What is the appropriate level to shell seal existing homes, in a cost-effective manner. Is it appropriate to target the sealing goal to current IECC or Energy Star standards for new homes or as DOE says in one publication to shoot for a 20 - 30% tightening (CFM50) level? Time and money are limitations, I could spend a week completely trying to seal an existing leaky bucket but the cost would be atrocious. So what is a good, reasonable, and cost-effective target?
The true answer is - it depends. Certain style homes in certain areas one might say hey we can get X% or down to X ACH pretty reliably, but this model we might only get to Y.
As for going, well if we seal XYZ, we will get so much reduction doesn't fly either. For example, my own house - after taking out the whole house fan & fixing a few other easy areas still leaves me at 12 ACH which is essentially where I started - why? Air seeks the path of least resistance so just because you may have a certain sized hole which you think will account in so much doesn't guarantee it. for more - check out Allison Bailes blog on his attempts: http://www.energyvanguard.com/blog-building-science-hers-bpi/how-i-...
Until you know what maybe the best answer for you - consider setting up a we will work on your house for X hours, using up to Y materials to start getting your baseline for your area. For some tips: http://thehtrc.com/2014/faq-are-all-air-leaks-equal
Yes, It all depends on the Client's Goals and to some degree budget. Blower door assisted air sealing is the Gold standard along with seasoned professionals willing to take the time and get into the difficult places to make a difference to the building shell. The client also needs to be willing to understand the "Limits" to what you can do and what may need to be opened to find those thermal bypasses. And when we make the house" too tight" ( my colleague , Walter Money says that's a Good thing.) we install a HRV (heat recovery ventilator, up here Oh.) so now we are spending more $ but doing a better job. I have found that it is always best to have a budget discussion up front so as to not waste time or resources later.
"It depends" is definitely the rule! I live in the MidAtlantic region so I deal with low Winter temps, high Summer temps, and relatively high humidity. I have implemented many projects to reduce air infiltration and improve shell insulation with good results. My brother and sister-in-law live in a microclimate in the Bay area, in a 1950s ranch house with an open crawlspace/attic and heating supply/return ductwork entirely outside the living area. But their climate is so mild that few recommendations to improve air sealing are cost-effective.