I'm trying to figure out how to properly insulate the ceiling of one room in my house. Specifics:

  • The entire roof is low pitched
  • Half the house has cathedrals ceilings. (The images in this post is roughly the same idea)
  • The back of the house is NOT cathedral ceiling. The room in question joins the cathedral part to the back of the house with normal (8'?) ceilings.
  • I'm in a cold climate that receives a good amount of snow

I do know typically you'd allow venting through the soffit, however in this case the soffit for all sections of the house looks to be airtight. The back of the house does have venting at the peaks. The room in question, I can see that the back of the house they've added insulation (fiberglass?) directly on top of the drywall and its packed further than I can see up. Presumably there is some section above this that is vented to the outside. In this particular room where I've removed the ceiling, the only insulation is the 8" or so that would exist immediately under the exterior roof (basically how it would be insulated in the previously mentioned post/pictures).

I'm mostly trying to figure out if I should:

  • add fiberglass to a depth of a couple feet, leaving a sealed airpocket. Effectively doing: ceiling, insulation, air pocket, insulation, exterior roof.
  • fill entire void with insulation, thus: ceiling, insulation, exterior roof.
  • other?

I am well aware that the proper way to fix this (assuming the cathedral ceiling) would be to spray foam it (again, as mentioned in the answer in the previously mentioned post). However this room is not cathedral, but I dont have access or a way to ensure there is any venting directly above this room.

The quick backstory on this room (bathroom): the contractor/owner before us created a bathroom by splicing part of the room that had a cathedral ceiling. They installed non-IC, halogen lights and added a shower exhaust vent (that evidently become detached from the exhaust hose). The roof ice damms and leaks in the later winter/early spring. Obviously because the 7-8" of insulation in the roof is not enough against the house heat and halogen bulbs pumping out a lot of heat into this space. I've ripped out the semi-vaulted ceiling, have replaced the halogens with IC/damp rated LEDs and re-positioned the exhaust fan and ensured the vent hose stays attached. I will be finishing the drywall/ceiling flush across. I want to ensure I don't repeat the situation of allowing my roof to warm up and creating more ice dams.

  • There are only two ways to insulate a roof: vented or unvented (a.k.a. "hot roof"). You never have an air cavity between layers of insulation. It's not clear what your challenge is from the description. You'd need to provide a sketch or some good photos. Also, the backstory isn't really important and mostly adds clutter to your post. Simple and clear are your goals at SE.
    – isherwood
    Nov 1, 2017 at 17:57
  • That sounds like "the" answer; fill the entire void with insulation as you don't want to leave an air cavity?
    – KHibma
    Nov 1, 2017 at 19:22
  • Probably, but I don't have enough information to form a confident opinion. I also don't have a ton of experience with hot roofs. One I did was similar to yours, but it was 15 years ago. Another was a modern log cabin with double 3" iso sheets under the roofing, also 15 years ago.
    – isherwood
    Nov 1, 2017 at 21:16


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