A well insulated building envelope is the fundamental prerequisite for an energy efficent house. A "tight" home uses dramatically less energy to keep it comfortable in any season. Insulation reduces the flow of heat into or out of a home by trapping air in loose-fill fibers or particles (glass, cellulose, mineral wool, cotton), sprayed in place foams, or rigid board products.

Weather barriers form a secondary drainage plane (the first in most cases being the exterior cladding) to assist in keeping assembly components to the interior protected from bulk water. Note that the vapor permeability of concealed weather barrier products can vary widely and the desirability of low or high or no vapor permeability is always in the context of the primary direction of wetting, the primary direction of drying, and the vapor permeability of all the other components in the assembly.

Board insulation can be made up of glass fibers, mineral wool or rigid foam. Unlike most batt or blown-in insulation, rigid boards can be applied across the surface of walls, roofs, or foundations, reducing thermal bridging through the structure. Foam insulation products are all petroleum-derived with most foams require a blowing agent to create the foaming action. Only a few specialized insulation materials in the U.S. are currently produced without ozone depleting HCFCs.

Loose-fill insulations can include fiberglass, mineral wool or cellulose, but we prefer Cellulose insulation which has several environmental advantages. Cellulose insulation contains 75-80% recycled newspaper and nontoxic borate and/or ammonium sulfate fire retardants. The energy performance is comparable to high-density fiberglass batts at roughly R-3.7 per inch, but cellulose insulation generally packs more tightly so is more effective at controlling air leakage. Health concerns with fiberglass have not been substantiated, but it is generally a good idea to install loose-fill fiberglass only if the fibers can be prevented from getting into occupied space or air distribution systems.

Denim Cotton batt insulation is manufactured from post-industrial denim and cotton fibers. The product is available in R-13 (3.5 inch thickness) or R-19 (5.5 inch thickness) unfaced batts. It is treated with borate for pest and fire resistance.

In vented roof and attic assemblies, insulation baffles ensure that a properly sized cavity is maintained between the top of the insulation and the roof decking. Proper ventilation in attics and roofs is essential to maintaining insulation performance unless other measures have been taken to prevent heat-transfer moisture problems. Insulation performance suffers when moisture, trapped under the roof sheathing, condenses on the insulation below, and escaping heat can cause ice-damming. The highly conductive nature of water renders wet insulation much less effective and wet insulation can also result in rotting wood framing members and mold growth. Insulation baffles can be made from corrugated cardboard with sizing, foam, or plastic.

Polyurethane spray foams are inert cellular insulation and waterproofing products. With foamed-in-place insulation it is relatively easy (though more expensive) to fill wall and ceiling cavities completely, providing high R-values (3.6 to 6.5 per inch) and blocking air leakage very effectively. Installation requires special equipment and must be done by licensed contractors. Most foamed-in-place insulation products are high-density (2 lbs. per cubic foot) closed-cell polyurethanes. Open-cell, low-density polyurethane foams have been produced with water or carbon dioxide as the blowing agent. Compared with closed-cell polyurethane, open-cell products use significantly less material, making them attractive from a resource standpoint, but also lower in R-value per inch. Some of the low-density foam products are made in part from bio-based raw materials in place of petrochemicals.

When radiant barriers face a heat source, they work by reflecting heat. When faced away from a heat source, radiant barriers function primarily by virtue of their low emissivity, which reduces the amount of heat that radiates from them. Radiant barrier products can be foil-faced kraft paper, foil-faced polyethylene film, foil facings on rigid insulation or wood-fiber sheathing, or aluminized paints. If the radiant surface is touching another material it won't work--an air space is required on at least one side of a radiant barrier in order for it to function as designed. Radiant barriers in attics are most beneficial in reducing cooling loads; their effectiveness in reducing heating loads is more limited.

Weatherstripping and gaskets are very important in achieving airtight, low-energy buildings.