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Insulating a 1 1/2 story brick veneer Cape Cod attic and kneewalls

patrickoneal | Posted in Energy Efficiency and Durability on

I live in Portsmouth, VA(Climate Zone 4), in a Cape Cod style house with knee walls and slanted ceilings upstairs. I pay quite a bit to heat and cool my home. I crawled around the attic to get a good idea of what’s there, and there isn’t much. The floor joists between the first and second floor are not sealed. The area between these joists(in the lower attic) have a thin layer of blown in fiberglass, and no insulation in some places. Same goes for the slanted ceiling, and the top attic. The slanted ceiling does not have insulation baffles or chutes installed either, so if there was decent insulation there, ventilation between the upper and lower attic would be questionable. The knee walls do have neatly installed kraft faced fiberglass batts, but no air barrier on the back(attic) side. My plan is this:

1. Obtain Dow Thermax or JM CI-Max foam foil faced insulation board. Both of these products are rated for exposed attic usage, whereas the stuff at Home Depot and Lowes must be covered up.

2. Plug all floor joists at the base of the knee walls with foam board, and spray foam to seal.

3. Seal all accessible penetrations into the attic with spray foam.

4. Cover the back side of the knee walls with foam board, sealed with spray foam and aluminum tape at the joints.

5. Install insulation chutes in the slanted ceiling, block the bottom of the cavity up to the baffle at the top of the knee wall with foam board and fill the cavity with blown fiberglass from above.

6. Blow fiberglass into the upper and lower attic floors.

My questions are:

1. The short slanted portion of the ceiling is framed with 2×6 rafters. This leaves little room for insulation. How deep should the baffle/chute be? That is, is a 1″ space between the insulation and roof decking enough to allow airflow between the upper and lower attic?

2. What material should those baffles/chutes be made of? Should I build them from something breathable like masonite, or use something impermeable like foil faced foam board?

3. Could I just stack foam board in that area from the ceiling wallboard up to 1″ from the roof decking , and forego the chutes? The thing that concerns me in that approach is that the foam board is a vapor barrier and could lead to condensation problems in the slanted ceiling. It would allow for a higher R-value as 4″ of CI-Max foiled faced foam is R-26. Although it is expensive.

4. The stairway from the first to second floor also has a slanted ceiling. It’s framed with 2x8s or 2x10s. This framing is several feet away from the roof decking, so access is no problem. Should that ceiling be treated like the knee walls, rather than a ceiling? Can I back it with taped and sealed foam board and shoot it full of fiberglass?

Sorry for being long winded, I want to make sure I do this right the first time. I can post pictures if that helps.

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds like you understand most of the challenges associated with insulating a Cape-style house. Here is a link to an article that you may want to read: Two Ways to Insulate Attic Kneewalls.

    You seem uncertain of the best way to insulate the sloped ceiling areas, and that is understandable. You are correct that 2x6 rafters don't provide enough depth for a ventilated channel plus an adequate thickness of insulation.

    For more information on all of the ways you can insulate a sloped roof assembly (a type of cathedral ceiling), see How to Build an Insulated Cathedral Ceiling. The article includes ideas for the types of materials you can use for site-built ventilation baffles. It also discusses ways to thicken rafters when they are too shallow to provide enough room for insulation.

    One of the methods you are considering is called the "cut-and-cobble" approach. I don't recommend using the cut-and-cobble approach for unvented cathedral ceilings, because of reports of moisture problems resulting from this type of retrofit. For more information, see Cut-and-Cobble Insulation.

    Here's the bottom line: the best way to tackle the problems you list is to install a thick layer of rigid foam above the roof sheathing. Of course, this approach means that you will also need new roofing. While this type of retrofit work is expensive, it's always the best way to go.

  2. patrickoneal | | #2

    I appreciate the information.

    The cut and cobble I was suggesting would end up being a vented solution, that is 4" or so of polyiso between the rafters directly against the wallboard, leaving a 1.5" gap between the bottom of the roof decking and the top of the polyiso. Should I still avoid this approach?

    The sloped ceilings make up a relatively small portion of the structure, so less than adequate insulation may still be a great improvement over what I have now.

    I started looking for articles about rigid foam over a roof deck, and I have a few questions about that if you're able to answer:

    I assume attached porch roofs should be isolated from the house roof, and the porch roof doesn't need the foam?

    How do you handle narrow dormers? I was trying to find a picture, but it seems like the house could turn out pretty strange looking with 6" of extra roof decking on top of a narrow(3-4ft wide) dormer.

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