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

[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 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 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/r...20is%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.