Our development company has been working with several builders, including our own parent company, to determine the factors that affect our blower-door test results.
The following list includes things we have learned to do, and things we have learned not to do, to achieve an optimal blower door test result. (We aim to achieve the Passivhaus standard of 0.6 ach50.)
Ten dos and don’ts
1. Do not install any kind of fireplace — gas, wood, freestanding, or otherwise. They leak, and you will not be able to overcome that amount of leakage. They are not designed to be completely sealed, and they may even fail if they are completely sealed.
2. Do not use sliding windows, sliding glass doors, or sliding vents in your window frames. They leak. You may be able to overcome the effects of one sliding glass door, but not more than one. Casement and awning windows, of virtually any brand, are much tighter fitting than sliders or single-hung windows.
3. Do use an all-SIP or ICF wall structure, and eliminate rim-joists where possible. It is possible to get good results using spray-foam insulation, or using rigid foam sheathing on the outside, but it is much more difficult, and less repeatable on a day-to-day basis.
4. Do use a balloon-frame approach to building exterior walls that are parallel to the floor joists. You can balloon-frame the SIP wall to go all the way from first floor to roof. Simply screw the first joist to the SIP wall from the outside, using an appropriate SIP screw. If using rigid foam sheeting, be sure your foam sheeting crosses the rim — that is, that it does not connect over the rim.
5. Do use an all-SIP roof. It is nearly impossible to completely seal up every crack in a typical ceiling, whether it be stick-framed roof or trusses. Every wall line is a leak point, as is every lighting box, recessed can, plumbing penetration, etc. A SIP roof gets you down to just a few plumbing vents and perhaps a bath fan or two — but that is it.
6. Do try to avoid attached garages. A typical attached garage will involve a number of penetration issues, including I-joist penetration of the building envelope, and mechanical and electrical system penetrations including wires, pipes, and ductwork.
7. Do a careful job of sealing the cracks between I-joists and wall plates. A surprising leak point is underneath the I-joists themselves, where they sit on the mudsill or top plate. Even when the solid blocking is thoroughly spray-foamed, the bottom chord of the I-joist can be warped just enough to allow some leakage. A typical garage-to-house connection will have about 2” of air leakage just from the bottoms of the I-joists alone, which would account for about 20 cfm. That would bump you from 0.6 to 0.66. If you do have such a penetration, try gluing the I-joists to the plate they sit on as you install them; that should seal off the leak.
8. Do keep your designs simple. Just as surface area is your enemy when it comes to energy loss and cost, corners and angles are your enemy when it comes to leakage. Even if you manage to make a more complex home airtight at the completion of construction, there will be more potential failure points over the life of the structure.
9. Do select bath fan and kitchen fan vent hoods with dampers that actually close tightly. I have seen many that cannot even be made to close tightly, while some can be tweaked to fit tightly. These can lead to some huge leaks if they are not tight.
10. Do make sure that all plumbing traps are filled and that all air-gaps, such as those below reduced-pressure backflow preventers, are sealed before conducting a blower-door test. If your dishwasher is using a “Johnson tee,” seal it for the test. Floor drains and water heater drain pans are often causes of air leakage, as the drain-pipe simply penetrates the wall, turns down, and ends, leaving a ¾” round hole in the building envelope.
We will continue to add to this list as we discover additional leak points. We have probably left off most of the most obvious ones, because they have not been a problem to us, but we welcome your comments and additions if you care to contribute.
For the record, most of our recent homes have come in around 1.3 ach50, and all have contained some of the issues mentioned above. Our most recent home, constructed by TC Legend Homes of Bellingham, Washington, scored a minuscule 0.56 ach50! Almost all the problems named above were eliminated in the design process. The rest were eliminated in the field by our outstanding contractor.
Ted L. Clifton is a designer, a builder, and the founder of Zero-Energy Plans in Coupeville, Washington. His previous blogs include Get Rid of Your Gas Water Heater! and Home Energy Efficiency Pays Steady Dividends.
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