission, or fewer plies of membrane on the roof. Any of these can result in reduced envelope performance and potential future intrusion of moisture or water-related problems.

By making adjustments at the end of SDs, the project team should be able to complete the design through every future phase without risk of a poorly per forming building envelope.

Liquid water

Liquid water causes the great majority of mold and mildew problems and the largest volume of claims for damages. We have read numbers ranging anywhere from 65% up to more than 85 percent of all damage is caused by water in the liquid state. This is why most of our efforts in developing details for buildings are located at the skin. Beginning with the roof and other relatively flat surfaces, such as balconies or amenity decks, we develop wall sections and details to con vey water down and away from occupied spaces. At the intersections of roof planes and wall surfaces, we supply enlarged details, providing guidance for each building trade to follow. On walls, we have multiple sheets with plan and section sketches for preventing water intrusion, especially at openings. Window and door openings, louvers, grilles, etc. are the most important holes in the wall where water can be introduced. Then we go on down to the wall-to-floor intersection and floor systems. Each of these conditions has challenges. Each has potential problems that must be addressed carefully and successfully for the building envelope to perform well in normal rain events. We have to step it up a few notches and do extremely well to protect against extreme weather events such as hurricanes and high-wind-speed rain events.

Water normally is expected in the form of rain. Rain typically falls down from the sky in a more or less vertical direction. Traditional design and construction methods perform fairly well in these conditions. Overhangs were introduced thou sands of years ago to keep rain from coming in open windows and to shade walls.

As buildings got taller and materials became more expensive, fewer windows and doors were protected by overhanging roof planes (overhangs). With more windows and doors exposed to the elements, more leaks occurred. When the rain (or snow) gets blown by high winds, gravity is overcome, and the particles come from every direction. Some rain intensities can exceed design values. Wind driven rain can build up against vertical planes such as walls or roof parapets, causing unique situations that may not have been planned for. Some maintenance procedures cause water to be blown at high velocity and pressures at buildings from every conceivable angle. How can we prevent problems from all these scenarios? Our principles are based on three simple words: gravity, geometry, and technology. We will show you our proven methods for first selecting the right kind of wall for your project and then executing appropriate detailing methods and following them up as the project gets built.

Water has many different ways to move into buildings. The two most common forces moving water are gravity and air. We have already briefly discussed rain fall and water moved by gravity. Whether it's one drop or 2 feet of accumulated water on the roof, gravity is the most constant motive force. We stop it with sloped roofing products that rely on gravity to move the water off the roof.

Airborne water

Let us now discuss the second motive force-air. Airborne water can be moving in any direction. Typically, it moves at less than 45 degrees from vertical.

Rain falling at an angle can strike every building surface. Most buildings are designed to convey water down and away from the occupied spaces. The smallest holes in the building envelope can permit gallons of water per hour to enter the envelope. Unfortunately, if those holes occur where horizontal surfaces meet vertical ones, as they typically do, the depth of water can build up, and water pressure can increase, allowing even more water in. We therefore must pay special attention to these conditions.

The best way to prevent airborne water is to stop the air. There are many ways to do this, but the most successful methods are based on a relatively airtight building envelope. We use materials that air can't move through as an air barrier. Metal and glass are very good; concrete block and wood are not as good.

We frequently use a combination of products depending on the condition.

Building paper and asphalt-coated building sheets or felts have been used for centuries to reduce moisture and air infiltration.

Water vapor

Water vapor is not so different from airborne water droplets except for size.

When the air is warm and dry enough, water stays in the gaseous state. As the air cools, the vapor condenses. The more it condenses, the closer packed the vapor molecules become, until they join one another and form larger molecules. If the temperature is below freezing, it forms solid water (snow or ice).

If the temperature is above freezing, it makes drops of liquid water. This takes place in the walls, just like in clouds. When water freezes in a wall (or any other cavity), it expands. This can result in voids in the building envelope.

When the ice melts these voids remain which can create pathways for water to enter the cavity. Water vapor can move through solids and air (gas). Vaporous water molecules are so small and light that they can be moved easily. Vapors can move even if no wind is present. They move as a result of temperature differential, pressure gradients, or capillary action. This is referred to as vapor drive.

Even if there is positive pressure in a building, vapor may be drawn in just by temperature. Energy moves from cool to warm, so if the vapor is cooler than the inside air temperature, it will move inward. It is always trying to reach equilibrium state. Just like water on the roof, it has potential energy based on the height above the ground. It wants to reach the relaxed state with no potential.

Water vapors are more difficult to stop than liquid or airborne water because of the size of the molecules. There are many materials that resist the transmission of liquid water but allow water vapor to move through easily. Materials that permit moisture to move through them are called hygroscopic. The more easily moisture moves through them, the more permeable they are. Permeability is measured in perms. A perm is a unit of measure equal to the amount of water in grams that moves through a material at a certain temperature and pressure.

Glass and plastic have a perm rating of about zero. Construction materials that have ratings lower than 1 (1.0 grams) are referred to as moisture-reduction barriers, or vapor barriers. Table below lists the perm ratings of common materials.

===

Table Perm ratings of common material: Number Product Thickness Rating 1 Aluminum Foil 1 mil. 0 2 Built-up roofing (Hot mopped) 2 ply 0 3 Polyethylene Film 6 mil. 0.06 4 Self-Adhesive Membrane 0.03 0.08 5 Fluid Applied Elastomeric Topping 36 mil. 1.08 6 Urethane Foam Roofing 30 mil. 2.9 7 Acrylic Elastomer Coatings 10 mil. 8.7 8 Varnish on wood 6 mil. 0.4 9 Enamels on Plaster and Stucco 8 mil. 0.75 10 Latex Paints 8 mil. 5 11 Hot-Melt Asphalt 3 oz/s. f. 0.1 12 Kraft Paper na 0.4 13 15 pound asphaltic felt na 6 14 Roll Roofing 0.03 0.25 15 Extruded Poly Styrene board per inch 1.2 16 Expanded Poly-Styrene board na 3.75 17 GFRP , glass reinforced polyester board 48 mil. 0.05 18 Plywood, fir, exterior glue 1/2" 0.4 19 Plywood, fir, interior glue 1/2" 0.95 20 Gypsum Board 1/2"38 21 Plaster on Lath 3/4"15 22 Concrete Block 8" Nom. 2.4 23 Concrete per inch 3.2 24 Brick 4" Nom. 0.95 Averaged from several manufacturers' product data sheets along with architectural graphic standard ().

===

Condensation

Condensation is the last of the water-related building envelope issues we are going to address. Condensation as it relates to our built environment takes place when air cools to the point where, much like it does in the clouds, water vapor within it changes state from gas to liquid. Water droplets are formed on cool surfaces or on the face of membranes. This can be the inside painted face of a wall system in the winter or the cold side of insulation. Condensation can form on air conditioning ducts, grilles, windows, or wherever the temperature is below the dew point. This water can lead to mold growth, just like any other form of moisture. This is why designers must calculate anticipated conditions in buildings and provide adequate insulation, the right membranes, and temperature-control devices. In some conditions, you only need to reduce the temperature of the air or a surface a few degrees to reach the dew point. The more moisture there is in the air to start with, the greater is the volume of water that will be formed by condensation. This is why we try to reduce moisture moving through walls, as well as reduce the humidity of the air in the space. Rooms in which the building heat ing, ventilation, and air-conditioning (HVAC) system supplies conditioned air that maintains less than 50% relative humidity (RH) in the space have been shown to prevent mold growth as compared with rooms that have gone above 60 percent RH for extended periods of time. Keeping rooms relatively dry helps to draw moisture out of the walls and ceiling cavities. This reduces conditions that support mold growth in the walls.

Early in the design process, we must establish the parameters for operation and calculate how to achieve them. Minimal insulation thicknesses need to be established in conjunction with mechanical system sizing and air distribution schemes. Window leakage rates, U values, and emittance are determined.

Methods of ventilation, building pressurization, and exhausting are decided.

Once these essential decisions are made, design can proceed to the next stage.