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

[Kory Watson]

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!

[Maurice Mcmurry]

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.

[Ralph Boumenot]

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.

[roger wiegand]

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.

[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.

[Jim Becker]

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.

[Kory Watson]

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.

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.

[Andrew More]

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.

[mike stenson]

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

[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

[Maurice Mcmurry]

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

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.

[Kory Watson]

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

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.

[Kory Watson]

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.

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

[Andrew More]

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.

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.


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.
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.

[Cliff Polubinsky]

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

[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.

[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/threads/roof-leaking-i-think-from-condensation.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.

493952

[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.

[roger wiegand]

Polyiso is much more expensive then the styrofoam types. r

It depends. Turns out that when commercial rubber roofing is replaced it is typical to also replace the polyiso sheets that underly it. There is a whole industry now based on re-using that foam. It comes in a wide variety of conditions, based on the age and condition of the roof it came off of. When I helped my son re-do his house it had a vey low slope, difficult to insulate roof. It also needed a new roof. We were able to put 10" of used polyiso foam on top of the roof for about $1/sf., then a rubber roof on top of that. Can't beat that with a stick! We opted for an "expensive" grade that was in very good shape. Polyiso loses some R-value in the first few years it is installed, so your initial R value is not as high as with new material, but it stabilizes after a while. The material is faces with a tarpaper like substance and comes in 2-4" thick sheets, not necessarily 4x8 so you need to piece them together a bit.

[Holmes Anderson]

I guess i'm late to the party on this thread but you may have more to worry about than choosing insulation. If that was unconditioned space and you are converting it to conditioned space, then you should consider ventilation. You say there are no soffits and I don't see a gable vent. Is there a ridge vent? Spray foam insulation against the sheathing is great for stopping airflow and heat/cooling loss, but it is hell on the shingles. Without cooling from ventilation on the underside of the sheathing, the shingles get too hot in the summer sun. Maybe heat is not a big problem in Seattle, idk, but condensation could be and even rigid foam or glass batts without baffles for ventilation will shorten the life of the shingles. You could attach sheathing to the trusses to create an attic and then put insulation on the attic floor and add ventilation. Powered roof vents are easy to install and that would rid you of excess heat and moisture without killing your shingles. Also, spray foam is hella expensive so I wouldn't apply it to a roof that is in poor condition and doesn't meet code.

[Andrew More]

I heard on the Green Building Advisor site that a few studies have shown the temperatures for shingles are not really that important, and the conventional wisdom of venting is problematic. While it's true high heat might shorten their lifespan, studies show that it's in the 5-10% range. In the mean time adding venting often increases the complexity of the building envelope, and makes it more likely there are more air leaks increasing heating and cooling costs.

[roger wiegand]

Here's a fairly extensive study (http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf) that says on average, in Florida, shingles on an unvented roof are only 2-3% hotter than on a vented roof. This compares to roof color, which can make a 20-30% difference. There's no data I'm aware of that says there are huge differences in roofing lifespan. In more northerly climates or places with a lot of overcast these effects will be much reduced compared to a place like Florida. In a new installation one might opt for optimal summer insulation effectiveness by laying sleepers above the insulation and venting directly under the roof sheathing. Given the cost of a second layer of sheathing this may or may not be cost effective.

Anecdotally, it's been 10 years since we spray foamed the under side of our house roof that had 15 year old shingles on it. At age 25 they are still a ways from needing replacement, on a partially shaded house in MA. No evidence of disastrous consequences.


I guess i'm late to the party on this thread but you may have more to worry about than choosing insulation. If that was unconditioned space and you are converting it to conditioned space, then you should consider ventilation. You say there are no soffits and I don't see a gable vent. Is there a ridge vent? Spray foam insulation against the sheathing is great for stopping airflow and heat/cooling loss, but it is hell on the shingles. Without cooling from ventilation on the underside of the sheathing, the shingles get too hot in the summer sun. Maybe heat is not a big problem in Seattle, idk, but condensation could be and even rigid foam or glass batts without baffles for ventilation will shorten the life of the shingles. You could attach sheathing to the trusses to create an attic and then put insulation on the attic floor and add ventilation. Powered roof vents are easy to install and that would rid you of excess heat and moisture without killing your shingles. Also, spray foam is hella expensive so I wouldn't apply it to a roof that is in poor condition and doesn't meet code.

[Jim Becker]

I guess i'm late to the party on this thread but you may have more to worry about than choosing insulation. If that was unconditioned space and you are converting it to conditioned space, then you should consider ventilation. You say there are no soffits and I don't see a gable vent. Is there a ridge vent? Spray foam insulation against the sheathing is great for stopping airflow and heat/cooling loss, but it is hell on the shingles. Without cooling from ventilation on the underside of the sheathing, the shingles get too hot in the summer sun. Maybe heat is not a big problem in Seattle, idk, but condensation could be and even rigid foam or glass batts without baffles for ventilation will shorten the life of the shingles. You could attach sheathing to the trusses to create an attic and then put insulation on the attic floor and add ventilation. Powered roof vents are easy to install and that would rid you of excess heat and moisture without killing your shingles. Also, spray foam is hella expensive so I wouldn't apply it to a roof that is in poor condition and doesn't meet code.

I have to disclose that at our previous residence, the local jurisdiction forced our contractor to ventilate the roof deck of our 2200 sq ft addition before the spray foam insulation envelope was installed. That resulted in a huge expense replacing the roof just a few years later because of condensation completely rotting it out to the point my foot went through the 50 year shingles while I was doing leaf maintenance up there. The foam was supposed to be on the deck by manufacturer and installer specifications, but alas, the (not pleasant) local inspector had his way and it cost me twenty grand. Roof and soffit Venting is not typically used with closed cell spray foam envelopes for this reason other than on high end structures that have a layered roof system, typically in the south...a double deck if you will. The living space is unvented other than ERV for air quality that does heat exchange to insure that the temperature (warm or cool depending on the season) is maintained while the humans have comfortable fresh air to breath. The shingles don't care if they are a quality product as Roger pointed out in his own reply to your comment.

[Holmes Anderson]

Here's a fairly extensive study (http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf) that says on average, in Florida, shingles on an unvented roof are only 2-3% hotter than on a vented roof. This compares to roof color, which can make a 20-30% difference. There's no data I'm aware of that says there are huge differences in roofing lifespan. In more northerly climates or places with a lot of overcast these effects will be much reduced compared to a place like Florida. In a new installation one might opt for optimal summer insulation effectiveness by laying sleepers above the insulation and venting directly under the roof sheathing. Given the cost of a second layer of sheathing this may or may not be cost effective.

Anecdotally, it's been 10 years since we spray foamed the under side of our house roof that had 15 year old shingles on it. At age 25 they are still a ways from needing replacement, on a partially shaded house in MA. No evidence of disastrous consequences.

The study says that the temps at the bottom of the roof are 23 degrees F higher at peak with spray foam than ventilated. It also says that there is a much greater temp difference at the underside of the shingle than the top and even a 2 degree F difference reduces shingle life by 11%. Doesn't sound like a vindication of spray foam.

[Bill Dufour]

California as of five years ago or so required asphalt shingles had to be rated as "cool roof". Not exactly sure what it means but they run quite a bit cooler in the sun, reflect more heat. no real observable difference. I think it is just the gravel minerals. They still sell black.
Bill D.

https://www.owenscorning.com/en-us/roofing/tools/how-to-meet-cool-roof-code-requirements-in-california#:~:text=State%20of%20California%20COOL% 20Roof%20Code%20Requirements&text=For%20zones%201%2D9%20and,of%20the%20roof%20i s%20removed.

[Dave Roock]

Hello Kory, Use R-15 fiberglass insulation. Then install some 1/2" -1" hard foam insulation boards over, followed by some 7/16" OSB nailed/screwed in over that. No soffit or baffles needed as there is no ceiling/attic space. It will stay warm in colder weather.

[Tom Bender]

Hi Kory
I'm concerned with your roof structure. Looks like 1 out of 3 rafters is supported by trusses. Take a look at it from the outside on a ladder. Does it sag any between trusses? If any it will only get worse. It may not be difficult to reinforce it before you insulate.

If the ridge sags you must beef up the ridge beam. If the rafters sag they must be sistered. You would need to jack up each before fastening.

To reinforce a beam you could fasten a 2 x 6 on one or both sides or you might screw/nail and glue a 2 x 4 to the bottom either on edge or flat.

[Daniel O'Neill]

Hello Kory, Use R-15 fiberglass insulation. Then install some 1/2" -1" hard foam insulation boards over, followed by some 7/16" OSB nailed/screwed in over that. No soffit or baffles needed as there is no ceiling/attic space. It will stay warm in colder weather.

Kory, lot's of good advice overall. Here's what I plan on doing with my roof which has 2x8's so not as "thin" as yours but the idea still works. I'll buy a bunch of Recycled foamboard and use that between the bays with a spray foam gun to "glue" it in. The foam will be set to have a >1" gap between the bottom of the roof deck and the foam such that the foam will be flush with the bottom of the roof rafters. (In your case I'd get 2" or 2-1/2" foam board and your gap will be set). Then I'll put one more layer of foam attached to the bottom of the joists to reduce thermal bridging. At the peak there will be a small triangle of rafter ties so that the air can move from one side to the other of my gable roof. Then I'll cover with some type of wall board or drywall.
All that said I'd take Tom's advice and look at beefing up those roof rafters.

[Kory Watson]

> I'm concerned with your roof structure. Looks like 1 out of 3 rafters is supported by trusses. Take a look at it from the outside on a ladder. Does it sag any between trusses?

No sag at all! The trusses themselves sag a little bit. That said, this building is nearing 80 years old. At this point I'm pretty confident in the roof considering the lack of sag.

All that said, there's some extreme sag in the header above the garage door. The garage seems to have been original designed for 2 doors and it was converted to 1, resulting it 2 inch sag in the middle of the door. I will replace that header with an LVL at some point.

All that said, we don't want to invest in this space too much. When we sell this property, townhomes will be built in the garage's place. We're in the middle of a pretty dense part of Seattle. So long as the roof lasts us 10-15 years, we're golden. Only reframed the walls because they were literally falling apart from extreme rot

[Maurice Mcmurry]

Adding collar ties to every pair of rafters would add strength to help carry the little bit of dead load your insulation and covering will be adding.

[Kory Watson]

Adding collar ties to every pair of rafters would add strength to help carry the little bit of dead load your insulation and covering will be adding.

I may indeed do this regardless, as we have 3 roof vents that could ventilate a "mini" attic space.