Deciding exactly where to place the thermal envelope, where the insulation and the air sealing will be installed in contact to each other, is one of the most important decisions made when planning an exterior renovation or major addition.
In theory, the air barrier must be complete and continuous on all six sides of all cavity insulation. While it is easy to accomplish six-sided air sealing in walls, the insulation is typically left exposed above the ceiling and below the floor because installing an air barrier in thee locations is often impractical and not very cost-effective. Furthermore, the location of the building envelope is often flexible. For example, the building envelope may be designed to include or exclude the basement, crawl space or attic.
Starting from the bottom, homes with basements have the option of insulating the bottom of the first floor or the walls of the foundation. When the foundation can be kept dry, insulating the basement walls provides for the best performance in most climates.
If there is a risk of flooding or water infiltration through the foundation, then the insulation and air sealing should be installed on the underside of the first-floor framing, placing the basement outside the thermal envelop. If the house is on a slab, the perimeter of the slab should be insulated in all climates; in cold climates and wherever a radiant floor heating is installed, the entire underside of the slab should also be insulated.
In all homes, the thermal envelope is located at the exterior walls. Choosing exactly where within the wall assembly to install the air sealing and insulation has a significant effect on the building performance.
Insulated sheathing on the exterior of the wall structure helps reduce thermal bridging at the framing members — a particularly effective strategy in cold climates where it can reduce or eliminate condensation within the wall structure while serving as an effective air barrier.
Cavity insulation placed between the studs, combined with well-sealed drywall, provides an alternate method of creating a thermal envelope. With this strategy, however, gaskets or other sealants must be used to seal the drywall at the top and bottom plates and around all receptacles, lights, and switches.
The rim joists must also be sealed with caulk or spray foam to provide a comprehensive air seal. Using spray foam installation in the stud cavities alleviates the need for most of the air sealing on either exterior sheathing or interior drywall; however, the gaps around windows and doors and between framing members must still be properly sealed.
The top of a house often poses the biggest challenge to completing the thermal envelope. In unconditioned attics above flat ceilings, a thick layer of properly installed batt or blown insulation in combination with ceiling drywall can provide a good thermal barrier, provided all the details are properly addressed, including soffits, attic accesses, heat registers, light fixtures, whole-house fans, and the line where the top of the drywall meets wall-top plates.
Critical areas to address include interior soffits and attic knee walls, which are the vertical walls that separate a home’s conditioned space from the unconditioned attic. Soffits must be properly air sealed to avoid creating a thermal bypass. Knee walls that separate conditioned space attic areas must have an air seal on the unconditioned side of the wall as well as blocking below the wall. An alternative location for the top of the thermal envelope is to place it in the roof, creating a fully or semi conditioned attic.
This piece was excerpted with permission from the forthcoming book “Green Building: Principles and Practices in Residential Construction” by Abe Kruger and Carl Seville. The book is available this month at Amazon.com in print and e-book formats.