How to Get Good Blower-Door Results
The top 10 dos and don'ts for building a really tight house
Our development company has been working with several builders, including our own parent company, to determine the factors that affect our blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. 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 PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. 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(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. or ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure. wall structure, and eliminate rim-joists where possible. It is possible to get good results using spray-foam insulation, or using rigid foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. 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 envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials., 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 plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double 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.
- Ted Clifton
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