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Is there a way to calculate depressurization values from blower door test results?

GBA Editor | Posted in Energy Efficiency and Durability on

OK, this question is for you building scientists and certified blower door testers . . .

Given a theoretical house size, volume, and airtightness result, is there a way to convert a flow rating into a pressure rating?

Example: Two story house, 30′ x 30′ footprint, 20′ wall height (this would include sealed crawlspace and floor joist area)

* Surface Area = 3,300 sq. ft. (Walls and Ceiling)

* Volume = 18,000 cu. ft.

* Blower Door Test Result = 0.77ACH50

What is the effective depressurization if the bathroom and kitchen exhaust fans are pumping out 150 CFM?

I know this is a very simplistic assumption. I know that it depends on the inside / outside temperature difference, outside wind velocity / direction, stack effect, etc. All of you realists, please humor me and assume that all those other variables go away for a moment! 😉

Can you estimate the depressurization at 150 CFM exhaust? Is it 3Pa? 5Pa? 10Pa? I understand the rules of thumb regarding the conversion of ACH50 to ACHnat, but it seems like this question is more definitive.

This question relates to natural draft appliances (read woodstoves) and airsealed houses. It’s not a standard, but the Canadians have used 5Pa as a house depressurization limit for buildings incorporating a naturally drafted woodstove. I’m trying to determine if there is way to calculate depressurization based on known active exhaust fans (given the specs of a certain house), to see the effect on a woodstove.

I recently read several articles discussing the question of outside air supply for woodstoves. It seems their is no consensus. The Canadians have been studying this very question for 20+ years – scientists in the US have too. Codes have swayed back and forth. “Expert” opinions vary. The fact is we don’t have much data . . . and it’s all very confusing.

There’s no question in my mind as to whether or not direct vent appliances (gas water heaters, furnaces) are necessary for airtight houses. But I have yet to see a woodstove that couldn’t spill or backdraft – whether it had an outside air supply or not. The saving grace? We ignore our water heaters and furnaces for months at a time, but we actively tend (or massage, or watch, or fuss) our woodstoves. That and our sensitivity to woodsmoke . . .

Builders of “tighter” houses need to be more careful. That’s what I’m trying to do here – get a better understanding of this whole game – at the planning stage.

I know some will question the need of a woodstove in a well sealed, well insulated house. That’s OK. I’m not against hearing those ideas too 🙂

Thanks for your input!

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  1. user-626934 | | #1


    If the blower door test result is (or theoretically is) 0.77ach50, then cfm50 is:

    0.77ach50 x 18,000cf / 60 min = 231cfm50

    231cfm50 is in the same ballpark as the exhaust fan airflow of 150cfm, so if the exhaust fans actually move 150cfm, then you could theoretically depressurize the house to close to 50Pa...perhaps 30Pa to 40Pa as a rough estimate.

    That said, I would question your exhaust fans' ability to actually exhaust 150cfm in a house that tested at 0.77ach50. While the fans might be rated at 150cfm on paper, the actual airflow in a very tight house (assuming no passive air inlets) will be much lower. You won't know the exact airflow until you install and test them.

    Others on the forum have a lot of experience with very tight houses and woodstoves. In fact, I think there was a previous thread (or two or three) on the subject.

  2. Riversong | | #2

    You're possibly looking at 15 to 20 Pa of depressurization, which is why it's insane to build a house that tightly and incorporate bath exhaust fans without passive make-up air, particularly when a woodstove is installed.

    That level of airtightness (PH standards) are more of a fetish than a necessary element of an efficient and healthy home. Tight is good. Hermetically-sealed is dangerous.

  3. Michael Blasnik | | #3

    The depressurization can be estimated as

    dP = 50 * ((CFMfan/CFM50)^(1/0.65)

  4. Daniel Ernst | | #4

    Thank you all for your answers. Michael's calculation puts the depressurization @ 26Pa.

    John - I agree with you on the airflow from the exhaust fans. I don't really see how a couple of small Panasonic fans could operarate at rated flow under any significant static pressure. Panasonic shows their Whisper fans operating at the rated flow under a 25Pa OR 62Pa static pressure (although they're expecting that resistance to be on the downstream side of the fan, in the form of elbows and pipe length). I'm not sure how they monitor and adjust the fan speed. From my experience that is a pretty complicated task.

    But how many people have actually tested the flow from their exhaust fans during commissioning? Michael Chandler noted here that a Panasonic fan operated at only 72% of its rated flow, though I couldn't tell if that test was under any type of static pressure:

    And I know that I'm beating a dead horse, but the answers are just not clear to me yet.

    My experience with HVAC is from working in a Class 100 cleanroom environment. There we created a pressure cascade, with the final cleanroom pressure at 37Pa relative to the very outside rooms. The flows at that pressure were HUGE. I'm trying to conceptualize the results of a negative pressure at the same level. I'm thinking it would make it difficult to even shut the outside door. ;)

    Robert - How tight is too tight? That's what I'm trying to understand. Dr. Lstibureck says the same thing about the Passivhaus standard, that "it is not worth going there effort wise." Worse yet, I think, is building an airtight house and then overventilating . . .

    For the HERS 46 house that is featured on Build It Solar, what were your blower door results? I know you recommend passive inlets for exhaust fans (and dedicated air supplies for woodstoves), but at what level of airtightness?

    Do you have any experience with masonry heaters? Right now I live in a "tight" house (the problem is that I don't have any numbers or tests to say how tight). I also have a masonry woodstove for winter heating. I CAN depressurize the house and cause backdrafting - IF I run a couple of exhaust fans while trying to start the fire and don't provide any make-up air for the stove.

    Here is a dated article. Norbert Senf also has a study out there that he conducted for the Canadian Mortgage and Housing Corporation. I would be interested in reading other newer studies:

    I'm moving and plan to build in my new location - so right now I'm working through the conceptual design. All of these posts and discussions are very helpful. One thing for sure, I'm going to assemble as much DATA as possible during this upcoming project - and plan to post it online.

    Thanks again.

  5. Riversong | | #5


    That house had a an ACH50 of 2.13 (ACHnat of 0.11) with the passive inlets taped, and ACH50 of 3.06 (ACHnat 0.15) with the inlets open. That means that natural stack effect air change would not be quite sufficient to meet ASHRAE 62.2 standards, but would likely provide some air exchange.

    My Panasonic Whisperlite fans, with short duct runs with two elbows, were moving 94 cfm for the 110 fan (86% of rating) and 74 cfm for the 80 (92% of rating).

    My judgement is that a tight house should still exchange 2-3 ACH50. But breatheability has as much to do with moisture exchange as with air exchange. Healthy homes naturally exchange both, and use mechanical ventilation, both spot and whole house, for additional fresh air and moisture control. Obviously, the more toxic the house or the occupants, the more air exchange required.

    The masonry heater experience I have is not in very tight homes. A friend of mine was one of the pioneers of custom masonry heaters and cookstoves in the 1970s (wrote the first FHB article on the subject). I helped build a masonry heater last year. Depending on how the flue path is oriented, its length in relation to the chimney height, and the size of the firebox door, a masonry heater should be less susceptible to backdrafting than a typical woodstove. But, as you know, it doesn't require much negative pressure to backdraft a cold stove when the door is open. Outside air supply, I believe, is mandatory for both woodstoves and masonry heaters in any relatively tight house.

    What we've done in the efficiency movement, is to take a good thing (reducing the excessive air leakage) and taken it to a ridiculous and unhealthy extreme. Any good thing becomes counterproductive in the extreme. We did not evolve biologically to live in hermetically-sealed plastic boxes that require life-support systems.

  6. Riversong | | #6


    How is that equation derived? Or, do you have a link to it's source?

  7. Michael Blasnik | | #7


    The equation comes from a little algebraic re-arrangement of the basic building leakage curve.

    Q = C * P ^ n

    where Q is the air flow, C is called the flow coefficient (and is related to hole size), P is the pressure difference and n is the flow exponent which is related to hole type but is remarkably close to 0.65 in most homes (laminar flow n=1, turbulent n=0.5). The CFM50 from the blower door test is the value of Q when P=50. If you assume n=0.65 you can solve for C and then calculate any P given Q or vice versa.

    Of course, the leakage equation is empirical and doesn't make much direct engineering sense.but it works remarkably well at pressures above 1 or 2 pa.

  8. Daniel Ernst | | #8

    Now that's what I'm talking about! Awesome. Thank you Michael.

    Robert - Thank you for the numbers and detail on your modified Larsen Truss house.

    If I rework the numbers of my theoretical home and give the blower door test a result of 2.0ACH50, then the depressurization comes to 6Pa . . . very close to the Canadian HDL of 5Pa.

    Norbert Senf's study (I can't link it here for some reason) showed that masonry heater spillage occurred when room depressurization reached 10Pa, whether or not the stoves had an outside air supply or not. His explanation:

    "Negative house pressure is, by definition, negative relative to ambient outside pressure. Since the
    effect of an outside air supply is to bring the firebox to outside pressure, it cannot be demonstrated to prevent spillage."

    He also conducted tests to show that even well crafted stoves are not airtight. The door leaks provide the path for the spillage:

    ". . . a door that leaks 5 L/s at 10 Pa is considered to be quite tight. . ."

    Finally, he also stated:

    "In a typical R-2000 house, drawing 30 L/s of inside air would result in only about 3 Pascals (Pa) of
    depressurization, well within any house depressurization limits."

    If I solve Michael's equation using the 30 L/s and the 1.5ACH50 figures (from the R2000 requirement), then I get 2.46Pa depressurization, which is right in line with Senf's study.

    OK, it's starting to come clear for now ;)

  9. Riversong | | #9

    Since the effect of an outside air supply is to bring the firebox to outside pressure, it cannot be demonstrated to prevent spillage.

    This statement is absurd on several levels.

    The purpose of an outside air supply, with a woodstove as with any combustion appliance, is to isolate the combustion chamber from the interior environment. Isolating it from the interior eliminates the primary coupling to house stack effect or mechanical negative pressure and the primary route for backdrafting. Isolating the woodstove chimney stack from the interior also eliminates the competing stack effect which can cause backdrafting of other appliances (other woodstoves, e.g.) as well as diminishing the draft of the chimney.

    If this were not true, then there'd be no reason to require sealed combustion, direct-vent appliances in tight homes, as building codes and Energy Star do.

    Unless you're deliberately using the woodstove as an exhaust fan for "free" whole-house ventilation (as I've done on a couple of superinsulated homes), then you don't want an additional exhaust appliance adding to the exfiltration flow and increasing the danger of excessive infiltration and its consequent moisture problems.

  10. homedesign | | #10

    Now that's what I'm talking about! Awesome.
    Thank you ROBERT.

    Thanks for addressing the the closing remark in Daniel's question.

  11. Daniel Ernst | | #11

    John - I'm not sure about the context of your statement (or mimicry).

    Michael provided a very specific answer that was well stated. It's a joy to me to find such intelligent and experienced people on this forum. I hope that in expressing my enthusiasm I wasn't showing partially. I'm very appreciative of Robert's insight and experience and his willingness to share details! Thank you Robert!

    Perhaps I have misrepresented myself. I don't know. I'm not arguing whether or not an outside air supply is necessary for woodstoves. I don't have enough experience or credentials to argue the relative merits of either system. I'm simply trying to understand the risks associated with woodstoves in tight houses. That's what it is in my mind, a risk.

    It seems to me that all woodstoves will spill at some point. That point depends on a LARGE number of variables. I think we can all agree that the tighter the house the higher the risk for spillage, no? And given a very tight house, even a woodstove with an outside air supply can spill, no?

    Michael and Robert's answers have given me a way to play with the numbers (so to speak) and perhaps calculate that risk.

    Robert - I don't plan to use the woodstove as a passive exhaust. Norbert Senf's study showed that 30 L/s was about the maximum airflow into a masonry woodstove, and that number came from testing a couple of "underfire" models (air intake below the wood charge). Most "overfire" models were in the 20 L/s range. As you likely know - users of masonry heaters close the chimney after the fire is out with either a guillotine style or chimney top dampers. Personally I use the chimney top model, which has a silicone rubber gasket. Since the stove is only used for ~ 2 1/2 hours a day, an open chimney would waste most of the heat produced and stored in the masonry.

    My concern for depressurization is only during the time when the fire is burning. Usually that's in the evening when kids are taking showers and something is on the cookstove - and you have competing exhaust streams.

    Somewhere on this forum I've seen your email address. I'll try to send you Senf's study (for shits and grins anyway ;). Thanks again. Daniel

  12. homedesign | | #12

    I know.....
    I am fireplace and woodstove phobic.
    I tend to think in extremes and worst case conditions.

    I thought Michael Blasnik and Robert's comments were both excellent.

    In the closing to your question you also welcome comments from those who believe that woodstoves may not belong in tight homes.

    You seem to be looking for an equation that will justify your desire to have a fire inside your home.
    If the odds are good enough .. you will pull the trigger

    It looks like you have done your homework and you may have found a way to manage the risks and perhaps not waste so much energy.

    I prefer no risk when there is an option and less waste.

  13. Daniel Ernst | | #13

    John - Now I get it . . .

    I'm going through an internal debate right now. I see these Passivhaus projects using extremely small quantities of heat. If I run the calculations it seems like a woodstove is overkill. And like you said, it has risks and problems.

    That points me in the direction of a small heat-pump. Clean, no risks, efficient, etc.

    Yet I go back to thinking that in the end, I'm going to end up burning something anyway (whether I have PV or not). Somewhere, somebody, sometime will be tending a steam-powered turbine that's driven by coal or oil.

    Wood is local, renewable and doesn't have the inefficiencies of our existing electrical distribution system. It does have its own share of problems.

    So I go round and round and round. Like you, I think I take all of this stuff way too seriously (I saw that you called yourself obsessed in a couple of places!).

    The debate is still very much open in my mind . . .

    Thanks for the feedback!

  14. Daniel Ernst | | #14

    In the interest of sharing, I found this post on the Passive House forum:

    Katrin has some detailed suggestions for dealing with woodstoves in a PH.

  15. homedesign | | #15

    Thanks Daniel,
    that is a good link

    I was not born with an attraction to fire....
    But I realize that many, many people really,really like fire

    I also realize that some people have access to all the wood they care to gather up.
    that makes sense

    Around here (in the burbs)folks buy firewood or those synthetic "logs" from the 7-11

  16. Riversong | | #16

    The PH discussion about fireplaces and pellet stoves further supports my contention that such sophisticated technological life support systems with so many potential failure modes belong in outer space, not on Earth.

  17. Expert Member
    ARMANDO COBO | | #17

    With all due respect, you can analyze a home to death but make sure you have fun with it. I’ve designed 2 homes for PE & PhD that by the time their builder was done with the house, they were miserable. One of the PhD’s wife told me that her husband suffers of RPA… since I was not aware of such illness, I fell for it and ask, “What’s RPA?”… Her answer: “Royal Pain in the AZZ”. They live happy together as far as I know.

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