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

[Tom Bain]

Spray foam is the best option, in my view … how is the condition of the roof? I would want to make certain the roof doesn’t need to get replaced anytime soon if you are going to spend the $$$ for spray foam.

[Kory Watson]

Kory,

With a hot roof installation you completely seal the underside of the roof so there's no air movement in or out. Then the insulation batts are placed between the studs and then all of that is sealed with drywall or whatever so there's no air movement in or out from the shop. It's an accepted construction technique, often used for cathedral ceilings. Do a little research on the technique and it might work for your shop. As I mentioned, I did my shop that way 10 years ago and have had no problems. I like the more open feel it gives an can store things on top of the rafters

Cliff

Did some research on this. How do you seal the underside of the roof? Do you pay a closed cell contractor to spray an inch of foam then fill the rest with rock wool?

[Kory Watson]

Spray foam is the best option, in my view … how is the condition of the roof? I would want to make certain the roof doesn’t need to get replaced anytime soon if you are going to spend the $$$ for spray foam.

We just replaced the roof. Adding insulation on top of the sheathing would have been too expensive as we would have needed to re-do all of the fascia.

[Kory Watson]

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.

I'm curious why you wouldn't do foam sealing. Is there not a risk of condensation without a solid air seal?

[Jim Becker]

Did some research on this. How do you seal the underside of the roof? Do you pay a closed cell contractor to spray an inch of foam then fill the rest with rock wool?

"Flash and Batt" is a common way to get the sealing and other benefits of closed cell spray foam while keeping cost more reasonable. So yes, you could do that in your structure no problem provided you can and are willing to do what's necessary to support and hold up the Rockwool to the underside of the roof deck after the foam is in place. Rockwool is very easy to work with and doesn't burn so you don't need to cover it. But you do need to keep it in place.

[Maurice Mcmurry]

spray an inch

In central Missouri the research shows that an inch of spray foam is not enough to prevent a moisture trap. Our Insulation professionals use the foam + batts or blown but they insist on 3 inches of spray foam. I have rigid foam + batts in my shop attic but I have an air space, vented soffits and a vented ridge. I experimented with rigid foam pressed tight against the underside if the roof deck. On a sunny 95 degree day the temperature between the foam board and plywood was 170 degrees. The Foamular is rated for 165. We have days as hot as 105. I sacrificed some R value and created the air space. Kory is in a mild climate and has purlins under the OSB roof deck. I suspect foam board would stay below 165 degrees. I would put in collar ties across the rafters and create a ceiling half way up the vault and add two gable end vents then use foam board below on the now smaller slope, and batts or blown above the new ceiling. Kory, your rafter spacings appear to be unusual which adds to the challenge. I admire you for saving your neat old building!

[Bill Dufour]

@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?

Polyiso is much more expensive then the styrofoam types. But all I have ever seen has foil on one face, important in a roof in a desert sun like here. The 2" polyiso I used is r13. Polyiso has the highest r-value per inch of any insulation. Not much room in 2x4 space for insulation so I went with the good stuff.
If you use fiberglass get the high density stuff for 15% higher r-value and no more cost. Frankly I do not know why they still allow r-13 and r-19 to be sold. The r-15 and r21 are special order only here. I believe it is the law in Canada
Bill Dufour

[Bill Dufour]

My dad in the San Francisco bay area built a carport. When I was around 8-10 he pulled the rolled tar paper roof and installed aluminum corrigated sheet roof. He removed about 1/2 of the 1x8s that were the roof deck, to reduce the weight and remove some rot. On cold foggy days the nail points that were exposed would drip water onto the cars and make little spots. Very not good inside a house attic. He ended up putting putting little squares of homsanite over each nail.
Bill D

[Jim Becker]

In central Missouri the research shows that an inch of spray foam is not enough to prevent a moisture trap. Our Insulation professionals use the foam + batts or blown but they insist on 3 inches of spray foam. I have rigid foam + batts in my shop attic but I have an air space, vented soffits and a vented ridge. I experimented with rigid foam pressed tight against the underside if the roof deck. On a sunny 95 degree day the temperature between the foam board and plywood was 170 degrees. The Foamular is rated for 165. We have days as hot as 105. I sacrificed some R value and created the air space. Kory is in a mild climate and has purlins under the OSB roof deck. I suspect foam board would stay below 165 degrees. I would put in collar ties across the rafters and create a ceiling half way up the vault and add two gable end vents then use foam board below on the now smaller slope, and batts or blown above the new ceiling. Kory, your rafter spacings appear to be unusual which adds to the challenge. I admire you for saving your neat old building!

At a specification of 3" of closed cell foam, you'd already be at or better than R21 because it's hard to stop exactly at 3" and for the OP's situation, they would effectively be "done" at that point without adding additional insulation for a small accessory building like this. My new post frame shop building has 3" up top...it's more like 4"+ in many places, just as the walls at 2" are between 3 and 4" just because of how it's sprayed on.

[Cameron Wood]

I'm curious why you wouldn't do foam sealing. Is there not a risk of condensation without a solid air seal?

It's a double edged thing. Air sealing is imperfect, so you can end up with water vapor getting in but unable to get out. While it's partly quality of the wood, older buildings with less air sealing have a fraction of the rot and moisture issues of newer, more tightly sealed, ones. If 95% of the gaps are sealed with foam caulking, vapor pressure will still drive quite a bit of moisture through the remaining 5%.

Bulk spray foam has some risk of problems (possibly serious) from poor mixing/installation, is difficult to modify, not very reversible, and can lead to degradation of sheathing and roof structure in spite of, or because of more complete air seal. Also expensive, & IMO ugly.

[Cameron Wood]

This is a bit different, but shows a spray foam involved issue.

Apparently, moisture is condensing and freezing in the attic, then melting and finding it's way through 2" of spray foam to drip through the ceiling.

The moisture may or may not have come from the living space, but still illustrates my point about sealing/vapor pressure since the water finds it's way past the foam by gravity.

https://www.contractortalk.com/threa...sation.448233/

[Tom Bain]

My shop that I built several years ago is spray foamed (open cell). It’s a cathedral ceiling with no venting and I’ve had no issues. It is a conditioned space but with a mini-split the heating and cooling bills are very low. My rafters are much larger than 2x4s, so closed cell is probably a better option for this situation but also considerably more expensive. There is all kinds of info out there on spray foam from both reputable sources and hacks. I would suggest checking out SprayJones on YouTube … there is wealth of good information on his channel.

D97AC88E-7034-4863-BF43-2E281B409AD0.jpg

[Dick Strauss]

Cameron, can you tell us if the spray foam that allowed water through is open or closed cell insulation? When talking about spray foam insulation it is very important to note closed versus open cell because they are very different.

[Cameron Wood]

Cameron, can you tell us if the spray foam that allowed water through is open or closed cell insulation? When talking about spray foam insulation it is very important to note closed versus open cell because they are very different.

It looks like closed cell from the pics in the link. From a current post on another site.

[Jim Becker]

Sounds weird to me that closed cell would allow any moisture to pass buy...it's one of the reasons it gets used because it's an excellent moisture and infiltration barrier. Now if wood above it is getting wet from external sources, that wet isn't going to come into the building, but that wood could rot because it's wet and outside the envelope. Sometimes one has to be very careful with what's "out there". There are legitimate issues and there are folks with agendas, like like anything in life.