Vapor barrier on interior with rigid foam exterior
I asked a similar question a while ago but got a little off-track and broad with my questions:
My question is this:
I am in zone 4 and have the wall cross section as shown in attached picture:
1. If I use sheetrock (painted with latex paint) for the interior wall covering, is a vapor retarder over the cavity insulation a benefit or a detriment?
2. If I use plywood for the interior wall covering, is a vapor retarder over the cavity insulation a benefit or a detriment?
3. Kraft paper has a perm rating of 1.0. This is about the same as 1″ of XPS foam (perm 1.1). Would these work interchangeably underneath the drywall or plywood? I would not mind the extra R value of the foam if a vapor retarder is needed anyway.
Thanks in advance.
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If I remember correctly, you are in zone 4. In your climate, you don't need any retaders with the amount of exterior rigid you show.
It doesn't hurt to use faced batts, but I would not spend any extra on it.
Painted drywall or CDX are fine on the inside. Plywood is hard to attach to metal framing, so if you do go for wood, make sure to get the right screws for it, Simpson has these for subfloors but will also work just as well for a wall.
Good catch, I added zone 4 to the original question. I appreciate your response: it sounds like I don't need any kind of barrier. So my next question is this, if I do want the added R value, is it reasonable to put 1" of XPS foam under the drywall or plywood?
It would be better to keep all of the rigid foam on the exterior so that interior has maximum drying potential. You'd be better off using EPS strips on the stud edges Bonfiglioli-style, then filling the stud bays with more fluffy insulation. You need to keep your interior:exterior insulation ratios in the safe zone for your climate zone if you do this though.
Bill
Since you are still in a climate with more HDD than CDD you can have a vapor barrier on the inside which is what the foam will be. Best is to use something that is not faced which has some permeability which will allow for a bit of drying.
For your assembly, I don't think the interior rigid is worth the effort. I'm with Bill, if you want more rigid, put it on the outside.
Bill, it sounds like you would also advise against even kraft paper covering my wall insulation?
This thought of putting foam over just the framing was also considered. I have access to 6.3" R19 insulation for the cavities. Would I need to be concerned about a small (3/4" +/-) dead space in front of the fiberglass batts and the wallboard like I show in my picture? I am in zone 4, any tips on interior:exterior ratios?
Kraft paper isn't a vapor barrier, it's a vapor retarder, and a reasonably "smart" one at that (i.e. it's more permeable under higher moisture conditions). Kraft paper is essentially an old-school smart vapor retarder, and won't be a problem here in your assembly.
That interior side air space isn't a problem, just make sure the exterior side does NOT have an air space. It would be better to full fill the cavities though, and note that that interior side foam in your detail really isn't buying you all that much in terms of overall R value, especially with the air spaces in the rest of the cavities.
Bill
just did a simulation with ubakus.net, you need an inside vapor retarder for 1" external foam and like 4-6" of fiber glass bat. The fluffy "R" is too much for 1" of foam, even with polyiso as foam, the surface of the foam gets too cold and will condense.
You have to be careful interpreting Ubakus results. The standard condensation controlled assemblies we talk about here don't have zero condensation but the amount of condensation is small enough that the assembly will dry quickly when it gets warmer.
If you look at code tables for Zone 4 assembly with only painted drywall, for a 2x6 wall you need R5 exterior. The OP has this plus the studs in this case are steel so the effective R value of the fluffy is about 50% of wood stud wall, far from anything that could be considered risky assembly.
Ok, so let me paraphrase some conclusions from the responses, know that I have access to a pile of unfaced 6" R19 insulation that will fit between girts:
1. I can treat finished/or bare plywood and painted drywall about the same in terms of what I do with the insulation.
2. This unfaced R19 insulation is fine with my wall assembly.
3. It is NOT worth adding a kraft paper/other vapor barrier over the insulation but underneath the wall covering.
4. Sounds like putting additional foam on the inside is a iffy choice as either
a.) full sheeting of foam may inhibit interior drying, or
b.) foam strips over the girts won't add much r value unless i add get rid of the dead air space by adding a small 1" strip of fiberglass to fill the cavity.
All of this sounds correct? If so, I plan to go with the original wall cross section (no interior foam, no interior vapor barrier).
I appreciate all of the feedback.
1- Yep, they're pretty close as air barriers.
2- Yep, no problems here.
3- If you are using kraft faced insulation, adding a secondary vapor retarder isn't gaining you anything and just adds an extra step and expense. The only thing I would consider if you want that vapor retarder is to use unfaced insulation and something like MemBrain over the entire wall. Sometimes it's easier to install the vapor retarder as one big sheet over the entire wall. Either works in terms of functionality though.
4a- Any rigid foam on the interior WILL inhibit inward drying.
4b- I don't think foam strips is worth the effort here and I wouldn't bother with that.
Bill
Thanks Bill,
3- I have a good supply of UNFACED 24" R19 insulation I got as cutoffs from a local insulation supplier. Do you think it is worth adding the vapor barrier (MemBrain, or otherwise)? You worded this as "if you want that vapor retarder": I don't necessarily want the vapor barrier (in fact, at this point, it would just be extra time/work), I just want to do what is best for moisture control in my insulation.
It is probably also worth mentioning: almost all of this this area will be kept at "working" temperatures: 45-55 in the winter. Non-air conditioned during the summer except while being used.
Any ideas on my last question Bill? Akos? wastl?
I think you're asking about the interior side smart vapor retarder? I like to add that for some cheap insurance (I use CertainTeed's MemBrain product), but in your warmer CZ it doesn't gain you as much. When you have enough exterior rigid foam for your CZ, you don't really need an interior vapor retarder, but using one doesn't hurt -- it only makes the assembly more robust.
Bill
Yes, I was asking if I should add a vapor barrier. With the steel framing, installation may be a little more troublesome than it is worth, particularly if you don't think it will affect performance much.
Thanks