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Unvented garage roof with contained mudroom

jaccen | Posted in General Questions on


Thanks, up front, for any and all input.  I was wondering if someone could review my idea on how to insulate my attached garage.

Link to the current drawings:

I have resigned myself to the fact that I lost the battle in these regards with “she who must be obeyed:”
-garage is attached
-dormer for an eventual room above the garage
-mudroom contained within the garage

Sorry, Martin.  I tried, but failed.  Now I need to get “creative,” unfortunately.

Things You Do Not Need

Due to the complications of ventilating, basically, a cathedral ceiling combined with a dormer I have chosen to try to convince my Building Official (BO) on an unventilated ceiling (my train of thought on justifying this provided at the end of my post).

1).  Even if I follow best practices outline on GBA (ie. 6 inches of polyiso on top of the roof sheathing, no vented soffits, 7.5 inches of roxul tight against the sheathing underside, membrane to satisfy BO, fire barrier, etc.) do I still need to worry about interior moisture?  Ie. due to humidity, air movement, etc.?

I ask because this is a garage, not a house.  The air sealing around the vehicle doors will not be as robust due to them being a garage doors.  Outside elements will have a much easier time entering as the doors will be opening and they cannot be sealed as well.  We do plan to have a motion sensor mechanical exhaust fan (ie. Panasonic bathroom fan) come on for ~30-60 minutes whenever there is activity in the garage.  Is it actually in my best interest to have a ventilated setup?  To me, ventilating with the dormer and the cathedral will just be a nightmare.

2).  Will the mudroom ceiling/roof composition be “risky?”  It’s currently the following (top to bottom):
-11.25″ roxul
-1.5″ furring strip/service cavity

No pot lights, wiring, etc. above the membrane aside from 1 bathroom vent which will be taped/sealed to the membrane.

This composition is the hardest thing I have to wrap my head around simply because, until the spare room is placed, the ceiling (to me) is more of a “roof.”  After the spare room is placed, it’s a ceiling/floor.  However, the mudroom is contained within the envelope of the “cathedral” garage so, for now, it’s a “roof” within the roof.  The spare room will eventually have a floor similar to the mudroom composition listed above.

My aside for discussion/future reference for my fellow Ontarian’s.

Unventilated cathedral ceiling in Ontario:

First, why it needs to be justified.

OBC Required Venting

(1) Except where it can be shown to be unnecessary, where insulation is installed between a ceiling and the underside of the roof sheathing, a space shall be provided between the insulation and the sheathing, and vents shall be installed to permit the movement of air from the space to the exterior.  (emphasis mine)

1). I currently live in Zone 6.

Climate Zone Map Including Canada

2). Per the IRC 2018, Section R806.5 “Unvented attic and unvented enclosed rafter assemblies”
We meet Items:
1. unvented attic space completely within building thermal envelope (everything is under the conditioned barrier)
2. No interior Class I vapor retarders installed on ceiling side (we’re using drywall)
3. (not applicable as we’re not using wood shingles)
4. Air-impermeable insulation shall be a Class II vapour retarder (1″ foam is 1.1 perm, 2″ foam is 0.55 perm, so 6″ will definitely exceed Class II).

 and 5.1.2 with 5.3  Impermeable, foam insulation directly above roof sheathing and permeable, roxul directly below roof sheathing.

3). Per the IRC 2018, Table R806.5

We have a semi-conservative estimate of R-30 (R-5 X 6″) polyiso at so we exceed Zone 7 and almost meet Zone 8.

4). Per that same IRC Table, we roughly meet the minimum of 51% for total R-value coming from the exterior, non-permeable insulation.  OBC requires R-60 w/ the last update for a roof.
Exterior = R-5 X 6″ = R-30
Interior = R-4 X 7.5″ = R-30

30/60 = 50%

Value of polyiso taken from the article with Straube.  I realize it could drop to ~4.5, but in my Zone I’m not particularly worried about it as that’s only for a couple weeks.  I also, perhaps mistakenly, take some solace in the article that speaks to Canadians only place R-5 foam on OSB walls and things being ok.

Polyiso chosen simply because I have a source on left-over/factory toss-offs (ie. no metal applied).

Mesa (DM40)

5). Also, we exceed IRC Table R702.7.1 of 36% for minimum exterior rigid foam of a wall (see calc above).

6). The sheathing will be airtight with taped seams and with a roof underlayment of asphalt felt.  Rigid foam will be staggered, with taped seams to maintain impermeablility. With all these options, we meet the R-60 minimum roof insulation per the updated OBC. Mechanical ventilation can/will be specified to meet any additional concerns.  It will be my responsibility to find a metal roof/asphalt shingle provider that allows a “hot roof” installment.

I, personally, couldn’t find anything within the OBC itself to justify it.  I’m hoping that’s enough to convince them.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Even if I follow best practices outline on GBA ... do I still need to worry about interior moisture? Ie. due to humidity, air movement, etc.?"

    A. Worrying is unproductive, so I guess my answer is no. Any building can develop a moisture problem, so you should always be alert to signs of mold or rot -- but alertness is different from worry.

    Q. "Will the mudroom ceiling/roof composition be risky?”

    A. This sounds like a ceiling to me, not a roof assembly. (Above the ceiling is an unfinished room -- right?) The assembly is not risky -- although I have no idea what type of "membrane" you are installing between the mineral wool insulation and the service cavity. Ideally, you would include an air barrier between the service cavity and the insulation.

  2. jaccen | | #2

    Hi Martin,

    Your direct and pragmatic replies are always appreciated.

    Your first answer alleviates my concerns.

    Your second answer reassures. Sorry, auto-correct changed MemBrain to "membrane." It will be my air barrier along with sealing the plywood seams above.

    Side note--the meeting with my Building Official (BO) went well. He said he didn't see a problem with the design as long as the air sealing was done well. He said he might need to come out for an extra inspection, but I'm quite happy over the ordeal (so far).

    Thanks again.

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