“I don’t get it why would we want to make the house so tight that you would need ventilation?”

“I don’t get it why would we want to make the house so tight that you would need ventilation?”


I was asked this question by a potential customer. I think that this is certainly valid question. I answered it to the best of my ability within the short amount of time. I explained that the mantra of our industry is to seal it tight and ventilate it right. I explained the positive impact on IAQ and reducing the amount of infiltration from the dirty attic. I said our goal was make the house safe and comfortable and we would naturally become more efficient. That our repairs would work 24 hours a day hot or cold rain or shine.

I explained that the house was already tight and our measures would make it tighter and ventilation was necessary if the work was to be performed. ”but I keep windows open all the time” she replied. I explained that the ventilation would still be needed in case the windows were not open. And that filtered fresh air is a good thing. I explained that this is not a new thing as commercial buildings have been employing ventilation needs for decades. I also explained that current codes here in California have ventilation codes.

“Well I could see it if I had small children but I just don’t see the benefit.”  Our customer base is sometimes willing to listen and sometimes defiant. The fact is that these principals for a performance home are difficult for some folks to accept.  They do not understand nor see the benefit. Nor do they want change.

It is easy for most to grasp the concepts of insulation, duct sealing, windows (as misguided as much of the information is) high efficiency HVAC systems and Photovoltaic’s.  The highest hurdle for most of our customers to understand is Ventilation. The fact that we state we can save you energy and then require a device to be installed that was not currently in the home is counter intuitive. You can’t visibly see stale air or infiltration.

While good ventilation might have the highest positive impact it is in my opinion the hardest concept to sell.

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I have been asked that question since we first required house tightening and mechanical whole building exhaust in the Northwest in 1983 and I have spent the past 30 years trying to answer it.  Several commentors above hit the concept right on the nose - do you want to depend on Mother Nature and the wind/Delta P or the Delta T to provide you with your "fresh air" or do you want to control it?  Having a house at 7 or 10 ACH50 with operable windows works well in a windy temperate climate with no outdoor particulates or odors (or crime) and 70 degrees year around.  The vast majority of the country is not that simple.  The leakage rate of a house changes daily and even hourly.  How much "natural ventilation" do you get when there is no wind and it is 50 to 75 degrees outside?  Virtually none because there are no driving forces to bring in outdoor air through those open windows or through the building envelope.  That annual leakage rate estimated by your blower door software is exactly that - an estimate of an average.  That means there are just as many days that the home is underventilated as there are days that it is over-ventilated.  That also means that there is probably little or no ventilation in the spring or fall when there are not driving forces.  A tight enclosure with mechanical ventilation will almost always provide better IAQ with lower energy bills and more comfort than a leaky house depending on the house to "breathe".  Houses don't breathe - people breathe and they do it 24/7 year around.


I am glad to see so many commentors taking the position that mechanical ventilation is very necessary.  I like to follow the doctor's creed that says "First do no harm".  Ventilation is a health and safety issue, period, both for the occupants and for the building. Deal with combustion safety issues first.  Get rid of that old waterheater or wall furnace or other old combustion device.  Remember, the most efficient fireplace is one that is welded shut (and sealed).  Replace it with a flat screen or a plant... Provide mechanical ventilation second, preferably following ASHRAE 62.2-2010.  Then you can tighten as much as you like for energy savings.  If your climate is very hot or very cold, consider heat recovery or energy recovery ventilation (HRV/ERV). Othewise, follow the KISS rule (Keep It Simple, Stupid). Make it continuous, efficient, inobtrusive, and quiet.


We have just finished a guide to retrofit ventilation for the California Energy Commission that will go live as a website under LBNL next month.  It includes a simple handout to help you answer those pesky homeowner questions about why they need ventilation, how much to provide, and how to install it so it works.  I will post the website when it goes live.  For reference, I am the Chair of the ASHRAE 62.2 committee and have worked on ventilation programs, products, standards, and codes since 1983.

Good morning Don, will it have ventilation strategies for homes that are hydronic (radiators) with window A/C units?



Yes, the Guide addresses why we need to ventilate, how much to provide, pros and cons and comparative costs of various strategies (exhaust-only, supply-only, and balanced ventilation), examples of the calculations based on both the ASHRAE 62.2-2007 adopted by California in the 2008 Title 24 requirements and on the ASHRAE 62.2-2010 edition with the 2011 supplement of addenda and the two addenda adopted this winter, and drawings of 16 systems on the same sample house.  The 62.2-2010 with addenda requirements are what CEC adopted this past Spring for the 2013 California Title 24.  While the Guide was written for California, it is applicable in most parts of the US.  We breathe the same amount everywhere in the world and 62.2 gives direction on how much ventilation you need; no matter where you are or what type of heating or cooling you have, the needed ventilation rate is the same.

What nobody has addressed is that controlled ventilation is just as important to the health of the house as to the health of its occupants - and the two are intimately related.

Uncontrolled ventilation means uncontrolled moisture movement, since most moisture migration is cause by convection. This is often not a major problem in leaky houses, since the envelope can often dry out on its own from the excessive convection. But, once we tighten up a house by adding both insulation and air sealing, we reduce the two forces that allow a building envelope to dry on its own: heat flux and air flux.

When we tighten a house to reduce energy consumption, we also risk making the building envelope vulnerable to moisture problems, including mold (a major health issue) and rot. Excess indoor humidity can also increase dust mites and other allergens, damage electronic equipment and accelerate the outgassing of formaldehyde from common building materials and furnishings. If the house is in an area prone to ground radon, then radon accumulations can also increase in a tight house that is not properly ventilated (though separate radon mitigation is more effective at reducing this risk).

It is a really good discussion so far: Of course, how airtight is tight enough is a question of the local climate. But: In most climates it pays of having really tight building envelopes.

(1) The most important cause is: To avoid construction damage by condensation of humidity. That's the most frequent cause of structural damage in Winter cold climates - and certainly also in Summer humid climates. We can avoid such damage by using airtight constructions. That's the main unique reason we demand that Passive Houses have to be airtight (there are more benefits: no drafts, full function of the insulation, no additional infiltration losses, no external sources of indoor air pollutants).

(2) Now: Drafts from really leaky envelopes will not be accepted by most customers (at least in Europe - may be not in the US?). So, new built buildings are already much tighter than they should be if to be sufficiently ventilated by infiltration. Consequence: Already in a "normal" new built you would need mechanical ventilation (in some countries this is acknowledged and therefore demanded in the building code: Finland, Sweden, France). No question that you need it in an airtight energy efficient house - I do not want to be responsible for health issues related to bad indoor air quality.

All US model codes and the IRC require mechanical ventilation, at least spot ventilation for source removal and often whole-house ventilation, as does Energy Star and similar efficiency standards.

The question of "how tight is tight enough" is a different discussion and John Straube of Building Science Corp. and I disagree with the PH mandate of 0.6 ACH50.

John Straube: In the cold climate anecdotal experience of Canada, houses over about 3 ACH50 tend to have a risk of interstitial condensation. Rates over about 5 or 6 tend to be dry.  Houses under 2 ACH50 tend to perform quite well and only gross local errors cause condensation problems. When the rate falls under 1.5 we note problems with high winter RH. 

I agree. From my 30 years of super-insulation design and building experience in the cold New England climate, I have found that 2 ACH50 is the ideal to aim for. Tighter than that is not cost-effective, not necessary for energy-efficiency, and can make the house less resilient in the event of a power outage.

Clearly, you and I will disagree.

So, US is got it in the issue of ventilation.

Yes, we do disagree on air tightness - but that is what makes the discussion helpful. (1) We have a lot of trouble from "not tight enough" envelopes everywhere in Germany and Austria - and those have been reported from the superinsulated homes in America, too. (2) It's almost no extra investment at all after you learned how to do it to go for some 0.4 ach n50. That is the average, new Passive Houses from experienced teams show. (3) To go for the nearly zero energy efficiency it wont work without the < 0.6 ach, infiltration losses will be to high


Zero heating energy is not possible in our cold 8,000+ HDD climate, and "net-zero" using PV is too expensive. So chasing the zero point is chasing a chimera.

With an exhaust-only ventilation system, infiltration losses are only what is required by ASHRAE for a healthy indoor environment.

There is a point where excellent is good enough, and perfection becomes the enemy of the good.

Yes, Robert, I agree with your last statement. But this is not about perfection - and we have been through that discussion a lot of times. The success of the built examples demonstrates it, this is reality. You can only make the experience if you try - and if you do not reject it before trying.

I even agree with that "zero" or "net zero" is not the point. "Nearly zero" is; and, the discussion should be about how near to zero is "nearly zero". My answer: So near to zero, that the arguments of Bill Gates "why we still need the traveling-wave reactor" becomes obsolete. That is the case with passive house efficiency.

We can go on for ever and we will not agree. 

There are many other factors that are just as important as the narrow metric of energy efficiency, including use of local materials and labor, resource-efficiency, global warming contribution, life-cycle costs and recyclability, toxicity and embedded energy of materials, ease of maintenance and repair, simplicity and affordability.

It's clear, from the recent unamicable divorce of PHI and PHIUS (the US organization) that you don't understand American conditions and building traditions and should keep your opinions to your side of the Atlantic.

Why so agressive? Yes, I agree again, it is no fun to answer to a statement like this one. But, I am quite sure, that the rules of logic and of a decent conversation are also welcome in America.

I think Katrin Klingenberg would ask you the same question (and I'm no fan of hers). Logic is merely a method to apply to knowledge of local conditions and traditions. The same logic will result in different conclusions in one culture as in another.

From my understanding, PHIUS attempted to fit the PHI standards, tests and protocols to the American building environment and you refused to allow them that necessary flexibility, demanding they follow the gospel according to Feist. But I'm really not interested in rehashing this breakup here, and that's an entirely different topic.


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