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Community and Q&A

Bonus room insulation

Walter Strobl | Posted in Energy Efficiency and Durability on

I have a bonus room (i.e over garage) that I would like to finish.  I would prefer not to use any type of blown in foam.  The roof is framed with 2×6’s (12/12 pitch) and the floor to the garage space underneath is 2×8’s.  The space is located in a cold climate (6).  The roof is not a hip or valley roof and there is clear access from the walls to the ridge.  There are colar ties about 11′ up and that is where a flat ceiling would go with the remaining 6′ above being “attic”.

I plan on using mineral wool insulation for the floor (R28 batts) and comfortboard 80 (1″) under the floor joists then strapping it with 1×4’s and 5/8″ drywall.

The roof I would like to keep as a cold roof so I would like to build site built baffles using either xps or eps foam.  The issue is that if I build the baffle (1″) and keep the desired 2″ air gap I cannot use batt insulation for the remainder of the 2×6.  Compressing the batt this much would not be desirable. Do I fur out the missing 1″ or do I cut and coble something into this space? I would also like to use the comfortboard 80 (2″) underneath the rafters to increase the R value or would using a rigid insulation be a better approach?  How much of an R value would I realistically get?  Is there a better approach not using CC blown in insulation?  The remaining flat section of the ceiling would be insulated with R60 batt insulation.

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  1. Charlie Sullivan | | #1

    If you use batts made for 2x4 walls, your proposed design would compress them from 3.5" down to 2.5". 2.5" is not a lot of insulation, but the compression isn't necessarily a problems. Compressing fiberglass that much may be easier than compressing mineral wool. My bigger concern is the total R-value. R-4 for the EPS plus R-10 for compressed fiberglass, plus R-8 for the 2" underneath only adds up to R-22, vs. a code requirement of R-49. Solutions could include putting some of the insulation above the roof decking, furring down the rafters substantially, or using more layers of insulation boards under the rafters. That may be a good place to use polyiso for its higher R-value per inch.

    Don't use XPS. It's made with a gas that has more than 1000X worse climate impact than CO2. EPS is fine.

  2. GBA Editor
    Peter Yost | | #2

    Hi Walter -

    another approach you might consider is adding rigid insulation to the interior surface of your assembly, giving you (if sealed or taped) a conntinuous interior air barrier.


  3. Expert Member
    Zephyr7 | | #3

    Are you building the ceiling flat to the collar ties? If so, you can probably use blown cellulose in that part, with baffles at the edges to keep from blowing over the end of the collar ties.

    Do you have knee walls and some angled ceiling areas like a cape cod? I’d try cut and cobble polyiso in those areas, and make up any shortfall in R value with some continuous polyiso on the inside of the rafters. As long as you have vent space above the cut and cobble, you should be fine.

    I’d use polyiso over either XPS or EPS. you get more R per inch with polyiso, and you have limited space so that’s important.


  4. Doug McEvers | | #4

    I think a 1 1/2" airspace will suffice and then I like fiberboard as a baffle, fairly rigid, inexpensive and vapor permeable. Then add batt insulation of your choice to fill the rafter space to the bottom (room side) of the rafters. On the room (warm) side attach Polyiso sheets with plastic cap nails to the bottom of the rafters and foil tape the seams. I used this system for my neighbor's cathedral ceilings in Minneapolis last winter because of major ice dam issues. This winter after a heavy snowfall, not an icicle anywhere, it works. We also reduced their natural gas usage by 20% by more than doubling the R-value of the ceiling and greatly reducing air infiltration through the ceiling assembly.

  5. Walter Strobl | | #5

    I plan on adding as much as I can under the rafters, but space is limited where this room meets the 2nd floor of the house. The bonus room is 3 steps lower so there is an issue of head room at the entrance. I was planning on using the Comfortboard 80 instead of the XPS or EPS for environmental reasons. Plus with 2 people with asthma in the house I thought it prudent.

    Zephr7, yes I plan on building flat until the collar ties. I could blow it in if I could get to it afterward, but it is completely blocked front and back hence probably going with the batts.

    I am making knee walls in the conditioned space, but the space currently doesn't have any per se, but it has outside walls that act as knee walls since the floor of the room is dropped down about 24" from where it would have normally sat (flush with the 2nd floor). It still gives me 10' of height in the garage underneath, but the walls are balloon framed (2x4) up to the rafter plate, so I will have to add some fire blocking.

    Doug, thanks. I am being tempted using 1 1/2" instead of 2". I had thought of using polyiso on the warm side. How much more R value would that give me over the Comfortboard?

    Charlie, Going over the roof isn't an option since the garage is pulled forward and then you would be into the valley to the rest of the roof where there is dormer very close to the valley that already is giving me issues. Things would quickly snowball. It really wouldn't be worth it. Plus the roof is relatively new.

    I think the code for our area for cathedral ceilings is R-31. The rest of the house has R-20 in the cathedral section of the cape cod as well as all of the walls. Insulation wasn't that important when the house was built (mid 90's).

  6. Doug McEvers | | #6


    I think there is a provision for R-38 ceilings vs R-49 in zone 6 when they are cathedral. I guess I should look at the code book occasionally to brush up. In my retrofit/ ice dam remediation project also in zone 6 my roof assembly was as follows.

    The rafters for the cathedral ceiling are 2"x10" with 1/2" roof decking. The roof was new (home built in 1977) so working from the inside was the best option. I ripped #3 pine strips at 1 1/2" and fastened to top of each side of the 2"x10" rafter, tight to the roof deck. Fasten horizontally into the rafter, avoid any possibility of going through the roofing. I like a 1/2" crown sheathing stapler for this task, quick and staples are reasonably priced. To the bottom of the firing strips (room side) I attached 1/2" fiberboard sheathing ripped to fit tightly between the rafters using again the 1/2" crown x 1 1/2" staples. Extend the air chutes out into the soffit area so there is a dedicated space for high R insulation over the wall plates of the exterior wall. You also want a wind wash barrier as an extension of the exterior sheathing to the bottom of the fiberboard air chutes. Getting all the R-value and air tightness you can in this area is critical for eliminating ice dam formation. I used leftover pieces of high R foam sheathing cut to fit and some minimal expanding foam to fill in any voids. You can extend the high R foam sheathing up inside the bottom of the rafters a bit to ensure high R value near the shallow birds mouth area of the rafters. If you are adding layer(s) of Polyiso to the room side this will cover the very critical exterior wall/roof connection area.

    Once the air chutes were in place and windwash/top plate work completed I reused the original fiberglass insulation that was in the rafter bays. It was an odd 6 1/2" R-22 batt with kraft facing which I removed. To this I added additional fiberglass to fill the rafter space fully to the room side. I then attached 3" of Polyiso foam to the underside (room side) of the rafters, foil taped the seams and sealed the edges to the existing drywall wall. This gave me about R-50 for the cathedral ceilings and very airtight.

    In my ongoing energy monitoring of this house and others I need to get back with the energy rater I work with and review the before and after air infiltration rates. I believe we reduced air infiltration for this house by about 20% with the insulation upgrade if memory serves me. In my initial gas meter readings before and post retrofit I am seeing about a 20% reduction in gas usage. The house was at 5.5 Btu/sf/hdd before the work and is now somewhere in the mid 4 range.

    An airtight ceiling assembly goes a long way to counter the stack effect. Stop the flow of warm air up high in the structure and you will find the lower levels will be more comfortable by reducing infiltration at the lowest level in the building.

  7. Walter Strobl | | #7

    "I think there is a provision for R-38 ceilings vs R-49 in zone 6 when they are cathedral"
    In my area the code requires R31 as far as I can figure it out. Instead of using fiberboard I was thinking of just using EPS as this will already give me some R value for what it's worth, but using the fiberboard would be a lot easier as I can just staple it instead of dealing with glue or foam. Could I use OSB instead of fiberboard?

    Thank you for the excellant write-up on the details. I wish I had 2x10's, but 2x6's is what the builder used.

  8. Doug McEvers | | #8


    OSB would also be a good choice of material for the air baffle. Fiberboard is more vapor permeable, likely less expensive, maybe a bit greener and has a higher R-value than does OSB.

    1. Walter Strobl | | #10

      Doug, just to clarify when you say "fiberboard" you are referring to MDF and not particle board?

  9. Brendan Albano | | #9

    Here's some assembly ideas for you based on the comments of other folks in this thread:

    - 1 1/2" Air Gap
    - 1" Foil-faced polyiso baffle, foil facing the gap (R-6 nominal for the foam, + about R-1 for the foil facing the gap)
    - 5.5 Mineral wool batt (R-23)

    This is about R-30.

    This adds up to 8", so you will need to fur down 2.5" beyond your 5.5" stud.

    You can do this by making "Bonfiglioli" strips by gluing a 2" thick piece of foam to a 1/2" thick piece of plywood/OSB.

    Accounting for the Bonfiglioli strip breaking the thermal bridge of your stud, this will perform better than the code-min R-31 that you've cited in your area.

    Making the Bonfiglioli strips might be a little fussy, but seems like it might be worth it in your situation: (the article is talking about new construction walls, but a retrofit ceiling is perhaps an even better use of the technique)

    Another assembly:

    - 1 1/2" air gap
    - 1/2" fiberboard (R-1.3)
    - 3 1/2" mineral wool (R-15)
    - 2" Polyiso, foil facing the straping (R-12, + R-1 for the foil)
    - 1x4 Strapping
    - Gyp

    This is R-29.3, but again, if you do the calcs for the fact that the foam is breaking the thermal bridge of the studs, this should get you where you need to be. Or you can just add another 1/2" of foam.

    Of course neither of these get to the R-38 or R-49 required in other locations, so if you can afford the head height to fur down even further, that's even better!

    1. Walter Strobl | | #11

      Brendan, you suggest using polyiso as the baffle, but is polyiso not vapour impermeable with the foil? Would this not create issues of condensation if it can't dry to one side or the other?

      Using the "Bonfiglioli strips" might be a way of furring out further and using a higher R value insulation in between. I would have to do the analysis of cost vs space.

      1. Brendan Albano | | #12

        Opinions differ as to whether it matters if your baffle is vapor permeable or not. For what it's worth, there are commercial cathedral ceiling baffle products made out of plastic, which is clearly impermeable:

        That said, a vapor-permeable baffle (fiberboard, osb, EPS, etc.) definitely works, and may be a more conservative choice if you are unsure about the impermeable baffle approach.

        Also, the "fiberboard" folks are referring to isn't mdf. It's designed to be used as wall sheathing in place of plywood/osb if you don't need the strength of those materials. You'll get better google results if you search "fiberboard sheathing". Here's an example:

        1. Walter Strobl | | #13

          Thank you for the information. In my area this is what I can find. I think this is comparable? Let me know your thoughts. Thanks.

          1. Brendan Albano | | #16

            Yeah, that looks like the type of fiberboard folks are referring to in this thread.

            Vapor permeable, asphalt coating, about R-3/in are all typical properties of fiberboard sheathing, so that spec sheet seems to check out.

  10. John Clark | | #14

    Does the OP really need an air gap of more than an inch for such a small section of roof? Lets assume the cathedral portion of the ceiling is 4 feet long.

    OP do you have drawings of the framing in this area?

    1. Walter Strobl | | #15

      It's closer to 10 or 12 feet in length.

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