Good morning! Any suggestions on how best to insulate a shed roof over my shop. Cold climate overall,with some scorchers in the summer(SW Mont). 2×10 rafters. Wasn’t thinking about venting it. Kinda sold on the expandable foam, but I’ve been following the debate on how thick to spray. Maybe rigid first and then spray? Thanks.
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Not sure why you would do rigid, then spray ... why not spray the whole thing? I'm not aware of the debate on how thick to spray/foam.
I read something about the cost effectiveness of spraying. Beyond a certain thickness the r-value doesn't increase that much?
But combining spray w/ rigid doesn't make sense for similar reasons ... and it is a lot more effort. One thing is for certain, install what you can afford ... 'cause when energy goes up, ADDING more will NOT be cost effective THEN.
The other guy is right ... need to cover foam w/ e.g. drywall for fire protection. Actually, you would be REQUIRED to do so (i.e. codes).
I read of one method spraying a number of inches of foam and then using batts. What I got out of that description, was that after more than just a few inches(4,5,6?) the cost-effectiveness of foam dropped. I like the idea of foam to block air movement, but it does seem to get pricey.
Well, there's no denying that foam, besides insulating, is a really good sealant.
By way of contrast, I recently opened a wall in my 1940 house. The fiberglass insualtion had a significant amount of dust and (it seemed) gravel-like grit in it ... plus, of course, sundry insect nests, empty cocoons, spider webs, and mold spotting.
This is arid Nevada - not a place really friendly to mold. I was not reassured by my being unable to actually SEE the openings where all this stuff entered; maybe I had closed them off when I gave the house it's very first coat of paint a few summers back. Or, maybe there are still thousnads of tiny, indirect openings there.
One way to make certain that the walls were sealed would be to cover them with foam, and count on the expanding mix to find it's way into every nook and cranny. Even a measly 1/4" covering would do this.
How much air leaks through walls? Well. let me put it this way: before I painted the house (I also caulked the cement wall shingles at the time), my curtains would move on windy days - with the windows closed and sealed! The house was not a shack, nor was it poorly constructed.
I strongly suspect that this is common to most of our houses. Sure, we have better housewraps and sheathing these days, but I'll bet there's still a lot of air flowing through our walls. Spraying foam will make a big difference in this problem.
One caution, though: It's better to have a 'leaky' wall than one that traps moisture within. If even my 'well ventilated' walls had some signs of mold, imagine what can happen when the moisture is trapped.
Sounds to me like maybe we shouldn't have faith in e.g. Tyvek. Maybe the traditional way of installing it isn't very good (I know I've seen some REALLY BAD Tyvek installs).
Aside from that, I surmise that foam is also a vapor retarder. That being said, you install just a little for air sealing purposes and then install FG batts and another [poorly installed] vapor retarder and you have a cavity that can trap moisture, don't we? I'm not criticizing, but bringing up some points of concern; thinking out loud (I know, you can't hear me ;) ).
Sounds like we're in agreement.
Those who remeber the '73 "oil crisis" and Carter's presidency also remember the way mold became a major issue in the '80's. The desire to make homes 'airtight' to save energy led directly to the mold issue.
I see much the same happening today, for the same reasons - plus the added push to be 'green.' I fear that they'll get the wrong shade of green.
Likewise, there is so much stuff placed in our walls these days that I think we'll have to change the way we make walls. These days, there's hardly a stud cavity that doesn't have something in it interfering with the insulation.
It sounds like you/they are combining the diminishing economics ("cost effectiveness") of more R-value with cost of a product. I would think if you mobilize to install a product, install it; don't do half a job and then change to another product. Rigid foam is also expensive. I'd think a lot of the cost of foaming is mobilizing to do the job.
You don't drag out a cellulose blower and install 2 inches and then start an install of batts.
An R-10 cuts heat loss by say 80%. An additional R-20 (R-30 total) cuts heat loss by say 95% ... you only get 15% for an R-20, not an additional 160%. There are diminishing returns for high R-values. But that is not an argument for using only an R-10. And then as energy costs rise, you don't come back an install an additional R-value 5 or 10 years down the road.
I say, pick a material/budget and install what you can. Meet the minimum of the energy code, though, try to exceed it. Also plan to fill the cavity full. Pick one material and use it (foam sheathing over cavity filled w/ batts would be an exception).
If you are going through the trouble of foaming ... do it. Just seems to me that it doesn't make sense to do a little with one material and the rest with something else. The second or fourth inch of foam costs a lot less than the first inch. Not sure if I'm conveying my idea clearly in the manner I think I want to (stupid brain; instead of saying what I want, it says what I think).
Yea, I think I get it. Makes sense.
The only 'debate' regarding spray foam is one common to all insulation: how much is cost effective?
One inch of foam will make a huge difference over no foam - both in comfort and utility bills. Yet, there's not going to be much improvement if you apply 21" rather than 20".
For summer heat, a vent and a "cold roof" (second roofing layer over the existing one, air space to allow free air flow, purpose is to keep the roof in the shade) are two things that make a huge difference. No heat radiating down, and the hot air has a way to escape.
While it may be tempting to 'spray and go,' any foam really needs to be fully enclosed in drywall. This will protect it from fire, physical damage, and vermin.
i did a shed roof with full fill cellouse.
put up 2 runs of sheetrock [8' wide] shot it full of insulation ,put another 2 up filled them. i have been very happy with the results.
with 2x10's i think your r value would be around 28.
the older i get ,
the more people tick me off
Sounds like the same problem I had. Are the joists exposed? In my case sheet rock covered the joists but I had access. I went to Lowes and rented the blower and enough bags of cellulose insulation and blew it all full, took a day including the running.
Made a big difference, AC turns off (never did before) heat in winter also turns off. Area feels much better all round.
Total cost $225. Foam would have been at least $ 1500 cause of minimum charge. Some simple math and you know what you need.
My foam guy advised that anything over 5-5 1/2 inches didn't pay. Still, thats R 30 plus with no leakage and I've been real happy with it. If you want to use rigid in combination, I'd suggest placing it below the rafters and sheetrocking over it. My roof shows thermal bridging through the rafters here in northern NY. I kind of wish that I'd done something to prevent that. There would then be a cavity and I don't know if that would be an issue, seems you could work around that. A shop would have decent air turnover and that might help with any moisture issues.
Yea, that's kinda what I've been hearing(5 1/2" of spray foam). I'm starting to think more about "thermal bridging". I never paid it much mind. Maybe strapping 1x3 like they do in the Northeast would help?
Yep, the joists are exposed right now and I"ll sheetrock afterward of course. I guy I worked for swears by the blown-in cellulose. I've seen it settle some (in walls) when I've remodeled.