GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

ceiling insulation from inside

MarieMar | Posted in General Questions on


I have an upstairs room that’s attic inaccessible in an 1854 house, so will be adding insulation from the inside.

Right now it’s:

attic with fiberglass(probably r13-19 – I’m in zone 5), ceiling lathes, a heavy plastic vapor barrier then a drop ceiling.

Was thinking of adding polyiso rigid boards to the lathes.

Would I remove the heavy plastic vapor barrier?

If I use 2 layers of rigid board, what sides should be foiled? Should two layers be used?

This is the first of several insulation projects, but it’s a start.  If you have other suggestions, please advise.

Thanks – Marie

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user-6623302 | | #1

    What are you going to use as a finish ceiling? Do the batts have paper/foil facing? Is it up or down?

  2. Expert Member
    BILL WICHERS | | #2

    Will you be completely removing the interior ceiling to do the work, or will you be working through a temporary access hole? If you'll be working through a hole, I'd forget about any rigid board type insulating materials.

    If this is a vented attic space, you can still use loose fill insulation (fiberglass, cellulose, or mineral wool). Those are usually the easiest insulating materials to install in areas where they can be used. The downside here is that you're only doing one room, so you'll probably not be buying enough material to get a free rental machine, so you'll have the additional cost of renting the blowing machine. You might check with insulating contractors though -- sometimes you can get this work done very cheaply, and they'll have their own blowing machine which might up costing similar to a DIY project.

    If you'll be removing the entire ceiling, loose fill insulation is not an option, since you won't have anything to support it! In this case, I'd use batts. I just did such a project myself, and what I did was to use non-faced R19 batts perpindicular over the top of the ceiling joists, then R21 (high density fiberglass batts for 2x6 cavities) between the ceiling joists. I used kraft faced batts, mostly just because the facer also supported the batts. Last I put up a layer of 1/2" polyiso mainly as an air barrier, since the finished ceiling is T+G pine. Total finished R value is about 43, which is a BIG step up from the poorly installed R30 batts before with lots of thermal bridging between them.

    The advantage to laying the batts perpindicular like I did is that you insulate over the tops of the ceiling joists, which massively cuts down on thermal bridging. You can use whatever thickness batts you need to get up to the R value you want. Current code for attic insulation is R49 in most areas, so that's a good target to shoot for. I did less, because I couldn't fit enough out at the eaves and I had to add vent channels too.

    You probably don't need that poly vapor barrier, but it doesn't hurt if this is a vented attic. If it's in the way, I'd just remove it. If you can fit a layer of rigid polyiso under the ceiling joists, you can use that as an air barrier like I did if you won't be using something like drywall that is essentially it's own air barrier as far as finished ceiling materials go. You DO need an air barrier.


  3. user-6623302 | | #3

    Just a thought, if you take down the ceiling, could you make a temporary access point. Blown-in would be a lot easier than what you are planning .

  4. MarieMar | | #4

    Thanks everyone.

    I think opening up access makes the most sense.

    Thanks again!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |