1.5-inch-thick polyisocyanurate insulation on cinderblock walls
I am working on a cinderblock house (every forth cell is solid) with plaster interior finish applied directly to the cinderblock. We are replacing windows, doors, and siding and the idea is to fur out the cinderblock and place polyiso insulation between the furring strips, then .5 inch OSB over the Furring and insulation, then house wrap/stucco wrap, and stucco part of the house and wood clad other sections of the house.
We are in Colorado at 7600 feet with a very sunny exposure. The cinderblock cells are not filled with insulation. The client does not want to use EFIS, and my insurance will not cover any EFIS work.
The OSB allows us to use a hard coat stucco (where we are using stucco), but I am a bit concerned about moisture forming from inside out with the hollow cinderblocks and no interior vapor barrier with the exception of the paint and plaster.
Also, anything better than tap cons to apply the furring? I have been forced in to the remodel world and I do not trust all that is drawn, so I am checking to find out if this is a poor solution to our otherwise non insulated walls.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Why not stick the rigid foam right to the blocks, then use the firring strips to hold it?
Our jambs are already 10 inches, not sure they would want to go even deeper, but I will take this in to consideration. I am more concerned with dew points and moisture. With this system, I know I can provide good flashing and sealing at penetrations, but I am a bit nervous about condensation and the lack of interior vapor barrier.
John: First off, I am not a pro. I'd be more concerned about HAVING a vapor barrier, as I don't think the polyiso is particularly vapor open, so the wall would not be able to dry to the outside. Just a thought; a more experienced voice is soon to add in here. Too, I guess I am confused by your not wanting an EFIS and yet having exterior foam.
On the face of it not having continuous insulation looks like the weak point in your assembly and the most likely aspect to get you into vapor and condensation issues, so I'd second John K's advice.
And while they're unfamiliar in a stick-framed world, tell your clients that deep window jambs are a delight.
John,
You need to rethink your plan.
1. You need a higher R-value in Colorado than R-10 -- especially if you are interrupting the insulation with furring strips.
2. You absolutely should be installing a continuous layer of thick rigid foam, followed by the furring strips. Don't cut up your foam into narrow strips.
3. If your foam is thick enough, the foam will keep the concrete walls warm, and there won't be any worries about condensation. Thin foam is riskier than thick foam. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
4. Stucco over OSB is a very risky type of cladding -- almost any siding choice (including properly installed water-managed EIFS) is better. For more information about the risks of stucco over OSB, see To Install Stucco Right, Include an Air Gap.
I would like to thank John, James, and Martin for the info and the related links.