I need to insulate a section of wall… studs are 3-3/4″, exterior sheathing is 3/4″ CDX with 30# felt and cedar shingles over. I could stuff 3-1/2″ fiberglass in there and be done with it, but the discussion here makes me wonder if foam would be a better choice. This side of the house faces the prevailing wind. It appears that I can get more R’s from 2″ of foam than I can from 3-1/2″ fiberglass… or I could go with 3″ of foam and really kill it. Q’s are… where would the air space go, since I’d have some air space in the 2″ or 3″ foam scenario? I could either push the foam out against the sheathing or hold it into the building against the sheetrock. I can’t get anyoone over here to install spray-in foam insulation without major expense and hassle. Opinions?
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dave.. depends on the scope.. if it's a big wall, i'd blow DensPak cells....
small wall i'd use pieces of 1" eps foam ,cut it loose , push it in against the sheathing, put a couple of roofing nail toes in to hold it in place...
then...
foam the edges with gun foam to lock it and air seal it
There's about 25 LF of 9' wall, minus two doors and a window.
Not sure what DensPak is... but if I need a sub then it'll be hard to get done. A job I trimmed recently had a blown in cellulose behind plastic netting, and it made me so allergic that I ran for my life.
And, only 1" foam in the wall? I've got 3-5/8" of stud space, minus some obstacles like blocking and a few electrical boxes. In the olden days I would have just put in FG but foam sounds better.
no... i meant 3 layers of 1" foam.. 1" is easier to come by than 3"
and you don't have room for 2 layers of 2"
i just find fiberglass to be a waste of time and money..
one thing about blowing cellulose, even with a mask.. it sure cleans out my sinuses..
lately we've been buying 100% borate cellulose, which i like better than the normal sutff which is amonium sulfate based..
perhaps your allergic to one and not the other... the amonium sulfate makes my eyes water a lot more than the borates Mike Smith Rhode Island : Design / Build / Repair / Restore
Foam is the best if you can afford it. http://www.fomofoam.com has do it yourself kits which for a small app like yours might be worth it. Denspak is cellulose insul blown into the wall under presure so that it becomes dense packed. Higher R value that way. You have to put stretch resistent plastic across the studs first,otherwise it can bow out drywall.
Cellulose has the best cost/benefit ratio, particulary for the full height cavities.
Rigid foam would be somewhat better, though more costly.
The way I fill cavites with foam board is to cut it just a little short, then I foam around the edges with my foam gun. My gun has a needle tip, so it requires very little clearance (perhaps 1/16) to inject foam in. If I were using can foam, I would want about a straw's width on each edge. Detailed carefully, the results will be just as good as sprayed foam, often for less money. Plus no special equipment or subs to schedule.
Even if you use cellulose, foamed-in-place foamboard is great for small and awkward cavities around openings and such.
If I had to have an air space, it would be on the inside. Easier to install, less foam gets cut for electrical boxes, and as a bonus you get a great place to run wiring or water lines.