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Insulation Question

GregKu | Posted in Green Building Techniques on

First of all, great site. I wish I would have found this sooner.

I’m doing a home addition in region 4 (Northern Virginia). The house is about 70 years old. I’m pretty clear on insulating most of the house. We’re using the Zip r-5 product on the exterior, sprayed cellulose inside the walls, and blown cellulose in the attic. Also, I plan to do the rim joists with open cell spray foam.

There are three tricky areas I can’t figure out.

1. The basement walls – The basement already has been framed with 2x4s. There’s an inch air gap with the concrete. In hindsight, I would have liked to do rigid foam, but I think that ship has sailed. Am I stuck with closed cell spray foam as the only preferred option. Is open cell not recommended for basement walls?

2. While most of the house has an attic, one bathroom is just drywall directly under the rafters (sloped 3:12). Can I still use the blown cellulose there? Or is there a mold/rot concern with moisture from the bathroom and the difficultly of ensuring an air gap between the roof and insulation? The roof is just standard plywood, paper, and shingles if that matters.

3. One small room is being built on the slab of an existing concrete porch. To match the floor level in the rest of the house, the contractor built the porch floor up with 2x6s on end. These are supported with periodic flat 2x6s on the concrete floor. I can easily put several inches of rigid foam here between the 2x6s. Can I, or is there a risk of rotting the 2x6s that will be covered with the foam? The porch is 4 feet above grade. It’s bricked all the way around so I don’t know what’s under the slab, dirt or air.


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  1. GBA Editor
    Martin Holladay | | #1

    First, an aside to GBA readers: If you are planning a build a house, don't wait until the last minute to finalize your insulation plans like Greg did. You need to finalize your insulation plans before the excavator starts work on your foundation hole. For more information on this concept, see Plan Ahead For Insulation.

    1. You have two choices for your basement walls. Either (a) demolish the stud wall carefully -- you should be able to reuse the studs -- so that you can install rigid foam, or (b) use closed-cell spray foam. You can't use open-cell spray foam for basement walls -- it's vapor-permeable.

    By the way, removing the studs will go remarkably quickly. Just tap them sideways near the plates with a hand sledge. Most of the studs will be salvageable.

    2. You have to follow the rules for cathedral ceilings above the bathroom. No cellulose unless there is a vent channel between the top of the insulation and the underside of the roof sheathing; the vent channel must be connected to soffit vents at the bottom and a ridge vent at the top. Remember also that you need to include code-minimum levels of insulation (probably at least R-38).

    If you can't vent, you should use closed-cell spray foam. More information here: How to Build an Insulated Cathedral Ceiling.

    3. You should have planned how to insulate the floor assembly you describe before it was framed. Ideally, you would have included at least one layer of continuous horizontal rigid foam under the 2x6 framing (along with a layer of polyethylene). If this were my house, I'd probably remove the floor framing and do it right.

    -- Martin Holladay

  2. GregKu | | #2

    Thanks, Martin.

    Really appreciate it. Yes, I agree. I wish we educated ourselves on much of this earlier in the process. Frankly, it's surprising to me that so few contractors seem to understand (or care) about this stuff.


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