XPS Foam in Cathedral ceiling instead of Spray Foam
I have a home in Northern Michigan (Traverse City area) that I need to redo the attic insulation. I had a steel roof put on last winter and had grace ice/water installed on the entire roof deck. It was a home built in 1977 with 2×8 rafter construction, with a 2.5/12 pitch cathedral ceiling on 12” centers. I need to upgrade the ceiling insulation to reduce the possibility of any moisture damage from condensation on the underside of the roofing deck. Also, because of a low ceiling height, I am not furring the ceiling to a thicker depth. Anything I do will be a significant improvement over what is already onsite, and my energy bills are already very low. The entire purpose of the project it to avoid future damage to the underside of the roofing deck.
I had been leaning towards spray foam, but I have some logistical and timing issues that has been seeking an alternative. I would like to use the following assembly, and am hoping some wise members can talk me out of, confirm I am on the right track or offer suggestions to improve.
Directly under the roofing deck, I would like to install a double thickness of 2” XPS foam insulation (that will be cut in 10 1/2” strips to fit in the 12” centers), and then add 3 1/2” batts on top of that (not sure yet if it will be rockwool or pink). Around the rigid foam insulation — am I better to leave an 1/8” gap around the perimeter to later fill with spray foam? Or should I gut the foam where it slides in with a very tight fit?
While I initially hoped to avoid ceiling penetrations, it does look like I will have 4 or 5 recessed CANs. Finally, the ceiling may end up being drywall or a wood T&G. If I omit drywall, and do a T&G ceiling — I am guessing I should maybe opt for rockwool insulation to act as a fire barrier before the foam insulation. In my primary house, all my walls are flash and batt and after MUCH discussion with energy experts, we decided against installing plastic on the inside of the wall assembly between the drywall and studs — and at this point, I presume we will handle the cathedral “modified” flash and batt approach the same. Thanks in advance for any shared wisdom.
Replies
Reading through a few typos.
https://www.finehomebuilding.com/membership/pdf/7319/021228066.pdf
worth a read.
I would get a pro foam gun and run a bead of expanding foam alongside the XPS as you install it, one thickness at a time. you can use stiff metal wires sold to hold fiberglass insulation to hold the XPS in place as the foam cures, then re-use them for additional layers, taking care to fully fill the gaps between the XPS and the rafters.
Know that the rafters will vary as to thickness and spacing. leave enough room to access the gap with foam. It will be impossible to fit the foam close enough to avoid air circulation to the roof deck.
the fiber insulation you are considering is rock wool, not steel wool.
Why not fill the cavity with XPS? even 7.5 inches at R5 per is only R37, still well below code.
Try hard to skip the cans. You can get LED pucks to surface mount and give good results.
I made the correction from steelwool to rock wool, not sure how that happened so I can only presume I need to blame siri! thanks for pointing that out.
I am not opposed to adding additional layers of XPS foam, but was thinking the rockwool may work better around the penetrations and to act as a fire barrier and would also be easier to run my electrical. If I was to stack two, three or even four layers of XPS foam -- could I glue those together on the ground and then insert the 4"-7" assembly directly into rafter spacing, hold it in place and then use the pro gun just once around the entire perimeter?
Also, if I was willing to give up 1/2" of headroom, would there be any value in spanning the entire ceiling with 1/2" XPS foam to create a thermal break because there are so many rafters? It's a second property that we keep at 60F when we aren't there in the winter and my gas bills are about $40/month, so this is not about reducing utility costs. Everything being completed is about protecting the roof sheathing from premature failure.
I just completed something similar with 2x10s and from the top (drywall and room below was perfect and replacing roof so I ripped off plywood and put metal on after new plywood and hi temperature ice/water).
Without any doubt, you either should have closed cell spray foam installed onto the underside of the plywood, or you need to vent it. Go to GreenbuildingAdviser and you will see countless articles about this, but if you try to cut and cobble or do anything but an absolute airtight installation, moisture-laden air is going to work up there and have nowhere to go once it hits the plywood.
As I had 2x10s, I used polyisocianurate foam board to get R-45 between the rafters. Three 2" and one 1" strip, canned foam to seal all around after install, and I still had about 1.5" on top for the vent channel. It was not fun. It took a long set of 3-4 hour pushes over three days, but holy hell I cut my electrical a/c usage in half instantly. Polyiso is about twice the r-value over xps or eps. I got it for similar cost at big box store (it was Johns Mansville as the manufacturer).
As for the install: I cut a bunch of pieces to fit tight and it was a PITA to put in, pieces broke, the spacing between rafters was all different by 1/8". It was much better to leave it small and then have ample room to run the foam can nozzle in. If they are loose, you can use little pieces of foam as wedges to hold it snug.
I am a little confused with your post as at the beginning it says to use spray foam, but then I think you describe exactly what I am trying to accomplish. Do you have any pictures of the install? It does sound like you left a 1.5" top vent channel - but do you have a ridge vent or did you just keep the gap for plywood drying purposes?
It was a shed roof (half of a cathedral). It was already vented at the soffit and the peak (although the 2x10 was stuffed full of old fiberglass, and there was full height blocking at the midpoint, so the vent was not previously effective).
What I mean is that (as I read and learned about from the experts) you can only make this an unvented assembly if you use closed cell spray foam. There are tons of articles and discussions on GreenBuildingAdvisor about the dangers of trying to make an unvented assembly with other methods, like cut and cobble (taking XPS or other board foam and stuffing it in and spraying foam around it). I could do cut and cobble effectively and safely *because* I had a vented assembly (so if some moisture works through, it will just go up the vent channel. There are articles about using dense-pack cellulose and one or two other options, but they are not common and many say they are risky. I called a spray foam company, the best in the area, and the estimator/owner tried to first sell me filling the whole assembly with open cell. I cited the GreenBuildingAdvisor experts and she quickly shifted to closed cell as the only option.
The problem, as I read, with cut and cobble in an unvented assembly is that you can make the cut/cobble perfectly airtight, but then the dimensional lumber expands and contracts and warm moist air finds small channels and that ends up under the sheathing and stays there to rot it out.
It sounds like my roof is virtually an identical situation with the exception I have 2x8’s I am contenting with. I am only doing half the cathedral and the soffit is vented BUT the ridge is not vented. The cathedral ceiling does have a ridge joist (and beam) so there is not air movement from side to side. I will eventually get over to the other half of the cathedral ceiling but it isn’t pressing at this point.
I wonder if I was to leave a 1/2” gap before putting my rigid foam if that would work. There would not be a lot of air movement, but still would be a little given the open soffits. I would think if there is only a very small amount of moisture that would work its way through, that the plywood would at least have a direction to dry towards.
If the rigid insulation is connected to the 2x8 via canned spray foam - I really can’t see how that would be any less air tight than spray foam. I have used both in the past and have probably seen more cracks in CC spray foam than I have in canned spray foam. So I guess I am back to considering the entire cavity with CC spray foam (not my preferred), or rigid against plywood sealed as tight as possible, or a 1/2” gap followed by rigid sealed tight and also the idea of a 1/2” rigid board across the bottom of my entire rafter assembly before putting on drywall or T&G. I have read a number of the green building advisor articles, and its really amazing there is not a clearer consensus. Glad to hear you were in a similar situation.
a ridge vent is needed to have circulation.
UncleMike is correct, you'd need a top outlet for getting airflow. With a solid beam at the top, that will be hard. If I were you, with only 7 1/4" and northern MI cold, I'd fill it up and not think twice. I had about 900 sq. ft. and got quoted $6600 for 5 1/2" of closed cell. I ended up spending about $2200 for the polyiso, $220 for canned spray foam, and three years of my soul for the days of labor with my crew (all of whom thought I was nuts to waste the time to do this). But my a/c compressor run times are running about 1/2 what they were the weeks before, and it's been hotter and sunnier since I completed the project. Remarkable change. Good luck.
I have a similar situation, shallow pitch cathedral ceiling, 2x8 rafters, fairly low, so not much room for additional insulation depth. I've read so many how-tos, and waded through lots of on-line arguments. One point that has stuck with me: The possibility of a poorly insulated hot roof causing major problems, so I've decided to do a cold, vented version. In my case the ridge is already vented and there are soffit vents. Just curious why you can't get up on your roof with a skilsaw and rip both sides of the ridge and install a ridge vent, is there something in the way? You can cut enough to the sides of a ridge beam, depending on how wide it is. It's pretty quick and easy, I've done a couple. If that's not possible do you have any horizontal ceiling near the top of the ceiling, and gable exterior walls on both ends, to be able to put in gable vents? As for the possible 1/2" vent space between insulation and sheathing, that's way too small, it should be at least 1", at least. Check out the square foot area recommendations for roof venting to confirm. I think I've decided to rip 1 1/4" wide (approx) strips of 1" XPS to attach to the sheathing along the rafters to create the vent space, then fill the bays with 7-8" of XPS, possibly using 2" polyiso for the last inside layer, leaving enough gap on both sides to fill with spray foam. If using 8" of foam leaves enough of a gap under the rafters I may piece in foam and spray to fill that. The reason I'll be using polyiso only on the warm side is because of all the reports on the R value testing that says it loses R value the colder it is, the tests show its high R value at 70 degrees (I think is was 70), but diminishing quite a bit as it gets colder. I won't use any batts, rock wool or otherwise for the limited space. My 1 1/2 cents.
Cutting pieces of foam to fit the very irregular voids between rafters while working on your knees in a hot attic will give you a new appreciation for hired labor. If you put a fair price on your own time sprayed foam will be less expensive and far better than cutting pieces out of sheets and trying to seal the edges. The spray foam falls out of the cracks almost as fast as you can spray it in.
Florida speaks the truth. I decided I needed to do the cut/cobble because I was working from above, had some odd angles, and couldn't do a strip off and have everything clean and ready for the spray foam company, and guarantee I wouldn't have the afternoon deluge thunderstorm that we get in the DC region. As it was, I had 2.5 long, wet, terrible days, but saved about $4000 (which is probably about what I'd value my time for that 2.5 days :)
I'm willing, and happy (while sitting here at the computer anyway), to do the grunt work to save $4,000. Ask me if I feel the same afterwards though. :-)
The OP's post says attic insulation, but then goes on to call it a cathedral ceiling, which is what I based my comments on. If it's a cathedral ceiling then there's no "on your knees" or "hot attic". :-)
There's "above your head trying to hold panels into irregular spaces and have spray foam glop down all over you and everywhere else." Having just done this from above, I have to say if I was doing it from below, knowing what I know now, I'd pay the extra for the spray foam.
No holding panels in place, no wildly irregular pieces, all rectangles, they get pinned sideways to the rafters with finish nails underneath each row of sheets. Except for the very top there'll be no "underneath", but there'll be a lot of "next to". :-) All worth it to me to save thousands of dollars, but to each his/her own. I own all the scaffolding (and wheels) so that makes it easier.
“[Deleted]”