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Replace fiberglass batts with cellulose or just add more insulation on top?

Bill Bertera | Posted in General Questions on

I have a great room attached off our main house in Central Massachusetts. I’m not sure what it is called, but its got normal walls for about 10 feet, then cathedral walls angled up for another 5, and then flat across the top for 10 feet or so. I’m pretty sure the room could use more insulation on the top, but I’m waffling on the best way to do it.

The Facts:
I have relatively easy access to the attic, through an access door off a 2nd floor bedroom. The attic puts me onto the flat part of the ceiling, and I can see down the cathedral rafter-bays, but don’t have access to them too easily (they are 9″ high). It is unconditioned attic, there used to be an Air-handler and duct-work up there, but it broke (because the space was unconditioned). I have since replaced the heat/AC work with a ductless mini-split in the great room, and just removed the old air handler and all the duct work. I have also air-sealed the attic/ceilling, which included the 4 HVAC supply registers, 1 big return, 4 recessed lights and a fan. The joists and rafters are 9″. there is fiberglass bats currently down the rafter bays and across the floor. I cannot confirm the bats go all the way down the 5′ rafter bays, they probably do, but the way they are jammed down there makes me think they are either compressed and/or not filling all the space. The bats on the floor seem to be in decent shape. There are also gaps where the duct work used to be. there are baffles from the soffit vents up the rafters that end a couple inches above the floor joists. The top part of the rafters are not insulated.

I have several different things I’m considering related to each part (baffles, rafter-bays, floor), please let me know your thought on how I would best do this. I am not considering conditioning the space, mainly for cost reasons. When I mention rafter-bays below, I’m talking about the bays that go down from the attic floor, not up to the ridge.

1. Should I extend the baffles all the way (or closer) to the ridge vent? Currently I could add insulation a few more inches and still not block them.
2. Remove all bats, and blow cellulose in bays/floor up to the baffles. (or higher if I extend the baffles).
3. Remove all bats, blow cellulose in bays/floor up to the joists, and put rigid foam on top of joists — this would leave a “gap” where the floor meets the rafters, as I dont think I would cut foam to fit horizontally in the rafter bays to meet the baffles.
4. Leave Bats (adding to where the ducts used to be), and just add rigid foam (or more unfaced bats) on top of joists.
5. Remove bats from rafters-bays, blow cellulose down bays up to joist level, leave bats in floor joists, and add rigid foam (or more bats) on top.

Thoughts? #2 has been my action-plan from the start, but now I’m questioning the approach as it gets closer to actually do it.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Bill,
    If you are able to remove the fiberglass batts from the sloped sections of ceiling, and if the ventilation baffles appear sturdy, you should be able to install dense-packed cellulose between the drywall and the ventilation baffles. The trickiest part of this job will be ensuring that the ventilation baffles don't get crushed. If they are flimsy polystyrene baffles, this approach won't work.

    Once you've figured out what to do with the sloped ceiling, you can tackle the flat ceiling. The usual approach for this type of flat ceiling is simply to blow cellulose on top of the existing fiberglass batts. (Ideally, you will have finished your air sealing measures before you add the cellulose.)

  2. Bill Bertera | | #2

    thank you for the reply. unfortunately they are the cheap pink polystyrene baffles, and I don't think I can get down there to replace them. Would loose packing with cellulose be better than leaving the bats?

    Just adding to the floor with loose makes sense, I'll need to extend the baffles higher, since I could only add a couple inches where they are now, but that's not a problem.

  3. GBA Editor
    Martin Holladay | | #3

    Bill,
    Sloppily installed fiberglass batts under polystyrene baffles represent one of the worst-performing ways to insulate a cathedral ceiling.

    Fixing this problem properly will be expensive. Only you can determine whether you want to invest the money to insulate this sloped ceiling properly. The best job will require full access -- in other words, removing the existing drywall. Unless you are plagued with annual ice dams, I'm guessing that you don't really want to invest the money to do a careful job.

  4. Bill Bertera | | #4

    thanks again. I have just spent a lot of money to fix the even worse insulation/roof problems in the main part of the house, and was hoping to be able to the great room attic cheaply but also still properly. Sounds like maybe not possible. Maybe I'll just add more unfaced bats to the top as a stop-gap until I'm prepared to spend the money to do it right. I don't want to be blowing cellulose if I have to remove it 2 years to do it all properly, but a few rolls of bats I have no problem with.

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