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Community and Q&A

How best to insulate the roof/attic space of a Cape Cod?

fgmei35nCV | Posted in General Questions on

Hi Martin,

Thank you for your article on radiant barriers. I hope you can help me. I
have a (hot in the summer, cold in the winter 2nd floor) approximately 1,300
(about 650 per floor) sq ft Cape Cod on Long Island, built in 1956 w/2 x 4 16″ oc construction. I have already re-insulated w/R-13, vapor barrier fiberglass and added new drywall to the 1st floor. The 2nd floor has 2 bedrooms separated by the stairs in the center of the house. The 2nd floor
has an attic or crawl space, behind the knee walls, and though recently
insulated with R 19, is quite drafty due to the non sheathing and board built
exterior walls wrapped in tar paper. I am dormering the back of the 2nd floor
to add space (bedroom and bath). My contractor has said he will eliminate the
ridge vent and build a roll roof. As you know and as I have read, the later
will invite wind driven rain/snow into the attic. I will purchase a gable fan
to help eliminate heat in the summer. I was going to have a radiant barrier
to help keep the 2nd floor cooler during the summers as it gets quite warm up
there, (partly due to the old/original insulation), but after reading your article I’ll no longer
be doing that. I will also add a vented sky light at the top of the 2nd floor stairs to help exhaust summer heat and cooking heat/odors (humidity?) as the heat goes up the stairs to the 2nd
floor, as the stairs acts like a chimney.

My question is how to best insulate the 2nd floor? My contractor will be
wrapping the entire house w/Tyvek (over the existing asbestos? shingles – is this a good idea?)
and adhering to it ¾ inch DuPont “blue board” under vinyl siding. I am
leaning towards blowing in insulation in the walls and adding between the
roof rafters, rafter vents, to run from the 1st floor exterior
overhang vents to the ridge. Should I also add insulation around/underneath these
vents? I want to exhaust heat through the gables, hoping
to help keep the roof cooler and 2nd fl warmer in winter. Also, though the 2nd floor is finished (1960), I’m ripping out the existing drywall and old insulation so I can blow in
new insulation into the walls. What’s your opinion on how best to proceed?


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  1. GBA Editor
    Martin Holladay | | #1

    1. Since you are asking how to insulate your house, here's my advice: instead of adding only 3/4 inch of rigid foam, add 2 inches. I suggest you use polyisocyanurate instead of blue Styrofoam (XPS), since polyiso is more environmentally benign. That way you can add R-13 to your walls instead of R-3.7.

    2. It's good that you decided not to install a radiant barrier. Here's something else you can get rid of: the powered attic fan you want to install in your attic gable. There have been many studies that show that such a fan will use more energy than it saves.

    3. If you are gutting the upstairs, you have an opportunity to insulate with a wide variety of materials. If you like blown-in insulation, I suggest you install cellulose. Choose a contractor familiar with the dense-packing method.

  2. fgmei35nCV | | #2

    OK, thanks. I think I would have to "build-out" my window frames to accommodate the extra 2 inches of outside insulation, as the new windows are already installed everywhere else? Also, is it OK that the contractor is putting it on top of the existing siding?

  3. GBA Editor
    Martin Holladay | | #3

    Yes, your carpenter will have to install exterior jamb extensions at your windows, and new window sills.

    I recommend removing the existing siding before installing rigid foam. The question was discussed recently in another Q&A thread. You can read it:

  4. fgmei35nCV | | #4

    Do you suggest I don't add a ridge vent to the dormer? I'd like too, as there's one now, which will be gone due to adding the dormer. And should I use rafter vents, w/ or w/o insulation under it adhering to the rafters? Thanks again.

  5. fgmei35nCV | | #5

    Is it more effective to use R15 then R 13 in 2 x 4 walls?

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Do you suggest I don't add a ridge vent to the dormer? I'd like too, as there's one now, which will be gone due to adding the dormer. And should I use rafter vents, w/ or w/o insulation under it adhering to the rafters?"

    A. Insulated sloped roofing assemblies can be either vented or unvented. Either type of roof assembly can work well, as long as it is well detailed and as long as the R-value of the roof assembly is adequate. For more information, see How to Build an Insulated Cathedral Ceiling.

    Q. "Is it more effective to use R-15 than R-13 in 2 x 4 walls?"

    A. Yes. The higher the R-value, the better the performance of the wall. If you can find a way to build an R-20 or R-30 wall -- even better.

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