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Preparing for follow-up blower door test! Should the dryer vent and intake and exhaust for the HRV be sealed?


The follow up EcoEnergy audit on my house is tomorrow and I was wondering what the proper practice is for the blower door test with respect to exterior vents through the building envelope.

Should the the HRV intake and exhaust vent and dryer vents be sealed?

Asked by Anonymous
Posted Sep 29, 2009 10:09 AM ET


23 Answers

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Here is a Web page with blower-door instructions:

The instructions note, "Do not seal combustion flues, dryer vents or ventilation system exhaust or intake vents."

Answered by Martin Holladay
Posted Sep 29, 2009 11:48 AM ET


Perhaps..not standard procedure...but I would like to see results both ways
One test standard
and One test with all "intentional openings" sealed up
Isolating the Enclosure from the Intentional openings seems like useful info to me.
Especially if you are doing leak detection or thermal imaging

Answered by John Brooks
Posted Sep 29, 2009 12:05 PM ET


Thanks Martin and John,

It makes sense to leave the "intentional openings" unsealed for a blower door test. Though the HRV openings combined are rather large, and must end up increasing the overall air leakage, which will affect the score and ultimately the rebate that the program calculates based on among other things the blower door test.

Since the air leakage out of the HRV vents is planned and a good portion of the energy is recovered it would seem contrary to be penalized for the air leakage that the blower door test detects from the HRV vents.

Maybe HOT 2000, or whatever program ecoEnergy is using to evaluate the house, already accounts for the HRV and resulting air leakage out the vents.

Thanks for the link. I'll read it at lunch.



Answered by Andrew Henry
Posted Sep 29, 2009 12:24 PM ET


Andrew, it's not at all "unfair" to include the air leakage of both the building envelope and its combined mechanical systems, since that measures the as-built air change rate. When the HRV is not running, it will contibute to the building's natural air exchange. And, in fact, when it is running in a theoretical balanced mode, the rest of the building leakage (including vents and ducts) will contribute to a pressure imbalance which will create negative and positive pressure differentials and consequent infiltration and exfiltration.

I also feel it's useful, at least to the builder, to know the air leakage rate of the isolated envelope (with all ducts and vents sealed), and to know where that leakage occurs.

Answered by Robert Riversong
Posted Sep 29, 2009 5:17 PM ET


Thanks Robert,

Philosophically I agree with you. It was my deep down, less virtuous side, interested in the maximum rebate possible for all the sweat equity I put into the house, who was musing about it being "unfair". : )

Thanks for straightening me out.


Answered by Andrew Henry
Posted Sep 29, 2009 10:14 PM ET


Just a follow up after my ecoEnergy second evaluation and blower door test.

The auditor, who was a pretty keen young guy, was kind enough to do a blower door test with the HRV vents sealed up after doing the normal blower door test. There wasn't any difference in air leakage with the vents open, as they should be, and with them sealed. Perhaps the HRV has a damper that closes when it is shut off, preventing air leakage through them.

The auditor had gone to carpentry school and been disillusioned by his experience framing tract homes. He was interested in building energy efficient homes and I guess has discovered that most production built homes aren't designed with energy efficiency in mind. Which is why he was now doing home energy audits

I told him about Passive House, GreenBuildingAdvisor, and Building Science Corporation which got him pretty excited. He was keen to volunteer on any green building projects I know of, so I gave him a couple of names of Passive House and Green Building guys I know in my area. It's heartening to run into young guys who truly want to build energy efficient houses. A guy with a blower door in his car would be a handy volunteer.

Disappointingly, he did mention that most of the other auditors at the company he worked for weren't actually all that interested in energy effiicient building. Just another job. Oh well!

Thanks again.

Answered by Andrew Henry
Posted Oct 1, 2009 2:27 PM ET


That's surprising that there was no measurable difference. I can't imaging the HRV dampers are that tight (surely the intake damper would be drawn open under negative pressure).

Do you want to share your results?

Answered by Robert Riversong
Posted Oct 1, 2009 4:34 PM ET


What were the results?
Did you get any clues from the test where any remaining weakness might be?

Answered by John Brooks
Posted Oct 1, 2009 4:45 PM ET


Robert and John,

The auditor said there was no "noticeable" difference with the vents sealed. I was outside, having just sealed the vents, when he ran the test. I was in a hurry to seal them, as the auditor was doing me a favour running this supplementary test. I think I did a pretty good job of sealing the HRV exhaust vent but I am not so sure about the intake, though I did my best.

As for ach@50Pa,I don't have the number. The ecoEnergy program doesn't give out that information it just provide a rating out of 100, 100 being the best. I asked the auditor if he could send me the ach@50Pa result, he said he'd email it to me, we'll see.

That said at 50 Pa the air leakage was 750 cfm. The result of the first blower door test was ~1500 cfm. The first test highlighted some obvious air leakage which I worked on resolving. The other way I identified much of my air leakage was a form of unintentional "fog machining" of the house. Let's say I ran out of time and to some extent and money, so we had to move in with the house not quite done. (The siding still isn't on, but I am working on it, full time job and two kids under two) . The heating system and HRV were up and running of course last winter and so the air leakage turned to very obvious hoar frost wherever it escaped. I don't recommend this approach to identifying air leakage!

My house is around ~1700 sqft, nominally with another 200 sqft loft, that doesn't count in the municipal calculation of the houses interior area, and a 450 sqft conditioned mini basement, only half of which I can stand up in.

What is the formula for calculating ACH@50Pa. For that matter, how do I calculate volume for a house that employs warm roofs. Is the volume only the interior living space or does the volume between the ceiling drywall and the underside (interior side of the insulation) of the warm roof get counted into the houses volume?

Answered by Andrew Henry
Posted Oct 2, 2009 3:01 PM ET



For heat load calculations, the house volume would be all the space inside the thermal boundary. If that is at the roof plane, then it would include the attic. If the foundation is insulated, that would include the basement.

For air exchange rate, the house volume would be all the space inside the air barrier. If that is at the upstairs ceiling, then it would exclude the attic.

So calculate the volume within the air barrier envelope, change CFM50 to CFH50 by multiplying by 60 min/hr (750cfm x 60min/hr = 45,000 cfh), and divide that result by the air volume of the house to get ACH50. To convert ACH50 to ACHnatural, divide the former by a factor of between 15 and 20 depending upon your climate zone, number of stories, wind-shielding and leakiness factors (15 is often used as a cold climate standard).

Answered by Robert Riversong
Posted Oct 2, 2009 3:27 PM ET


Or you could normalize your air leakage in a more sensible way: Divide the cfm50 by the square footage of the building envelope. Leaks don't happen within the volume. They happen at the surfaces, so it makes no sense to bring volume into this measure. ACH50 and ACHnat also happen to be biased toward large houses since the surface area increases as r^2 and the volume increases as r^3. Why do so many people in this industry insist on using ACH?

Answered by Allison A. Bailes III
Posted Oct 4, 2009 9:24 PM ET


Actually, the old 0.35 ACH standard was biased AGAINST large houses, which is why ASHRAE and the IRC shifted to a ventilation standard based on floor area and occupancy load.

But the reason for the emphasis on ACH is that the purpose of ventilation codes is to guarantee indoor air quality. Air barrier standards (which are envelope-based) are focused on energy-efficiency and reduction of moisture-related problems.

Answered by Robert Riversong
Posted Oct 5, 2009 3:53 PM ET


Yes, Robert, I'm aware that ASHRAE 62-1989 resulted in ridiculous discrepancies between the number of people rule and the 0.35 ACH rule for large houses. But I don't get your point. You point out the problem with using ACH for ventilation and then you seem to say that using ACH guarantees indoor air quality, if I'm reading you correctly. Are you saying that ASHRAE 62.2 isn't as good as 62-1989 because it's gotten away from ACH?

It doesn't really matter, though, because the point of the original poster's question was about Blower Door testing, not ventilation. The purpose of such testing is to determine how leaky a house is. Yes, that's related to energy use, but it has a pretty big effect on ventilation as well, especially if a lot of the leakage is coming from moldy crawl spaces or dusty attics or polluted garages.

Answered by Allison A. Bailes III
Posted Oct 5, 2009 5:49 PM ET


I said the purpose of the 0.35 ACH ventilation standard was to guarantee indoor air quality, not that any arbitrary ventilation standard actually guarantees anything. Obviously, exchanging indoor for outdoor air is beneficial only if the outdoor air is of better quality. Which standard makes sense depends largely on the sources of indoor air pollution. If the source is human occupancy activity, then it's logical to apply a per capita standard. But if the primary source is toxic building materials and radon and mold intrusion from poor building practices, then an ACH standard would be more appropriate.

If the goal is to achieve absurdly high levels of air tightness (the hermetically-sealed house) and to get LEED points, then measuring air leakage as a function of envelope surface area would be appropriate. If the goal is to ascertain a minimal level of indoor air quality for livability, then the whole house air exchange rate is the most appropriate measure.

We should be building homes for livability and health (including the health of the earth), not for brownie points.

Answered by Robert Riversong
Posted Oct 5, 2009 11:52 PM ET


Hear, hear. I'm with you on that last statement, Robert. I'm not a big fan of LEED myself, nor of doing anything just for points, brownie or otherwise. I AM, however, a big fan of sealing houses as tight as we can get them, to the level of what you call "absurdly high levels of air tightness (the hermetically-sealed house)." I built a SIP house to 0.14 cfm50/sfbe* (1.7 ACH50 if you want a volume-based measure) and could easily get it lower were I to have another crack at it. I don't believe that 'fresh' air comes through random leaks, so I'd rather see a tight house and intentional ventilation.

But even that should come only after you've done all you can to eliminate the sources of pollutants and to separate those that you can't get rid of. When you've done all that, then ASHRAE 62.2 provides too much ventilation for the house, so a controller, such as the AirCycler, allows you to dial in how many minutes per hour you want the outside air coming in.

Infiltration is one of the biggest problems in most houses, so minimizing it is good for health, livability, comfort, efficiency, durability, and, as you say, the health of the Earth. Every kWh saved in the home results in about 3-4 kWhs saved in the power plant (at least for those of us who get our electricity predominantly from coal).

Anyway, Andrew, let's get back to your original question, I don't think anyone ever mentioned it, but if your Blower Door test is for a HERS rating, the rules about what gets sealed and doesn't are spelled out in the HERS Standards, Appendix A, pp. A-26 & A-27. Dryer vents are left as is. Intermittent ventilation gets left as is. Continuous ventilation, like your HRV, gets sealed. You can download the HERS Standards at the RESNET website:


*sfbe stands for square footage of the building envelope.

Answered by Allison A. Bailes III
Posted Oct 6, 2009 2:22 PM ET



We seem to agree on "a tight house with intentional ventilation", but clearly disagree on both the proper level of "tight" and the means to accomplish it. We also, I suspect, disagree on what is required to achieve "intentional ventilation".

I consider 2 ACH50 to be a very tight house (equivalent to 0.10 - 0.12 ACH natural), and I believe that an exhaust-only fan system with strategically-located passive make-up inlets to be an ideal fresh air system, which will operate to some extent even during power failures and has the greatest likelihood of preventing the condensation damage caused by exfiltration because the living space remains under negative pressure.

Contrary to your assertion, infiltration is NOT "the biggest problem in most houses", in a heating dominated climate and in terms of moisture problems and their consequent impacts on building durability and human health. Infiltration does not cause condensation, since outside air is warming as it passes through the envelope and hence dropping in relative humidity. In fact, the most innovative approach to ventilation and heat recovery is the Dynamic Insulation System, using an air-permeable envelope that allows infiltration, filtering and heat recovery through the building envelope, controlled by an exhaust-only powered vent.

Building a hermetically-sealed house, particularly if composed of plastic in any form (poly vb, foam insulation), is analogous to putting a plastic bag over one's head and then inserting a powered snorkel into it in order to maintain life. In addition to the utter absurdity of creating a "dead" space and then relying completely on mechanical ventilation to allow life to exist within it, such a building has no moisture buffering ability (breathability) and a dangerously-low negative ion count.

Well before PassivHause, another German approach (all but ignored in the US) was Bau Biologie, which has as a primary tenet the need for a building envelope to breathe, like the cell semi-permeable membrane of any living organism.

Answered by Robert Riversong
Posted Oct 6, 2009 6:41 PM ET


I am confused about where you stand on airtightness.
I remember that you have reported better than 1.5 ACH50 on at least one project....
Is that too tight ?
Would you consider going back and drilling holes until you are back at 2 ACH50 ?

I agree that infiltration is not a condensation issue during the heating season.
Exfiltration can be a problem during the heating season.

I agree that vapor tightness is risky business.
I do not understand how extreme airtightness is a problem as long as ventilation is provided.
Exactly where do you think our buildings should be leaking air?
Should every building cavity have a prescribed air leakiness factor?

It is possible to build airtight without foam or plastic.

Answered by John Brooks
Posted Oct 6, 2009 7:15 PM ET



The only project of mine that was blower door tested, a modified Larsen Truss house with air-tight drywall system, measured about 2 ACH50 with make-up air inlets taped and about 3 ACH50 with them open. In this 8500 DD climate, that equates to approximately 0.11/0.15 ACH natural.

And, yes, I did build reasonably tightly (eliminating the common bulk leakage points), and then "drill holes" to make the envelope deliberately leakier (passive make-up air inlets - Airlet 100s).

Extreme airtightness (á la PH) is not a problem any more than building a hermetically-sealed spaceship is a problem as long as the life-support engineering is flawless and never ceases to function as designed - and the intention is complete isolation.

But a spaceship is a habitation designed for an environment that is inimicable to life. Why would we design a spaceship habitation on the one planet in the galaxy that has conditions perfectly suitable to the flourishing of life?

The problem with the entire Technological Program, upon which the edifice of modern civilization is built (like a Tower of Babel), is that it is in perpetual mortal combat with nature in order to subdue and conquer and overcome it - all for the illusion of safety, comfort and convenience (which, by all measures, we have less of today than in the past).

Bau Biologie (and perhaps to some extent, biophilic architecture) is based on the alternative concept of flowing with natural law instead of against the current.

One of the corollaries of the Technological Program is that if a little of something seems to be good, then a lot of it must be better. We've come a long way from the 3-6 natural ACH houses of our recent past (really just relearning the common sense of our ancestors who regularly rechinked their log cabins to keep the wind out). But it doesn't logically follow that, because moderate tightness is good, absolute tightness is better.

The procession of technological "progress" has been a linear progression from complete immersion in the natural world to complete isolation from it. It is no wonder that 1 in 10 Americans are taking anti-depressants - we've completely removed ourselves from everything that sustains life and gives it meaning: nature, community, spontaneous engagement with mystery and the infinite variability of life, and an understanding of ourselves as integral to the web-of-life.

Answered by Robert Riversong
Posted Oct 6, 2009 7:53 PM ET


As someone who grew up in the farthest extremes of northern Vermont, I read this with amusement:

"But a spaceship is a habitation designed for an environment that is inimicable to life. Why would we design a spaceship habitation on the one planet in the galaxy that has conditions perfectly suitable to the flourishing of life?"

There aren't many folks who feel that February in Vermont is perfectly suitable to the flourishing of life. It usually feels downright mean spirited.

The sign it was -20 degrees out was when we were waiting for the school bus and you felt like your eyeballs were freezing over.

Answered by Jesse Thompson
Posted Oct 6, 2009 9:57 PM ET


OK, this is (probably) my last post here on this topic. First, Robert, I'll recommend you read Martin's recent blog on the top 10 most worthless products for his take on passive air inlets. You're probably already familiar with his opinion on those because I know that's come up here before.

Second, I'll reiterate my position, which you seem to kind of agree with, that random leaks are not the way to get fresh air into the house. Controlled ventilation is the way to go. In a cold, one-way climate like Vermont, negative pressure ventilation is OK. In the South, where I live, bringing humid air into the building assemblies is the last thing you want to do so I always recommend balanced or positive pressure strategies.

Third, a tight house doesn't depend completely on high-tech mechanisms to keep the inhabitants from suffocating. They do include, you know, a certain type of modulating device that allows the people within to bring fresh air in whenever they want to. They're usually rectangular, spread throughout the house, and even allow for views to the outside. Yes, I'm talking about windows.

Answered by Allison A. Bailes III
Posted Oct 7, 2009 10:27 AM ET



I don't imagine the Abenaki had the same perception problems as you do about winter in Vermont. I know the Inuit don't consider subzero winters inimicable to life (the threat to them is the opposite - global warming).

And that's the point: the hairless apes evolved in the tropics but, once they began to exploit technology to control their environments, they spread over the entire earth. Over time, their dependence on technology became so complete that they had to isolate themselves from any deviation to a narrow band of "comfort" - and they perennially complained about the weather.

Negative 20° can be balmy to those who are accustomed to it. I took people dogsledding in northern MN when I worked for Outward Bound, living and sleeping outdoors without a tent in weather between -20 and -40. Everyone not only survived without injury, but had an opportunity to learn that it is mostly our attitude which determines whether we enjoy or hate our environment.

The more we isolate ourselves from the world around us, the less tolerance we have for any life which is not entirely artificial. And some have the audacity to call this trend "green" building!

Answered by Robert Riversong
Posted Oct 7, 2009 11:12 AM ET



Yes, I'm not only familiar with Martin's critique of passive air inlets, I've debunked his debunking eslewhere in this forum. The study he relied on was of admittedly leaky, poorly-built and poorly-operated homes. His criticism of these is similar to his critique of programmable thermostats (which are demonstrably energy-saving devices): he dismisses the device because of operator error, ignorance, apathy or misuse.

And yes, we agree that random leaks are an insufficient (and potentially dangerous) way to get fresh air into a house that attempts to be climate-controlled. My approach (with strategically-placed passive inlets) is anything but random. This is a cold-climate strategy which, combined with minimal natural envelope leakage and negative-pressure mechanical venting equipment (bath fans on programmable timers), creates a fresh-air system that not only prevents moisture problems but allows for some minimal natural convection even in the event of a power failure.

If the incremental advantage of HRV central ventilation equipment is energy-saving, then it certainly defeats the purpose to open windows for several days in the dead of winter during that ice storm which knocked out the grid. And if we can't trust occupants to program a simple set-back thermostat, then we certainly shouldn't trust them to monitor the air flow through open windows in the winter.

The obvious solution: eliminate the occupants and our high-tech homes will operate flawlessly.

Answered by Robert Riversong
Posted Oct 7, 2009 12:05 PM ET


I don't see any problem with the PH's extreme air tightness requirements. The reason for the said requirements are to reduce energy consumption, not to encapsulate the occupants. I would think that almost all PH envelopes include operable windows which can be open as much and as often as the occupant desires, as long as no energy is used for heating or cooling...

I also see no difference between being surrounded by an air barrier made of drywall, OSB, or plastic. All can be made to be extremely airtight with painstaking detailing. All require proper ventilation. All walls need to be vapour permeable in at least one direction to allow the envelope to dry if necessary.

Answered by Garth Sproule 7B
Posted Oct 7, 2009 7:16 PM ET

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