REMOTE wall retrofit – the next steps have me puzzled
I live in Anchorage, Alaska (zone 7) in a early 70’s split entry home. The original construction consists of 2 x 4 walls, but an addition to the house 2 years ago was built with 2 x 6 walls.
I spent the last 3 months installing 4 inches of XPS foam from the footers on up to the bottom of the soffit vents, using a mix of bituthene and 10-mil plastic behind the foam and liberal amounts of Tremco sealant to seal edges and overlaps. 1 x 4 furring strips and 7″ long screws into existing studs keep it all in place and siding will go up after winter is done – I have covered the outside of the house with Alaskan siding (Tyvek) to protect the foam until that happens.
Question #1 – From reading Martin Holladay’s article “Calculating the Minimum Thickness of Rigid Foam Sheathing” I assumed having R-20 over the 2 x 6 walls was more than adequate, but from every other source of information (dew point calculations, Cold Climate Research Center, and other people with such knowledge) they say I should reduce the batt insulation from R-19 to what would be in the 2 x 4 walls, i.e. R-13. I am getting ready to install drywall and would like to feel confident that I am doing the right thing. Should I reduce the amount of batt insulation or not in the 2 x 6 portion?
Question #2 – Just about every article I have read about installing rigid insulation on exterior walls on GBA has stated, in effect: “All foam-sheathed walls must be able to dry to the interior. That means you don’t want any materials with a very low permeance — especially polyethylene — on the interior of a foam-sheathed wall.”
Then Martin posted this on 11/28/11: “Tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and these Canadian homes are not experiencing wholesale failures or problems.”
I was planning on removing the interior poly when I installed the new drywall, but my question is this – is it a “must do” or a “might do”?
Question #3 – Part of the addition to my home 2 years ago was a 16 x 20 foot expansion of my garage, now a tandem style. A deck is all that will be on the roof of the addition. The way it is constructed is as follows: decking (not yet installed), 4 inches of XPS foam, rubber membrane, 3/4 inch treated plywood, 2 x12 glue lam joists with a 2″ bevel cut (sloped, for drainage), R-30 batt insulation and poly. The architect and contractor said to not have the insulation against the bottom of plywood, but to leave a 3″ gap between the two. This is a hot roof as there is no venting to the outside.
Is this a proper set up? Do I leave the poly up or does this create a double vapor barrier?
Thanks in advance for all answers!
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Replies
Jon,
Q. "Should I reduce the amount of batt insulation or not in the 2 x 6 portion?"
A. I think it would be safer to reduce the amount of insulation in your 2x6 walls. The building code rules which I described in Calculating the Minimum Thickness of Rigid Foam Sheathing are not intended to be guidelines for REMOTE or PERSIST walls. Rather, they refer to conventional framed walls with rigid foam on the exterior side of the studs or wall sheathing.
What's the difference? REMOTE walls and PERSIST walls usually include peel-and-stick membrane on the exterior side of the wall sheathing -- a step that you have done. This membrane layer makes the sheathing more vulnerable to moisture accumulation and rot, especially if the sheathing is cold (due to the use of insulation between the studs). That's why I wrote, in my article on PERSIST construction, "Instead of leaving stud bays empty of insulation, some REMOTE builders fill them with fiberglass or cellulose. This practice is potentially dangerous, however. The more insulation in the wall, the greater the possibility that the wall sheathing will get cold enough to allow condensation to form. That’s why most PERSIST builders prefer to keep framing bays empty, with all of the home’s insulation outside of the rubberized membrane."
One final point: when it comes to Alaskan conditions and Alaskan construction details, I always defer to the experts at the CCHRC. If they are telling you to reduce the amount of insulation between the studs, listen to them. They know what they are talking about.
Q. "I was planning on removing the interior poly when I installed the new drywall, but my question is this - is it a ‘must do’ or a ‘might do’?"
A. If you are installing new drywall anyway, let's call that step a ‘must do’ -- OK? Why take chances when you understand the building science? You'll sleep better without the poly.
Q. "The architect and contractor said to not have the insulation against the bottom of plywood, but to leave a 3-inch gap between the two. This is a hot roof as there is no venting to the outside. Is this a proper set up?"
A. When building a hot roof that combines rigid foam on top of the sheathing and fiberglass batts under the sheathing, the fiberglass batts should be in contact with the sheathing above, and the rigid foam should meet the minimum R-value requirements set out in section R806.4 of the 2009 IRC. That code provision calls for at least R-30 of rigid foam above the sheathing in Climate Zone 7. So, your builder appears to have made two mistakes: your rigid foam isn't thick enough, and there shouldn't be an air space between the fiberglass batts and the sheathing.
More information on hot roofs can be found here: How to Build an Insulated Cathedral Ceiling.
Martin-
Since there is no room to add another 2 inches of rigid foam under the future deck, do you have any suggestions/recommendations on how I should proceed?
Jon,
It's hard for me to give advice on how to proceed without a site visit. If you can't add insulation above the sheathing, you can either:
1. Leave everything the way it is, and hope for the best.
2. Open up the roof assembly from below, and add rigid foam or spray foam to the underside of the roof sheathing.
Martin-
Since I have not put drywall up on the ceiling yet, installing rigid foam on the underside of the sheathing seems like the easiest and best option. With this configuration do I use a layer of poly under the drywall or not?
Jon,
No poly.