Concept

Some homes lose as much heat energy through air leaks (cold air infiltrating and conditioned air exfiltrating) as they do through all surfaces combined (roof, walls, windows, doors, and foundation). Incredible that infiltration typically receives so little attention during the building process.

ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) establishes infiltration rates necessary to assure good indoor air quality. This rate has been refined over the years as we learn more about how the overall house works with a tighter shell. The ASHRAE standard (62-89) for determining the necessary infiltration rate, the Building Airflow Standard (BAS), currently adopted by the Building Performance Institute and NYSERDA is requires infiltration of .35 of the volume hourly under natural conditions or 15 CFM per person (number of bedrooms +1), whichever is greater.

You heard the expression, "A house has to breathe"? Well, believe me houses that exceed the BAS breathe pretty well. As a mater of fact, they breathe over 3X what a well air sealed house should breathe.

Building scientists are trying to convey to builders, "Build it tight, AND ventilate it right" as the new motto to replace, "A house has to breathe." And for good reason, while meeting the BAS will probably assure good indoor air quality, it leaves a lot to be desired (gained) on energy efficiency.

Application

An underlying theme on my house has been, build it tight. ICFs (insulated concrete forms) are filled with continuous, reinforced concrete. Any penetrations are foamed tight.

The rim joist (connection between concrete and sill plate) is sealed with closed-cell foam.

The walls are made tight from the outside using the Zip System sheathing and tape. The windows and doors are installed. Then we install open-cell, spray-in-place foam on the outside 3 inches of the exterior walls to further air seal. I then test the house with a calibrated blower-door to track down any leaks. It is much easier to fix them now than it is to wait until after the drywall is installed.

The windows are casement and fixed windows by Accurate Dorwin with very good leakage properties. After installation, we carefully use low-expansion foam around each window to assure no air leaks in that trouble prone area.

The roof assembly is made of spray-in-place foam between the rafters and two layers of staggered and taped polyisocyanurate above the rafters.

The bottom line is we are "building it tight" – 0.1X BAS (Building Airflow Standard) or less. This house is holding its breath. See the next section on "ventilating it right".

Previous Concept    Back to Concepts Index    Next Concept