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

I've been renovating a 1940s detached garage.

The walls were reframed & insulated, and new electrical run. The last step before drywall / ply is insulating the ceiling.

My problems here are:
* the ceiling is composed of 2x4 rafters
* no soffits, so soffit vents are not possible
* the structure is non-conforming and hence can't be raised in height
* replacing roof to add external insulation is cost-prohibitive for a variety of reasons

I'm curious how you'd go about insulating such a space. Is closed-cell foam my only option? Do I need to sister all of my rafters with 2x6s to hang bats?

P.S. I know the framing is pretty ugly. It passed inspection mainly because it is significantly safer than the previous state of this building, so please hold your judgement :)

Thanks!

Thats a challenging undertaking. I assume creating a ceiling is un desirable? Is the roof asphalt shingles? I capitulated to having a "hot roof" in a similar situation and sprayed on DIY foam it green, followed by net and fill cellulose. The old framing makes your situation complicated. Creating a ceiling or partial ceiling will have some benefits.

P.S. The DIY spray foam is not great. Next time I need a vault foamed I will hire professionals. Spray foam has a good maximum service temperature. Foam board is only rated to 165 degrees. Creating a moisture trap is also a concern. Design so that the dew / frost line will be within the foam.

I would put one layer of foam board insulation in between the rafters and another layer at 90 to that one across the rafters. Screw sheet rock to that or leave it exposed.

In a similar situation with an unventable roof I went with spray foam. Works great, but not cheap. The foam must then be covered with a flame retardant material. I did quick and dirty drywall as my roof rafters were a little deeper than yours, but you can apply more foam and use intumescent paint-- also not cheap. The spray foam rendered my 1910 barn completely tight and quite easy to heat and cool.

A few options as mentioned but they depend on how you want to finish the space and the local code. You mentioned inspection, what does your local code say about exposed insulation? No bueno here inside city limits, just fine a couple miles out though. That will drive what you do, but I'd guess they require a 30 min fire code, ie drywall. In that case, a ceiling and cellulose or fibreglass on top of it is standard. Curious about that code, then we can probably throw out some good options.

Since venting is an issue...I'd personally do the closed cell foam and be done with it. Not only will it seal and provide the insulation benefit you need, but it will also add some structural rigidity without adding more structure, as it were. And you can keep things open. As much as I hated spending the "yuge" amount of money to do this in my own new shop building, the end result is very pleasing...open, airy, comfortable. It also allowed me to put certain things like wiring, air and dust collection above the ceiling level so that there was zero blockage of lighting.

As an alternative but with reasonably good results, you could use rigid foam insulation board (two layers of 2" will get you about an R20) and closed cell foam spray from cans to seal cracks while keeping your vaulted ceiling. It requires a lot of careful measuring and cutting, but can work if necessary. But there is still cost there...each sheet is still about $50 each, although it varies with geography.

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Originally Posted by Steve Rozmiarek

A few options as mentioned but they depend on how you want to finish the space and the local code. You mentioned inspection, what does your local code say about exposed insulation? No bueno here inside city limits, just fine a couple miles out though. That will drive what you do, but I'd guess they require a 30 min fire code, ie drywall. In that case, a ceiling and cellulose or fibreglass on top of it is standard. Curious about that code, then we can probably throw out some good options.

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We need to do the insulation inspection, and will close up the walls afterwards with ply. Inspector has OK’d this plan. Drywall for fire recommended but not required as it is a detached garage.

Insulating the ceiling to local energy requirements basically isn’t possible without reframing. Around here you don’t have to insulate garages though, so we’ll work with the inspector. I’m sure he’ll be fine with what is reasonable given our constraints.

My guess is he’ll ask for a firewall if we decide to spray the ceiling , for obvious reasons. I’m calling tomorrow to ask.

Spray foam is an option, and I don't find the DIY kits any better or worse than the commercial options. Not sure why you're need additional fire retardant, all the ones I'm aware of come with it. I'd go with a closed kit if I went that direction. Also do not make the same expensive mistake I made, they have two types: sealing and insulating. The "sealing" kit does not rise as much as the insulating kit, despite being the same thing.

If you do NOT want to go with the spray or extruded foam you could rip thin (1/4-1/2") strips to hang every 2-3' or so, perpendicular to the rafters. This will enable you to install fiberglass bats to whatever size is necessary. I'd go with the paper faced personally, just to keep things from coming down.

I did 3 layers of 2" closed cell foam sheet on my detached building, with corregated steel roof and post and beam construction.

Kory,

Take a look at doing a hot roof using rock wool. You can install the rock wool yourself and then close it up with drywall. I did that when putting together my shop 10 years ago and it's been doing fine.

Cliff

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Originally Posted by Andrew More

Spray foam is an option, and I don't find the DIY kits any better or worse than the commercial options.

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The DIY kits have a disposable spray gun and no pump to pressurize the liquid or liquids. The foam comes out in a low velocity splatter. A good portion of the spray never makes it to the surface you are shooting at. It is a messy and frustrating undertaking. A pro rig is high pressure, two parts, mixed at the gun. A pro with a pro rig is very different than a DIY kit.

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Originally Posted by Cliff Polubinsky

Kory,

Take a look at doing a hot roof using rock wool. You can install the rock wool yourself and then close it up with drywall. I did that when putting together my shop 10 years ago and it's been doing fine.

Cliff

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Do you have ventalation concerns? If I installed rock wool, it would need to be touching the roof sheathing directly. My understanding is that will do major damage over time.

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Originally Posted by Maurice Mcmurry

The DIY kits have a disposable spray gun and no pump to pressurize the liquid or liquids. The foam comes out in a low velocity splatter. A good portion of the spray never makes it to the surface you are shooting at. It is a messy and frustrating undertaking. A pro rig is high pressure, two parts, mixed at the gun. A pro with a pro rig is very different than a DIY kit.

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

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Originally Posted by Maurice Mcmurry

The DIY kits have a disposable spray gun and no pump to pressurize the liquid or liquids. The foam comes out in a low velocity splatter. A good portion of the spray never makes it to the surface you are shooting at.

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I think we're talking about different things. The pressure is in the tanks, or at least that was the case with the two I've used. For the first one the company is no longer in business, but I've also used the DOW kit, and it was not as you're described. I did not have an issue with the majority not getting to the surface, which in three applications was a roof.

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It is a messy and frustrating undertaking. A pro rig is high pressure, two parts, mixed at the gun. A pro with a pro rig is very different than a DIY kit.

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I'll agree it's messy and nasty, not to mention hot and sweaty, because of the full body plastic suit and required temperature. However, I believe this is the case for both the professional and the DIY kits. I'd say the same about installing fiberglass bats, and I'd guess Rockwool, which I have yet to install.

I have not used a pro rig, it's true, but the results appear to be identical to me, from what I've been able to see of both professional as well as my own DIY applications. You could be correct that there is more pressure with a pro-rig, but it doesn't seem to matter in my experience.

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