In a Northern climate, I am going to install R10 4×8 sheets on a 2×4 walls. What is the best method of attachment? Trim out the windows, doors and sill plates first? House wrap goes over the foam or under? Vapor barrier on the inside still? I have heard of problems with mosture getting trapped? Should the seams of the foam boards be taped? Use the foam with the foil side out?
Thanks
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LOL, there are a lot of ways.
First thing to know is that in a northern heating climate, the foam belongs on the inside of the walls.
On either side, tape the joints and you need no other VB except at openings.
You did not say if you are using what kind of foam panels, and knowing your degree days for heating is helpfull too.
If foil faced and you want to get any benefit of the radiant barrier, it needs to face the direction you want to reflect the heat back to, and it must have 3/4" or 1" of air space in front of it.
So what I do in heating climate is to place the Thermax foil face into the interior across the face of the studs on the interior, then strapping horizontally, then sheet rock.
Now if you already built the house and did not allow for this wall thickness re door locations, electrical placement, cabinet layout, etc, you would have a problem.
If you simply MUST put iot on the exterior, use an air barrier or vapour retarder, NOT a vapouir barrier, so any moisture driven into the wall assembly from the interior can have a chance to escape in the summer.
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I'd agree with most of what you said, except for putting the foam on the inside. I'm in Northern MN, and foam goes on the exterior, mostly. This isn't something I dreamed up. It's very common here. The reasoning is that adding foam to the exterior warms up the stud cavities so that condensation is less likely. How much foam to add is an interesting question. When I did my own house, I read that 1" of extruded polystyrene had a high enough perm rating that the structure still could dry to the outside. Other people will come along and say 1" isn't enough and that you are risking condensation in the stud cavities. My house has worked well since I foamed it. I even had to demo a wall (for an addition) and there was no sign of moisture. I omitted building paper, though I was very anal about detailing it to prevent water intrusion. In the jobs I've seen around town, most guys aren't using building paper. It's an insurance layer, and I don't think it matters if you put it over or under the foam. What you do need to think through is how you will flash the windows and doors. That might influence your choice of building paper.I taped the joints with a foil foam designed for taping foam.
Most of the discussion I've seen of it says that 2" of foam out to be enough even in our northernmost US. Climes. There is a way to calculate whether the warm side of the foam will be above the dew point. Riversong has posted it in the past, so a search would probably find it.
As you suggest, I should install the foam inside over fiberglass and studs. Foil side facing in. Just tape it up and no vapor barrier is needed. Housewrap the outside. The electrical boxes will all need extension, plumbing rough needs extensions, doors need thicker jams, windows need extension, lose a few inches of living space and etc. But this will not cause and condensation?Thanks
in a heating climate, placing it inside of the studs will give you less condensation than on the outside of same wall.Fill those stud cavities with cellulose or chopped FG instead of batts and you are yet another step ahead.Controlling your infiltration is a large step often ignored also. seal around all the penetrations and at base and sills well. I am working on a house right on the shore facing a north wind that has a lot of glass and only R11 FG batts in walls, but it is amazingly comfortable.
I think one reason is because they have sprayed foam insulation sealing the sills and rim joist against any infiltration, and same around windows. Roof is tight with 4" spray foam in cathedral.
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Spraying is another option. I always worried about off gasing. I heard there is now "soy" based sprays but do not know the brands or seen any installs. Thanks
I hope soy based foam spray doesn't "melt" like the soy based "packing peanuts".
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see there is one of the variables that he was not clear on - EPS, XPS, or foil faced????
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