We have an old concrete block house that was built in the 50’s. What is the best way to insulate and side it?
mtguttergal
| Posted in GBA Pro Help on
My grandpa built our house and used a concrete block for the exterior. I don’t believe there is much insulation between the drywall and the block and I don’t believe that the block was filled with concrete. It is most likely hollow.
We want to add new windows and place siding on the exterior and were thinking of adding furring strips and foam board underneath the insulation but I am worried about the moisture factor. Please advise. (The basement is also uninsulated, unfinished, and is concrete block).
We are located in Montana.
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For cinder-block & CMU houses it's usually possible to add continuous rigid foam on the exterior. Changing out windows at the same time is good since it allows you to flash the windows properly. If the drain-plane is going to be between the foam & blocks, use a crinkle-type housewrap, and lap the flashing properly to it. Bulk water is by far the biggest moisture issue, and getting the drain plane & flashing details right is job-1.
The type of foam matters. XPS is fairly damaging to the environment relative to EPS or polyisocyanurate. Polyiso is higher R/inch than EPS, but only for layers where the average temp through the layer is going to be above 35-40F. If the existing wall stackup only comes to R4-5 total, 2" of polyiso next to the wall, with 2" of EPS between the polyiso & siding would work pretty well (and better than 4" of polyiso) since the EPS keeps the polyiso in it's performance zone. EPS gains performance with falling temps, and the average performance of a 2" + 2" stackup will be about R20 in your climate, plus whatever the current wall's performance is. An R20-ish wall will meat or beat IRC 2012 code min for an MT zone 6 or 7 climate since with the block wall structure it becomes a "mass wall" with more than half the R on the exterior. The R on the inside of the existing wall will make it that much better than code:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
Hollow blocks have an inherently better capillary break compared to poured concrete or filled blocks, but they have air leakage issues. If it's possible to at least air seal the top of the cores you can reduce the convection losses by more than 90%. The details of how the top of the wall works with the ceiling-joists/attic floor joists matters- it has to be air-sealed between the blocks & attic insulation.
In the basement you have a couple of options. With 2" of EPS (seams & edges sealed) against the foundation you can then safely add a 2x4 studwall insulated with unfaced R15 rock wool or fiberglass, ending up with an ~R20-ish "whole wall R" for the basement, which is still a financially rational number for MT climates, even if that ends up being higher-R than your above grade walls. (See Table 2, p10: http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones You are in either zone 6 or 7.)
Dana has given you great advice. I'd like to add that you may have a non-foam option, too: rigid mineral wool boards. These boards would be attached and externally furred just like foam boards, but give you the advantages of never being vulnerable to insect infestation, yielding extra sound insulation, and being vapor-permeable to permit outward drying and excellent drainage. If you're going to use a cladding that's not stucco or adhered stone or brick, rigid mineral wool could work great. There's some debate about whether you even need housewrap in a wall with mineral wool boards and a ventilated rainscreen gap between them and your siding, though it's cheap insurance to be sure. But perhaps even less important with a block wall that's not inherently moisture-sensitive the way a wood framed wall is.
Thank you for such detailed information. We are sub-contractors and our builders all have different ideas which is a bit confusing and worrying so we want to make sure we do it right. Not a fan of stucco and stone and brick not an option cost wise, so it will definitely be some sort of wood or fiber board siding. We have closed off rooms during the winter to save on heating and have had some moisture issues inside the rooms on the drywall so we want to make sure we don't have any condensation issues when we side. Thanks again.
The only problem with rigid rock wool is that you can't quite get to code-min with it.
With a hollow cored block wall you'd still want the housewrap (detailed as an air barrier), whether the exterior insulation is foam or rock wool. While rock wool dries quickly, I personally wouldn't count on it for being the drain plane for the window flashing either, even though you might be able to get away with it.
Closing off a room makes those closed-off rooms very cold in winter, but the door is not a perfect air seal, and there will be slow steady convection between the cold room and the rest of the house. The dew point of 30% relative humidity 70F wintertime conditioned space air is about 40F, but in a closed off unheated room the surface temp of the exterior walls of a barely-insulated cinder block structure will likely average below 40F in those rooms. That is one likely mechanism for the moisture issues, assuming they can't be traced to bulk-water wetting due to absent or poorly implmented flashing, or direct wetting of the exterior of the wall due to minimal roof overhangs, etc.