As explained in section 1, insulation standards have increased dramatically since 1973.The quantities and types of insulation needed to facilitate solar heating of a home are no longer considered unusual or “alternative.” As house construction becomes tighter and insulation standards rise, the danger of causing water damage through condensation increases. and by sealing fresh-air vents to the outside, we risk jeopardizing the indoor air quality. We will need to be very careful not to “over-do a good thing” by completely sealing up a home. Our homes need to have adequate fresh air. Just as overgrazing will cause overheating problems, we can cause air quality and maintenance problems by not providing proper ventilation for the well-insulated and tightly constructed solar home.
WHAT IS VAPOR?
Vapor control is probably one of the most misunderstood principles in home design. In order to properly design a highly insulated solar home, we must first understand how to control vapor. We have all seen water condense on the outside surface of a glass filled with ice water on a hot summer day. The warm, moist summer air is full of water in the form of vapor—a gas. When this warm, moisture—laden air strikes the cold surface of the ice water glass, the water vapor changes from a gas to a liquid, and drops of water appear on the outside surface of the glass. The conditions existed for condensation to occur. These conditions are a combination of temperature, moisture content, and vapor pressure. Similarly, under certain conditions dew will form on the late evening or early morning summer grass, when chilled air makes con tact with warm blades of grass and water vapor condenses to liquid droplets.
Water vapor will migrate toward cooler areas, and without proper use of a well- sealed vapor barrier on the living-space side of the walls, insulation will gradually collect moisture, rendering it eventually useless.
In winter, our homes are full of warm air, which has moisture in it. With the outside temperature being very cold, the conditions for condensation will sometimes occur within the wall and /or roof cavities. If moisture—laden air is allowed to enter the wall or roof cavity, and if it condenses there, the result will be water damage, just as if a leaky roof or burst pipe had flooded an area that is supposed to stay dry. First, this condensing water vapor will ruin the effectiveness of fiberglass insulation, and then it will cause rot and mildew. The irony is that the more insulation that’s placed in the walls and roof, the greater the danger of creating the conditions for condensation within a wall or roof cavity.
WE NEED FRESH AIR
The remedies for such vapor problems are providing good fresh air make-up to the home, and providing positive vapor barriers in the walls and roof. We should maintain the fresh air replenishment of our homes at no less than two-thirds of an air exchange per hour; that is, two- thirds of the entire air volume of your home should be replaced each hour. The ways in which this can be accomplished include the measures enumerated below.
- VAPOR BARRIER
- RIGID EXTERIOR
- INSULATION (TONGUE and GROOVE STYROFOAM
- “BLUE BOARD” OR EQUIVALENT)
- FIBERGLASS BATT
- INSULATION
- PLYWOOD SHEATHING
- 2 x 4 STUD
- CONTINUOUS 6 MIL POLY VAPOR BARRIER
- 3 1/2” FIBERGLASS BATT INSULATION
Two designs for layered wall construction. I recommend the design,
above, utilizing 2 x 4 studs and a continuous layer of 1-inch rigid insulation.
While both walls are R-20, the layering of the 2 x 4 wall with exterior
rigid insulation produces a thermal- break, thereby reducing framing losses and outside-air penetration.
- 6’ FIBERGLASS BATT INSULATION
- 2 x 6 STUD WALL
- 1/2” CDX PLYWOOD
- 1/2” CDX PLYWOOD
- 9” FIBERGLASS
- BAIL INSULATION
- 2 x 12 RAFTERS’
- FASCIA
- TRIM BOARD
- OUTSIDE SIDING
- OVER 1” TONGUE &
- GROOVE STYROFOAM
- “BLUE BOARD” OR
- EQUIVALENT
- 2 x 4 STUD WALL .1/2” CDX PLYWOOD
- ALLOW FREE AIR 1/2” CDX PLYWOOD
- 2 x 12 RAFTERS
- FASCIA
- TRIM BOARD
- CONTINUOUS 6 MIL POLY VAPOR BARRIER
- OUTSIDE SIDING
1. Provide ventilation in all bathrooms. Fans should be vented directly to the outside.
2. Where possible, provide ventilation in the kitchen. Fans that just recirculate and filter kitchen air are not as good as fans that are ducted to the outside
3. Be sure to vent a clothes dryer directly to the outside.
4. Don’t be concerned about the use of a woodstove. It will pull fresh air into your home.
5. When it comes to daily comfort, use your best judgment, and don’t be preoccupied with saving energy to the point that you don’t open windows to allow fresh air into your home if the house feels stuffy or stale.
Let there be no misunderstanding about where the fresh air make up is coming from. The walls and roof of your home should be very tightly constructed as shown in the details below. Fresh air will enter your home through controlled or deliberate openings, as previously described, not through gaps in the insulation or poorly sealed windows and doors. The amount of fresh air intake can be measured by independent testing agencies at a nominal cost. This service is pro vided in some cases at no cost by state agencies. One such testing method is called the “Blower Door Test,” where a fan is installed in an exterior door, and the rest of the house is closed up. By running the fan and measuring the overall volume of air, the number of air changes can be determined. If the rate is too low, you will need to increase the amount of fresh air intentionally introduced, possibly by adding an air—exchange or ventilator system. If the rate is too high, you can re duce infiltration by improving insulation, adding weather-stripping, or sealing gaps around doors and windows.
POSITIVE VAPOR BARRIERS
In heating situations, the rule is that a vapor barrier must be placed toward the heat, in other words on the heated or interior side of the structure. In normal wall and roof construction, the vapor barrier is placed right behind the drywall—between the drywall and the studs. This vapor barrier should be “positive” in the sense of being a discrete membrane, not incidental to the batt insulation, and it should be care fully lapped and sealed. Positive vapor control means the placement of a separate vapor barrier such as the 6-mil “poly” shown in the 2 x 4 stud wall detail below, which shows an R-20 wall and an R-32 roof section. In section 6 we will calculate these R—values (the R 46 / The Passive Solar Home value represents the resistance to heat transfer, therefore the higher the R—value, the less heat this material will transfer).
The preferred wall design shown below is the 2 x 4 stud wall with batt insulation and a layer of Styrofoam outside the exterior wall sheathing. This layering makes a tight wall and provides a continuous layer of rigid insulation outside the 1” plywood sheathing. Layering the wall construction in this manner reduces heat loss which occurs through the framing members (“bridging losses” are heat losses that result from studs transmitting cold directly into the home). It is all but impossible for outside air to penetrate a wall that has been layered in this way, since the seams between pieces of rigid insulation and the seams of the plywood will not coincide.
Although I don’t recommend doing so, the exterior layer of rigid insulation may be eliminated by the substitution of 2 x 6 studs with 6- inch batt insulation; however, with larger studs bridging losses will be more significant, and these additional losses should be considered when the framing lumber is in direct thermal contact between the inside and the outside of the wall unit. Although 2 x 6 framing has become standard, in most cases it isn’t structurally necessary to use 2 x 6s; a wall constructed of 2 x 6s 16 inches on center is probably overbuilt, and the bridging losses will be greater with 2 x 6s and no exterior rigid insulation. The use of an exterior house wrap is important with 2 x 6 wall construction to seal cracks and construction joints. Exterior house wraps (such as Tyvek or Typar) are designed to stop the wind but allow moisture to pass through (so that moisture will not be trapped inside the wall, but can exit to the exterior side). House wraps are not vapor barriers. House wrap is not needed with the 2 x 4 stud wall, since the outside tongue-and-groove Styrofoam serves as both additional insulation and a seal against penetration.
When selecting rigid exterior wall insulation, be sure to purchase closed-cell, extruded polystyrene insulation such as Dow Chemical’s Styrofoam “Blue Board,” or U.S. Gypsum’s Formula R. Less expensive open-celled alternatives are susceptible to insect damage, and degradation in R-value over time.
Note the placement of the roof insulation and the roof venting de tails in the drawing below. An ongoing free flow of air should be maintained from the cave to a continuous ridge vent. This flow of air above the insulation will keep the roof plywood from getting warm, helping to prevent “ice dams.” It will also keep the roof cooler in the summer. In high snow areas, a “cold roof” is often used, in which a separate vented roof is installed above the roof plywood. This design is useful where double protection from moisture and cold is needed. The “cold” roof is added on top of the vented roof construction. The original roof is covered with heavy felt or tarpaper, and the top of the cold roof is typically covered with a metal roof to shed snow. I recently noticed that all new construction at Sun Valley, Idaho, is built this way.
Remember that damp insulation loses its ability to block the loss of heat, and wet insulation is worthless. Pay particular attention to the continuous interior vapor barrier shown in the wall and roof details. Placing unfaced batt fiberglass insulation and then applying a continuous and distinct vapor barrier is a better solution than relying on foil—faced or kraft-faced fiberglass for vapor control. Positive vapor control will stop water vapor from migrating into the wall or roof insulation cavity.
It is not uncommon for a newly constructed and tightly insulated home to have excess moisture content in the air during the first win ter. This is due to the gradual stabilization of moisture content of all the materials used inside the home. As these materials dry, the moisture content of the air will slowly decrease. If there is excess moisture in the air, water vapor will condense on the coldest surface available— the windows. This is entirely predictable in the first few weeks of the first winter. The “cure” is to open a couple of windows and ventilate the home. If, however, this condensation persists, it means that there is a bigger problem, and the source of the excess moisture should be investigated.
A client once called me, sure that his ski house was “self destructing.” A 1" layer of ice had formed over some of the window surfaces, and water was dripping off the windows. The temperature was about 10 degrees outside. Upon inspection, a dryer vent was found to be venting to the inside of the house. As his clothes were dried, moisture was being pumped into the home. Since the home was properly constructed and had positive vapor control, the water vapor had only one place to go—the windows. The homeowner was instructed to “crank up” his woodstove and open a second floor window at each gable end to let the house vent. In a matter of hours, the house began to stabilize.
Another homeowner installed the batt insulation in the ceiling of his home but had never managed to install the vapor barrier. He was living in and finishing the construction of his home at the same time. After six weeks of the heating season, the ceiling insulation was completely saturated with moisture, rendering it useless. All of this soggy fiberglass had to be removed and replaced; this time he installed fiberglass insulation properly protected with a positive vapor barrier.
An old-time Vermont builder once told of installing a board ceiling in the second floor of a new home. Since he didn’t believe in vapor barriers, the insulation was placed with no vapor barrier between it and the square-edged board ceiling. Halfway through the first winter, the boards were all water stained. Both the boards and the insulation had to replaced at his expense. His rationale had been that he always used batt ceiling insulation with no vapor barrier on top of drywall so that the ceiling could “breathe.” Inadvertently, he was creating a dry wall vapor barrier. Drywall with two coats of latex paint makes a fairly effective vapor barrier; however, when square-edged boards were substituted for the drywall, moisture traveled through the joints, and the dew point was reached within the ceiling insulation layer, causing the water problem.
Solar Principle # 4
Insulate thoroughly and use well-sealed vapor barriers.
Build tightly constructed, properly insulated walls and roofs. Carefully install and seal discrete vapor barriers on the living-space side of walls, ceilings, and /or roofs. Incorporate an air-lock entrance.
R-VALUES
R-values have been mentioned several times. In order to specify the correct insulation levels for a passive solar home, we will need to understand what R—values are, how they are calculated, and how you can use the information derived from the calculations. All materials transfer heat at different rates, and R-value is the measure of the resistance of a given material to the transfer of heat. As explained in section 3, concrete transfers heat at a rapid rate, while wool sweaters with air trapped in their weave transfer heat more slowly. Appendix 3 shows a list of R-values for various materials.
“U-values” are the inverse or reciprocal of R-values. U-values are expressed in BTUs per hour per square foot per degree Fahrenheit. Btu stands for British thermal unit, and is the amount of heat necessary to raise the temperature of one pound of water one degree Fahrenheit (in this book, for the sake of clarity, we will use the nomenclature “BTUs” in text and equations when referring to these units in plural; true ASHRAE aficionados will note this departure from engineers’ normal practice).
The heat loss of a home is calculated by first determining the U-values for the walls, windows, and roof. Individual heat losses for specific areas are determined by multiplying square feet of surface area by the U-value. Then a calculation is made of the amount of energy needed to reheat the fresh air that is coming into and escaping from the building during each hour. The total of these losses represents the total theoretical loss of the building. This kind of calculation will be demonstrated and explained in section 6.
NIGHTTIME WINDOW INSULATION
Notice in the floor plans shown in sections 5 and 8 that thermo-shutters are shown on some of the windows and patio doors. In the three bedroom plan, the thermo-shutters are used on three south—facing patio doors as well as one window each in the east-facing dining/family room and west-facing master bedroom, on the first floor, and on two windows in the east- and west-facing bedrooms on the second floor. The combined area of these windows represents a total of 203 square feet of glass.
Insulating the windows and patio doors at night (especially the largest windows) will measurably improve their performance as solar collectors. A single pane of glass has an R-value of 1, meaning that single- glazed glass is essentially only keeping the wind out! The window companies have now developed better-insulated glass. High-performance glazing has selective coatings on various surfaces of the sheets of glass, and the air between the sheets of glass is replaced by gasses that are more effective insulators. and yet, although high—performance glass is better than ordinary glass, the R—value of even dual—pane glass pales when compared to an R-21 .36 wall. Remember, insulated dual- pane glass has an R-value of 1.92, whereas the wall is 21.36/1.92, or 11 times better. While architecturally attractive glass makes an excel lent solar collector while the sun is out, the winter nights are long and cold, turning windows and patio doors into thermal losers at night.
Thermo-shutter design for nighttime window insulation.
Thermo-shutter design for insulating typical 6-foot patio doors.
PLYWOOD; DOUBLE-FOIL-FACED URETHANE; STOP (ON SIDES and TOP); SURFACE-MOUNTED
4” “H” .HINGE (MUST FACE OUTSIDE); THERMO-SHUTTER; 1/4” V-GROOVE PINE PLYWOOD;
1” DOUBLE-FOIL-FACED URETHANE
In addition to transmitting heat out of the home’s airspace through thin panes of glass, uninsulated windows actually draw heat out of you. Have you ever noticed that it seems much colder to sit next to a patio door at night versus sitting next to a nicely insulated wall?
There’s more bad news. Warm air from the room will be drawn toward the glass, and as this warm air is cooled by the colder glass surface, it flows toward the floor, allowing more warm air to be drawn to the cooling glass. This is the same kind of reverse thermosiphoning effect that can take place at night with the Trombe wall described in section 2.
Most heating system designers locate heat grilles in front of windows and patio doors to provide a “bath” of warm air across the glass surface. This increases the inside surface temperature of the glass, which increases the temperature difference across the glass, which in turn increases the heat loss of the glass. One error compounds another, and so on.
As you have probably deduced, the solution to this problem is to add nighttime insulation to the windows. The illustrations on above show thermo-shutter details for a typical six-foot patio door and six-foot-wide window grouping. Note that the interior insulation of the thermo-shutter is 1 inch of foil—faced urethane. The interior foil face will reflect heat back into the room, even though it is sealed inside the thermo-shutter. With the thermo—shutter closed you may now comfortably sit next to a patio door on a cold night. The thermo shutter is providing added insulation as well as reflecting heat back into the room. The stop shown on the details allows the thermo-shutters to fit tightly, which eliminates reverse thermosiphoning at night.
The photograph below shows how the thermo-shutters may be deco rated with fabric, which may be changed seasonally. Construction of thermo-shutters takes the skill of a qualified finish carpenter. You could hire a cabinet shop to make them.
Thermo-shutters have a year-round benefit, as they may also be closed to keep out the sun. They are most beneficial in summertime on east- and west-facing windows since the sun enters more directly into the living space in the morning and afternoon. The outside foil face of the insulation contained within the wood veneers will reflect the sun’s summer heat back out the window.
Other Options for Window Insulation
There are commercial products made of fabric on the market that can be used to add insulation to windows and patio doors. Make sure that any product bought for this purpose provides both added insulation and a tight fit along the top or bottom edge (ideally both) to stop the nighttime reverse air flow.
Detail of thermo-shutter showing use of curtains as decorative finish.
When in the open position, the folded-back shutters are no more obtrusive
than curtains along the sides of a window or glass door. Likewise, when
closed, the window will appear to be covered by curtains, yet the insulation
value of the layered shutter is far superior to that of curtains alone.
Next: Basic Layouts and Floor Plans