How to go about insulating 2x4 vaulted ceiling in detached garage?

[roger wiegand]

Definitely no way I'd bother with DIY foam. It's nasty stuff.

I've used it a couple of times for sealing the rim joists in a basement, one kit gets me all the way around in a medium size house. Easy to use, kind of fun to apply, way cheaper for that application than bringing in a guy with a truck. For a whole roof I'd definitely hire a pro, no way the DIY kits are cost effective. In my hands essentially all of the material ended up where it was supposed to go-- but I've only used one brand, there may be variation.

[Kris Cook]

No experienced-based wisdom to add other than for those that have a lot of building experience (Steve R. etc.) keep in mind Kory lives in the Seattle area. It rains there - a lot.

[Maurice Mcmurry]

A good operator behind a pro set up is fun to watch. There is quite a learning curve for for being able to spray. I have clients who were some of the first in town to have their basement done years ago. They paid for 3 inches. In some places they got 13 inches. It looks bizarre.
I was thinking the Foam It Green was excellent while doing walls. Doing the vault was a rollercoaster ride of frustration and comedy. The end result is good. The house is so tight we had to add makeup air for the water heater and range hood.

[Cameron Wood]

I don't see why there would be any insulation standard to meet for a garage- it's not a habitable space.

I would use rigid foam between the rafters, as mentioned above. Cut butts from the sheets at the rafter spacing with the long blade of a snap-off knife and a sheetrock square for a press fit. Do not saw.
I would do just one layer of 2 or 2 1/2" , with no foam sealing and battens to hold in place. The foil face will help with lighting, but cover if you want- Tyvek is good. Sheetrock would be a bad struggle, batt insulation tends to trap moisture and rain down particles if not covered, spray foam is disgusting overkill.

The spaces at the skip sheathing etc. will allow moisture to dissipate- no showering, cooking etc. so moisture load is not great, but would not be enough for living space.

I've done this many times in various situations- there's little waste, little mess if you don't saw, the material absorbs very little moisture, and it's pretty easy to get tight fits since the foam compresses slightly.

[Jim Becker]

A good operator behind a pro set up is fun to watch.

I agree...the dude who did my new shop building was a "smooth operator". It was like watching a dance of sorts.

I don't see why there would be any insulation standard to meet for a garage- it's not a habitable space.

It really depends upon the jurisdiction where the building is, but I agree that in general, a detached accessory structure gets less scrutiny around things like insulation. For example, I have about an R15 minimum in the walls and about an R21 minimum up top with the closed cell spray foam in my new shop building. If this was a residence or had plumbing that could make it convertible to a residence, my jurisdiction would not accept the R21 up top because that's far below the current standard for ceiling/roof insulation for this geography. But that's not the case with my "residential accessory building"...and they don't even care that the foam is exposed and not coated.

[Andrew More]

I would use rigid foam between the rafters, as mentioned above. Cut butts from the sheets at the rafter spacing with the long blade of a snap-off knife and a sheetrock square for a press fit. Do not saw.

I've used my little Delta 14" bandsaw to cut rigid insulation. It makes a little mess, not a lot, has a fence, and easily cuts with a cheap blade. What's wrong with sawing extruded insulation?

[mike stenson]

Other than it making a royal mess?

I used an insulation knife, score, snap, cut the back side if it's lined.. done.

[Andrew More]

Other than it making a royal mess?

Okay, cool. I'm fine with this downside, didn't seem overly messy to me, but to each their own. I thought there might be some sort of issue with the saw, etc.

I like the fence, and ability to have better control over and custom cuts I need to make, like outlets. Also seems a little quicker than the knife.

[mike stenson]

I've tried both, the knife is much.. much faster, despite seeming not to be so. I used a drywall square for a fence...

[Cameron Wood]

I've used my little Delta 14" bandsaw to cut rigid insulation. It makes a little mess, not a lot, has a fence, and easily cuts with a cheap blade. What's wrong with sawing extruded insulation?

Just the mess, & those foam bits are not good to breathe.

[Andrew More]

Just the mess, & those foam bits are not good to breathe.

Huh? Let be really clear here, I'm talking about the extruded pink sheets, not the white balled stuff. The pink stuff doesn't produce any more mess than normal cutting on a bandsaw, and doesn't put stuff into the air. I was able to clear the saw in about 5 minutes with a shop vac.

I've tried both, the knife is much.. much faster, despite seeming not to be so. I used a drywall square for a fence...

Guess it depends on what you're doing. I found I spent a lot of time futzing about setting up the square, and then doing the cut. I was also batching things out, so setup the fence on the bandsaw, and then just feed stuff through. For the actual cutting it seemed to be about comparable, though I find being upright, instead of kneeing on the floor more comfortable, and I think it has a slight advantage in speed, because I could move at walking speed, rather than crawling on the floor.

[mike stenson]

It's really hard to batch things out when there's always variations, so using a fence it had to be reset over and over anyway. Also, cut on a table no need to kneel. Plus, any non-linear cuts were done by a knife anyway.

[Bill Dufour]

FYI Polyiso insualtion panel cost went up 75% in the last year or so. Polyiso cut clean on the tablesaw.
Check craigslist for used polyiso insulation from roofing companies. I see it at Sparks nv. Ebay often has it in Florida and Colorado.
Insulation of any kind is tied to fuel prices to make it..
Bill D.

[Andrew More]

@Bill, from what I can tell polyiso is mainly for high temp installs, like roofs. Have you had much experience with it? Is there some other difference between it and XPS insulation?

[Cameron Wood]

Huh? Let be really clear here, I'm talking about the extruded pink sheets, not the white balled stuff. The pink stuff doesn't produce any more mess than normal cutting on a bandsaw, and doesn't put stuff into the air. I was able to clear the saw in about 5 minutes with a shop vac.


Guess it depends on what you're doing. I found I spent a lot of time futzing about setting up the square, and then doing the cut. I was also batching things out, so setup the fence on the bandsaw, and then just feed stuff through. For the actual cutting it seemed to be about comparable, though I find being upright, instead of kneeing on the floor more comfortable, and I think it has a slight advantage in speed, because I could move at walking speed, rather than crawling on the floor.

I'm talking about foil face polyisocyanurate sheets. 30% higher R value than pink foam plus some radiant performance from the foil.

The pink foam is much more benign to work with.

AFAS crawling on the floor, sawhorses and a sheet of plywood works well.