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Q&A Spotlight

Does Spot Ventilation Work in an Ultra-Tight House?

A homeowner wonders where makeup air comes from

Measuring air leakage: A blower door test at Frank O's house finds amazingly little air leakage, which makes him wonder where makeup air for exhaust fans comes from.
Image Credit: Green Building Advisor

UPDATED: 12/9/10 with expert opinions from David White and Marc Rosenbaum

Frank O’s new house is tight — very tight. Tests by an energy auditor measured 0.13 air changes per hour at 50 pascals of depressurization (ACH50), meaning the house beats the very stringent airtightness target of the Passivhaus standard.

Frank O has installed a heat-recovery ventilator (HRV) to provide fresh air as well as fans for spot ventilation and a range hood fan rated at 189 cubic feet per minute (cfm).

Sounds perfect. So what’s the problem?

“The auditor and I also played with the spot ventilation fans and his manometer, and we were able to reach -46 pascals with all house fans on and the electric clothes dryer on,” Frank O writes in his Q&A post. “My concern is: with such a tight house, where does the air come from during spot ventilation and clothes drying?”

Are the numbers right?

J Chesnut’s first question is whether the auditor has calculated air leakage correctly.

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

“The tightest among Passivhaus builders in the U.S. 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).”

In a subsequent post, Frank O confirmed Chesnut’s suspicions. It turns out that his house tested at 2.14 ach50, not 0.13 ach50. So Frank O’s house isn’t quite as tight as his first post claimed.

Frank O provided additional details about how the house was constructed: blown-in cellulose in the scissor-truss attic, damp cellulose in the framed walls, EPDM gaskets under the bottom plates and at the top plates, foam along the rim joist and at building penetrations, and good doors and windows.

Makeup air comes from random leaks

To answer Frank O’s question, GBA senior editor Martin Holladay says that makeup air usually comes from “random envelope leaks,” such as under mudsills and around windows and doors.

“If you are worried, you can always test the airflow of your exhaust fans,” Holladay says. “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.”

Wrong, says Robert Riversong.

“You must not have read the post,” he says. “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.”

The result, he says, is that none of the spot fans will be able to pull “anything close” to their rated flow, and that the clothes dryer will run much longer than necessary because it can’t move enough 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,” Riversong says.

Finding a source of makeup air

Riversong’s recommendations are what Frank O is hearing from his energy auditor. Are there “smart systems” that will do the trick? he asks.

First, Riversong says, a “big arse” range hood fan isn’t a great idea for a house that tight. Try something with a maximum rating of 150 cfm.

Second, he says, it’s “crazy” to build a house that tight to begin with. Although this is an old debate on this forum, Riversong thinks 2 ACH50 “is more than adequately tight in most climates.”

And as to the makeup air, he adds:

“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 ft. 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, fail-safe systems,” he adds, “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.”

Like many topics discussed on GBA forums, the question of passive air inlets is debated often, and consensus is elusive. In his comprehensive blog surveying ventilation options, Holladay explained why passive air inlets are usually unnecessary.

If a tight house includes a powerful range hood fan, however, a powered makeup air unit often makes sense; Holladay provides advice on the topic in his recent blog, “Makeup Air for Range Hoods.”

David White of Right Environments and engineer Marc Rosenbaum weigh in:

David White:

For passive houses (and any air-tight house) my approach is:

1. Try to eliminate exhaust appliances

Not only because they force an air exchange with the outdoors, but also because they penetrate the envelope.

2. When a penetration is necessary, use a dedicated inlet the delivers the air directly to the device.

For instance, for a gas dryer, I have two 4-inch lines. One is the dryer exhaust while the other is a dedicated outdoor air line that dumps behind the dryer. I have heard that there are pre-manufactured kitchen hoods like this as well. For wood stoves, a dedicated outdoor air line of course.

3. Seal the lines.

I’m not sure how best to do this, but at this point I use motorized dampers wired to the dryer, on both lines. You could use backdraft dampers, but I’ve had difficulty finding one that doesn’t open too easily (allowing infiltration) but opens easily enough not to excessively de-pressurize the space. Ben Southworth used a sliding gate valve by Valterra on his outdoor air line in one project to positively seal the line when not in use.

I’m assuming that ventilation exhaust is not the subject here.

I always do a balanced supply/exhaust with heat recovery, and even if heat recovery isn’t involved, it’s best to balance supply/exhaust in my opinion.

Now, Marc Rosenbaum:

I guess the question is, what impacts are we worried about? The big one is depressurization causing trouble with combustion devices. Sealed combustion (that is really sealed!) is a necessity.

My preference in houses that are Passive House tight is no combustion devices, period.

Once there is no combustion, then the issue is where does the make-up air come from? I’m not sure I care all that much for transient loads like a range hood or dryer. I think you need to verify that the fans you have installed have a sufficient pressure capability to provide the exhaust you need (I haven’t looked at dryer fan pressure vs flow curves, if you can even get that!).

Germans use condensing dryers, no exhaust.

If a house is 100 CFM50, then a 100 CFM range hood will depressurize the house to 50 Pa at 100 CFM, so it needs to be able to move 100 CFM at 50 Pa (not a problem I think).

My colleague Mike LeBeau has a clever set-up for small exhaust flows.

He puts a couple of motor dampers in the exhaust of the HRV and when the kitchen range hood comes on he sends the exhaust of the HRV back into the house to balance the flow.



  1. Jan Juran | | #1

    Makeup Air
    Hi Scott: the ACH data for Frank's house was ambiguously worded, as Frank later noted:
    11. UPDATE: 2.14 ACH50, .13 ACH natural. Thanks all for your feedback...too large of a range hood has been addressed.
    Answered by Frank O - Nov 13
    --At ACHnat of 0.13, a 2700 SF house with 8 ft ceilings has 21.6K CF which ventilates naturally at 2808 CF per hour or 47 CFM (albeit more on windy days and less on calm days), enough to handle those spot ventilation fans. As you correctly point out, tighter houses and larger exhaust fans can pose imbalances requiring makeup air. I recall Dr. Wolfgang Feist once proposed a solution whereby a clothes dryer exhaust could be run through an HRV, recapturing 80%+ of the heat. Dr. Feist, perhaps could you help us out re the specifics of this idea; how does it balance the air flows?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Jan Juran
    Jan Juran,
    Thanks for your comment. I have edited Scott Gibson's article to reflect Frank O's correction.

  3. David Meiland | | #3

    Recovering copy editor notes that
    Frank O becomes Robert O in the fourth section of the article.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to David Meiland

  5. John | | #5

    HRV inlet becomes the make up air supply?
    I thought I asked a similar question a little while ago and someone commented that the HRV will just leak (ie not be balanced) and supply the make up air?


  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to John
    Strong depressurization can unbalance an HRV system and cause the HRV inlet to leak. But an HRV system is not designed to provide makeup air, and should never be depended upon to supply makeup air.

    HRV manufacturers are explicit about this fact in their instructions. They advise installers that if you need a makeup air system (for example, to provide combustion air to an atmospherically vented appliance), then you need to install a separate makeup air system, since HRVs are NOT designed to provide makeup air.

  7. Jan Juran | | #7

    Dryer Exhaust and HRV
    Dr. Fiest alluded to this idea generally in a post in May but more detail re air flows balancing implementation would be interesting:
    12. Martin: Yes, I often asked myself whether one can top the German extensive use of the wash-dry-wear-cyclus. You just told me: Yes, one can. But, doesn't matter, with a cabinet a bit larger than the one shown, you can do what you need (:-). And another idea: The propan/butan heaters in your dryers - these will produce some 1 to 3 kW of heat output. That is just as much you need for the whole heating of a passive house. So, you could save the money for the whole heating system and use the dryer exhaust air (run it through the HRV) to heat a Passive House. You might laugh: Such ideas have already been tested by one of the "extremist" passive house engineers in Europe.

    Answered by Wolfgang Feist - May 4 10

  8. Bruce Glanville | | #8

    Tight homes and ventilation.
    There is no blanket answer to the issue other than to consider the "Whole House" point of view. Start with 'do no harm" and consider combustion saftey as your priority. Once we know there are no CO backdrafting issues then the make-up air problem can be addressed. If the place is really tight (<= to 2ach) then the air out = air in concept is relevant to exhaust vent performance. There are several balenced sytems on the market from Honeywell and Panasonic. Climate Zone becomes an issue as the moisture control question come up. I live in a mixed-humid zone and there have been some tight houses here that have not performed well due to humidity control, or lack there of. The point is, we need to look at each home as a seperate case and not expect there to be a one size fits all solution. Stick to the KISS principle and use common sense and your blower door.

  9. Roy E. McAfee | | #9

    passive make up ducts!
    Are we really going to go to the trouble of designing, installing and charging for "passive make up ducts?" At what point doesn't a well designed home (attention to window and door placement) and traditional crafted windows (long lasting windows made from sustainable materials like wood and glass) make just as much sense? Our ancestors where neither dumb or ignorant. Their designs and techniques came from millennium of experience. Start from that asumptioin if you wish to build a better home.

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