Attic: Fluffy or Foam?
user-980774
| Posted in General Questions on
Climate Zone 5, Flat ceilings, ventilated truss attic (no storage),new construction, only R30 required by code (2006 IRC). Vented Crawl Space is R30 and Walls will be R28 with R6 Sheathing plus 5.5″ Open Cell Foam.
For the same price I can get R50 Blow In Cellulose OR R40 Spray in Open Cell Foam. I know R50 is better than R40, BUT spray in place foam really seals things up, will not blow around, will not settle and isn’t R40 in the ceiling overkill any way.
What would you do???
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Replies
Do a good job of air sealing the "old fashioned" way (caulk and canned foam), then use blown cellulose. This is usually the cheapest option, and it does a good job. I'd go for R49 too, which is current code in most areas -- your area is using a really old code revision!
I would not use huge amounts of spray foam in a conventional vented attic, it's not really necassary. Some will do a thin 1 or 2 inch layer for air sealing, then blow loose fill insulation over that, but targeted air sealing does a good job too with a lot less material (greener, cheaper).
You mention walls will be "R28 with R6 sheathing plus 5.5" open cell foam". That's probably around an R56 wall? Why build it like that? A double stud wall with dense pack cellulose is likely an easier and cheaper way to get to really high R values like that in a wall, or just use batts and exterior rigid foam to get up to the R30+ level. If you're using batts AND exterior rigid foam AND open cell spray foam, you're really going a bit overboard on that wall.
Bill
It looks like he is doing R6 exterior insulation, plus open cell in cavities to get him to R28 total.
I agree though: air seal at penetrations, and then do blown in.
Yeah, that makes more sense regarding the wall R value :-) I probably should have thought of that...
Bill
The open cell foam is R4.5 / inch plus R6 Zip Panels equals about R30 for walls.
Spraying 1.5" plus R45 Blow for a total of R50+ was about $600 more.
I agree this is the better combo.
Caulk and canned foam does a "good" job but is very labor intense.
Too damn old to crawl around in an attic.
Spray foam does a "great" job.
Spray foam does a great job only if it's installed well, and it uses a LOT more material than the old fashioned way. I agree it's a tradeoff though, and you do save a lot of labor going the spray foam route.
Be sure to put baffles in at the eaves prior to spray foam going in so that the spray foam will lock them in place (and the baffles help keep the spray foam from filling soffit vents this way too). Place the baffles in a way that the spray foam can still cover the top of your exterior wall top plates though since that's an area that you want to be sure to seal and insulate.
Bill
What Bill said- the quality of the air seal depends a LOT on the experience and expertise of the foam installer. That's generally true for closed cell foam, but triple-true for open cell.
Regarding "...and isn’t R40 in the ceiling overkill any way."
Uhh no, it's not even IRC code minimum for Zone 5. R40 of open cell foam might be overkill on your wallet compared to R50 cellulose, and may never "pay off" on reduced heating/cooling energy bills within the lifecycle of a house in a low energy price scenario (you may be better off applying the foam funds elsewhere in the house) but it's not anything like a high performance attic, or a cheap way to air seal the attic.
A competent open cell installer can get pretty good air tightness with only 3-5" of foam. But even at 8-10" it's better to test the house with a blower door and theatrical smoke generator to find all the leaks before they pack up the equipment and head on to the next job. The guys that foamed my attics missed a ~2" x 10 foot gap below a beam leading out to an insulated unsealed porch roof attic space. Without benefit of blower door it took me over a year of living there (and an hour of crawling around doing contortions in a multi-level oddly framed kneewall attic under a hipped roof corner) to figure out where the big air leak was. As awkward as it was to get in there it was easy to see how they missed it. With a smoke test it would have been glaringly obvious.