If you could please confirm whether or not this is a good idea, I would really appreciate it...

My home is in the Pacific Northwest (north Oregon coast). The climate here is fairly mild. I would like to insulate to better hold in the heat in the winter (as opposed to cooling in the summer).

The house is two story, concrete/masonry block construction. No insulation in the walls currently (hollow blocks).

What I am trying to eliminate is the 'heat sink' that the walls are acting as right now, literally pulling the heat out of the interior thru the walls during the heating
season.

I inquired here awhile back, the general concensus was to insulate the exterior, rather than the interior. I do see the advantages to that method, but for several reasons, we will be doing it on the interior.  (Later on, will there be any problem with adding insulaion to the exterior, if we proceed as planned on the interior?)

Here is what I want to do:

  (1) insulate the entire exterior walls (from the inside) with poly-iso rigid insulation,

  (2) Add 2x3 studs laid flat to provide drywall backing, as well as create a 1 1/2" airspace behind the drywall.

  (3)add drywall tothe studs in the usual manner.

The Poly-iso will be glued to the concrete so as to hold it in place until the studs are attatched on top, with fasteners going all the way thru into the blocks. This will hopefully give me a thermal break.

As a note, I have purchased the Poly-iso insulation already. It has a foil backing on both sides.

The house is 1900sf with an unfinished basement.

What do you think? Any suggestions / changes you would make?  

Thanks alot

Mark

I would, if I were you, use InSoFast. It is a simple, fast, cost effective way to insulated CMU walls. If you want more than 2" of EPS insulation do multiple layers. Google InSoFast.

How thick is your iso board? Have you calculated enough clearance to be able to install electrical boxes without having to breach the iso, creating an air leak?

FYI: 1.5" iso with no breaks outperforms 2x4" studwalls w/R11 batt insulation. And 2" iso with no breaks beats 2x4 w/R15 batt insulation. But if you're looking at an inch or less, you'll need more, in the ~5000heating-degree-day climate of Astoria<-->Tillamook. Blowing cellulose into the 1.5" gap formed by your 2x3" furring strips gives you another ~R6 and is well worth doing. Counting on the aluminum facing & gap for the additional rated R3 or so is folly- if you have the space, fill it with cellulose. That will stop convection within the thin cavity and provide a reliable R-value. If you turn the 2x3s on edge rather than flat you'll give up another inch of living space but you'll be able to garner ~R10 as cellulose, and have enough room to hang the electrical boxes without punching holes in the iso.

Tape the seams in the ISO with FSK tape (2" aluminum) for minium air leakage, and caulk both top & bottom for a good vapor-retardent air seal. Take care to seal the edges of the insulationl well around window/door openings (low-rise "window& door" spray foams works.)

In 2 story houses sealing the attic and the basement is important for keeping the "stack effect" from driving excess air infiltration, particularly during the heating season. Sealing all plumbing & electrical penetratons with expanding foam is an important first-step. Even if you have some attic insulation, if it's a couple inches of vermiculite or R19 batting etc, a 6-12" thick overblow of cellulose makes a huge difference.

Insulating the basement walls at least down to the frost line (probably only a foot or so below grade in coastal OR) is also huge- it can be as much as 20% of your heat loss, even more after you've sealed & insulated the living space walls & attic.

If you later add insulation on the exterior, don't use foil-faced board goods or you'll have moisture-trapping issues. EPS or fiber-faced iso (sold in commercial building as roofing insulation) would work though.

FYI: The DOE insulation estimator indicates retrofitting R15 on masonry walls, R38 in the attic, and R11 on the basement walls is cost-effective for zip codes 971_ _

http://www.ornl.gov/~roofs/Zip/tmp/results27229.html

R15 would be either 2.5" iso, or an inch of iso plus 2.5" of cellulose. R11 (for the basement) would be 2" of iso or 2.5-3" of EPS depending on density (the InSoFast solution is probably "good 'nuff" at R10, but probably more expensive than straight-ahead sheet goods, if you're not planning to fully finish the basement.)

Great suggestsions, thanks!

The poly-iso is 2" thick, by the way. It is also foil faced on both sides.

"Moisture trapping issues" as mentioned, is one of my strongest concerns. Our climate is rather mild, but very wet, with a lot of wind driven rains.

The house has 30" of roof overhang on all sides to help keep the walls dry, but when that wind is blowing...

I'm not sure about moisture buildup / trapping issues. Is the moisture in question comeing from the interior? or exterior? Not being familiar with CMU construction, (as compared to typical stud framing), I want to avoid the dreaded MOLD (Shhh!) problem common with improper building technices.

Given the info so far, do you recomend perforating the foil backing on the poly-iso to make it vapor permeable? Or tape and foam everything as good as possible as mentioned?

Whadd'ya think?

Thanks again

Mark

If it was me, here is what I would do.

First, apply Thoro-seal (or similar) waterproofing to the block wall. Then glue the foam to the wall exactly as you described.

You mentioned using 2*3's, but the last job I did with 2*3's, we had to cull about 50% of them because they were twisted and warped, while we only culled 10% of the 2*4, thus the 2*3's ended up being more expensive. You might have a better supply of lumber in your area, but just be wary.

I would install the 2*4's (or 2*3'S) on edge, not on the flat. Why? Because the boards are much less likely to be warped/ warp later along the 3.5" side than the 1.5" side, as well as opening up a nice cavity to put some fiberglass into. If you are already going through the trouble of installing them, might as well get some R-value back for it. You can put in a top and bottom plate to make fastening easier, and eliminate the need for your fasteners to penetrate the foam, eliminating another thermal break.

Your 2" polyiso is probably R-12 to 14, and if you put your studs on 2' centers with plates, you can probably get R10 to 11 for the stud wall, netting you a R- 22 to 25 wall.

If you are still worried about moisture (although with Thoroseal I wouldn't be), you can use unfaced fiberglass batts and the green drywall that is moisture resistant. (I am using green as in it is the color green to signify moisture resistant for bathrooms, not green as in eco friendly)

I know I am showing my ignorance, but... does the Thoroseal go on the outside or the inside? I don't know the mechanics of vapor bariors vs vapor permeable surfaces. When there are moisture problems in a home, where is the moisture comeimg from? Can it get trapped inside the walls? Or is the problem on the room side of the walls Sorry for the questions, and thanks for all the great informaton given here. Mark

It would be OK to put the Thoroseal on the interior, but with foil-face iso I don't think it's worth it. Putting it on the exterior would create a vapor trap in the block wall when you have foil-faced iso on the interior, which will lead to degradation of the mortar during frost cycles. Even though marine climate of NW Oregon is quite damp, with an interior layer that's very vapor-retardent (the foil facers) the structure needs to be able to dry toward the exterior (and it will.) Masonry takes up and gives up moisture freely- the humidity in the masonry will track the weather, so on those winter nights when it drops below freezing, the water content within the masonry will already be quite low, and large ice crystals can form within the cement block causing it to spall. Moisture sealing it in from the outside and leaving it exposed is just plain asking for trouble, since moisture trapped within the wall sufficient to form larger ice crystals will cause it to flake & crumble a layer at a time.

The 30" overhangs are good for minimizing the flow of bulk-splash into the block wall, but there's no hope of actually keeping it DRY dry, eh? If rain-driven wall wetting is common, 1/2-3/4" thick furring strips between the iso and the wall making a masonry "cavity wall" type structure is a good idea (if it isn't already a cavity wall- which it might be.) That allows any bulk moisture that finds it's way in via any crack to drain to the floor rather than infuse into building materials. Cavity walls are centuries old tried & true. Most brick-faced structures built in the US in the past 50 years are cavity walls, whether the interior half is cement block, brick, or studwall.

In the PNW (as in any heating-dominated climate) it's moisture from warmer air within the building leaking into cooler materials and condensing that's potentially the problem (particularly when the condensing surface is wood or some other moisture-intolerant material.) Fortunately most of the time the building itself is above the dew point, but it's often close to it. It's a bigger problem for areas where it's substantially colder, and building components nearer the exterior can be below freezing, but you're not immune. Making the more air & vapor-retardent materials toward the interior side of the insulating layer keeps this warm moist air from leaking into the colder bits and forming liquid water within the wall.

There's probably more than you ever wanted to know about the subject on the document linked to here:

http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design/view

If you foam seal/caulk/tape all seams gaps & penetrations the foil facer on the interior of the iso becomes a very good vapor retarder. Whether 2x3" or 2x4" you'll still be OK in that climate with insulation in the cavities and the vapor-barrier on the exterior side because the insulation value of the ISO keeps them warm: Since your 2x3s are wood, it's nice that they're on the inside of the structure where they'll always be above the dew point, so no condensation will form (or if it does on extreme cold days, it won't persist.) Using blown insulation between the studs is preferable- it will outperform batts by typically 15% due to it's near-perfect coverage. There are always gaps & voids in batt insulation installatios- wall cavites aren't die-cut, to perfection, but fiber-fill blown under pressure finds every gap nook & cranny, fills in around plumbing & electrical with ease, etc, and the net result is measurably better.

Having insulated the interior, pay close attention to air-sealing the penetrations to the attic, particularly where the joists are supported at the top of the wall. Any air leaks from conditioned space with condensation there could lead to rot issues over time. Familiarize yourself with foam-sealers (I'm guessing a case of spray-cans of Great Stuff won't be quite enough, but it might be.) Foam sealing guns with replaceable tips that use the bigger 22ounce cans are cheaper & easier (available at most contractor supply houses, but not typically found in the box-store home centers.)

The three main sources of humidity in walls in the heating season are:

1 Bulk water (rain, plumb leaks, etc.)

2 Interior air infiltration leaking into the wall & condensing

3 ground water vapor permeating up through the foundation floor & walls

If you're going to use something like Thoroseal, the basement slab is the right place. But for the basement walls you want to allow some inward drying or the ground moisture will wick up to the above-grade portion causing efflorescence (and possibly spalling during freezes) above grade. If you seal the basement wall too well and you see efflorescence getting started, plastering on a sacrificial coating of parging on the lower 12-18" of the wall will usually keep the problem from becoming structural. The moisture still wicks up, but it only destroys the parging, which can be replaced every decade or three. If there's efflorescence on the basement walls you probably DO want to seal it with something, and apply a skim coat of parging over that.)

BTW: I'm pretty familiar with your area & climate, even though I haven't lived in the area for more than 30 years. My father had a place in Ocean Park WA (on Willapa Bay, a bit north of the mouth of the Columbia) and was in the construction biz. I worked projects with him in Tillamook, Manzanita, Cannon Beach, Seaside, Astoria, going as far back as the late 1960s, as well as some in the Willamette Valley (Salem/Monmouth) and a couple around Portland.