We just moved into a new home after relocating from overseas. House is obviously built to code, but definately not superinsulated, etc. Basement is unfinished there is very good access to the RIM joist which just has fiberglass batts stuffed up in there. My question is: What is the best "bang for the buck" in tightening up this house? My thoughts are to first spray foam around the perimeter under the first floor. For spray foam, what thickness is recommended? What R value, etc. Since the batts are here, should I put them back in the cavity after the spray foam? I was quoted $4.25 per linear foot for closed cell foam at 3" thick. Is this reasonable? Thanks in advance. Ian Upton

yes, that is a reasonable price. It is typically 1.25-1.50/board ft so that is within that range. 3" would be valuable at the rim joist; and it should cover the sill and lap onto the foundation. Be sure they do not spray onto the fiberglass as that creates air bubbles & gaps in the foam. You can reinstall it if you want; won't make much of a difference. You will need to cover the ccsf (closed cell spray foam) with an intumescent paint. You should have them do 2" onto the basement walls also if you truly want to make a difference in the house.

Thanks Bob. Do you mean do the entire basement walls with 2" or bring the foam down 2" onto the concrete poured wall to properly seal everything up? We will finish the basement at some point and are planning on 2X4 walls offset from concrete by 1 to 2 inches. Then dense pack cellulose fill the cavity. Ian.

To recommend an optimum thickness of the foam and the foam/fiber R-ratios the local climate matters. Where are you?

3" of closed cell foam is about R18 or more, and expensive R at that. For basement walls it's cheaper & greener to use some thickness of EPS or polyiso (sealed with can-foam at the edges & seams) trapped to the foundation with a fiber-insulated studwall.  Rock wool is somewhat preferable to cellulose in sub-grade applications if there's even a REMOTE chance of ground moisture seepage or minor flooding, since the rock wool can dry quickly, and celluose can't, but it depends on the site conditions/setup/climate.

The foam/fiber ratio needed for basement & crawlspace walls by climate zone are roughly the prescriptive levels for above grade walls found in the IRC, by climate zone:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

Assume R15 max fiber-R for 2x4 framing, R23 max fiber-R for 2x6 framing.

The D.O.E. climate zones referenced in the IRC documents are as delineated in this map:

Just north of Detroit. So 5A. For the 3" of foam, I was just thinking of rim joist for air sealing. Ian.

You cannot use cellulose or fiberglass against a foundation; has to be either polysio foam (or XPS) or spray foam. 2" Sheet foam is about $1.00/SF + installation and air sealing (doesn't do any good if it isn't sealed around the edges); closed cell will be around 2.80, installed. You can build your stud wall inboard of that.

Posted By ian_upton on 13 Feb 2014 04:51 PM
Just north of Detroit. So 5A. For the 3" of foam, I was just thinking of rim joist for air sealing. Ian.

You can air-seal the rim joist with as little as 1"/R6 of closed cell foam, and do the rest of the insulation at the rim joist as R15 rock wool (cut very snugly)  which will outperform 3" of closed cell foam.

In Zone 5 you only need R5 of air-impermeable (read "foam") against the foundation is sufficient to allow a 2x4 studwall w/rock wool or celluose cavity fill.  The cheapest (and probably best) approach would be to use 2"/R8.4 of Type II (1.5lb density) EPS  against the wall, with the studwall snug up against it. Put an inch of EPS under the bottom plate of the studwall as a thermal & capillary break as well, and TapCon the bottom plate to the slab.  At 2" EPS has a vapor permeance of about 1.5 perms, which is a sufficient vapor retarder against ground moisture at the wall, and with a foam/fiber ratio of R8/R15 you'll be well-protected against wintertime condensation/frost accumulation inside the studwall portion, as long as the interior gypsum is reasonably air tight and painted with standard latex paint (3-5 perms.)  You could also use 1" of closed cell foam (between 1-1.5 perms, and about R6) against the foundation making it continuous from the slab to the top of the band joist, which would also be a sufficient R-ratio to be protective in your climate.