Insulation / moisture
Hello, I am buying a concrete block house. I will have to do some extensive renovations. I have brought a structural eng. to the site . He has told me that the exterior concrete block walls and foundation are fine, as well as the attic system. The rehab needs to be done inside. Due to previous water leaks inside the interior footings,girt beams and floor joists have to go and be completely redone.
The problem I am having regards the insulation system. This house is located in Ma., and due to the significant rehab, has to be brought up to current insulation code. The walls of concrete block go from ground level to the ceiling/wall tie height. These walls not only need to be insulated to R-19 but they are going to need some sort of vapor/moisture barrior as well . I know that the concrete block will conduct moisture inward toward the living space. I am trying to come up with the best way to bring these walls to R-19 and stopping the moisture at the same time. After much speculation with some of the insulation pro`s in the area I`ve come to the conclusion of some kind of combo between rigid and fiberglass should do the trick. My original thought was to frame 2×4 walls along the exterior block walls, like a house within a house. Some of these pro`s have said I can use 1 inch of rigid up against the concrete block then construct the 2×4 wall up against the rigid. Others have said I`m going to need 2inches of rigid up against the block to stop moisture. What would be the best and most cost effective way on insulating and controlling the moisture of these exterior walls without stealing too much living space from the inside of this house? The exterior has a brick finish on 1 1/2 sides so attaching anything outside won`t be easy.
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Replies
R-values in MA are currently governed by IRC 2009, and is entirely inside of zone 5 (see: http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_sec002.htm ) In zone walls have to be insulated with R20 (not R19) if thermally broken by studs and framing, but only R17 if it is continuous insulation on the interior side of a CMU wall, or R13 if it's continuous insulation on the exterior side.
An inch of foam and a 2x4 wall 24" o.c. with a single top plate rather than a double (since it's not structural) will get you there if you use foil-faced polyiso for the foam and R13 or higher batts. That's the R13+ 5 option in the code for framed wall construction, but if you go that route it's worth going with R6 foil faced polyiso and R15 rock wool batts. The foil facer is a true vapor barrier, and the foam thickness doesn't matter- there won't be any moisture coming or going through two foil facers. But with half-inch gypsum that takes up a full 5.5" of interior space.
Some other options for insulating on the interior side:
To hit an effective R-17 while blocking moisture from the masonry you have a couple of rigid foam options that work without eating up too much space. You could get there with 3" of poly iso held to the wall with 1x furring through-screwed to the CMU with TapCons 16" o.c., then hang the wallboard you'd be there in less space than a 2x4 batt-insulated interior studwall, and that only takes up 4.25" of interior space. It would also out-perform the R13+ 5 solution by a bit in both R-value and infiltration.
A higher performance version would be to install 1.5" of EPS against the CMU, with another layer of 2" of foil-faced polyiso on the interior side of that, taping the seams of both layers. With the staggered seam 2 different foam approach you take advantage of the up-rating curve of EPS (that increases in performance with lower temperatures), and maximizes the R/inch of the polyiso. The R-value of polyiso rises slightly from it's rated R when it's exterior side temp is ~35-40F but begins to fall dramatically at temps below 25F. With EPS on the exterior side it keep the iso in it's maximal range, whereas if it were all polyiso that last outer inch would only be good for R2 at 0F outdoor temps, whereas the. exterior inch of EPS would be bumping on R5. That stackup would take up 4.75" of interior space,. but would deliver ~10-15% better R value than an all polyiso solution over the winter season, since both the EPS and iso would be performing better than their labeled R-value.
Both EPS and foil faced polyiso are vapor retardent, and foil faced polyiso is EXTREMELY vapor retardent. With taped seams and 2-layers it completely protects the interior finish wall from high moisture drives from wet masonry.
If insulating on the exterior side, reverse the stackup relative to the wall- the polyiso goes against the brick, the EPS goes over that, held in place with furring onto which you hang the siding. You can meet the R13 continuous insulation code with as little as 2.5" of foam on the exterior, but if it's cheap enough (see below) it's worth going as much as 4" on the exterior, after which it becomes a bit awkward to drill & screw into masonry. The longest 1/4" shaft Tapcon's are 6"( http://www.confast.com/products/tapcon-concretescrew.aspx ) , and you need to allow room for a fender washer (to keep from splitting the furring) and an inch of penetration into the masonry, so with 1x furring and a washer you're pretty much limited to 4" foam using that method.
In MA there are multiple vendors of reclaimed roofing foam that can take the wallet-sting out of an all-foam approach. The best known is Nationwide Foam Recylcing Inc (formerly called The Insulation Depot) at 703 Waverly Street in Framingham:
http://www.nationwidefoam.com/
But there are others- search your local craigslist for "rigid insulation" and you'll find some of them:
http://worcester.craigslist.org/search/maa?zoomToPosting=&catAbb=maa&query=rigid+insulation&minAsk=&maxAsk=&excats=
Some will deliver, others not, so factor that into your costs if you have to supply the truck but it's still dirt-cheap foam.
Any roofing iso (off white or pale yellow center, with paper, fiberglass, ashpalted felt or other facers ) is going to be at least R5.6/inch, and assume any EPS or XPS (the pink or blue or green stuff) will be R4-R5/inch. You can and should patch any screw holes or large dings in the stuff with can-foam as you install it- some is in better shape than others (and priced accordingly.) The pretty-good stuff is still usually under 30% of the price of virgin-stock. The most I've paid for 3" roofing iso (close enough to R17 to call it that) from any reclaimer was $20 for a 4x8 sheet, which is about 3.7 cents per R per square foot. Most of it comes in around 3 cents/R-foot, which is cheaper than high-density batts.