Two questions: Rigid foam in attic space
1. I have some knob and tube wiring running about 3 inches above the 1st floor ceiling, behind a kneewall. After having an electrician look at it I’m going to box it in with plywood, leaving a good size air gap. This will decrease the R value for this area. Is it ok to put 1 inch rigid foam on top of the plywood? This would make from bottom up: 1st floor ceiling, wiring with air gap, 1/2 plywood, rigid foam, cellulose, open attic and then roof.
2. What about putting rigid foam behind a kneewall, on the vertical kneewall? I have R13 fiberglass which is all that will fit, then plywood. If I put 2 inch rigid foam I can almost double the R value. I’m concerned about condensation, I’ve read both that this is ok and not ok on other websites. To be clear, doing so would make it be: drywall, R13 in the stud bays, thin plywood, rigid foam. Also wondering on the benefit, as the sloped ceiling above the kneewall would still be R13, but the attic above that will be more in the R60 to R70 range.
For either I’m leaning towards polyisocyanurate from HD which has foil on one side and a covering on the other, but could also use Polystyrene from Lowes, which is just foam, no covering.
This is all in NE Ohio. Thanks!
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Replies
Jeremy,
1. On the knob-and-tube question: talk to your local code inspector to make sure that your plan will be accepted.
Visible knob-and-tube wiring is easier to inspect and easier to replace than hidden knob-and-tube, so I don't really like your plan. It would be better to hire an electrician to rewire the circuit in question.
If you want more information on this controversy, read the comments posted below the following article: Borrowing a Cellulose Blower From a Big Box Store. The short version is that there isn't much evidence that covering knob-and-tube wiring with insulation causes house fires; however, covering the wiring is still a code violation in most jurisdictions, and doing so could cause hassles if you ever try to sell the house.
2. Concerning rigid foam on kneewalls: Yes, you can do that, and No, doing so should cause any moisture problems. For more information on this issue, see Two ways to insulate attic kneewalls.
Again, it would be a good idea to talk to your local building inspector on this issue as well. In some jurisdictions, local inspectors require rigid foam to be covered with a layer of drywall. In most jurisdictions, however, the drywall isn't required for rigid foam facing a small triangular attic behind a kneewall.