Insulating Dormer Ceiling

Old 01-02-19, 06:44 PM
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Insulating Dormer Ceiling

Hello all,

I am remodeling a bedroom and am hung up on the best way to insulate the ceiling. The room previously did a poor job of keeping the temperature consistent, as I needed to have a dedicated window AC to keep the 14'x18' room cool enough in the summer to be in. There are a couple of HVAC vents and a return already, but being the highest room in the house it just couldn't stay cool enough.

After tearing down the ceiling, the previous insulation was similar to option 1. My concerns with this solution was that the blown in insulation was packed in all of the air space so that the vent on the roof was useless. Do I even need this vent though if I pack the entire airspace between the ceiling and the roof with blown in insulation? The area between the ceiling and the roof is already very narrow with 21" being the max height on the end. Living in the Kansas City area with a Zone 4 recommendation of R38 to R60 in the ceiling, I would like to get as much insulation as possible, but I can't exactly afford blowing in 6" of spray foam.

Any suggestions on the best way to proceed? Maybe a combination of some rigid foam insulation sheets to give me more density with the limited area between the ceiling and the roof? I also have some radiant barrier sheets I bought initially with the thought of installing it on the 2x6 on the roof and leaving 6" airspace.

I've attached a pdf with a relatively accurate depiction of my situation.
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Ceiling Insulation.pdf (60.3 KB, 50 views)
Old 01-02-19, 06:51 PM
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Rigid panels will be pricey and much more labor intensive to install. Ensure you have adequate fresh air intake (soffits vents) and a ridge vent. How old are your windows? New windows can make a world of difference.

Last edited by mossman; 01-02-19 at 07:32 PM.
Old 01-02-19, 07:05 PM
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The r value of 1" of foam is around R-5. The r value of 3 1/2" of fiberglass is around R-13. So fiberglass is around 3.7 per inch. So foam has at least 50% more r value than fiberglass. Some rigid, like Super Tuff-R, approach R-7 per inch... almost TWICE what fiberglass is.

Your problem is that your ceiling joists and rafters do not provide adequate space for insulation. The best thing to do with poorly designed narrow roofs is suck it up and go with spray in insulation in the rafters, with no ventilation... aka hot roof.

If you aren't willing to do that, the next best thing is probably to install chutes from your soffit up the rafters (to maintain ventilation) then fill the ceiling joists with rigid foam. However 5.5" of rigid foam only gets you to maybe r-27.5 to r-39, depending what product you choose, but you could always blow in cellulose or loose fiberglass on top of that. Air sealing all the edges of the rigid foam is a must to prevent air leakage.

Radiant barriers are (IMO) in most cases a waste of time and space, especially if they can't be installed correctly which would be the case in your roof due to the way its constructed. (Not enough space) If you want to use a radiant barrier, you should install it on cleats (between each pair of rafters) so that the radiant barrier would be about 1.5" below the roof decking.
Old 01-02-19, 09:10 PM
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Windows are newer and higher efficiency.

There is no ventilation/vents on the shallow side, no soffit, so the air above that room does not circulate well with just the one vent on top every ~6ft.

Looks like the hot roof approach would probably run about $2200 to fill the rafters with 6" of closed cell. Going that route, would you still sheetrock the ceiling and just have an insulated air space above the ceiling? This will be a bedroom so visible rafters/joists might not pass the wife test.

I could install rigid insulation between the ceiling joists. I'll be doing the labor so will just take lots of cutting. Blowing in insulation on top of that, would I need to maintain any air space at all or could I just fill the entire cavity with insulation? This is close to the previous installation, with fiberglass batts instead of the proposed rigid. I'm sure even just the air sealing properties of rigid foam will make a large difference.

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