Condensation Concern with Intello Membrane in Middle of Wall
We’re renovating a very old house in northwest MA that currently has no insulation. Planning to put recycled rigid foam in real 2×4 stud bays. Contractor proposed putting Intello membrane over studs to air seal, then building 2×3 wall with offset studs to thermally break and filling with batt insulation, which would be easy to put around electrical wiring. Question: would having air barrier in middle of wall allow moisture in the winter to go through the 2×3 wall and condense on the exterior wall where the membrane is located, thinking those exterior studs would be especially cold. Here’s the design from exterior inward:
cement shingles
wood shingles
plank sheathing
2×4 wall filled with rigid foam (sealed with can foam)
Intello membrane
2×3 wall with fiberglass batt insulation
drywall
The purpose for the Intello in middle is to make it easier to air seal and not worry about penetrations for electric boxes, etc.
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You'll get condensation on that membrane if it falls below the dew point of the air it's in contact with. That's really all that matters for condensation to form. The tricky part is figuring out when those conditions might occur, and that's actually not that difficult to do.
The important thing for you is "what is the ratio of insulation on the two sides of that membrane in the middle of the wall"? With that info, you can figure out the temperature in the middle. If, for example, you have R15 on the exterior side, and R11 on the interior side, the ratio is 15/(11+15) = 0.58. If you have 0F air on the exterior, and 70F on the interior, which gives a temperature of about 0.58*70F = 40.6F at the membrane location. If you know the humidity level of your air, you can find out if you'll see condensation or not by using a psychrometric chart.
If you have the entire 2x4 cavity filled with rigid foam, and the entire 2x3 cavity filled with what I'm guessing is a slightly compressed R11 batt, then you'll probably have a pretty safe wall assembly, assuming you have a decent air barrier on the interior side.
Bill
This is a very typical use of intello, except with dense pack cellulose instead of foam in the wall cavities, the ramifications of which others are much better qualified to comment on. I personally would dense pack the walls.
This page has tons of info on intello - https://foursevenfive.com/intello-plus/
The people at 475 are all very focused on building science, and based on my experience with them, would be very happy to discuss your contractor’s proposed usage of intello with you even if you are not buying anything from them directly.
In my comment I expressed doubts about the original plan, but switching to dense packed cellulose in the cavities of the old wall would solve the problem, as the humidity in that region would become very low, equilabrating with outdoors, and allowing any excess moisture at the intello a place to go.
The cold studs poses an interesting problem. Even without the Intello, the overall outer wall has low vapor permeability, so the humidity in the fiberglass batt region would be theoretically mostly established by an equilibrium with the interior conditioned space dew point. If the edges of the studs are cold, there could be condensation on them, at least in theory. In practice, it might be fine, if you have that newly added well cavity well sealed so air doesn't circulate through it--but if your plan is to be casual about that, maybe it's a little risky.
The intello wouldn't do much other than air sealing--if it got condensation on it, it would become vapor open. If you wanted it to help that issue, it should be just under the new drywall. But then you'd lose your ability to use that space for wiring without worrying about air sealing. Maybe just bite the bullet and plan to do air sealing with intello and careful treatment of each electrical box at the drywall.
Another option that might be easier is 1/2" or even 1" of foil faced polyiso where you had planned to put the intello. That can be taped to make an excellent air barrier and will also protect the edges of the studs from condensation. And you'll save the cost of the intello.
Stoltzberg, that seems like a good assembly to me. In fact I'm working on two projects now with similar details, though without the foam, but the R-values are comparable. I use the IRC for guidance--based on research by Dr. Joe Lstiburek, in climate zone 5 it allows only a class 3 vapor retarder (aka painted drywall) at the interior if you have at least 27% of your R-value toward the exterior. That keeps the condensing surface (the interior face of foam or the Intello, in your case) above the dewpoint temperature the vast majority of the time. The more insulation you add to the exterior, the safer the assembly.
https://codes.iccsafe.org/content/IRC2015/chapter-7-wall-covering#IRC2015_Pt03_Ch07_SecR702.7
In my earlier comment, I was thinking the wood studs might be too thermally conductive and the edges of them too cold, but I hadn't thought through the numbers. If they are maybe R-5 (R-1.25/inch), and the fiberglass gives R-9 (optimistic), that's 35% of the R-value toward the outside so you are right: it should be OK.
I still think the intello won't do any smart membrane action there, and could be replaced by a cheaper air barrier material, even tyvek, and that if you want the Intello to do its thing, it should be closer to the interior... Or if the cavity insulation was something permeable like cellulose.
+1 on using fiberglass or cellulose between the existing studs and then putting taped rigid foam in the interior side.
No Intello needed - but make sure that the result complies with the recommendations in Table 2 A or B here.
Intello is supposed to have no more than 30% of total insulation value on the interior side of the membrane, per 475.com.
Board insulation is harder to work with when you start wiring.
What about r15 rockwool/cellulose for 2x4 area, intello, comfort board 80 1 1/2 r-6 with the 2x3 perpendicular to the studs. That would be with the 2 1/2 width nailed to the 2x4 and the 1 1/2 thick for the comfort board.
Are you leaving the existing exterior siding in place and covering with cement shingle? I am asking because if you could do a vented rain screen on the outside, along with the suggestion of dense pack cellulose that is exactly the assembly recommended by 475 building supply. Take a look at their Wood Retrofit assembly in the e-book section.
As others have mentioned the ratio of insulation in and out will keep condensation from forming most of the time and if your able to create a vapor open assembly with ability to dry in or out you should be in great shape.
The house already has cement shingles over wood shingles and we cannot afford to replace the exterior siding as well as renovate the inside. I was planning to leave an air gap between the board sheathing and the rigid insulation in the 2x4 stud bays. Does that seem like a good idea or do you think I should just fill the stud bay completely with insulation?
No do not leave an air gap on the sheathing side, definitely fill the stud bays completely or at least tightly to the sheathing side. Any air space in there is an invitation for condensation and thus rot and mold. If you are doing the rigid foam in the stud bays check out the GBA article on cut and cobble foam insulation.