Airtightness of wall systems
mrbreadpuddin
| Posted in GBA Pro Help on
Can anyone produce a list comparing the average airtightness of the most popular wall systems?, tyvek and fiberglass, airtight drywall and fiberglass, taped EPS and fiberglass, sprayed caulk and fiberglass, same where applicable w cellulose, open cell, closed cell foam? I know everyone has their favorites, and there are a million variables,
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Replies
William
I believe that any of the above wall systems can be made airtight as long as the builder or subcontractors doing the work have the knowledge, diligence, and incentive to do the job properly. And there's the problem..."young men in a hurry" don't usually fit the bill. Some custom builders do specialize in this type of work and probably have their favorite systems. So, my advice would be to find the right builder first. And consider providing a cash incentive if the buider attains your desired ACH 50.
Garth is right, many wall systems can be made airtight. I would say 9 of 10 people in the homebuilding industry would give the "Hereford stare" when ach50 is brought up, the look of total incomprehension. You have to seek out building professionals dedicated to offering energy efficient and durable buildings.
With that said, I have been very impressed with many owner involved new and retrofit projects. The level of detail achieved leads to a very high quality product.. Some have contributed to the GBA forums with photos and cross sections.
William,
I'll support the above answers. The results are more dependent on the workmanship rather than the method. There are many good methods and you listed some of them above. A good method with poor workmanship will yield poor results. I do find that it starts with a design that takes air sealing into account as a requirement at the outset rather than an afterthought. If a good airtight envelope is the goal, consider between 1 and 2 ACH 50 to be excellent results.
William,
I agree with the previous responders: almost any type of wall can achieve excellent blower-door results if the builders care about airtightness.
On average, though, different construction details yield different results.
Bruce Harley collated some interesting data in 2005 that sheds light on some aspects of your question, although it doesn't address your question directly. Harley looked at the blower door results for hundreds of Energy Star homes in Massachusetts and Rhode Island. He separated the homes into subsets based on the type of insulation that was installed.
The bottom line, as I reported in the April 2005 issue of Energy Design Update: "Harley found that houses with walls insulated with spray polyurethane foam were significantly tighter than houses with walls insulated with cellulose, and that houses with walls insulated with cellulose were significantly tighter than those insulated with fiberglass."
Harley's results are summarized in the table below. (Click the image to enlarge it.)
I am a builder, and in my area it's 80s style fiberglass batts and tyvek for low end homes, and spray foam without extra ventilation for high end homes. The engineering is changing so much faster than what is happening on the ground (at least in Vt.- no residential code), that I'm just trying to keep on top of the trends. That's why I love your website!
Martin, on looking at the numbers in Harley's table, I see that the ACH values are given as "natural," meaning I presume a best approximation of expected actual leakage in cold, windy weather. Multiplying by a nominal 18-20 to get ACH @50 Pa (Bruce would have divided ACH50 by a factor like that to approximate ACH-natural), none of the numbers for FG and cellulose are particularly good, and even the best of the foam numbers wouldn't pass the 3 ACH50 bar set by the new 2012 IECC. Now of course the numbers reflect what was acceptable at the time for an Energy Star rating, but a new house built to 2012 IECC will be far tighter. I wonder what the differences will be in a similar table done some time in the future, for houses built to the new code, where added steps will have been taken to reduce the dependency of tightness on selection of insulation.
Dick,
Good points. It's amazing how sloppily a builder can build a house and still get an Energy Star label, isn't it?
But Energy Star and the code are both becoming more stringent. (As usual, Sam Rashkin's attempt to make Energy Star better than code always seems to lag, and by the time the Energy Star program gets builders to accept a new standard, the code has often overtaken Energy Star.)
I agree with your basic point (I think): a good builder should be aiming for a certain blower-door test result. The code will soon require this approach. And if you are aiming for a goal, almost any construction method will get you there, as long as you pay attention to air sealing along the way.