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

Attic insulation

CwTs68Z | Posted in General Questions on

Good afternoon!
I am getting new roof in a week. I have 4  feet of sloped ceiling that now is filled solid with fiberglass batt insulation. My roofer will pull up some of the 1 x 8 deck boards so I can install smart baffles. Rafters are 2 x 8 so that will only leave me with 5 1/2 inches for insulation. Will that be enough?
I am getting ready to air seal the balance of the attic today. The 700 square foot roof has soffit vents and a ridge vent but the soffit vents haven’t worked at all because of the rafter bays plugged with insulation.
 Thanks for any advice you can provide,

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. Expert Member
    Dana Dorsett | | #1

    With an inch of air vent you're looking at about R23-ish compressed batt in the remaining 6.25" for that sloped 4' section of roof, which isn't a lot, but way better than nothing. If you can add rigid polyiso insulation on the interior side of that sloped section the thermal performance can be improved a lot, at the expense of a few inches of headroom.

    What is the R-value of the rest of the attic?

    What is your climate zone/location?

    1. CwTs68Z | | #3

      I’m in climate zone 5, Chicago north suburbs. Rest of attic is empty right now, getting ready to air seal attic floor. Was planning on lots of cellulose insulation.

      1. Expert Member
        Dana Dorsett | | #4

        There's no definition for "...lots of cellulose insulation..." but at a code min R49 every square foot of the sloped section with R22-R23 has more than twice the heat loss of an R49 roof. With 3" of continuous polyiso (~R19) on the interior side of the slope it'll be a lot closer, and might even hit code-min on a U-factor basis.

        If installing 20"+ cellulose everywhere else the ceiling losses will be dominated by the sloped section, even if it were filled with 6" of 2lb polyurethane unless there is some rigid insulation on the slope.

        1. CwTs68Z | | #5

          Dana, for the rigid polyiso, do you mean inside the rafter bay against the backside of the plaster ceiling?

          1. Expert Member
            Dana Dorsett | | #6

            Behind the ceiling gypsum, but NOT between the rafters.

            If slipped between the rafters the thermal bridging of the R1/inch rafters would subtantially rob R6/inch polyiso of it's potential performance. Put the cheap stuff between the rafters. The 6.25" of available rafter-bay fill can be the same cellulose you're blowing elsewhere, and would perform at about R23 at that depth.

  2. GBA Editor
    Martin Holladay | | #2

    This article describes all your options: "How to Build an Insulated Cathedral Ceiling."

  3. CwTs68Z | | #7

    All 3 2nd floor rooms have plaster walls and ceilings. I’m not willing to take that down.
    This house is 68 years old. What other options do I have?
    The smart baffles that I was going to use are rigid plastic and provide a 2 inch airspace.

  4. Expert Member
    Dana Dorsett | | #8

    With a 2" baffle you only have 5.25" of space below the baffle. If you're not going to re-do the ceiling install R23 rock wool batts in that space. It'll only perform a R22 when compressed to 5.25", but that's still way better than nothing. If you ever re-do the interior you can improve on it at that point with interior side foam.

    For a huge amount of money and only marginal benefit you COULD install 5" of HFO blown closed cell foam bringing it up to R35 at center cavity, but barely moving the needle over what you'd get with R23 rock wool, due the severe R5-R6-ish thermal bridge through 5.25" of rafter.

  5. CwTs68Z | | #9

    Thanks Dana,
    I think the R23 rock wool batts is what I’ll use in the sloped ceiling area and 20 inches of cellulose in the remaining attic area. I really appreciate your help with this.
    Thanks much!
    John McLoughlin

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |