GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Cape / side-attic insulation

davidmeiland | Posted in Energy Efficiency and Durability on

Advance warning: hypothetical scenario… your chance to settle a flame war! Or not, if you don’t feel like it.

I’d like some opinions on how best to insulate the following areas, in a typical 1920s cape with minimal existing insulation. Exterior walls are 2×4, roof rafters are 2×6, second-floor joists are 2×8. There are small side-attic spaces on the second floor, and there are collar ties fairly close to the ridge that create an 8′ level ceiling and a low-overhead attic space above the second floor rooms. The ceiling in the second floor rooms is *also* partly cathedral… the 5′ long sections of rafter between the kneewall and the rafter tie are drywalled. Attic space is vented via gable vents, no eave/ridge venting.

Option 1 is to insulate exterior walls and roof, enclosing the side attic space within the envelope, using spray foam. No siding or roofing work is proposed.

Option 2 is to insulate the floor of the side attic, the kneewalls, and the attic floor with cellulose, and the rafters above the cathedral ceiling with rigid board, moving the side attics outside the envelope.

In both scenarios, the rest of the house also needs insulation. Assuming no budget limitations and no access/drywall demo issues, which would you choose?

Thanks for your indulgence.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Option 1 is preferred.

    Option 2 will work, but only with extraordinary attention to air-barrier continuity and thermal-barrier continuity -- between the perimeter top plates and the second-floor joists, between the second-floor joists and the bottom plates of the kneewalls (note that blocking is required between the joists under the bottom plates of the kneewalls), between the kneewall top plates and the rafters / rigid insulation, and between the rafters / rigid insulation and the attic cellulose.

    Most builders screw up these continuity issues, which is why energy experts always advise Option 1.

  2. jwyman | | #2

    I am in the process of the exact same job where rather than using spray foam, we framed the knee wall/attic space with 2 x 4's parallel to the slope and added 18" of cellulose. The new studs were fastened to the second floor and the back side of the knee wall allowing for a thermal break between rafters and new framing. We also spent some time airsealing the rafter seat area.

    The sloped roof was filled with vent channels, cellulose and 4" of rigid foam on the interior with drywall finish. We then capped the flat ceiling triangle with 18" cellulose and added a ridge vent.

  3. davidmeiland | | #3

    Martin, would you not be concerned about possible issues with foaming the inside of the wall sheathing where the WRB is not known or is non-existent? Seems to me you are taking a well-breathing wall assembly and changing it significantly... not automatically a problem but it could certainly turn into one over time. Or maybe you are saying you would use cellulose in the walls?

    Also, your answer seems to assume some level of builder error in executing detail #2. Maybe not an unreasonable assumption but the details don't seem that complicated to me.

    Jon, your approach sounds good and maybe better than either of the two I have described.

  4. GBA Editor
    Martin Holladay | | #4

    You're trying to ambush me. First you suggest Option 1 and Option 2, and ask, "Which one is better?" Then you point out the flaws in the option I chose.

    You're right -- there's always Option 3 (one you did not offer). That would include removing the siding, installing a WRB, installing rainscreen strapping, and then installing new siding. Much better.

    I haven't inspected the house, so I know nothing about the siding situation. Only you can evaluate that situation and decide how to remedy any problems with the WRB and wall flashing.

  5. GBA Editor
    Martin Holladay | | #5

    If it doesn't seem to you that it is particularly difficult to establish a continuous air barrier from the exterior walls, along the floor under the kneewall area, up to the kneewall bottom plate, along the kneewall to the rafters, and along the rafters to the attic, and along the ceiling of the attic and down the other side -- then answer me this: what materials and methods will you use to establish that continuous air barrier?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |