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Community and Q&A

Insulation plan for bonus room above garage

AsimM | Posted in GBA Pro Help on

I am in MN(zone 6), working on my new construction.
It has an attached garage with a bonus room above it. I don’t plan on finishing it right away but would like to plan how I am going to insulate it.
I have sheathed the roof deck with 7/16″ zip sheathing. It will be covered with Cedar shakes. I have already roofed almost half of the house and am slowly making my way towards the garage.
I am considering both vented and unvented assemblies.

One of the possibilities is:

Cedar shakes with 30 lb felt paper interlayed
1 1/2″ above deck ventilation
Zip sheathing
5 1/2″ Cut and cobble EPS stuffed in between the top chords(top chord is 2×6) , sealed with spray foam.
6″ of eps covering the top chord.

I would appreciate your comments/opinions on it.

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  1. GBA Editor
    Martin Holladay | | #1

    If you are building an insulated sloped roof assembly, nothing beats a roof assembly with rigid foam above the roof sheathing. Every other approach is worse.

    If you haven't started roofing yet, I strongly urge you to install one or more layers of thick rigid foam above the roof sheathing, followed by a second layer of OSB or plywood roof sheathing. You won't regret it.

    If you install cedar shingles without any rigid foam, and later try to figure out a good insulation plan, it will be too late to do it right.

    Here is a link to an article with more information: How to Install Rigid Foam On Top of Roof Sheathing.

  2. Brian Knight | | #2

    Agree with Martin. Or use Sips as an alternative.
    Also realize the dangers of attached garages and work hard in air-sealing, to separate the garage air from the living space air.

  3. GBA Editor
    Martin Holladay | | #3

    If for some reason you don't want to follow my advice, there are other ways to insulate a sloped roof assembly. Here is a link to an article that explains all of your options: How to Build an Insulated Cathedral Ceiling.

    The problem with most insulation schemes that attempt to insulate this type of roof from the interior is that they fail to address thermal bridging through the rafters. If a contractor does a good job of addressing thermal bridging through the rafters on the interior side of the rafters, he or she ends up lowering the ceiling height significantly, reducing the usefulness of the bonus room.

  4. Dana1 | | #4

    Put 7-8"of EPS above the roof deck, and 5.5" of 0.5lb open cell spray polyurethane on the underside of the roof deck and it'll be golden. It's less work, less money higher performance, even if you include the cost of a nailer deck on top of the exterior EPS through-screwed to the top chords. In zone 6 as long as at least 50% of the total R is above the roof deck you won't need interior side vapor retarders to protect the roof deck.

  5. AsimM | | #5

    Thanks Gentlemen,

    I have some "aesthetics" concerns using foam on top of the roof deck. However, performance wise the approach makes a lot of sense and I will try to manage the performance/asthetics balance.

    So would this be considered a vented or non-vented assembly? If vented, would the ventilation be above the top deck?
    Can I do 2.5-3" polyiso, covered with 2" of EPS(to keep the polyiso warm) to achieve the minimum r-25? I am thinking of ways to make the assembly as thin as possible.

  6. GBA Editor
    Martin Holladay | | #6

    If you want to vent this type of roof assembly, then the ventilation channel would need to be between the rigid foam layer and the roofing. Ventilation is optional.

    If you install 3 inches of polyiso (about R-18, if you keep the polyiso warm) with 2 inches of EPS (about R-8) on the exterior side of the polyiso, you'll end up with about R-26 -- so that would work.

    If you are worried about aesthetics at the eaves, there are ways to disguise the existence of the rigid foam. See the illustration below from Building Science Corp.


  7. Expert Member
    Dana Dorsett | | #7

    The extra overhang in the foam-over detail provided by Martin is also beneficial for the walls, reducing the amount of direct wetting the walls experience.

    With just 6" of EPS on the exterior it would take 6.5" of half-pound foam on the interior to hit the code-minimum R49, and it would just barely squeak by the 50% exterior rule with R25 on the outside. Most open cell foam can only be safely installed in 5.5" lifts, maybe 6" if you're stretching it, but 6.5" would require two passes of foam with an extended cooling period between. This is both a fire-safety and installation quality issue. The stuff heats up while curing, and too much foam in one go makes it cycle through higher (sometimes kindling) temperature, creating higher likelihood of cracking, poor-adhesion, or depressions. With 7" of EPS on the exterior (~R29.5) and 5.5" of half-pound foam on the interior (R20-R21) you have both more dew point margin, and the spray foam can be installed in one pass, using the top chords as a depth gauge to guide them.

    If the extra thickness is a problem you can go 3" polyiso + 2" EPS and still have sufficient dew point margin even with climate-derating of the polyiso would be marginal using a generic foam if you limited the interior side to 5.5" open cell, but both the mid winter performance and labeled-R value would be a bit less than R49, but may be above R50 during the shoulder seasons, not that the difference wold be easy to measure.

    If you really need to limit it to 5" and meet labeled code-min R, at 2" the 1.5lbs density graphite loaded Neopor/R-Tech Platinum for the EPS would deliver R9, and an R6.5/inch labeled polyiso such as Dow Thermax would be labeled R19.5 and 5.5" of and using 0.7lb open cell foam instead of half-pound spray foam (~R21-R22 @ 5.5") would get you there with plenty of dew-point margin.

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