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Community and Q&A

Insulation retrofit

bdrfab | Posted in Energy Efficiency and Durability on

I have a ’50s-era home that i would like to insulate better. It’s a conventional stick-framed roof with all of a 3-inch heel at the outside of the wall and 2×6 rafters. Even better, it’s a hip roof.

I’d like to remove the existing 4-6 inches of blown-in insulation to be able to seal everything up. What’s the best way to seal things, say at the interior of the wall/ceiling?

I was planning to the cut 2-inch iso roof sheathing to fit in the bays and going up the rafter bays, leaving a 1 inch airspace for venting, and sealing that with Great Stuff foam. To finish I was going to blow 18 inches of cellulose up there.

Does that seem like it will work? Sadly, spray foam is cost-prohibitive, as is adding foam to the roof deck. Any other thoughts?

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  1. Tim | | #1

    My 2 cents for what it's worth. You could limit the amount of sprayfoam used by having only 2-3 inches foamed on top of your ceiling (up and over the ceiling joists) with closed cell spray foam. This can create a decent air-seal and vapor barrier—assuming that you are in a heating climate where this kind of thing makes sense on the warm side of the assembly. At the eaves, you can install vent chutes that will extend beyond the top of the new insulation, and block off the air-intake area (in your soffits). Then, spray foam against the blocks and fill the area above the wall as good as possible with sprayfoam, only. If your desired R-value is in the neighborhood of 60 (which 18 inches of cellulose suggest), you may follow up the roof slope a few more inches with foam, right at the rake—basically making a "tub" of sprayfoam, which then can be filled with your desired amount of cellulose. Substituting the sprayfoam with cut-pieces of insulation and using great-stuff to secure them in place may be viable, but likely laborious and in the end less effective than a sprayfoam application. Since a good air-seal and vapor barrier are key to a safe and robust assembly, I would not sacrifice quality and performance on that first layer.

  2. Riversong | | #2


    If I understand you correctly (polyiso between both the ceiling joists and the rafter bays with a continuous 1" vent space), yes that will work fine and, assuming you're doing this yourself, will cost a lot less than having someone spray closed-cell foam.

    It will work even better if there are inlet and exhaust vents for the roof system. Air Vent now offers a Hip Ridge Vent specifically for hip roofs to complement their excellent ShingleVent II ridge vent.

  3. bdrfab | | #3

    Robert, thanks for the tip on air vent. I am going to be re-roofing due to hail damage, so i will definitely look into that. Yes I am doing the work myself, I would love to do CCSF, but there are so many projects and a limited budget. any recommendations for caulking?

  4. user-723121 | | #4


    Make sure to address the interior partitions and other attic bypasses in the attic like wires and plumbing vents. Make sure cabinet soffits if any are blocked and air sealed.

  5. Riversong | | #5

    Caulking what, where?

  6. bdrfab | | #6

    I was thinking I needed to caulk at the drywall joints at the exterior walls. Or is that not necessary?

  7. Riversong | | #7

    You had me confused because I thought you were asking only about attic sealing and insulating.

    If you want to use the drywall as the air barrier, then you'll need flanged electrical boxes or Lescco polypans behind them, and Tremco acoustical sealant on the top and bottom plates, window rough openings and electrical boxes. The drywall joints get air-sealed with the tape and spackle.

  8. GBA Editor
    Martin Holladay | | #8

    Aaron Vander Meulen,
    If you are using caulk to air-seal the ceiling from the attic side -- assuming you have removed the existing insulation to expose the ceiling -- then you want to be sure to caulk the crack between the drywall installed on partition walls and the partition top plates.

    Needless to say, all the other usual penetrations -- including electrical penetrations and plumbing vents -- need to be addressed with caulk or spray foam. Don't forget to install weatherstripping at your attic hatch.

  9. Tony Olaivar | | #9

    Using foam in the rafter bays seems unnecessary when cardboard chutes are cheap and available and can be installed with a slap stapler. Without a doubt, bypasses between the attic and the space below will be your main concern. The 50's era homes we find ourselves in usually exibit problems where above-cabinet soffits in the kitchen or bath connect with the attic. These can usually be filled with cellulose without need for additonal prep. If the soffit contains non IC rated recessed lights (painted white or tan) or if the soffit connects to wall stud cavities, you might need to seal the whole soffit from the attic with foam or sheet metal. Another common feature is the "scooby-doo wall" or open cavity between two plumbing walls. These and other cavities (around fireplaces, above split level features, pipe chases) need to be sealed with rigid materials and foamed into place. Tony Olaivar - HERS Rater and BPI Professional - Lanz Heating and Cooing

  10. bdrfab | | #10

    Thanks for the advice fellas. I'm using the Iso board for chutes simply because I'm trying to get as much r-value in the limited space I have, as cheap as possible. Hopefully I'll be able to see things better once I get the old insulation out. Definitely have a couple of can lights to fix, its incredible how much ice a can light can make. I'll probably end up replacing them with ceiling mounted fixtures. is there some sort gasket to cover an electrical box in the attic. could I cut a chunk of ice&water to cover it? (free, I'm really cheap) maybe set that in a bead of caulk?

  11. Riversong | | #11


    The foil-faced polyiso board will also act as a radiant barrier under the roof deck and help reduce summertime solar heat gain.

    The best thing for sealing up exposed electrical boxes is gun foam (or canned foam), but if you have much sealing to do it'll be much less expensive (and less messy) to buy an adjustable gun and foam cannisters (use low-expansion foam).

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