Confusion on Flash and Batt
I know there has been a lot written and a lot discussed about flash and batt. I thought I understood the pros and cons. The most import design criteria is to ensure the flash is thick enough to keep condensation point to the inside of the flash barrier. In the case of my location (17026-PA-climate zone 6), 2″ of closed cell foam should be thick enough. I have been talking to foam installers in the area and this is what I got back from one of them
“…we do not flash and batt nor will I flash and batt for anyone that is a mold night mare it will destroy you house in under 5yrs! “
The wall confirguration (as I described it in an earlier posting on this forum) is:
5/8 drywall
2×6 studs 24OC
3.5″ Roxul batts (R15)
2 ” closed cell spray (R12)
Zip wall R6 sheathing (joints taped & properly) (R6)
3/4 strapping
fiber cement siding
What am I missing here? Based on my research and feedback on this forum, this configuration will work without mold issues (as long as I control the humidity in the house).
Any help will be appreciated.
Thanks
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Replies
Jim,
As I am sure you know, contractors have a right to decline work if they feel that a proposed method has not yet proved itself through years of successful implementation.
There are many reasons why a spray foam contractor might prefer a full-spray job to a flash-and-batt job.
Ask the installer to tell you the minimum foam thickness he'd spray. Presumably he'd go full depth. Would he go less, and if so, how much less? Maybe he thinks that by "flash" you mean a half-inch or something like that. Seems like when I first heard about flash and batt, the foam was the minimum they could manage, just enough for some air-sealing. Since then, it's gotten thicker as people have realized the possible problems.
Jim,
You have clearly done your math and satisfied yourself that two inches of closed cell combined with the Roxul will work. I agree with you on that front.
But, if you install an honest two inches of closed cell and then go back and do it all over again with the Roxul with the necessary care and attention to detail, I doubt that you will realize any savings over an all-foam job. We are frequently asked to quote flash and batt and every time I run the numbers, I find that it doesn't make sense in walls. If I were you, I would ask my foam contractor for a quote for a 5" average fill using a medium density foam (about 1pound/ft3). This will give you an r-value that is essentially the same as your flash and batt, I believe the price will be competitive and it will certainly be a whole lot simpler. If he is unfamiliar with that type of foam, I will be happy to point him in the right direction.
thanks - I'll check it out. What is the R value per in?
The one we spray is R-4.8 per inch. It is significantly denser than open cell, easier to control during installation and therefore produces less waste.
Other sprayfoamers are also doing as Torsten using the 1lb types...
If you use it, let us know how all goes.
Thanks - I will follow this lead.
So is it open or closed cell then?
The manufacturer quotes closed cell content at less than or equal to 50%.