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.13 ACH home performance test results…is that too tight for spot ventilation, kitchen fan and electric clothes dryer?

Frank O | Posted in Energy Efficiency and Durability on

My new house was tested at 50 pascals and my reading was .13 ACH.

The auditor and I also played with the spot ventialtion fans, and his manometer, and we were able to reach -46 pascals with all house fans on and the electric clothes dryer on. My concern is with such a tight house, where does the air come from during spot ventilation and clothes drying?

I have a HRV which provides slightly postive pressure in low mode during normal use…but does not adjust for the extra load of spot ventilation – namely the kitchen fan.

Do I have to open up a window every time I turn on the bath fan or clothes dryer?

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  1. GBA Editor
    Martin Holladay | | #1

    Frank O,
    In general, makeup air for small exhaust fans come through random envelope leaks (under mudsills and around windows and doors, for instance).

    If you are worried, you can always test the airflow of your exhaust fans. If you have a 40 cfm exhaust fan, test the airflow. If it is moving 40 cfm out of your house, then by definition, 40 cfm is also leaking in, since airflow out = airflow in.

    So as long as your appliances are working, don't worry. However, if you have an atmospherically vented combustion appliance (like a water heater, furnace, wood stove, or fireplace -- something that is not a sealed-combustion unit), you should have a backdrafting test performed to be sure your exhaust fans aren't pulling outdoor air down your flues.

  2. Riversong | | #2


    You must not have read the post. This house is so tight that there are virtually NO random leaks. That is evidenced both by the insanely low ACH50 and by the fact that the house exhaust fans can depressurize the house almost as much as the blower door fan.

    That means none of the spot ventilation fans are able to exhaust anything close to their rated flow. That also means that the dryer is going to run much longer than necessary because it can't move sufficient air.

    This house absolutely requires additional make-up air, at least in the laundry room and perhaps in the kitchen if the range hood is rated at more than 100 cfm.

  3. Frank O | | #3

    Robert, that's exactly what the auditor you have a steer to smart systems where I can get the additional air as required? During normal operations (low flow on the HRV, no spot fans running), I have slightly positive pressure throughout the house.

  4. Christopher Briley | | #4

    Wait. When you did the blower door test and you came up with these incredibly tight numbers (congratulations by the way) you then tested the envelope with the appliances running, came to the conclusion that you needed more make-up air, did you then unblock the ventilation of the HRV?

    I should think that a good portion of the relief air that you'd need would come right through the HRV.


  5. J Chesnut | | #5

    Are you sure you calculated your ACH number correctly? It is not unusual for someone to determine the volume of the building incorrectly and therefore miscalculate the ACH from the cfm reading.

    .13 ACH is a very tight envelope. The tightest among Passivhaus builders in the US I am familiar with is .15 ACH and I heard this was possible to attain because it was a mid sized commercial building (apparently the smaller the building the more difficult it is to get a low ACH).

    At this level of air tighness you need to use the smallest fan (I forget the letters that indicate the sizes) not the standard size that a typical blower door contractor is equipped with for the blower door test in order to get accurate results.

  6. Frank O | | #6

    Chris - for the appliances, we used the testers handheld manometer (tubing sensor placed outside of door) and then turned on the spot fans and dryer ... numbers read -47 with all units running. With just the HRV running, it read 0 to +1 pressure.

    J Chestnut - the tester used a laser meauring device and did come up with a square footage number which seemed lower than my building plans (2700 v 2900 sfh). I will discuss with him. Thank you for the reference re passivhaus standard.

    House air seal and insulation: blown in cellulose in closed scissor truss attic w/ limited penetrations, damp cellulose in standard framed walls, epdm gaskets top and bottom plates, rigid blocking foamed in along rim joist (along with all penetrations), quality doors and windows. Ventilation: panasonic bath fans, big arse range hood, Venmar 189 cfm HRV

  7. Riversong | | #7


    I'm not sure what you mean by "smart systems". It's not smart to install a "big arse" range hood in a tight house. I never use one that pushes more than about 150 CFM.

    I think it's crazy to build that tight. 2 ACH50 is more than adequately tight in most climates. I always install a passive make-up air duct through the wall into a laundry room with a dryer, with a louvered cap on the inside and a screened hood on the outside. To prevent cold air thermosiphoning when the dryer's not running, I drop the duct 3' in the wall before exiting to the outside, since cold air cannot rise, and I weatherstrip the door to the laundry room.

    I prefer simple, failsafe systems, so I use American Aldes Airlet 100s as make-up air inlets for my exhaust-only ventilation systems using Panasonic bath fans and Grasslin programmable timers.

    I would advise changing out that range hood for a more appropriate unit and you could install an electrically-operable air inlet wired in parallel with the hood.

    The slight positive pressure from the HRV should not matter, but in cold weather negative pressure is preferable to positive to prevent exfiltration and condensation in the thermal envelope.

  8. Frank | | #8

    thanks for your recommendations and comments. Fortunately for me, the stove fan has 2 lower settings besides big arse (I save that one for my purchase...).
    Spot ventilation is just that...periodic, spot as needed. The majority of the time, the units are not running, and my HRV provides a balanced, constant supply of fresh air in the low mode.
    So, what is the risk of running periodic negative pressure in the house? High pressure infiltration? (I have no gas appliances)

  9. Riversong | | #9


    As I said, your exhaust fans including the dryer are going to run longer than necessary since they cannot push air out of the envelope unless air can leak in at the same volume and the same rate.

    This not only means excess and unnecessary use of electricity and inadequate performance, but also premature failure of those appliances.

  10. A non mouse neo-dadaist | | #10

    What is the R value at all these vent locations Robert? In another thread you are against recessed lights that are sealed inside foamboard saying no recessed lights is right. Btus lost per situation can't be that different. And a direct vent like your Airlets must have an R value of what 0? Choices eventually to be made of when to not mind R-0 and when not to mind a recessed witth R-14. The recessed is doing 14 times better than the Airlet. Yes...the vent is good for air to breath. The light is also good for personal reasons and for.... seeing. Another thing we all like to do.

    Agreeing to disagree with you in advance.

    By the way... pretty good advice IMO being given for this thread's too tight home.

  11. Frank O | | #11

    UPDATE: 2.14 ACH50, .13 ACH natural. Thanks all for your feedback...too large of a range hood has been addressed.

  12. Riversong | | #12


    Those numbers are more reasonable. That's a good target for air tightness.

  13. jude | | #13

    I'm confused about the issue of a house that's too tight, can anyone explain please? My house had a blower door test result 0.28 ie didnt reach the recommended 0.35 AChn and I was advised to get a WhisperGreen 80 CFM bathroom fan running at 18 hours a day. Does this make sense? If the house is tight will I get enough leakage inwards for this strategy to work? Am also installing a pellet stove insert with external air supply via the chimney, whose blower is 150 CFM. Could this be a problem too, even though it has external air coming in?
    Thank you!

  14. David Meiland | | #14

    Please clarify: your blower door test result is that you're getting .28 ACHn?

  15. Riversong | | #15


    Is this a new or existing house? Where is it located - what climate zone? What was the ACH50? And how big is the house, how many bedrooms, how many occupants? Was there no bathroom fan before?

    For most homes 0.28 ACH natural should be sufficient for routine air exchange, but you still should have and use exhaust fans in full baths and over the kitchen range. The stove insert uses outside combustion air and recirculates inside air, so it has no net effect on air exchange.

  16. jude | | #16

    Old house - family of 4, built 1965, in zone 2 New Hampshire (brrrr). yes the blower door test was 0.28 ACHn. I have some other numbers on my report: House size 2150 sq ft (excluding basement); CFM50=1200; CFM/ft2=0.56; ACH50=4.2; ACHn=0.28; leakage area sq inches=120; and leakage area sq ft=0.83. I have some more results called Air Leakage test Results in a different part of the report (not sure why the results are in 2 different places) where it says Measured leakage=54 sq in (1197 CFM @50 Pa); the est. annual air change rate is 0.16 air changes/hour (9.4 CFM/person); and estimated cost of air leakage is $403/year in heating. The upstairs windows get lots of condensation, and moldy - I wipe them 4 times a year with dilute bleach. We have 2 damp bathrooms each with 70CFM fans which definitely need upgrading. Appreciate the help! These reports can get a bit confusing even with explanations by the auditor!

  17. Riversong | | #17


    You're right that the report is confusing. The numbers don't quite add up, and you're either in climate zone 5 (from Concord south) or zone 6 north of there.

    The house is surprisingly tight for a 1965 vintage, and you clearly need more air exchange (15 cfm per person) and better bathroom exhaust fans. Bleach is actually discouraged for mold mitigation. Simple soapy water works fine.

    Installing an efficient bath fan, like any of the Panasonic Whisper series, and running it both on a demand timer in the bathroom and a programmable 24-hour time like the Grasslin KM2 ST, would make the house both dryer and healthier, though it would do nothing to save energy costs. The house should be leaky enough to supply make-up air.

    If that doesn' t take care of the upstairs condensation problem, then you need to look for another source of moisture, such as a damp basement and repair that.

  18. David Meiland | | #18

    At least some of the numbers appear to add up. The 'Zone 2' that Jude is referring to is probably off the LBL "N-factor" map that BPI auditors and others use to estimate ACHn from measured CFM50. New England and large parts of the country are in Zone 2 per that map. If the house is 2150 square feet with 8 foot ceilings and the second floor frame is excluded, there are 17200 cubic feet. 1200 CFM50 equates to 72000 CF per hour @ 50, or 4.19 ACH50. The LBL N-factor for Zone 2 is from 17 to 20, use 18.5 as an average and a 2-story correction factor of .81 gives you N=15... 1200 CFM50 divided by 15 = natural airflow of 80 CFM, or 4800 CF per hour = .28 ACHn.

    I've never paid much attention to the leakage area numbers. The Minneapolis BD software provides 2 significantly different figures. Suffice it to say that 1200 CFM50 in a 2150 foot house is a fairly small hole no matter what.

    Question for Jude: did the auditor measure your bath fan flow with the doors closed? I've seen quite a few that are rated 80 CFM but actually move anywhere from 30-50 CFM due to long and convoluted duct runs, use of flex duct, no undercut on the bathroom door, or some combination. If those aren't moving enough air to keep the room reasonably clear during a shower then they should be improved. There's enough leakage area in the house to supply them.

  19. GBA Editor
    Martin Holladay | | #19

    Besides the issues under discussion -- air leakage rate and mechanical ventilation (bathroom fans) -- it's important to discuss sources of moisture. Robert mentioned the possibility of a damp basement; you should investigate that possibility.

    Is there any possibility you are running a humidifier? If so, turn it off.

  20. jude | | #20

    Thank you all. The test did NOT include as check on the bathroom fans, tho' I do intend to replace those with bigger ones. There's always been a damp problem upstairs - when we bought the house 10 years ago it had NO bathroom fans at all (!) & we installed the 70 CFM ones which aren't big enough. My husband takes very long showers. But here's the next bit of the puzzle - yes we do run a humidifier downstairs (downstairs is open plan) because we have 2 wood instruments that will crack without it - piano & harp. The harp's sound box already cracked & needed repairs due to drying out about 4 years ago which is when we started using the humidifier in winter. I guess we may need to find a way to keep them in their own room & only humidify that room - difficult with our floor plan. The basement has a pump that's set off when the water level rises (we're on a hill & the snowmelt runs under our house), & we also run a dehumidifier down there. Sell up & move to Florida?

  21. GBA Editor
    Martin Holladay | | #21

    Bingo! I guessed right. You're running a humidifier, and your windows are dripping.

  22. jude | | #22

    If my new pellet stove insert blows 150 CFM from an external air source (that's how they're going to install it in the chimney), will that help with air exchange - in other words, isn't the warm air that's sent into the room going to be new air from the outside?

  23. Riversong | | #23


    The pellet stove should take its combustion air from outside, but it should recycle indoor air through the heat exchanger. I've never seen a woodstove that heats outside air.

    All the blower door determines is the air flow rate at 50 pascals. The conversion from ACH50 to ACHnat is theoretical. But, if the downstairs is so dry that wooden furnishings are cracking, then you have too much - not too little - air exchange. The only thing that dries out a house in the winter is infiltration of cold air which, when warmed, has a very low relative humidity. The leakier a house, the drier it is. A tight house tends to get too humid, and that's the primary reason for whole house ventilation.

    I would suggest tightening up the house, with particular attention to the basement and the top floor, and seeing if you can get rid of those humidifiers.

  24. Riversong | | #24


    In the old DOE climate zone map, New England was in zone 1 and 2.

    The LBL climate correction zones are not climate zones, since Vermont and Texas are in the same zones.

  25. David Meiland | | #25

    I'm surprised you mentioned Vermont and Texas in the same sentence. We'll let it go just this once.

  26. Jude | | #26

    Thank you all so much, this has been very helpful. Who'd have thought science (in the energy audit) could cause so much confusion!!

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