Last year energy audits in Unity revealed that many homes were losing every hour over 1/2 of the air they paid so dearly to heat. When it is zero degrees outside and 70 degrees inside, the air pressure of the warm air rising equals that of a hot air balloon. Under that pressure, warm air finds every possible opening in the top half of a home to leak to the outside. All of this escaping warm air is replaced by cold air from the outside, leaking in from openings in the bottom half of the home. In addition to this “stack effect,” winds blow in cold air from one side and draw heat out the other.
Energy audit calculations show that weatherization in the form of “air sealing” is the most cost effective retrofit available. Even $200 worth of caulk, foam, and weatherstripping (and a few weekends of work) can cut 10%-25% of a household energy bill. The best way to find these openings is with a blower door and an IR scanner.
The best time to air seal, however, is during construction. You may have seen builders putting down a gasket between the concrete foundation and the 2×4 or 2×6 sill plate. On house the size of TerraHaus this measure alone is equivalent to blocking an eight-inch diameter hole in the wall. If you don’t seal around your windows during installation, you may as well buy cheap single pane windows because you have nearly eliminated any measureable advantage to double-pane low E glass. Just a few years ago, contractors thought they had taken care of the air sealing issue with house wrap. Since then studies have clearly shown that house wrap alone misses almost all of the primary leaks and must be supplemented with caulk, foam and gaskets.
The Passive House standard for air sealing is very rigorous* but cost effective. Alan Gibson of G O Logic estimates that the extra air sealing measures used to achieve the Passive House standard can add $2200 to the cost of a home (over standard, code-compliant construction). This investment, however, leads to about a 27% reduction in fuel use according to his model so the payback period is quite short.
What are some of the air sealing measures used in TerraHaus? Thick polyethylene sheeting was placed under the foundation and extends up the wall where it will be sealed to the sheathing. Each of the seams between the SIPs is sealed. Scissors trusses sheathed on the interior with coated (Zip) OSB create a space between the bottom of the truss and the drop ceiling. This creates a chase for lighting, ventilation ducts and other utilities, eliminating every penetration of the sealed truss except for the bathroom stack. In fact, one of the biggest gaps for air leakage in most homes is the chase along chimneys, air space to keep combustibles from contact with the chimney. TerraHaus needs no chimney because it requires so little heat that a traditional heating plant is unnecessary. The goal is to achieve a completely air-sealed envelope.
In another post, I’ll look at ventilation, the flip side of air sealing, and how heat recovery ventilation (HRV) will be used to meet the ventilation standard of 35% air exchange per hour.
*The Passive House standard for air sealing: At the standard test pressure (50 pascals) measured with a blower door a Passive House must allow no more than 0.6 Air Changes per Hour (ACH). This translates to about 0.03 ACH under natural conditions, 17 times less air exchange than found in typical Unity homes. Mechanical ventilation that exchanges fresh air for stale air while transferring the heat energy from the stale to the fresh is necessary in passive house construction.
9/26/2011 update on the TerraHaus Stats: CFM50 = 132; ACH50 = 0.53. Using the energy auditors’ rule of thumb that your CFM50 divided by 10 is roughly equivalent to the total square inches of opening, the total opening would be roughly equivalent to a circular hole with a two-inch radius. Compare that to my home (pre-weatherization) with a CFM50 of 3180, a hole 318 square inches, or a circular hole with a radius of 10 inches!
An inch or so of spray foam in stud cavities is commonly used as an air sealing measure in stick frame construction. For best results, a third party blower door test and IR scan should be used before drywall is installed but while the spray foam contractor is there to fill the gaps. The photo below comes from an auditor who works with spray foam contractors. He finds the leaks while the contractor is still there and points them out with a laser pointer. The contractor can then fill the leaks. The dark areas in the photo indicate leakage. To the naked eye, no gaps existed, but the blower door and IR scanner indicated otherwise. We didn’t use spray foam for sealing on TerraHaus (other than a small amount of the foam you can buy in cans at the hardware store) because we wanted to avoid the high global warming potential of the blowing agents in spray foam.
Doug Fox, Director, Center for Sustainability and Global Change, Unity College