If I use Zip R sheathing against 2×6 framing with rock wool, will we get condensation on the interior face of the Zip panel?
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We are planning cabin construction in northwest Montana. We debated rigid foam sheets with housewrap but are leaning toward Zip R sheathing with 2 inch foam backing.
With the waterproof barrier integrated with the Zip system we are concerned that there may be condensation on the interior face of the panel. Is this a concern and how would it be handled? Would this dry to the inside?
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I think you would need to use the 2.5 inch panel or switch to a 2 x 4 inch wall, but let's see what other folks think.
Are you planning to include an air gap between the sheathing and the cladding?
Stephen,
You are in Zone 6, so the so-called R-12.6 panels from Zip-R (the ones you are planning to use) would seem to work. The only problem is that polyiso doesn't behave the way the label says it will in cold temperatures; these R-12.6 panels will behave more like R-9 or R-10 panels during cold weather. I'd say that a different approach (with thicker rigid foam) would be safer.
For more information on this issue, see these three articles:
Calculating the Minimum Thickness of Rigid Foam Sheathing
Nailbase Panels for Walls
Cold-Weather Performance of Polyisocyanurate
Thanks to you both. We have really appreciated the excellent information that GBA has provided and have read the article Martin mentions on rigid foam. One of the issues that has us hesitating to use the exterior foam is the problem of ants, which we know our neighbors struggle with in their conventional home and we read about in GBA, "If ants like rigid foam should we stop using it?". We are in the woods on a lake. We are concerned about not having enough insulation and creating the dripping moisture issues between hot and cold, but we're optimistically thinking perhaps the lower humidity of Montana might be enough to compensate for this. We will use rock wool insulation, and some sprayed in foam. We are hoping to achieve a near PGH, although this would be primarily a summer residence we may end up year there round eventually and we have an eye on that. Our winter heat, left on low, will be propane, which is not drying either, but the house will be cool, not warm. If we are there in the winter we will have split units, and some baseboard heat. We want to have a solid house, but not overbuild or create unseen problems with rot and mold. We may consider using 2x4s for the second interior wall in the end, but it is a smallish cabin. Could we sandwich another layer of rock wool board under the sheet rock, but against the 2x6 studs to add to the R value? From the outside in: Hardie Board, rain screen, 2" polyiso Zip R, 2x6 with rock wool, 1" rock wool board, Sheetrock? Would this be enough? Again, many thanks.
Stephen,
Here's the theory behind walls with a combination of exterior rigid foam and interior fluffy insulation:
1. You are worried about the dewpoint at the first condensing surface (either the exterior sheathing or the interior face of the rigid foam).
2. The ratio of the R-value of the rigid foam to the R-value of the fluffy insulation depends on your climate zone. Colder climates require thicker rigid foam to keep the wall safe.
3. Adding more fluffy insulation on the interior side of the foam (as you propose) makes the situation worse, not better. That changes the ratio the wrong way -- it makes the first condensing surface colder. You want that surface to be warmer, not colder -- so you need thinner fluffy insulation (or thicker rigid foam), not thicker fluffy insulation.
More details here: Combining Exterior Rigid Foam With Fluffy Insulation.
Would you consider exterior mineral wool instead of exterior foam like mentioned in that article you mentioned?
If mineral wool solves the ant issue (it sounds like some people think it does, but it's not totally clear), then you just follow the rigid to fluffy ratio guidelines that Martin gave you and you're good to go!
Thank you Martin and Brendan. We really appreciate your clarifications. We are trying to sort out all of these details from Kabul and have few local resources. The internet, source of all truth, has been really helpful as we plan the cabin build. We have discussed the external application of Comfort Board to reach the R value we are considering. Thanka again. Steve