Thermal Envelope

Critical Envelope Science 

It cannot be overemphasized how crucial it is to get the physics of a building’s thermal envelope right. The layers of any foundation, wall, and roof system can either protect or endanger a building from the consequences of moisture. Without a proper plan to account for both interior moisture generated from a building’s inhabitants and the need to shed exterior moisture, the foundation, wall, and roof systems can be quickly damaged by mold, mildew and rot.

Surrounded by Super Insulation

An Airtight Envelope

A building’s indoor/outdoor air pressure differential is like a balloon: the high-pressure indoor air is seeking a way to escape to the low-pressure outdoors. This is why sealing all possible air leakage points is so critical, especially in a Minnesota winter when the indoor/outdoor air temperature differential is the greatest. 

To prevent air leaks in the home’s envelope, we first filled larger cracks with polyiso low expansion Great Stuff spray foam. Then we sealed all the joints, whether plywood or EPS foam, with 3M 8067 All Weather Flashing Tape. 

Minimizing Envelope Penetrations

Minimizing penetrations is another way to prevent conditioned air loss through the building envelope. We opted not to install a fireplace or a wood stove to avoid the obvious hole and thermal bridge with a chimney and damper, as well as to avoid having to open a window to create the make-up air and draft that would be necessary to pull smoke and carbon monoxide out of the house. 

Instead of an exhaust fan over our oven and cooktop, we installed a recirculating fan and we control interior air quality with our heat recovery ventilator (HRV), boosting the HRV's fan speed when needed. Rather than adding another exhaust hole through the envelope, our condensing clothes dryer has a heat exchanger that changes warm moist air into water that is pumped into the clothes washer’s drain.

Besides windows and doors, the other openings in our wall insulation system are the supply and return for the HRV, two mini-split compressor lines, the electrical service entrance, one garden hose bib, and two GFCI receptacles. The underground water and sewer services and the supply and return for the in-floor heating system (which we have not yet used) pass through the insulated floor system and are sealed with caulk and tape. The plumbing and radon venting passes through the roof insulation system and is also taped.

Minimizing Thermal Bridging 

To preserve the integrity of all this tightly sealed insulation, it is essential to decrease thermal bridging: the rapid transfer of heat or cold through highly conductive materials (with a low R-value) like glass, wood, metal and concrete. This is especially important during our very cold northern winters when heat loss and condensation occur most. If warm, moist indoor air finds a pathway to exit through the exterior wall system, it will condense on any cold surface: the windows and their frames, the studs, sheathing and siding, for instance. Over time, this condensation will create mold, mildew and rot and compromise the integrity of both the structure and the energy efficiency of the envelope. For example, when loose-fill insulation like cellulose becomes wet, it will lose its R-value. 

A Continuous Insulation Layer

To minimize thermal bridging through the 2x6 stud walls, we installed a continuous layer of 4-inch, high-density expanded polystyrene (EPS) foam over the exterior plywood sheathing. The EPS foam prevents the studs from becoming cold-conducting strips, greatly increasing the envelope’s R-value. We then spray-foamed any cracks and taped every seam with the 3M flashing tape.

We chose EPS because it was more environmentally benign than other foam types, and its R-value increases slightly as the outdoor temperature drops—a plus in Minnesota.