I have reviewed, tested, studied, asked, and analyzed the many facets of whole house air leakage.  I am still somewhat unsure what constitutes a respectable level of air leakage to a home.  Newer homes built to higher standards are not a problem but when you get into older homes with varying degrees of air leakage depending on when they were built and by whom presents some interesting situations.  The recommended rates of leakage on older homes do not present the same situation as new homes as there are many areas of the home which are not accessible and to properly be able to reduce the leakage to ASHRAE guidelines would mean in many cases a major teardown and reconstruction to be able to seal the leakage to the "standards".  In addition, many of these homes were built on the premise that the leakage would keep the wood and structure dry, minimizing those types of problems.

If you have suggestions, references and guidelines, please let me know.  Thank you for your input. 

Roy Sakamoto

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This jives with my experience trying to air seal a very small house - lots of caulk and foam and not much of a result (only a few hundred CFM after several hours of work).  Bigger homes seem easier to reduce in my experience.

This is definitely worth investigating if you are an air sealer and are forced to use the ACH50 metric.

First look at a 1500 ft2 ranch on a slab at 50'X30' with an 8' ceiling.  It has a volume of 12,000 ft3, and a surface area, all-in, of 4280 ft2.  If we divide the surface area by the volume, we get .36 - this is a measure of the number of square feet represented by each cubic foot of volume.

Then look at 3000 ft2 2-story on a basement at 50'X30' with 9' ceilings above grade.  It has a volume of 39,000 ft3 and a surface area of 7160 ft2.  Divide the surface area by the volume and we get .18.

So each cubic foot of the volume of the big house represents twice as much surface square footage as it does in the small house.  To reach the same 3 ACH50, each square foot of the smaller house must be 2 X as tight as the ones on the big house.  Or the way I like to phrase it, each square foot of the bigger house is allowed to by twice as leaky as that on the small house.

You can do the same exercise with that small house on a basement, with a house that is "U" shaped, with a long rectangle, with a square, etc.  And you can even see that the big house with 8' ceilings instead of 9' has a ratio of .19 - so just raising the ceiling a foot gives you a 5%+ advantage on the final air leakage number!

In MD that has had the '12 code for a year and 9 months, just last week I spoke with the head inspector of one of the counties.  He told me they still have a 30-40% failure rate and they are all houses under 2000 ft2.

Wrong metric.  

Hope we aren't hijacking the thread, but this applies to old houses as well as new.

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