SIKA Poly Iso for cut and cobble insulation
My house is in zone 6 and is 125 years old with old growth true dimension lumber.
I followed the recommended retrofit, cut and cobble, insulation recommendations using rigid foam and 1×2 sleepers in the stud bays to keep the foam off the 1×8 old plank siding. I even tested it with a sheet of insulation against the siding, then pulled it off and found at least one plank saturated with water. I then reinstalled with the 1×2 sleepers refoamed and cut it away after 3 weeks of rain, and the plank was dry as a bone. (the exterior is generally very leaky so that 1 inch space (really 3/4) is probably getting lots of airflow
2 concerns I have though.
1) I used Sika poly iso, with foil backing to do the cut and cobble. I’ve since read that might be a problem due to lack of vapor flow.
2) I had a structural engineer look at the house for other reasons, and he saw the insulation and was concerned about the impact on the 2×6 exterior wall studs (not the plank sheathing). He said eventually the 2×6 studs would rot because of the foam. I’ve not read anything about the impact on the studs when using the cut and cobble technique. So any information would help.
So wondering if I have to rip it all out and start with a different product.
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The only time the facing of the foam matters is if you don't have enough R value for condensation control. Since you are reffering to 2x6 walls, I'm guessing you are looking to add 2x4 batts behind the foam. In zone 6, as long as the rigid is at least R7.5, the facing doesn't matter as the foam is thick enough for condensation control.
As for the foam rotting the studs, first time I heard of it. If there is no liquid water in the walls (which should be the case with a condensation controlled assembly plus the vent gap on the outside) there is no source of moisture to rot the stud.
I have done pretty close to your assembly (real 2x4 studs with 2x2 strapping, so 5.5" space) in zone5 with 1.5" rigid without issues.
Thanks so much for the note. I was planning on double up the poly iso to get R26 walls. But have read recently to go with fiberglass or rock wool. Any downside to doubling up the poly (I already bought a lot.
You can put in a double layer of polyiso without any issues. Just make sure you don't have an air space between the layers -- you want the two layers tightly pressed together.
Bill
Thanks Bill!
To everyone- the concern on the foam rotting the studs was related solely to vapor infiltration. The concern was that the vapor would enter the stud (depending on the season from inside or outside) and then condense 1/3 of the way in. The outside (both directions) of the stud would be fine, but the middle section would start to rot. This is the first I had heard of this, and wonder if anyone else has encountered this, or heard something similar.
Many thanks.
Vapor condenses on surfaces. You would get rot on what is known as the "first condensing surface", which would be the face of the stud. Which face would depend on which face was below the dew point of the air on that same side of the stud.
You can't get "condensation" INSIDE of a solid material. There is no way for that to occur. Condensation has to occur on a surface, since it is, by definition, water "condensing" out of the air onto a solid surface, when the air holds water vapor and the surface is below the dew point of that moist air.
Bill
Thanks Bill!
Thanks again everyone. I'm interested in any and all responding to the merits of the following idea. I hate to keep revisiting, but rot scares me more than fire.
"A wooden stud has a much higher perm rating than closed cell foam.
Hence water vapor will flow thru studs at a much higher rate than thru closed cell foam and will condense at the point inside the stud where the temperature drops to the dew point. The dew point varies as does the temperature gradient thru the stud. This allows condensation to sweep back and forth inside of the stud enabling rot and mold to occur across a broad section of the stud or at least the thickness/depth of the closed cell foam."
Moisture DOES NOT condense INSIDE wood framing. Moisture can and does MIGRATE THROUGH wood framing, and the moisture content of the wood can change (look up "adsorption", which is different from "absorption" -- note the very small spelling difference between the two words). What happens is the moisture content of the wood can and will vary slightly throughout the seasons, but that's OK as long as it can dry at some point too.
What you don't want is condensation, which is basically bulk water -- wetness, like droplets or a puddle -- to sit on the wood. Mold won't grow INSIDE the wood, but it will grow on the surface if the moisture content is high enough. Condensation is the enemy here, not humidity levels and adsorption by the wood. Water vapor in air DOES NOT move through the wood, because the wood is an AIR BARRIER. It's not the same process. This is the same reason why moisture doesn't condense out inside rigid foam materials, and that's one of the big reasons why rigid foam is a safe material to use in damp locations like basement walls.
The important thing to understand here is that water vapor water in air. The moist air doesn't move through solid materials that are air barriers, and it can't condense OUT OF THE AIR if the air isn't there with it. Mositure within wood IS NOT WATER VAPOR and there is no process for condensation because there is no vapor INSIDE the wood.
Hopefully that makes sense :-)
Bill
Thanks a million Bill for hanging in there with me. I understand some of what you outline, but am still not clear on some aspects. The moisture that you reference in the stud- where does it come from (if not water vapor in the air)? And is insulation (any type) going to make it harder for the stud to dry out? Or can I expect the moisture to be able to get out the same way it came in (presumably through the drywall or outside sheathing.
If it's easier to talk off line, my email is [email protected] But no problem if not. You've already done god's work. Thanks.
Tom