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When we make a house increasingly energy efficient with conventional materials, we reach a point at which the house can be called “super-insulated” — it's far more energy efficient than an ordinary house. Much of what we think of as super-insulating techniques are simply commonsense building methods carried out with an unusual degree of care and attention to detail. We suggest that you take a long look at both super-insulating and passive solar building systems, and use whatever techniques from both approaches that seem appropriate to your situation.
There is a rivalry of sorts between the proponents of super-insulation and those of passive solar construction, with each side trying to one-up the performance of the other. it's fair to say that each probably has something to learn from the other. While passive solar designers have tried to tune their designs to the variations of climate, super-insulation designers have developed a single universal solution for all climates. Both systems are constantly being refined, and the dialogue between them will ultimately help to perfect each.
All super-insulated homes have certain characteristics in common; they can generally be described as follows:
1. Very high levels of insulation are used throughout the building, often requiring thickened walls and other unusual building details.
2. There is less south-facing glass than for passive solar houses. Direct-gain passive solar is considered not crucial to overall building performance.
3. The combined window area for north, east, and west is 50 % that for south.
Michael Scott, an energy economist in Minnesota, recommends that the total glass area equal 12 % that of the total floor area.
4. There is no more thermal mass than in conventional construction.
5. The building is virtually airtight. This may require a mechanical ventilation system to prevent the buildup of high levels of indoor pollutants and humidity.
6. The small air change per hour (ACH) achieved in these houses is made possible by a continuous air/vapor barrier. If a plastic sheet is used for this barrier, it must be carefully protected during and after construction.
7. The super-insulated house requires careful workmanship.
8. If all of the above conditions are met, a small direct-vented heater may be the only heating system required.
9. Although these houses are initially more expensive than a conventional house, the extra costs are recovered quickly because of the ongoing fuel savings.
10. The house stays cool in summer if windows are opened at night.
The first thing most people think of when they think of lowering energy requirements for a building is increasing insulation levels. We’ve found it more useful to think in terms of an integration of a number of elements, all of which must be present and functional in order for the building to perform properly. These features include adequate levels of insulation, an airtight building shell, a controlled ventilation system, a properly sized and designed heating system, and correctly oriented, good-quality windows.
Insulating materials have a high degree of resistance to heat transmission. The relative resistance of a material is expressed numerically as its “R-value,” with higher numbers indicating greater resistance to heat flow. Common insulations such as fiberglass, cellulose, and plastic foam insulations usually have R-values ranging from about 3.0 to around 7.0 per inch.
Superinsulated homes have wall R-values as high as 40 and roofs insulated to as much as R-65. To give you some perspective, in many areas local codes only require R-11 walls and R-19 roofs. The insulation levels you choose will depend upon what’s available in your area, the degree of energy efficiency you’re trying to achieve, the local availability and cost of heating fuels, the economic payback, and personal considerations. We will discuss the types of insulation currently available in a following section.
ill. 4-4: This map shows the northern area of the United States where super-insulation may be most beneficial.
ill. 4-5: Air leakage in a typical house.
Reducing Air Leakage
In most conventional houses, air leakage is the single largest component of heat loss. Factors that influence air leakage include wind speed, the surface area of the house, the temperature difference between the inside and the outside of the house, and the area of cracks and other openings in the exterior surfaces. In conventional homes, it's not uncommon for the entire volume of air in the home to be replaced by outside air .5 to 3 times per hour (.5 to 3 ACH).
Strict quality control is essential to effectively reduce air leakage. One of the most common strategies is to carefully install an air/vapor barrier on the warm side of the wall (under the finish surface) to prevent indoor air and moisture from escaping. Historically, at least 6-mil (a mill is 1/1000 of an inch) polyethylene sheeting was used for this purpose, but any material that resists moisture flow will work. The relative resistance to moisture flow is referred to as “permeability” or “permeance” and is expressed in perms. The higher the perm number, the more permeable the material. Any material with a permeability of 1 perm or less is considered an adequate air/vapor barrier.
Whether you use polyethylene sheeting or one of the new cross-laminated air/vapor barriers, proper installation involves sealing every joint with a flexible sealant, usually acoustical sealant. Where splices must occur, overlap the sheeting a distance at least equal to the space between two studs, rafters, or joists. Seal carefully with acoustical sealant and staple the sheeting to the studs, rafters, or joists. Then the finish surface (such as drywall) can be installed.
Some kinds of insulation, notably sprayed-in-place polyurethane foam and cellulose sprayed in place with a binder, form their own air/vapor barrier, if properly installed.
Generally, the air/vapor sheeting can be eliminated if you plan to use one of these insulation materials.
In addition to the air/vapor barrier installed on the warm side of the wall, many builders install an air barrier on the outside of the wall, over the sheathing and under the siding, as extra protection against air leakage. This material should have high permeability, so that it will readily allow any moisture that might get trapped in the wall to escape to the outside. Products such as DuPont’s TSvek and Parsec’s Airtight White are used for this purpose. The choice of wall sheathing also can reduce leakage. Butt joints between sheathing panels can let outdoor air leak into the house. Insulative sheathing having overlapping joints inhibits this leakage yet permits the escape of indoor vapor that gets past the inside vapor barrier.
ill. 4-6: Close-up of an energy-conserving sill. Gypsum drywall; Poly vapor barrier; Batt insulation; Subfloor; 2 X 6 base plate; Caulk in all cracks; Floor joist; Rigid insulation; Header joist; Flashing; Siding; Foam sill sealer; 2” extruded polystyrene.
In frame houses, there are certain points in the shell that are difficult to seal. Fiberglass or foam sill sealer placed between the sill plate and the foundation wall is an inexpensive way to prevent air leakage at this joint. Caulk or foam gaskets used at joints in the framing will also increase the air-tightness of the structure.
Polyurethane foam caulking should be used at all penetrations, including electrical and plumbing penetrations. Take special care when applying foam around door and window framing—if too much foam is sprayed into the opening between the framing and the jamb, the expansion of the foam can distort the jamb, making the door or window difficult or impossible to operate.
Doors and windows should be weatherstripped. With the relatively small cost of an additional door, a double-door “air-lock” entry can be built to reduce leakage. Sliding doors are leaky and should be avoided; several companies now manufacture swinging “atrium” doors that have the appearance of the old sliders, but are much more energy conserving. Windows in general should be the swing-out type (casement or awning), because their locking mechanism seals the weather-stripping between the sash and the frame. Sliding type windows (sash and sliders) should be avoided unless their measured leakage rates equal those of casement and awning windows.
Other spots prone to air leakage include the places where interior partitions meet outside walls and ceilings. At these spots, pieces of vapor barrier should be secured before the partition is erected, and later the pieces should be lapped into the full wall vapor barrier. Exterior corners can be caulked (sheathing to framing) or taped.
Joist headers present a particular problem in terms of keeping the air/vapor barrier continuous. Some builders terminate the air/vapor barrier at the joists and install pieces of rigid foam insulation with a low permeability in each cavity between the joists. Another approach is to run the air/vapor barrier around the joist header and over the face of the floor below. it's critical that the joist header be set in from the edge of the wall and insulated on the outside, since the air/vapor barrier must always be on the warm side of the insulation.
Superinsulated Stud Walls
The following are some specific wall details that have been used in super-insulated homes. Whether or not you decide to super-insulate, the techniques shown here should be useful to help you think in terms of making your home as energy efficient as is feasible, 2 X 6 wall with foam sheathing: 2 X 6 construction is used with R-19 fiberglass batts and a foam insulative sheathing. The foam could be urethane, isocyanurate, expanded polystyrene (EPS or beadboard), or extruded polystyrene. The insulating sheathing covers the entire exterior of the wall, including the rim joist.
Variations on this wall include the “Styro House,” which features 2 x 4 frame construction with R-11 fiberglass insulation and 5.25 inches of EPS on the exterior. The air/vapor barrier is installed on the outside of the sheathing on the frame wall and the EPS is applied over that. it's safe to install an air/vapor barrier inside a wall as long as two- thirds of the R-value is on the outside so that the air/vapor barrier stays warm.
• The strapped wall: This is similar to the 2 x 6 wall described above, except that the wall is furred out with 2 x 3s applied horizontally to the interior of the studs. The air/ vapor barrier is installed over the studs and under the 2 X 2 strapping. The space created by the strapping can be used as a wire chase, eliminating the need to penetrate the air/vapor barrier for electrical boxes. The space can also be filled with insulation.
ill. 4-7: Strapped wall super-insulation construction.
• The double wall: This wall is actually two walls with a space between them. The air/vapor barrier is placed on the exterior of the inside wall, and the interior wall, the exterior wall, and the space between them are all filled with insulation. This allows an R value of 45 and creates a 3.5-inch cavity for plumbers and electricians to work in without violating the air/vapor barrier. The inner wall is the bearing wall (it carries the structural load), so the exterior wall needn’t be designed to carry any overhead weight and it can be built without headers.
• Staggered stud wall: A wall can be made extra wide through the use of studs staggered on wide plates: Some studs stand along the inner edge of the plate to support the interior finish surface, while other studs stand along the exterior edge for the exterior surface. Gene Leger from southern New Hampshire uses staggered 2 X 3s on 2 X 6 plates, and he fills the walls with sprayed foam. In this design, no vapor barrier is necessary since the foam acts as one. Sprayed cellulose also works well in this situation.
• Airtight drywall approach: This system uses the drywall interior finish of the house, applied carefully with foam gaskets located in the framing and between the drywall and the framing to create airtight seals. Most moisture that gets into walls is carried there through cracks and other openings. By carefully eliminating all such openings in the drywall surface and applying a good two-coat paint job, you should be able to keep both moisture and air from leaking into the wall. If you want the extra protection, choose a paint marketed as a vapor barrier.
• Foam-core panels: Foam-core panels were originally developed for use in the refrigeration industry. Consisting of a core of foam insulation (generally polystyrene or urethane) sandwiched between two layers of sheathing, they can be used in timber frame construction: The panels are attached to the outside surfaces of the posts and beams with construction adhesive and large nails. Urethane has a higher R-value per inch than polystyrene, and it acts as a vapor barrier, with a perm rating of about 1 for a 3½-inch core. it's also, however, much more expensive. Since polystyrene doesn’t act as a vapor barrier, you may want to paint the interior surface with a vapor barrier paint.
Wiring these walls can be handled a number of ways. Some panels come prewired; others have a chase or thin-wall conduit in the foam under the drywall that wires are fished through. We’ve also seen some attractive baseboards designed to accommodate wiring, and Wiremold makes a surface-mounted wiring product for applications like these.
ill. 4-8: Double-wall super-insulation construction.