Interior surface rigid foam application
PGOttawa
| Posted in Energy, Heating & Insulation on
I have early 60’s bungalow here in cold Canada. Walls are woodframe with brick exterior veneer. I’d like to remove the interior drywall replace the old fiberglass R8? with R14 roxul, a layer rigid foam on the surface of the studs, vapour barrier. I’ve thought about spray foam but I’d still like a thermal break to the studs and I could do this myself for about 1/2 the cost of an open cell product around here.
Do you see any potential problems with this wall design??? Other than sloped ceilings, I can’t find any info on interor wall application.. We are planning extensive renovations and an addition so now would be the time to do this.
Paul
Replies
The only difficulty with your plan is that you'll have to deal with outlets and door/window frames. Some will claim that having both the foam and a poly vapor barrier is a "double vapor barrier" and therefore verboten, but they'd be wrong. (Though you could skip the poly if the foam you use is an effective vapor barrier AND you carefully seal the seams.)
Thanks for the reply. The electrical boxes and window jamb extensions are not deal breakers. Actually it gives a bit more insulating space behind the electrical boxes which is not a bad idea.
By the time we add the addition, there's only about a 100' of exterior wall left, minus windows and doors so it's a good opportunity to do this.
Even if they were both vapor barriers...
Eeven if they were both vapor barriers they are both on the same side of the wall, so NBD.
When we did this in our house we only had to move about 3 existing outlets! 8-)
Depending on your thickness of foam, if you want to add outlets (which you likely do) you can make plywood "plates", say 6" square the same thickness as the foam. Cut out holes for the electric box then screw the plates into the face of the stud. Cut foam board to fit. You will have very minimal thermal bridging, but offset by the modern conveniences of electricity, internet, whole house audio, etc.
-Rob
Thanks for the suggestions. The plywood is a good idea. I'll probably be using 3/4" foam to make it easy to adjust jamb extensions so a few pieces of subfloor should take car of the plugs.
Paul
Your wall
will be roughly R19 assuming the Roxul is R14 and you use 3/4" of polyiso. Why not thicker foam? It sounds like you are interested in the convenience of using 1x lumber as extension jambs, but you could use thicker foam and get more R value with minimal extra work.
What are you going to be doing in the attic insulation-wise?
If you tighten up your building like this you should give some thought as to whether ventilation is needed. Gas furnace, water heater or similar inside the house?
I'm not set on 3/4"....maybe
I'm not set on 3/4"....maybe more could be used. There is a couple tight casings where the addtional thickness would mean having to trim them...a particular pet peeve of mine. And an additional window replacement is out due to existing brick openings etc. Maybe I should just learn to get over it and hang a curtain........
The attic I'll be topping up to R40 with cellulose.
My mechanical appliances are all direct sealed combustion. I'll be taking a wait and see approach over the ventilation in the first winter or two and see if more control is needed. I figure the basement has some air leakage and it may be enough to offset the addtional sealing of the main floor.
interior rigid foam
I have used Roxul on several projects some in 60s bungalows
Remove drywall
Fur out electrical boxes (there are not many anyway)
Install R14 Roxul
Horizontally strap 2x2 on wall studs with solid furring around door and windowopenings - cuts down on thermal bridging
Install 1 1/2" type 2 EPS
6mil vaporbarrier
this gives you better than 2x6 insulation values
Would another option be to . . .
I have a nearly identical situation to Paul, except that I'm renovating a 103 year-old brick rowhouse, so I have about 4" thick brick immediately around the window openings and the rest of the wall is about 12" thick. On the interior, I plan to put up a stud wall and a drywall finish surface.
I've been contemplating putting a layer of rigid foam against the brick wall before I put up the stud wall, then insulating between the studs with roxul or another material (haven't decided yet), then hanging a vapour barrier and drywall on top of that.
This approach is appealing to me because it seems the electrical would be easier, as would hanging the drywall.
But I can't find definitive guidance on whether the rigid foam would serve as a vapour barrier and, consequently, when combined with a vapour barrier under the drywall, possibly trap moisture within the stud wall.
Any advice on whether I can take the approach I've described, or whether I should instead sandwich the rigid foam between the drywall and studs and follow the advice in this thread for dealing with electrical boxes?
Thanks, in advance.
no worries
I used 1" interior polyiso on the interior stud faces when I finished my attic. It was all "new" construction, so moving electrical boxes was not a problem. In your case, instead of moving your boxes you could use box extenders. Easy.
Increasing jamb depth is not a worry. I do recommend offsetting the jamb extension laterally outwards from the window jamb, don't try to align them flush. I'll use a 1/8" to 3/16" offset, it creates a slight shadow line and a bit more visual detail to the window trim, and you don't have to worry about a flush line telegraphing through the paint if paint grade, or having grain patterns align or stain matching perfectly if it's stain-grade work.
A caution; if you attach the drywall directly over the foam (no furring strips) then be careful about overdriving the drywall screws. If you snug them too much you can compress the foam a bit between the drywall and the framing. Over time the foam can "kick back" a bit, causing the drywall to pop out a bit and the drywall screw to tear through the drywall's gypsum core.
When I hang the sheets of foam I gap the edges by a bit, say 1/4" or so, and then later foam the gaps with canned foam. Makes for a nice air tight installation. The ends of the foam sheets don't have to break on a stud. They can break mid-bay. No worries. Prior to foaming the gaps I peek through the gaps and mark the stud locations on the face of the foam sheet with a sharpie marker. Use those tic marks for locating the framing centerlines when screwing the drywall.