Insulating Roof Rafters

[TurboEuro88]

Now that I am pretty far along with my Garage Remodel, I wanted to address insulation... or at least the lack thereof. It's a 440sqft detached garage that has never been insulated. This makes it a glorified shed in the winter with next to no thermal barrier to keep things bearable indoors. The walls and garage door are easy with batt and foam board insulation, however it's the overhead portion that's got me a bit unsure of how to proceed.

I know the go-to solution for this is to install drywall overhead and blow in insulation on top. Easy enough, but not the route I wanted to go given how I wanted to use the garage. Specifically I was looking to keep the volume of the garage is as open as possible so that whenever I have the hood up on one of my cars, I can lift the car off the ground as much as I want letting the hood stick up into the "attic" volume. As well, I wanted to suspend things like my mountain bike and a Kayak from the roof joists for storage since I don't have any other indoor places to keep them.

The long and short of my question is this - what options do I have to insulate the roof joists properly? I've read I can install standard insulation batt with a rafter vent between the batt and the roof sheathing, but strangely the garage roof ridge and soffits are not vented at all, even though it and the house roof were supposedly replaced at the same time (house does have ridge venting). I've also thought about just cutting foil-faced foam board between the rafters, but wasn't sure if that'd be enough insulation for the cold winter months.

Open to hear what options I may have and what the right procedures and materials would get the job done. I've got some time to figure this out before the cold really sets in but I'm hoping I can get this done before then. Thanks!

 

[QwikKotaTx]

I am having the same dilemma. I would think rafter insulation would be less effective and possibly reduce shingle lifespan but I could be wrong. I too don't want to lose my ceiling joist storage and make the garage feel smaller. If I want to use my attic I have to deck the top of the joists anyway.

It's easy enough to add 3" holes to the soffit and nail on flat vents.

Sent from my VS987 using Tapatalk

 

[TurboEuro88]

From what I recall reading, shingle lifespan should only be impacted if there isn't a rafter vent behind the batt insulation. But I believe that would presumably apply if I had soffit and ridge vents, which I don't. There's also a question of a vapor barrier as well. Right now the garage doesn't have any vents, but I am going to be adding gable vents, one of which will have a fan to exhaust hot air that builds up during the summer. That said, insulating the roof rafters should help cut down on radiant heat.

Not really sure how to proceed given all this. Hoping someone knows the right solution for me!

 

[DC73]

Closed cell spray foam is probably the best answer but can be a fairly costly option.

If you choose to not use foam, you'll need a ventilation channel under the roof deck where each bay has an intake and exhaust vent. That likely means a continuous soffit and continuous ridge vent. If you have a hip roof, it's probably a no go to ventilate under the roof deck.

There are other options. If you just happen to need a new roof, there are options for installing enough foam board on top of the roof to prevent condensation and then you can get your remaining r-value from conventional insulation placed underneath the roof deck.

You can also install enough closed cell foam to avoid condensation (amount depends on climate zone) and then can finish with other less costly forms of insulation.

Don't worry about shingle life. When they first starting insulating the bottom of the roof decks, that was a concern but they've since learned that shingle color has more impact on lifespan.

Best to do some research on BuildingScience.com and GreenBuildingAdvisor.com

Both are excellent resources. There is a Q&A forum on GBA where some building science gurus hang out. They can best advise you on non-traditional approaches.

DC

 

[TurboEuro88]

Roof on the garage (as well as the house itself) is still relatively new, however the previous owners strangely chose to not install a ridge vent on the garage. I'd really like to not have to put money into the roof just to insulate it as that really adds expense I don't have budgeted for. Realistically just looking for the options/solutions I have available to me without having to change anything with the roof just yet.

Also, I am not planning to drywall after insulating the roof rafters - just going to leave them exposed. I was thinking installing a good batt insulation would be enough but I know ventilation between the roof sheathing and insulation was necessary, but really have no idea what works given my application.

 

[rlev11]

If all you need is a ridge vent, it probably wouldn't take a lot of time or money to just add one. Pop off the current ridge cap shingles, cut the slot with a circular saw, nail down that ridge vent mesh, and just re-cap it with new cap shingles that match existing.

 

[strutaeng]

Closed cell spray foam is probably the best answer but can be a fairly costly option.

If you choose to not use foam, you'll need a ventilation channel under the roof deck where each bay has an intake and exhaust vent. That likely means a continuous soffit and continuous ridge vent. If you have a hip roof, it's probably a no go to ventilate under the roof deck.

There are other options. If you just happen to need a new roof, there are options for installing enough foam board on top of the roof to prevent condensation and then you can get your remaining r-value from conventional insulation placed underneath the roof deck.

You can also install enough closed cell foam to avoid condensation (amount depends on climate zone) and then can finish with other less costly forms of insulation.

Don't worry about shingle life. When they first starting insulating the bottom of the roof decks, that was a concern but they've since learned that shingle color has more of impact on lifespan.

Best to do some research on BuildingScience.com and GreenBuildingAdvisor.com

Both are excellent resources. There is a Q&A forum on GBA where some building science gurus hang out. They can best advise you on non-traditional approaches.

DC

This is the best advice. Without tearing off your roof and installing rigid insulation, you are down to spray foam. You can also do a combination batts and rigid above roof deck.

I did all rigid insulation on my living space. Below is exposed trusses. I had to replace my roof, so it was getting ripped out anyways. I posted some photos a few months ago. The thread was about some ceiling fan question in this sub-forum.

I've got a detached garage with 2x6 rafter that I also want to leave open. Currently it was a low ceiling/attic for a portion. Ceiling is only 7' so I wan to open it up. I plan to do fiberglass and will add rigid insulation above (and new deck also.) It so happens that the roofs need replacement, LOL. I've got that flimsy 3/8" deck too, so will upgrade to 7/16. It's limping along...

 

[OldSoldier]

Spray foam isn't the only option. My shop was uninsulated when I moved in. It had 2x6 rafters, so I knew I wouldn't be getting thick insulation, but I figured out the best I could do was to install 4" thick polyiso (mine was used/recycled, I found it on a CL business ad; there are businesses that specialize in such things and it was fairly inexpensive) in the rafter bays. That required me to cut the foam to fit the bays, but I used my table saw and ripped the insulation pretty easily. I cut it about 3/8" short all around and used can spray foam from HD (the orange stuff mostly) to seal the perimeter. To make sure I had proper spacing for ventilation between the roof sheathing and the polyiso, I used scraps of 1" thick XPS that I'd used to build out the garage door insulation. I used a couple of nails to hold it all in place until the spray foam had set and then I removed the nails. I did have to install a ridge vent all along the rooftop, but that was simple to do like rlev11 said. On the inside of the rafters I nailed another 1" of XPS from HD and extra long drywall screws secured the drywall through the XPS into the rafters. I did this to mitigate the rafter wood short circuit for heat flow. Fairly light, built-in ventilation from soffit to ridge, with a total R value of 24 + 5. The fact that I bought the poly iso used helped the cost. My building is a 26x53 and it cost me about $2500 to insulate it (that's with walls also, I used 2.25" recycled polyiso there). Plus one heckuva lot of elbow grease.

 

[strutaeng]

Spray foam isn't the only option. My shop was uninsulated when I moved in. It had 2x6 rafters, so I knew I wouldn't be getting thick insulation, but I figured out the best I could do was to install 4" thick polyiso (mine was used/recycled, I found it on a CL business ad; there are businesses that specialize in such things and it was fairly inexpensive) in the rafter bays. That required me to cut the foam to fit the bays, but I used my table saw and ripped the insulation pretty easily. I cut it about 3/8" short all around and used can spray foam from HD (the orange stuff mostly) to seal the perimeter. To make sure I had proper spacing for ventilation between the roof sheathing and the polyiso, I used scraps of 1" thick XPS that I'd used to build out the garage door insulation. I used a couple of nails to hold it all in place until the spray foam had set and then I removed the nails. I did have to install a ridge vent all along the rooftop, but that was simple to do like rlev11 said. On the inside of the rafters I nailed another 1" of XPS from HD and extra long drywall screws secured the drywall through the XPS into the rafters. I did this to mitigate the rafter wood short circuit for heat flow. Fairly light, built-in ventilation from soffit to ridge, with a total R value of 24 + 5. The fact that I bought the poly iso used helped the cost. My building is a 26x53 and it cost me about $2500 to insulate it (that's with walls also, I used 2.25" recycled polyiso there). Plus one heckuva lot of elbow grease.

That's certainly a way to do it, but as you mentioned, a lot of work. The preferred method is to NOT cut the sheets, and install it in what the code calls, "continuous insulation." When installed in a continuous method, you eliminate "thermal bridging." Don't need venting in this case.

The site on the GreenBuilderAdvisor call this method "cut and cobble." They state someone had some issue with thermal movement and breaking the spray foam (Great Stuff), causing air leakage. I don't think there's any testing on this method, probably because it is only used by DIYs. Commercial jobs require C.I.

In any case, polyiso cut between rafters is certainly better than no insulation.

 

[OldSoldier]

Agree with all you say. I was OK with taking the ridge cap shingles off to install the ridge vent, but I didn't want to replace the remainder of the roof because it wasn't that old, which would have allowed me to then more easily do it continuously. I think the method I used would work for the OP, as long as he's willing to install ridge and soffit vents.

 

[TurboEuro88]

Having read the above replies, am I basically stuck with not being able to insulate the roof rafters until a ridge/soffit vent system is installed? While I am sure I could install a ridge vent myself, and by and large I am pretty capable, messing with my roof (specifically the part that keeps the water out) is not something I feel all that comfortable with and would mean I'd have to call in a pro. That obviously comes at a cost which I am not sure is in the budget for me right now.

This is about as good of a photo I have available right now to show what the roof/attic of my garage looks like:


I know throwing in some 2x4s and drywall on the joists would be the simplest/most effective solution but I'd rather keep the space more open and usable. I also don't want to do that since the ceiling is relatively low as well. Just hoping I can find an affordable solution for insulating the upper half of the garage for the winter so I can get work done out there.

 

[strutaeng]

Yeah, I think that's the best bet. I have wondered about installing the rigid C.I. BELOW the rafters(+fiberglass cavity,) but I have not seen that method. With seams sealed, I can't see how you would get condensation from air leakage. Might be a problem at the ceiling joist intersection with the roof. Perhaps post the question on greenbuilderadvisor? All literature states rigid has to be next to the roof deck.

How about you raise the garage? That seems to be a common question asked here, LOL. 5 or 6 courses of CMU and drop it back down. I know that's not what you are asking, but just a thought

 

[strutaeng]

Perhaps the rigid C.I. below the rafters IS an option:

I didn't look into the details closely, but look at post #7:
https://www.greenbuildingadvisor.co...nsulating-an-old-non-vented-cathedral-ceiling

I'd ask these guys. Non-vented assemblies are not well understood by the DIY crowd. These guys know their stuff...

 

[TurboEuro88]

How about you raise the garage? That seems to be a common question asked here, LOL. 5 or 6 courses of CMU and drop it back down. I know that's not what you are asking, but just a thought

Oh, it's a thought that's crossed my mind many times. The reality is that this is my first home, and while I love where I am I know this isn't my forever home. Either I'll build new or buy something that's set up better than this is. 7.5' clearance is enough... though I'd prefer to be in the 10' range.

I've been doing more research into the ridge vent option. Adding one looks straightforward, other than the daunting thought of cutting into my roof I am still going to get it quoted by a pro just so I have a sense of what that route would cost me, but do you know if there would be any immediate concerns with not installing the ridge vent before insulating the rafters? My thought here would be to press on with insulating as I had originally planned, but installing the ridge vent later on either a) when I feel comfortable with the job, or b) have the money to pay a pro to do it. This is on a scale of months at most, not years.

 

[TurboEuro88]

Started calling around to see what this kind of job would cost a pro to do. So far I've only got one solid estimate at about $450 fully installed. Doesn't sound awful but not cheap either. Anyone have experience on whether that's high? Waiting for other companies to return phone calls. I think I've settled on paying someone to install the vents - I'd rather the peace of mind of knowing a pro installed it and, worst case, having a warranty of some kind to back the work up.

Also... Would there be any harm in moving forward with insulating the rafters with batt insulation (with rafter vents between the decking and insulation) while I am doing the rest of the insulation and adding the rafter vent in a month or few? I know the purpose of the vent is to let out hot, humid air and don't want to cause issues in the relative short time the ridge wouldn't be vented.