Lightweight insulation/sheathing

[Pfhoren]

I am looking to insulate the garage in my new house so that it doesn't get so much hotter than the outside every day and possibly allow for adding a minisplit make it actually bearable on the summer. (Southern California)

The garage has no ceiling joists (just some full width tension beams) and very little to no trusses for the roof. From what I have read on this site, I would need to get a structural engineer out to see if the roof could support something like sheet rock over fiberglass directly on the rafters, or to see about adding a ceiling since the span is so wide.

I contacted a few engineers but nobody responded. I wouldn't feel confident adding sheet rock to the rafters due to thr weight, but I have been wondering if there is some insulation that doesn't need to be covered, or if there is a really lightweight product to cover the insulation.

I have been diving down the radiant barrier rabbit hole, and considering using it along, or even just using that as sheathing over the fiberglass since some of it is fireblock rated and it's so lightweight.

Does anyone have any thoughts on doing this DIY or any other suggestions?

Thanks!

 

[racecougar]

From the photo provided, there are no trusses present.

Is the garage attached? You mention "fireblock" rating, but from the framing, my inkling is that this is a detached garage without occupancy.

What is the span, side to side?

There isn't any roof/soffit venting present, is there?

 

[Colin Len]

Super common setup there, my garage is similar. Assuming you're not looking to reengineer the structure ($$$) then your best bet is to frame out a ceiling by adding ceiling joists on the wall top plates and then hanging drywall. You can then use the insulation of your choice (fiberglass/rockwool batts or blown in cellulose).

Your roof is not structural, so don't hang any weight from the ridge board or rafters. Those "full width tension beams" you mention are there to keep the walls vertical and keep the roof from sagging. I have a similar roof setup on my garage and a previous owner had added some overhead storage which ended up sagging the roof over 3" over time. If you add anything up there you need to make sure the load you're creating is transferred to the walls and not the overhead roof structure as it's not designed to take any additional load. If you frame out a ceiling like I mentioned above then you can easily hang drywall and, depending on the size of the framing members used, may even be able to store some stuff up there.

Lastly, if you add a ceiling/insulation as I've described you'll want to add some attic venting. I'm 99% sure that your roof doesn't have ridge or soffit vents, or likely any venting at all. You can try to add ridge and soffit vents but this may be more costly or more of a PITA than you prefer. I know with my garage it would be really tight trying to squeeze in any sort of soffit vent. I would consider using gable end vents or possibly even just whirly birds or other roof venting. Depending on your preference you could also consider using powered type vents rather than passive.

 

[Pfhoren]

From the photo provided, there are no trusses present.

Is the garage attached? You mention "fireblock" rating, but from the framing, my inkling is that this is a detached garage without occupancy.

What is the span, side to side?

There isn't any roof/soffit venting present, is there?

The wall behind me in the picture is 2/3 attached to the house. I am mostly worried about wildfire, not garage fire as I live in a high wildfire risk zone. There is no sheathing on the outside of the walls, just horizontal siding. I have been caulking the gaps in the siding for bug and fire reasons (embers coming in through the gaps), but still worried.

The span is 20ft side to side.

No roof venting, two small vents near the floor on the left side.

 

[racecougar]

With the lack of roof venting and the concern of wildfire embers entering any venting, sprayfoam on the roof deck might be the least intrusive method to follow here. *And I'm typically not one to suggest spray foam.*

 

[Pfhoren]

Super common setup there, my garage is similar. Assuming you're not looking to reengineer the structure ($$$) then your best bet is to frame out a ceiling by adding ceiling joists on the wall top plates and then hanging drywall. You can then use the insulation of your choice (fiberglass/rockwool batts or blown in cellulose).

Lastly, if you add a ceiling/insulation as I've described you'll want to add some attic venting. I'm 99% sure that your roof doesn't have ridge or soffit vents, or likely any venting at all.

My worry with the ceiling is that 20 ft is too long for ceiling members to cross the span from what I have read on other threads on this site.


My first thought with the garage was just to add some roof vents, but I was hoping I could add a minisplit to make the garage really usable in the summer. If I can't add a ceiling or insulation without spending $10k, then I'll probably just do the roof vents plus maybe some radiant barrier.

Unfortunately I had bought some fiberglass insulation before I realized my issues and I'll have to find a way to get rid of that...

 

[Pfhoren]

With the lack of roof venting and the concern of wildfire embers entering any venting, sprayfoam on the roof deck might be the least intrusive method to follow here. *And I'm typically not one to suggest spray foam.*

Thanks! I thought I had read that spray foam was flammable, but it looks like it's a fire retardant? So I shouldn't need to cover it? And it solves me having to worry about vapor barrier issues. I'll look more into it.

 

[Hank11]

Thanks! I thought I had read that spray foam was flammable, but it looks like it's a fire retardant? So I shouldn't need to cover it? And it solves me having to worry about vapor barrier issues. I'll look more into it.

No -- foam will burn.

I would find a copy of the local rules and regulations for fire prevention. While likely will be mostly common sense, there will be specifics regarding how best to prevent and slow the spread of fire.

Vapor barrier is one of the least important things based on where you live.

 

[Pfhoren]

No -- foam will burn.

Yeah, I wish I could find any non flammable insulation, that would be the easiest. I looked quite a bit. The only thing that seems to exist is radiant barriers.

 

[PCustoms]

Foam can be coated in special fire retardant paints and left exposed.

 

[racecougar]

Agreed that reading your local code is the place to start, as that will tell you what you can and can't do. If you're set on a completely non-flammable option, rockwool insulation is one path forward. That said, this thread wavers back and forth between cost conscience and full on "Cadillac" treatment. Are we after minimal/moderate insulation with the existing framing, or willing to add attic ventilation and frame in a ceiling?

 

[Colin Len]

My worry with the ceiling is that 20 ft is too long for ceiling members to cross the span from what I have read on other threads on this site.


My first thought with the garage was just to add some roof vents, but I was hoping I could add a minisplit to make the garage really usable in the summer. If I can't add a ceiling or insulation without spending $10k, then I'll probably just do the roof vents plus maybe some radiant barrier.

Unfortunately I had bought some fiberglass insulation before I realized my issues and I'll have to find a way to get rid of that...

20' should be possible, but of course you'll want to do your due dilligence. According to this span chart you could do 2x8's spaced 12" or 16" OC.



Here's another table that confirms 2x8 should work and with the right grade you could even do 2x6:
https://codes.iccsafe.org/s/IRC2021...ng-construction/IRC2021P2-Pt03-Ch08-SecR802.5

Spray foam was mentioned. I've researched it a bit and love the idea but you should consider the weight, I never looked into that. You may not want to add that weight to the roof structure itself so something to look into as your roof may not be able to support it. Additionally, the cost seems to be VERY high for this service.

Personally, for a garage where I wouldn't be loading up the ceiling/roof I'd be comfortable pushing the limits of the span tables as there's already some margins of safety added into those max spans. Worst case, you get a little sag over time and personally I could live with a little sag if that's what it takes to get a nice, comfy, conditioned space. Especially in a situation like this where the alternative would be a very involved and costly redesign of the roof structure. And if you were going that route I'd personally opt for a vaulted ceiling.