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Community and Q&A

Insulating my hip ceiling

Geoffrey Cook | Posted in Energy Efficiency and Durability on

OK.. 1880 foursquare, Hip Roof, Balloon Frame construction. I hated the low ceiling height so I’m almost done building a hip ceiling directly below the hip roof.

The outside of the rafters sit on a 2×4 ledge attached with 3/8 lag screws and the ceiling frame is built with 2×6’s. I will keep (god I wish I didn’t have to) or replace the old ceiling joists with 2×6 exposed beams to provide tension and keep the house from opening up with any snow load.

We just experienced a tornado system that ripped siding off and destroyed my window trim and the house was fine. I have a ton of 3″ EPS Blue Dow R-15 16″ wide foam and I was thinking of using that as a base for insulation either on top or between the rafters/joists along with fiberglass or rock wool.

There will be plenty of room for ventilation and I am installing a whole house fan at the top of the stairs (about where camera angle is from). The ceiling rafters if you will…are about 16 on center.

My question is: should I install it between the joists and rock wool over the top or install and tape it and install rock wool between the joists? I could load it with blow in but I’m afraid that won’t be good with the whole-house fan. Any thoughts? And thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    If you want to combine rigid foam insulation with blown-in insulation for this ceiling, I would install the rigid foam on the underside of the ceiling joists. I would secure the foam with 1x4 strapping, screwed through the foam to the joists. I would seal the foam seams with high-quality tape.

    Then you can blow insulation above the rigid foam. Cellulose is preferable to blown-in fiberglass, in my opinion.

    If you are installing a whole-house fan, make sure to surround the fan's rectangular housing with an insulation dam that is higher than the proposed depth of your blown-in insulation.

  2. Expert Member
    Dana Dorsett | | #2

    The blue stuff isn't EPS, it's XPS, which is much more vapor-tight at about 0.4 perms (compared to 1-1.2 perms for Type II EPS @ 3".) If it's 2lb XPS it's even tighter. No matter what at 0.4 perms or less it that would require at least some type of venting for the attic space.

    Putting it under the joists as Martin suggests is also the right way to go, as it the recommendation for cellulose over fiberglass, with blown rock wool in-between. Unlike fiberglass, cellulose is opaque to infra red radiation, and the cooling season performance will be measurably better. And even at open blow densities cellulose is far more air-retardent, and will not lose heating season performance at the temperature extremes. Blown rock wool is IR-opaque and denser than fiberglass, but still more air-permeable than cellulose.

    Putting high R/inch foam between joists is a waste of good foam since the R1.2/inch thermal bridging of the timbers robs it of performance. Putting that R15 above the joists would work too, and would not create moisture issues (provided the attic is ventilated) but you'd have to put more fiber on top of the foam as well as between the joists to hit R49 (IRC 2012 code min for most of the US.) With high density fiber between the 2x6s you'd get about R20, and with R15 above the joists you'd only be at R35 center-cavity, about R28 whole-assembly after thermal bridging.

  3. Geoffrey Cook | | #3

    How would I keep the blown in...or hand scattered cellulose from sliding down to the corners if I put it above the foam on top of the joists? I cannot put it below the ceiling as I wouldn't have any headroom. Would it be ok to run R21 fiberglass between the joists then the R15 foam, then another R21 above the foam in fiberglass or denim form? Would I need to tape the foam seams and would I need a vapor barrier towards the living areas?

  4. GBA Editor
    Martin Holladay | | #4

    It's hard to tell from your photo how steep your ceiling joists are. If you feel that the ceiling is too steep for cellulose, you can use two layers of fiberglass batts as you propose. However, cellulose can usually be installed on a slope. It tends to stay put.

  5. Geoffrey Cook | | #5

    It's an 18 degree pitch. Can the cellulose be blown in by me..I have about 10 bags of it in the basement. Will it interfere with the whole house fan which I will install at the top of the stairway...I will modify the ceiling there to be flat and that will be the only access to the "attic"


  6. Geoffrey Cook | | #6

    Also, any particular taping product that is better than others for sealing the seams in between the foam? Thanks

  7. GBA Editor
    Martin Holladay | | #7

    I know of only three tapes that can be used successfully on XPS: Siga Sicrall, Siga Wigluv, and 3M All Weather Flashing Tape.

  8. Geoffrey Cook | | #8

    Thanks...and about that vapor barrier?

  9. GBA Editor
    Martin Holladay | | #9

    First of all, there is no code requirement for a vapor barrier. Some areas have a code requirement for a vapor retarder (a less stringent membrane).

    Second, there is no need for a vapor barrier from a building science perspective. Paying attention to air sealing is far more important than slowing down vapor diffusion.

    Third, your rigid foam will be a strong vapor retarder -- almost a vapor barrier. So, whether you need one or not, you've already got one.

    For more information, see Do I Need a Vapor Retarder? and
    Vapor Retarders and Vapor Barriers.

  10. Expert Member
    Dana Dorsett | | #10

    Open blow cellulose is a dead-easy DIY job with a rental blower. Scattering it by hand leads to a very lumpy and uneven install, at a high average density but with a greater propensity toward efficiency robbing voids and sinks. The blowers have rotating tine wheels to break up and separate the fibers/flakes, which continue to separate as they bounce along the ~50-100' of 2" hose. (Short hoses can also yield a lumpy install, though not as crummy as just shoveling it in there by hand.)

    An 18 degree pitch is not huge, but wind in the eaves can also move the stuff around. If you use "stabilized formula" cellulose even if you dry-blow it the seasonal humidity cycling will usually be enough to activate the adhesive. But even standard dry-blown will stay put at the eaves if you lightly mist the surface with a plant-spayer, forming a weak but sufficient papier-mache' crust.

    At 3" XPS has about the same vapor retardency of the kraft facers on fiberglass batts. If there happens to be a non-scientific hare-brained local requirement for a vapor barrier in a vented attic, the XPS surely qualifies- it's a solid mid-range Class-II vapor retarder

  11. Geoffrey Cook | | #11

    Ok..Blown in on top of the XPS..what's the best for between the joists?



  12. GBA Editor
    Martin Holladay | | #12

    If you follow the advice given to you here, you will install your XPS under your joists, and you will fill the joist bays with cellulose.

    So the answer to "What's the best for between the joists?" is, "Cellulose."

  13. Geoffrey Cook | | #13

    If I put 3" under the joists my ceiling height will be lower than I would like. Also, how would one attach drywall then tongue and groove through 3" of foam?

  14. GBA Editor
    Martin Holladay | | #14

    As I explained way back in Comment #1, you secure the foam with 1x4 strapping, screwed through the foam to the joists with 4-inch screws. Then you screw the drywall to the strapping.

  15. Geoffrey Cook | | #15

    Would it be any benefit if I were to put one or two inch foam on the underside, then put the 3" xps, which is cut into 16" wide sections into the joist bays, then blow cellulose over the top? Or any combination of that?

  16. Expert Member
    Dana Dorsett | | #16

    It's perfectly fine to make a cellulose/XPS/cellulose sandwich.

    Do NOT cut up the XPS and insert it between joists. When put between the joists the R3.6ish thermal short of the joists at a 15% framing fraction brings the performance of that R15 XPS down to about R10, whereas mounted either over or under the joists without the thermal bridging you get the whole R15.

    With 5.5" of cellulose (~R20) between the joists that layer would run about R14 after thermal bridging.

    Taped & sealed continuous XPS atop the joists adds another R15, bringing the total to R29.

    Open blown continuous 6" of cellulose on top of that brings the total up to R50, which meets or exceeds code min everywhere in the US.

    The compressive strength of XPS is more than sufficient for handling even a couple feet of cellulose dead-load on top without needing an wooden deck underneath it, but you probably don't want to be walking on it without planking or OSB or something between the XPS and joists.

    If there's room for more without blocking the soffit venting, go for it- it's cheap stuff, and there's a lifecycle rationale for more than R50 in most of the US. See table 2 p.10 of this document:

  17. Geoffrey Cook | | #17

    Can I place a blocking on the bottom side of the rafters up a few feet to allow for more cellulose to keep it from getting in the soffits? Thanks

  18. GBA Editor
    Martin Holladay | | #18

    Q. "Can I place a blocking on the bottom side of the rafters up a few feet to allow for more cellulose to keep it from getting in the soffits?"

    A. Yes.

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