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Ceiling insulation

myoon595 | Posted in GBA Pro Help on

Have several questions.

1. I found a lot of rigid foam (2.5 inch) in the ceiling which is laid over the cellulose insulation. I thought that the moisture will be kept under the foam. Do I need to take off all the rigid foam? or just leave it?

2. I tried to install the door jamb. However, I realized the wall is not vertical but slanted. Therefore, I can’t put the trim around the door. If I nailed it, it will be twisted because the door jam is vertical but the wall is not. Therefore, I tried to install the door jamb along with the slanted wall and make the hinge grooves balanced so that i can open the door smoothly.. Does it make sense?

3. My soffit vent (it is a fake soffit vent) does not work. Therefore, I decided to seal all air leak as much as I can and put lots of insulation. is that OK?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Is the 2.5" installed in direct contact with the roof deck, or is there an empty space of air between the foam and the roof?

  2. myoon595 | | #2

    The foams are laid on the flat ceiling joist over the cellulose insulation. Therefore, there are plenty of space between the foam and roof.

  3. myoon595 | | #3

    none

  4. Expert Member
    Dana Dorsett | | #4

    So if I understand this correctly you have, from top to bottom:

    shingles ( or other roofing material)

    plywood roof deck

    air space(?)

    cellulose

    rigid foam

    gypsum board

    paint

    interior air

    Without a vented air space above the cellulose the drying path for the roof deck is through the cellulose, foam and paint, which may be a problem. If there is a vented air space, it complies with building codes.

    The type of rigid foam matters. If it's EPS (the white beaded-board type material) it's vapor permeance may be in the right range: Tight enough to limit the amount of moisture getting in over a cold winter, but not so vapor tight that it becomes a moisture trap. Even though it is at the gray edges of the code and not exactly in compliance, it's not super-risky. If the rigid foam is XPS (pink/blue/green board) or if it has any type of aluminum or plastic facer, it is too vapor tight and the risk of ending up with moisture problems is higher.

    If you remove a section to view the underside of the roof deck, does it show any signs of moisture, or mold, such as dark streaks or patches?

  5. charlie_sullivan | | #5

    I'm understanding differently--I think he's got a vented attic, with, top to bottom:
    shingles ( or other roofing material)
    plywood roof deck
    BIG air space--enough to walk around in.
    foam
    cellulose
    gypsum board
    paint
    interior air

    The question is whether moisture coming up from inside condenses on the bottom of the foam. 2.5" might be enough such that the bottom of the foam is warm enough that is not a problem. What's your climate zone?

  6. myoon595 | | #6

    My climate zone is 5. Chicago
    I am using the Polyiso rigid foam.
    but I dont have enough the polyiso to cover the whole attic area. Those. may cover 80 %.
    Therefore, I have two options

    option 1. Attach those to roof rafters by angle. and insulate the attic floor using cellulose and fiberglass.
    roof rafter
    rigid foam
    air
    fiberglass batt
    cellulose

    option 2. if the moisture is not trapped under rigid foam, I can do as following.

    roof rafter
    air
    flberglass
    rigid foam
    cellulose

    Which option is better?
    Thank you for advance.

    ------------
    Yes.. this is the order of the materials right now.

    plywood roof deck
    BIG air space--enough to walk around in.
    foam
    cellulose
    gypsum board
    paint
    interior air

  7. charlie_sullivan | | #7

    Your option 2 is unusual but should work well. Whether the 2.5" of polyiso is thick enough that the bottom of it is warm depends on how thick the cellulose layer is under it, but if it's not thick enough, when you lay more fiberglass on top, it will become warmer and be OK. To check more specifically, can you measure the thickness of your cellulose layer?

    That will work best if the foam is directly in contact with the top of the cellulose. If it isn't, you might want to put a little more cellulose to fill in where it's not in contact.

  8. myoon595 | | #8

    Charlie,
    I know that is unusual.. therefore, I was embarrassed.
    Thank you for the advice.
    Thickness of cellulose is 5.5 inch because it fills the 2 by 6 joist.

  9. charlie_sullivan | | #9

    You are fine... Zone 5 with 5.5" of cellulose requires foam at least R-7.5. (https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing) You've got about twice that so that is plenty.

  10. myoon595 | | #10

    Charlie,
    That is for the wall.. not for attic.
    Does it work for the attic also?
    Thanks. Ben

  11. charlie_sullivan | | #11

    The physics is the same for the wall vs. the attic. The reason the information is for walls is that the question almost never comes up for an attic, because it's cheaper to blow cellulose thicker than to buy foam boards.

  12. Dana1 | | #12

    The physics isn't exactly the same for walls and attics. Nighttime radiation to the sky and daytime solar gains are far higher for roof decks than walls. Roof decks will often drop to the outdoor air's dew point on clear nights, whereas the incident radiation temp on walls is roughly the outdoor air temperature. That's how you can end up with frost on the roof on nights that stayed well above the freezing temperature.

    The IRC prescriptive minimums for exterior insulating sheathing reflects these differences. For US climate zone 5 it takes R20 out of the presumptive total R49 for dew point control on unvented roofs, or about 40% of the total, whereas in walls it's R5 out of a total R20 (R15 cavity, R5 sheathing for 2x4s) or 25% of the total for walls.

    So with R20 between the rafters, you'd need at least R13 above the roof deck or above the fiber insulation for dew point control if the foam is on top. If the foam is on the bottom and there is a vent space above the fiberglass and below the roof deck, it doesn't really matter what the R-value of the foam is, but the vent space has to be open to the outdoors.

  13. charlie_sullivan | | #13

    Except that it's none of those situations. The insulation is all on the floor of the attic, with none at the roof. The foam is on the top. There's a giant, big enough to walk around in space between the foam and the roof deck.

    It's true that the roof deck might get colder than the outside air, but the air in the attic probably still won't get colder than the outside air.

  14. GBA Editor
    Martin Holladay | | #14

    Ben,
    No one has bothered to answer two of your questions, so I will.

    Q. "I tried to install the door jamb. However, I realized the wall is not vertical but slanted. Therefore, I can't put the trim around the door. If I nailed it, it will be twisted because the door jamb is vertical but the wall is not. Therefore, I tried to install the door jamb along with the slanted wall and make the hinge grooves balanced so that i can open the door smoothly. Does it make sense?"

    A. You are encountering a typical problem that arises when working in an old house. There are three possible solutions: (1) Install the door plumb and hide the problem as best you can by installing custom casing (either thick casing with tapered thickness, created with a power plane, or a two-piece casing in which the casing is backed by a tapered shim); (b) install the door parallel to the finished walls (I don't like this solution, because the door won't operate smoothly -- but you have evidently taken this approach and tweaked the hinges to improve the door functioning); or (c) a compromise between a and b.

    Different finish carpenters use different approaches, depending on how out-of-plumb your walls are.

    Q. "My soffit vent (it is a fake soffit vent) does not work. Therefore, I decided to seal all air leaks as much as I can and put lots of insulation. Is that OK?"

    A. It's kind-of, sort-of OK -- as long as your attic shows no signs of moisture problems. In most cases, the advantages of attic venting are overstated. The most important thing you can do to keep your attic dry is to make sure that you have no air leaks at the ceiling under your attic. For more information, see All About Attic Venting.

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