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How to Get Good Blower-Door Results

The top 10 dos and don'ts for building a really tight house

Image 1 of 2
Know the location of common leaks. If you do your best to seal vulnerable areas in your thermal envelope, you're more likely to get the blower-door test results you're hoping for.
Image Credit: Ted Clifton
Know the location of common leaks. If you do your best to seal vulnerable areas in your thermal envelope, you're more likely to get the blower-door test results you're hoping for.
Image Credit: Ted Clifton
Since rim joists are a notorious source of leakage, it's a good idea to seal these areas with spray polyurethane foam.
Image Credit: Ted Clifton

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.


  1. user-723121 | | #1

    Tight Homes

    While I agree with a few of your points my experience has shown conventional stick frame buildings can be made very airtight. Satisfactory ach50 results can be achieved by good air-sealing details, typical building plans and construction details can be tweaked to greatly reduce air infiltration.

    There is no need to switch to exotic wall and roof systems to which most builders are uncomfortable or unfamiliar to get good ach50 test results.

  2. Ted Clifton | | #2

    I stand by #3 above

    I believe I said that in #3 above. Yes, you can get good results with other methods, including stick-framing, but the number of seams and joints needing to be sealed up make it a daunting task. My experience is that it is less expensive to use SIPS than it is to fully air-seal a stick-framed structure.

  3. Mike Eliason | | #3

    i'm confused - you are aiming
    i'm confused - you are aiming for passivhaus levels of airtightness, but using exhaust fans?!?

  4. Ted Clifton | | #4

    Exhaust fans, and other good products...
    To me, air tightness is not only about energy efficiency, it is primarily about a healthy structure, with no possibility of mold growing in or on your walls. Exhaust fans are a vital part of indoor air quality. The right fans are a factor of your climate zone. In the coastal Pacific Northwest, our climate is relatively mild, with average winter temperatures around 40˚F. It is hard to justify the use of a heat recovery ventilator in these circumstances. Not so in Eastern Washington, where the summer temperatures are much hotter, and the winter temperatures are much colder.

    In either case, I recommend bringing in your fresh air through a HEPA filter. It will take out mold and mildew spores, pollens, and many other common pollutants. A powered HEPA filter can be used to balance the air pressure when your kitchen fan is working.

    Now about the Passivhaus levels of air-tightness...
    While there is much to be learned from the Passivhaus model, I believe they have lost sight of the forest for the trees. A tight house is good, very, very good. Elimination of the heating system, just to not have a heating system, is moronic. Everything in the house should be done to the degree that makes good economic sense. If a small ductless mini-split can provide the heat needed in a colder climate at 1/3 the cost of heating with a small electric resistance heater, then that is the system that should be used. If that system is only run a few days per year, instead of the 200 or so days it would run in a "normal" house, it will simply last longer than the 20-year "normal life", and can thus be amortized over a much longer period.

    Heating with incandescent light bulbs is moronic. Light with compact fluorescents or LEDs, heat with the heat pump, or whatever else may be your most efficient heat source. When you need the heat the most, it is night time, you don't want your lights burning all night. Exhausting your range hood into the house, with all the moisture and mold-food, is moronic. Get that stuff out of your house, and live a healthier life. We are achieving passivhaus-like numbers, at far lower initial cost, and going way below net-zero with our homes, even in the colder (zone 5) eastern Washington and Oregon regions. Our Painted Hills house has produced enough surplus energy in its first eight months of operation to power a Nissan Leaf (electric car) more than 15,000 miles. Even with average overnight lows in February around 20˚ F, the ductless mini-split was never employed. The passive features of the house kept the minimum inside temperature above 65˚ F. Do you know of any certified Passivhaus that can boast of that kind of performance?

  5. Mike Eliason | | #5

    Now you seem to be


    Now you seem to be confused...

    Passivhaeuser have heating systems - in the States, uber-small mini-splits are routinely used.

    I don't know of any Passivhaeuser that use incandescent lights for heating - LEDs and CFLs are the norm to avoid blowing the consumption 'budget'.

    Lots of Passivhaeuser are able to maintain temps in low-mid 60s without the utilization of space heating in cold, cloudy climates that don't get much sun in winter. When I lived in Freiburg (climate is slightly colder than Seattle, about same insolation) - we visited a PH that had only needed the heater on 7 nights in the 3 years it had been built. A couple of doctors we know recently retrofitted their house to PH, haven't used heating. You can search online and find German and English blogs where folks talk about not utilizing heat at all, or only needing it on the coldest nights of the year.

    You claim you are doing it for far lower first costs, but at $200/sf - that's more than double what the Smith House cost ($94/sf) and 60% over the Salem Passivhaus ($127/sf). The Salem PH would only need 3kW PV array for site net zero, and it's an 1,885sf house. Were it in Eastern Oregon with all that extra sunshine - and reduced to 1000sf - 1.5kW PV for site net zero.

    There's a good case study of the Salem PH here:

  6. Ted Clifton | | #6

    Glad to hear it...
    Mike, I am happy to hear that builders of passive homes are not listening to the drivel I have been reading put out by the "sellers" of Passivhaus technology. There is nothing wrong with the Salem example you sent me the link to, thank you.

    Please do not confuse the cost per square foot of a project built for the US Government, with all their "prevailing wages" and expensive procurement procedures with the cost of any of our very real homes built for private individuals. Our average cost of construction has been running under $100 per square foot, even in the pricey Seattle market. Our recently completed Ballard house cost just $179,000 for a 1,915 sf conditioned area home (including contractor mark-up). The 6.7 KW of PV on the roof cost around $32K installed on top of that, but it will probably power the owners car in addition to running the house. We are looking forward to monitoring this over the next few years.

  7. Brian Knight | | #7

    fireplaces and bath exhaust
    Thanks Ted, great article. I agree with #1 but constantly get clients who insist on fireplaces. They can understand the drop in efficiency but are not willing to sacrifice the idea of it. I did get a SIP house down to 1.25 ACH50 with a wood stove flue that was stuffed with a fleece jacket but now that the owner installed a stove I wonder what it would test at...

    If I were to build my personal house I would still take a hard look at wood stoves and would be interested to hear about readers opinions of the tightest wood stoves available.

    Care to divulge the manufacturers or models of your preferred bath fans?

  8. Ted Clifton | | #8

    Wood stoves & fans
    I had hoped this article would induce more builders to share their own tips on how to build the tightest envelope, but we seem to be talking about everything else...

    The tightest envelope is not always the only objective. Wood stoves have their place, in fact two or our three most recently completed homes had wood stoves, and they achieved about 1.3 ACH 50. I do not know who has the tightest stove, but all the wood stoves seem to be tighter than the gas models. I do not know why that is.

    For fans, I like several of the Panasonic Whisper Green models, depending on what type of controls you need. If I am coupling them with a powered HEPA filter on a speed control, I am using FanTech remote-mounted fans. Either way I am getting the right amount of air moved, using a very small amount of energy, with virtually no sound. There are other good players in the game, and about the time I recommend one brand or model, somebody else comes in with something even better, so keep your eyes out for whatever is the current best on the market. To stay on top, always be willing to change!

    One final note on fireplaces, I can show you how to generate more useful energy with a fireplace that never has a fire in it than you could generate with a fire. Anybody want to pick on this one?

  9. cussnu2 | | #9

    Blower Door Test
    So when you are done with your test and you take the blower door out of the do you know how tight the door is that you took it out of?

  10. Ted Clifton | | #10

    How tight is the blower-door?
    I have yet to see a blower-door test frame that is as tight as a properly fit exterior door. I am am sure there are exceptions, however. For a house that leaks "to code", it probably wouldn't make much difference; the door would have to leak several square inches to make a noticeable difference. When you are getting down into the rarified air of passive house like numbers, it could make a substantial difference. The only way to really know would be to do the blower door test from two different locations on the same house. Even doing that, however, you still don't know exactly how many cfms were coming from each of the doors, vs the rest of the house. All you really would know is if the leakage from the two doors was equal, or not.

    This brings up another good point for the list of things to do to get a tight blower-door test: If your house has some in-swing, and some out-swing doors, use one of the in-swing doors as your test door. That is because as the house is de-pressurized, the outswing doors will tend to pull shut tighter, whereas the in-swing doors will be pulled away from the weatherstrip. Your first reaction may be that this would be cheating, but not really. Depending on the orientation of the house relative to prevailing winds, any given side of the house could be under positive pressure, rather than negative, as measured from the inside.

    Following this logic, to make a house really tight in the real world, use out-swing doors on the "weather" side, in-swing doors on the leeward side. This way the prevailing winds will make the house tighter, not looser.

  11. dickrussell | | #11

    0.8 ACH50 with woodstove, range vent
    Ted, you wanted some input (post #8) on tight houses with woodstoves. Mine is a new double wall superinsulated house in NH, with a small Quadrafire Millenium 2100 woodstove in the basement. It has the outside air adapter in back, and air is ducted in from the outside to it. I chose that one for its small size and the fact that it has a welded firebox, not a cast stove bolted together with a lot of gaskets, plus an outside air duct could be attached to it directly, so as to avoid having outside air dumped into the vicinity of the stove.

    For the preliminary blower door test, we got 0.65 ACH50, but that was with the woodstove air inlet and chimney sealed off and the range hood duct also sealed off. The final blower door test came at about 0.80 ACH50, still considerably below the 1.0 I had hoped to achieve.

    For air sealing, we used a lot of acoustical sealant between framing elements and other places. I decided to establish the primary air barrier at the inside. I used MemBrain for vapor retarder, tape sealed to strips of poly that were run under all exterior inner wall frames and over all upper level walls (inner exterior frame and interior partition walls).

    Edit: for the range hood, I selected the Broan 639 4x10 rectangular wall cap, which is as good as any I'd seen, for its rigid construction and fairly tight sealing flapper; The ductwork to it goes down and out rather than straight out or (worse) up and out. I actually contacted the mfg of the Heartland 2100 dryer vent, which I used for the dryer, to see if they made a larger version that might serve as a range hood wall cap (they don't). By the way, I like that Heartland dryer vent for its tightness against backdrafting. The preliminary blower door test was run in January, with that vent installed but the inside end of the duct completely open, and I couldn't feel a thing coming through it.

  12. Ted Clifton | | #12

    Thank you, Dick Russell!
    Dick, this is exactly the kind of input I was hoping to encourage with this article, thank you! Anyone else? Don't be shy, I know you are out there!

  13. Lizzieplants | | #13

    Size, Cost to build etc.
    Dick, How large is that house in NH and what did it cost to build (lot not included)? Who designed the house?

    We are looking at land now (Saratoga Springs, NY area) and want to build a double walled tight house. We love our Woodstock wood stove and want to have a wood stove in the new house. Power outages are becoming more common and having an grid independent source of heat makes sense to me.

  14. xHkZBSHpJL | | #14

    avoid attached garages?
    I understand that the connection to the house creates some leak challenges. But, in northern hemisphere heating climates putting a garage on the north side is a great insulator from cold north winds...putting it on the west side is good for cooling climates...I see way too many houses where the location of the garage is bass-ackwards...seemingly arbitrarily determined by street layout.
    Attachment...I know I will be glad my garage is attached tonight when I get home in the snow storm we are having today in Wisconsin!

  15. Ted Clifton | | #15

    Attached garages
    Joe, thanks for your comments. When we do use an attached garage, we take the effort to assure that it has negative pressure, and the adjacent house has positive pressure, so there is never ay transfer of fume-ladden air from the garage to the house. I agree that in cold climates there can be a benefit to having the garage attached, not only for convenience, but also for the extra separation of the building envelope from the cold outdoors. If we all went to electric cars, and stopped storing nasty paint thinners and lawn mower fuels in the garage, this would not even be an issue.

  16. user-444644 | | #16

    I'll bite on the fireplace question....
    "One final note on fireplaces, I can show you how to generate more useful energy with a fireplace that never has a fire in it than you could generate with a fire. Anybody want to pick on this one?"

    What is it? an electric faux fireplace insert?

  17. Ted Clifton | | #17

    Fireplace as Thermal Mass
    Greg, you asked, so here it is!:

    You build a fireplace that looks like a fireplace, but does not have a chimney or a base that penetrates the thermal envelope. You place the fireplace in a location inside a window where the sun will warm up the thermal mass each morning. In most climates, you will receive more useful thermal energy this way than if you build the fireplace to actually have fires in it. The only option that can beat this is the Russian style fireplace (there are some commercially available imitations that work also), but they need to have a fire in them almost continuously if you are to overcome the energy that is being conducted and convected out through the chimney and the base. In smaller, tighter homes, you would cook!

  18. qofmiwok | | #18

    What about a sealed direct vent fireplace with a single penetration out the wall?

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