Thermal Envelope: An air-tight, leak stopping seal surrounding the living part of a home that can leak warm or cool air to the outside. A well-maintained thermal envelope reduces energy waste and overall heating and cooling costs. A house with a poor thermal envelope is like a ship with small holes all over.
The following describes our first thermal envelope air sealing project in Menomonie in 2008. We intend to maintain a regular relationship with the customer as we study the changes set into motion: how the home heats/cools and how their energy use changes.
We started in the attic where it was blown full with fiberglass insulation. To the untrained eye, it looks pretty good, level and uniformed. However, we know from experience and calculation that generally about 90% of heat in a home is lost through the ceiling. In this project, with most of our efforts in the attic, we stopped 53% of the air leakage. Fiberglass insulation does NOT STOP air leakage.
When the insulation is pulled back from the ceiling, walls and penetrations, we can see gaps in joints and crevasses; each is an air leak. And most attic moisture is transported through these leaks, as compared to diffusing through the sheetrock itself. 100 times more in just one square inch of space.
To close the leaks, after the joint or penetration was exposed, we applied a two-part polyurethane expanding foam into and over the joints. Along every inch of the ceiling and wall joint, gaps were sealed. Any missed gaps discovered during the retest were sealed again. Sealing the few gaps we found pushed our results from 52% reduction to 53% reduction. After the retest and fixing the misses, a 12” layer of blown cellulose insulation was applied over all the now damaged fiberglass insulation. Cellulose is ideal because over time, it more impervious to air leakage.
Moving into the basement, we found air coming from the sump pit and through all the floor band joists. To stop these air leaks, we removed the band joist fiberglass insulation. We foamed the perimeter of each space, coating the joints between wood sheathing and farming and the joint between plate and foundation. Once completed, we reinstalled the same fiberglass insulation.
On the main living level, we found air leaks at wall/attic intersections, around and through all electrical outlets, light fixtures, windows and doors. The outlets had been sealed on the back during construction but we found large gaps between the wall and the outlet box, as well as air leaking thorugh the previously foamed plugs. We pulled the cover plates off and applied a filling bead of latex caulk around it briding the box to the drywall. Additionally, we reached in with fingers and caulked the slits at each unused knockout.
Windows were not only leaking around, but also through the units. Some of the windows did not have enough room to insert our foam gun, so we verified that each window and door was packed with insulation. Then taped all around the window and door casings, this did not completely resolve the problem, so we fashioned some foam plugs to fill the gaps in the bottom tracks. These plugs made of white foam, can be removed for service on the window at a later date. Poorly detailed and assembled windows and frames are a main cause of air leaks. You can see the frost build up on the inner surfaces of the window on the 'warm' side between extension jamb and window. We resolved this with foam plugs at the end of sill cover and a complete sealing of the window/extension jamb joint and other openings through to the living part of the house.
Unlike improvements in mechanical systems, which rely on maintenance, electricity and fuel, air sealing of the attic, basement, and walls has minimum to no ongoing maintenance costs and will provide savings and comfort for the life of the home. Also, the current (and future added) insulation will be more effective because of these improvements in the thermal envelope of the home.