It is my understanding that you need an airtight envelope (let's say less than or equal to 0.6 ACH) along with super insulation (whatever R value you need) for best energy performance. However, a building science specialist in charge of energy efficiency in Berkeley states that air tightness is (1) not important and (2) would make a house feel uncomfortable (even after a HRV/ERV system). I would like to invite professionals (building scientists, energy experts) to evaluate these views. As a non expert myself, I would like to know the truth.

Thank you!

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Hi Kaushal,

To a point, air sealing saves energy.  Beyond that point it becomes more of an air quality issue as all homes and people need a certain amount of fresh air.  As to where and how much air is ventilated through a home, a tight home leaves that option up to you instead of random leakage through unknown locations with unknown contaminants.

IMO, we need to be more generous with the fresh air and distribute it to all parts of a home.  H/ERVs are an ideal opportunity if the collection  and distribution points are many and well selected.

Just wanted to get my opinion in before the crowd arrives :).


In the UK we often used to estimate that about 40% of heat loss in the average traditional poorly insulated house was due to air loss. Where the climate is more equable the percentage may be less but as insulation levels are increased the proportion of air loss becomes more significant and the effectiveness of increasing insulation is reduced. There is little point insulating to passive house standards without controlling air infiltration. It follows that HRV is also needed. In my experience for most of the time the air quality provided by a balanced HRV system is better than natural ventilation.

Thanks William; I wish folks here were equally enlightened on the significance of airtightness and thermal performance. 

Older buildings typically  are 6-15 ACH50   worst I have personally tested was 17 ach50.  The energy loss  scales ~ linearly with ACH50 so If you go from  17 to 1.7, the heat loss from air leakage is reduced 10 fold.  I usually find the air leakage contributes 300-1000$ per year in heating costs.

Generally speaking when I model building heat loss, I find  10-40% of the loss is from leakage. (Duct leakage can make it worse)

If you start with 40% of the energy loss from infiltration and cut that to 4%  ie 17 to 1.7 ACH50,  you saved 36% now if you cut that to 0.6, you save 38.6% from your original bill

Code for new construction is 7 and this still sucks  Next code cycle will be 3. Better, but not great.  Passive house is 0.6.

It matters a lot where the leaks are, for comfort and building durability.  One of the big deals about lowering air leakage is reducing moisture transport into the walls that leads to condensation.  One of the practical aspects of 0.6 was the very reduced likely hood of moisture accumulation in the wall.   Highly insulated walls dry slower so they need to be more airtight.   Is this a problem in your climate?  Less so than in a colder climate unless the humidity is very high in the winter. 

The second point is that the ACH50 is relative to Volume of the house, where as air leakage is relative to surface area of walls, ceilings, and slab.  Think of a cube-  a small cube has a lot more surface area: volume than a large cube.  If you have a ranch it has more surface area : cubic ft than a 2 story colonial.   It gets even worse if you make the house longer and narrower  for the same square ft.  Ever notice that most passive houses are 2 story cubes? 

The long and short of it is that it is easier to make a big  square, 2 story house 0.6 than a small long ranch.

I figure that if you are airsealed to the point that the structure is durable, and the yearly cost of leakage is negligible, and the temperature variations in the house are not noticeable- you are done.  I don’t think 0.6 is always necessary, but if you can get there, go for it.

You also need to do things proportionally, a house with no insulation, but perfectly airtight- think shipping container would still suck to heat and live in.

You will need fresh air, preferably balanced ie HRV distributed throughout the house

When I work with people, I model their existing house after testing it, then look at the heatloss by individual components, then play what if games to figure out what the best overall solution and balance between expense and savings, there is no sense overdoing any one thing.





1) not important; air leakage can introduce pollutants to the building (dust, moisture, gases, etc.). Moisture can damage a building when it condenses on surfaces, and you get mold, or rots. High indoor RH (relative humidity) can increase off gassing, dust mites, and increase risk of moisture problems. And of course you may have to heat, cool, or dehumidify the air.

2) make the house uncomfortable; is a drafty house comfortable? It takes a building of a certain level of airtightness to be able to mechanically ventilate effectively.

The CA Air Resources Board kept us from having a 3.0 ACH50 prescriptive requirement in the 2013 Energy Code, due to IAQ (Indoor Air Quality) concerns.

Hogwash, the evidence indicates that air tight buildings can be more durable, comfortable & have better IAQ, not to mention lower energy use / cost.

How airtight does a building need to be? Some experts say 3 ACH50 is the maximum, but that lower does not save energy (based on needing mechanical ventilation). and that lower than 1.5 ACH50 is not cost effective or reduce moisture problems.

I'd say 3 ACH50 is maximum, 1.5 ACH50 is the minimum, and lower is better.

In the mild CA climates the 0.6 ACH50 Passive Haus standard is tighter than needed (Dr. Feist has told us so, but the Standard is 0.6 ACH50).

Sounds like he wants to sell you what is easy for him, not what you need.

Try opening 2 windows, leave them open all the time.  In fact, remove ve the screens.  Now you certainly have a leaky house.  Let us know how that expert advice works out for you over the next year...

Do you want control of your environment and energy bills?  If so, find another "expert."

Close the windows.  Get below 3 as George suggests and, if properly designed, really nice things start to happen.  Houses become comfortable without fiddling, without adjustment. If you can get to below 1.5, great, but there may be geometric increase in cost for significantly diminishing returns. 

Yeah,  and he should also suggest logging the relative humidity in the house...  wrong climate and you've just made your mold problems worse.   Just opening the windows can be a bad move -- in some regions... especially if we've got reasonably efficient conditioned air spaces, with clean filtered air.

First off what expert and a link to what he is talking about would be helpful. The two statements appear to be false but in what context was the expert speaking of is not clear. Basic Building Science would refute those statements.

You reference Passive House which is one of many paths to Home Performance and not the only choice nor method.

Truth can be elusive and subject to interpretation seek validity instead. 

The dude is right. Open a window and let nature in. If we truly to emulate nature in our new design thinking, Open and passive ventilation is the best approach. 

"building science specialist in charge of energy efficiency in Berkeley "

The specialist may have been speaking specifically to the local Berkeley area.  LBNL has some pretty good studies that indeed show that air tightness is less important in COOLING dominated environments.  It is extremely important in heating climates.

In cooling climates - moving the HVAC unit and the cold ducts out of the attic -- probably would save more than tightening up the building envelope.

So the question that should be asked... was the building science specialist referring to local impacts and priorities... ?

Hi Dennis,

Thanks for your response. I would say local climate of Berkeley, CA is quite mild and pleasant year around. The HVAC system here is heating dominated, even though we get a couple days (2 to 6days) a year that we wish we had an air conditioner. Therefore the advice from the energy expert that air tightness is not important does not make sense for Berkeley. 

In heating dominated environments it  makes sense and is important.  It is possible to loose more energy through the air leaks than you can through the walls.  If you are retrofitting a house - you need to have an energy audit done with a blower door and possibly a thermal camera.  You can quickly find the problems and focus on those areas for the most impact.

In most cooling dominated environments - removing the latent heat is the big load.  If you are in a dry desert (AZ), the air leaking into the house might be warm... but it can be dealt with (and if fact you might cool the house by adding moisture as part of the air conditioning process).  In those cases its the location of the HVAC equipment and ducts that is often the more critical issue.   Danny Parker (Florida Solar Energy Center) has some papers written that cover many of those issues,  as do the researchers at Building Sciences Corporation.

ACEEE also has some info on their sites,  and Build America has a very good repository of information.

LBNL has their research on their website to look at:

Ian Walker has a project that I believe is about to be released (based on e-mails from DOE),

One of the links on the site is for the Residential Envelope Leakage database,  a study made across the use of residential air tightness.

In the next study, they did a pretty comprehensive comparison of tightening homes in different climate zones and ventilation impacts.  Tighter homes nearly always improve -- unless you are in a climate zone with nearly constant temps 72F year round.  But it varies by region.  I believe I've seen the paper elsewhere were you do not need to buy it from ScienceDirect.

Their concluding paragraph: 

"We used a physics-based modeling approach to assess the energy impact of envelope tightening on the U.S. housing stock. Envelope tightening alone has the potential to reduce the residential sector site energy demand by 2.9 quads (3.1 EJ). However, this would require the leakage of all homes to be reduced to the level specified by the Passive House standard which is not reasonable for the existing stock. Current levels of tightening seen in WAPs and energy efficiency programs could reduce the energy demand by 0.7 quads (0.74 EJ). We estimate that advanced methods of tightening could potentially double that energy savings, achieving half of the savings that could be achieved with stock wide application of the Passive House standard. Substantial additional energy savings are possible by improving air sealing practice to what has to be regarded as an achievable goal – to get all homes up to the current 90th percentile performance level of homes of the same type. This analysis considers the characteristics of the home that may limit air tightness and compares each home only to homes of the same age, type, and income class. There is a clear need to develop and apply the most effective methods of envelope tightening in home retrofits.", J.M. Logue, M.H. Sherman, I.S. Walker, B.C. Singer

There are also many good papers and articles on the measured impact of duct sealing.  If the ducts are outside of the conditioned space... and they are very leaky,  it is possible to loose more energy that way than through the "building envelope".  The duct leaks would be measured and seen in a blower door test - as an envelope problem - but that could be retested specifically and the loss from the ducts to the outside could be quantified.

Lot of research,  it really comes down to what was the context in which the person you were talking with or that you've mentioned.  They may have been trying to stress that in the newer houses... they are mostly air tight and its the other things you need to deal with.  Setting priorities for example,  a house with AHU in the attic that is already 3.0ACH50, would probably benefit more by moving the AHU out of the attic -- but that may not be possible....So then it might be can you convert the attic into a conditioned space, and is it worth it.  If the energy bill is just a few hundred dollars a year - you could spend a lot of money trying to save almost nothing.  Context is very important.

I'd go back and check with them.  Most authors / speakers are also approachable by e-mail if you have questions.


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