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Blower Door Results — room for improvement?

humm9er | Posted in Energy Efficiency and Durability on


I had a blower door test performed as well as thermal imaging taken.

My home is 3300 sq ft of finished living space, 1600 sq ft of unconditioned basement and about 1200 sq feet of unconditioned attic (the remaining roof is cathedral ceiling).

My blower door test came back at 3050 cfm. The tech said that to get down to 2350 cfm would be “perfect” and that I was about 30% above optimal but that it, in his estimation, we were at the point of diminishing returns outside of caulking some areas we identified as leakage sites.

I read this article and want to confirm my math / understanding here is accurate:

Units — cfm 50 per 100 sq ft of building envelope / per square

Total building envelope — 6100 sg ft / 61 squares

3050 cfm 50 / 61 squares = 50 cfm 50 per 100 sq ft of building envelope

Am I tracking correctly? Thank you!

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  1. davidmeiland | | #1

    Not sure if your technician used software to run an automated test, but if s/he did, it's very easy to enter both the cubic volume of the house and the surface area, and get both results. I rarely hear anyone use the surface area-based number that Allison argues for. What was the ACH50 you got? And, given that your tech did IR, where are the leakage areas?

  2. BillDietze | | #2

    The article you reference states this:
    "3 ACH50 — translates to about 0.25 cfm50 per square foot of envelope, or 25 cfm50 per hundred square feet of envelope. Since roofers have already abbreviated 100 square feet as 1 square, I like the latter form the best. It gets it into a whole number form and is easy to remember. Get your blower door number down to 25 cfm50 per square (or below), and you've got a tight house."
    So to get to "tight", per the article (ACH50 = 3), you'll need your cfm50 at (6100/100) x 25 = 1525 cfm. Some pretty good house people aim for ACH50 = 1, three times tighter.

  3. humm9er | | #3

    Thanks for your reply.

    Tech has not sent me the report yet so I am not sure on my ACH50 number.

    My home was built in the late 17th century and remodeled in the 90s. The primary leakage was around the post and beam framing in the corners of the home and where the drywall meets the old framing (framing is exposed for aesthetic). 2 ceiling fan ceiling fixtures were leaky. A few of the Windows also had some leakage, as did basement door, attic door and stove vent outlet hole.

    Additionally, while I gasketed all the outlets, the slit for the sliders on my lutron dimmers were pretty leaky. Not sure what my options there are outside removing the boxes and air sealing from behind?

    The good news is my new led recessed light retrofits were tight - smoke stick reveled no leaks around them which was a topic I discussed here so I will follow up on that thread for others considering that upgrade.

    No significant air leaks to attic beyond minor gap in attic door gasket. His only pitch was spray foaming fieldstone foundation and Dow thermax on concrete foundation but he seemed reluctant to give me a payback estimate there in terms of years to ROI.

    I will address what I can above but am curious if my math above is right.

    Thanks everyone for your feedback.

  4. humm9er | | #4


    Thanks for your feedback. I get that my home is not super tight by green building standards, but is my number super leaky? 17th century construction ...

    Auditor told me that every 1" square air path sealed reduces CFM number by 10, so a reduction to say 1500CFM from 3000CFM requires finding and air sealing the equivalent of a 10"x15" hole throughout the home.

    Is he correct in that loose estimate?

    Thank you

  5. BillDietze | | #5

    Some people use cfm50 = 1000 for a 1 sq.ft. hole. That's 6.9 cfm50 per of hole. Note that the size of the hole affects this, so the above is an estimate and your technician basically has it right.
    As to your question, is your 17th century house super leaky at (approximately) ACH50 = 6? I say you've done well, but "leaky" is in the eye of the beholder.

  6. davidmeiland | | #6

    Sticking with ACH50 here...

    I'll hazard a wild guess that your house has a cubic volume of 32,000 feet. 3050 CFM50 translates to 5.7 ACH50. Your technician is suggesting that 4.4 ACH50 would be "perfect".

    My experience is that houses at 5-6 ACH50 are perceived as a bit too drafty by their occupants, 6-7 is problematic, and above 7 is definite cause for action. If you can get into the 3-4 range you'll probably have no complaints, and might possibly have to address ventilation in a serious way.

    Still curious to know where the leakage was found. Hopefully you get a report with a bunch of telling images.

  7. Dana1 | | #7

    Bill/Justin: A slightly more accurate (but still crude) estimate is to multiply the CFM/50 by 0.13 to come up with the approximate hole size in square inches. Thus, 1000 cfm= 130 square inches, which is 10-11% smaller than a square foot.

    Leakage of 3050 is roughly 400 square inches, or about a 20" x 20" square. That's like leaving as small double-hung window in an otherwise perfectly tight building wide open.

  8. humm9er | | #8


    I thought we include the basement in the air volume calc? If so, given some of the home has cathedral ceilings, my cubic volume is closer to 44,000.

    44,000 cubic volume
    3050 cfm x 60 = 183,000
    183,000 / 44,000 = 4.15 ACH??

    Regardless, going back to someone's question, the primary air leak sites he found are:

    Basement door to outside, basement door to garage
    Front door
    8' section of rim joist I have yet to fill with 2' XPS and spray foam (closet in the way -- will remove and seal)
    Framing gaps between the old timber frame & significant (1/8 - 1/4") gaps wherever drywall meets old timber framing
    4" hole between bath vanity and whirpool tub that leads to garage
    The SOLE recessed light I have not converted to LED (will now convert)
    Outlets that feature lutron dimmers -- the slit the dimmer slides up/down
    Both bath fans, even though they both have 2 dampers
    4 window sashes (20 year old double hung harveys)
    Small area of attic door where gasket wasn't tight
    Kitchen stove fan outlet hole in sill (couldn't access without removing closet -- will remove)
    Ceiling fan electrical box in cathedral ceiling
    Small air gaps in rubble foundation in crawlspace -- will be sure vapor barrier is 100% sealed and spray foam where I can

    Related to above, couple questions:

    Since the mechanical bath fan dampers seem imperfect, what options exist for truly stopping airflow out bath fans? Are these good instead of the flappers and if not, is there a better option?

    What options exist for stopping air flow through dimmer sliders, outside of removing electrical box from drywall and taping the box?

    How to air seal where ceiling fan enters ceiling in cathedral with no access from above? Foil tape over box with slit for wires to enter fan?

    Will use silicone and/or acrylic on drywall and framing gaps where I can. Remaining holes are pretty straightforward.

    Thank you!

  9. davidmeiland | | #9

    If the basement is not heated and the basement ceiling is the insulation line, I would not include it in the volume calculation.

  10. humm9er | | #10


    I understand your logic. In this case, with 18" pine flooring on 1st floor with no sheathing/sub-floor beneath, I can see between the cracks to the basement in certain rooms if the basement lights are on which is why I was inclined to include it in the envelope. House to basement door is also board and batten and by no means a barrier.

    That was also a reason I was contemplating insulating the basement walls, since outside of rigid board under floors I think keeping the basement outside the thermal/heating envelope will be a real challenge?

    Along those lines, I see 2" blue rigid foam outside my house poking out in front of foundation down at the ground in 1 spot of house...was exterior rigid insulation common practice in the early 90s? I can dig to determine but am assuming that's evidence of such.

    Thanks everyone your input is much appreciated!

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