Spray Foam Insulation for Garage Roof Deck

[garretts]

Thanks to everyone for the help I got a few months ago - based on your feedback I'm going to use spray foam on the roof deck. My estimates are coming in around $3700 for 3 inches of closed cell or $2000 for 5 inches of open cell. I found a company that will do the closed cell and paint all of the roof deck and joists white with fire block paint for about $4,000 all in (and close off the ridge vents).

They did say open cell would be ok at first, but I'm thinking I would get a better result/look given the 2x4 rafters (although I guess no one will be looking up that high so it doesn't really matter that much). I think the R value is similar for either.

I'm also planning to rip out non-essential wiring/fans/lights before they show up - anything else that I may want to do beforehand?

thanks!

https://imgur.com/a/wvMjFRX

 

[pembol]

If you are going to have an unventilated attic/cathedral ceiling then you really need to use closed cell foam, assuming the building will be sealed/conditioned. With open cell foam, moisture will pass through the foam until in hits the coldest surface, which is the roof deck, and will condense there, rotting the roof deck. With closed cell foam, hopefully your ceiling will be warm enough not to condense, but if it does it will be on the foam, not the roof deck.

Lots of details here: https://basc.pnnl.gov/resource-guides/unvented-attic-insulation
and here: https://buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates

 

[Hank11]

Open or closed cell for roof decks depends to some extent on where the building is located. Beyond that, a roof leak with closed cell won’t show until its done significant damage. Open cell will let the moisture show. The other advantage is that open cell is softer and easily removed with your hands. Closed cell is hard and tuff stuff so lots more work to dig into so changes or repairs are more difficult. Both foams have uses where they excel so you should think it through and also try to ascertain local practice to decide what is best for your needs.

I have 2” of closed cell where it was needed for air sealing and 7” of open cell under the roof deck. Both put in place on purpose for different requirements.

You didn’t ask, but I’d do my best to put up some kind of real ceiling for appearance and fire safety.

 

[PCustoms]

Original thread

Rafter insulation

I just became the proud owner of a new (to me) garage in central Florida - I'm planning on putting in framed walls and a mini split. I currently have a 12K BTU AC in the window and it works fairly well except on the hottest of days. I know the big roller garage is a huge cooling loss no matter...

www.garagejournal.com

 

[billconner]

In Florida, I think you'll be fine with the open cell. I'd only ask the contractor if the fire retardant coating actually meets code, but sounds good. Looks like a very good plan.

 

[pembol]

In Florida, I think you'll be fine with the open cell. I'd only ask the contractor if the fire retardant coating actually meets code, but sounds good. Looks like a very good plan.

I did not realize this was Florida - in which case open cell is fine.

 

[garretts]

I have 2” of closed cell where it was needed for air sealing and 7” of open cell under the roof deck. Both put in place on purpose for different requirements.

I have been watching "Spray Jones" videos and am considering 2" also since this is a garage and the gains in BTU efficiency going to 3" really isn't much!

You didn’t ask, but I’d do my best to put up some kind of real ceiling for appearance and fire safety.

If I did that, wouldn't that be an issue with an insulated roof deck and then a ceiling? If I went that route,wouldn'tI just be better off putting up R-13 batt and a ceiling and leaving the roof vented as-is?

 

[billconner]

Your climate is important. Almost regardless of that, I believe insulating at the ceiling instead of roof deck is safer. R13 is close to 2" of foam, coupled with less volume to heat or cool is good.

 

[Hank11]

Yes, leaving the roof vented and insulating the ceiling would probably be the way to go.

 

[billconner]

If I did that, wouldn't that be an issue with an insulated roof deck and then a ceiling?

I see I didn't really answer your question. Apologies reread whole thread to be sure.

No, adding a ceiling under the foam insulated roof does not cause problems as far as moisture, heating and cooling, etc.

I'm still ok with pie cell spray foam on roof, no "attic" venting, so long as you are OK with looks and fire retardant treatment. It's pretty darn simple, which I like. Look up and see painted foam.