Rigid foam over existing drywall?
NorthJersey
| Posted in Green Building Techniques on
I’m planning on converting half my garage to living space, and I would like to add three inches of rigid foam insulation and another layer of drywall over the existing drywall.
The walls are 2×4 on 16″ centers with fiberglass batt insulation, plywood sheathing (or possibly OSB), house wrap, and vinyl cladding. Can you think of any reason why this technique might pose a problem in my northeastern climate?
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Replies
Ben,
Where in the northeast? Is the wall you want to cover an interior or exterior wall?
I'm in northwestern NJ (climate zone 5). I want to cover interior walls. I was thinking 3" EPS and then 1/2" drywall over the existing drywall. If I leave the existing drywall intact, the 3 inches of foam is deep enough to permit me to install in-wall speakers without compromising the air barrier.
I wouldn't because you could trap moisture within the drywall and generate mold. Then you have electrical issues (wiring running through flammable foam).
Just tear out the drywall or are you worried that a neighbor is going to call a code official if they see the construction debris?
Interior foam (even over pre-existing drywall) can work just fine from a moisture point of view, but the ability to dry toward the exterior matters. Vinyl siding being inherently back ventilated lowers the risks considerably, but other aspects still matter. How deep are the roof overhangs?
Is the sheathing plywood, or is it OSB?
The type of foam also matters. Polystyrene (EPS & XPS) melts and flows while burning, whereas polyisocyanurate and polyurethane would char in place, and have a somewhat higher kindling temperature.
I'm not aware of any code or other problems with running wiring through rigid foam insulation (unless it's antique knob & tube wiring.)
Thanks John and Dana. I confirmed the sheathing is OSB. The overhang is maybe a foot. The foam will be covered with drywall. I'm open to polyiso, but I'm unsure about the permeability of polyiso and the availability of unfaced product.
My wall composition will look like this:
Vinyl cladding
Housewrap
OSB
Fiberglass batts ( between2x4s at 16" OC)
1/2" painted drywall
3.5" polyiso (1.5" paper faced plus 2.0" paper faced)
1/2" painted drywall
You could have the moisture content of the wall modeled with WUFI by a professional or try it yourself (it's pretty tricky) with the free version of WUFI. I don't know if that is necessary but perhaps it would provide piece of mind or direct your design.
Thanks, Keith. I'll look into it.
Ben,
Adding interior rigid foam over the existing drywall is fine. It won't create moisture issues.
There is no scientific or technical justification for John Clark's statement that "you could trap moisture within the drywall and generate mold."
Keith's suggestion -- that "You could have the moisture content of the wall modeled with WUFI" -- is unjustified. That would be a waste of money.
For more information on these issues, see these two articles:
Walls With Interior Rigid Foam
WUFI Is Driving Me Crazy
I realize this post is 3 years old, but I have a very similar issue, and just wanted to confirm that this strategy could also work for me.
Here's my situation: I recently purchased a very old home (~110 years) in Minneapolis (climate zone 6). In all 4 corners of the 2nd level there's a 4' high knee wall (approx 6' wide) followed by a slanted ceiling that's approximately 6'x6'. I've attached a picture of 1 of the 4. Currently there's no insulation in the house except for a trace amount in the attic. [sigh] As you can imagine, the ice dams are intense! I do plan to insulate the attic with cellulose this fall, but cellulose isn't a good option for these spaces between the slanted ceiling and the roof decking because there's is not much (if any) more than 3.5" (2x4 rafters) between the lathe&plaster and the roof decking.
I will replace my roof soon, and my solution to this problem had been to to apply closed-cell spray foam on the exterior side of the lath & plaster when the shingles & decking were removed. But recently I had the idea similar to the OP: adding interior rigid foam over the existing slanted lathe & plaster ceilings. So that would basically be applying a 4 6'x6' squares. I'm leaning towards it. Convince me otherwise! ;) To be clear, this would be the resulting construction:
living space | drywall | horizontal strapping | 2" rigid foam insulation | lathe&plaster | rafters (providing attic ventilation) | roof decking | steel shingles
One concern that I have about the spray-foam option is that it reduces the air flow into the attic (the 3.5" between the 2x4 rafters in all 4 corners of the house is the *only* place where air currently enters the attic). I wouldn't imagine moisture to be an issue because there is no insulation in the walls and the isn't any OSB. I also worry that the spray foam doesn't actually achieve higher r-values than interior rigid foam when you factor in the space that would need to be left open for ventilation and the thermal bridging through the rafters.
Note: the highest point on these slanted ceilings is *basically* the edge of the attic that will be covered in cellulose.
thoughts very appreciated!
If you can vent the existing knee wall space and the mini attic above, you best bet is to take your SPF money and seal up the existing assembly. This would be the attic floor of the ceiling bellow, the floor joist of the 1/2 story and the back of the knee walls. 1" to 1.5" of closed cell SPF will seal these up quite well.
For the rest install batts and loose fill cellulose.
As long as both knee wall and mini attic are vented (eaves vent+gable vent for knee wall, gable vent+ridge vent for the mini attic), you can dense pack the tapered roof section. No need for any SPF or interior foam there. You could install the interior rigid insulation but since it is a small surface, it won't really make all that much difference for energy use.
The most important part when retrofitting these older homes is air sealing. As long as everything well sealed, even R10 walls are surprisingly comfortable.
thanks Akos. If I follow you, it sounds like you're proposing that I install gable vents instead of using the existing ventilation pathway above the slanted ceiling? Do you know if this is a common strategy?
I'm just a novice, but I'd be concerned that the roof above these slanted ceilings might heat up without the ventilation (and with only R10) - perhaps exacerbating the ice damns?
One of the BS* Beer episodes had a section that suggested similar approach. As long as each section is individually vented, there is no need to vent the sloped section. Any moisture in there can diffuse through the insulation and be vented either above or bellow. The venting for each section can be any combination of ridge/eve/gable/mushroom as long as you meet the building code free area requirement.
Roof venting is about controlling condensation under the roof, not about roof temperature. Venting any roof does very little for reducing attic temperature. R10 is not great, sometimes you have to work with what you have.
For example, if you have say 10' wide room with 5' tapered ceiling, so 100sqft on two sides. With a typical shingle roof temp can get up to 160F, with an R10 assembly over a 75F room, the heat gain is (160-75)*100/10=850BTU, which is not that much.
P.S. Most ice dams are caused by warm air from the interior leaking into the roof, this is another reason to focus on air sealing.
I get a little nervous about packing those slanted-ceiling sections with insulation and hoping the moisture diffuses. Can others (like Martin and/or Dana) weigh in?