Ceiling insulation for flat roof detached garage

[DetachedFlatRoof]

Hi All,

First post - thanks in advance. I'm in Canada with a just framed single detached garage 16 x 24 with ~12 foot ceilings. It is a flat roof.

My drawings specified a "warm roof". 2 layers of rigid insulation above the roof to insulate the place, with nothing in between the ceiling joists/under the roof deck.

My builder missed this detail, and did one layer of rigid insulation with intention to get the rest of the insulation between the joists to make up for the missing insulation up top.

I've read a bit about warm roofs and sounds like this isn't the ideal situation. I reviewed with my architect and he said it should be ok as long as the insulation is stuck right against the roof deck.

My main question is - should I be concerned? It's a detached garage, cold climate, it will be insulated in the walls and ceiling, but heated only intermittently and I doubt we will ever actually park in it. I do intend to have a heat source of some sort in there - we ran a gas line.

In this cold climate with a garage that is mostly cold, does it matter that much? Or can I just go ahead with the way its being built in terms of roof instulation

 

[billconner]

What kind of foam in the roof and how thick?

 

[DetachedFlatRoof]

“closed cell graphite-enhanced insulation”R10 2 1/8 inch

 

[jack stand]

"It should be ok" would worry me.
What if it isn't? Whose going to correct this next year after problems this winter.
Presumably snow will accumulate on the flat roof and with the designed insulation, will stay as snow and actually provide additional insulation for the building. Now if enough heat escapes, especially where it's only 1 layer at the framing members and ice starts to form...and build....and get very heavy....
This is just my common sense thoughts and I'm not qualified for an "official" approval or rejection but I'd dam sure ask both the builder and architect about the above reasoning.
And welcome to GJ

 

[billconner]

How deep are roof joists? If 2x8 or deeper, I tend to think you'd be ok with fiberglass and a vapor barrier. If only 2x6, I'd want an inch of spray in and then fibreglass and vapor barrier.

 

[rustyjames]

I would use Roxul between the joists to make up the difference in R value. That stuff is far superior to fiberglass.

 

[DetachedFlatRoof]

Roof joists are 2 x 10 so there is space

 

[billconner]

R10 on top and R30 or so inside should be fine. Very good vapor barrier on inside.