r/buildingscience • u/ecg86 • 1d ago
Insulation advice
I am looking to insulate the ceiling of this diy pool house. Originally I planned rockwool but with the I joist depth and width spacing standard batts won’t fill the room and if I understand correctly that can lead to moisture issues.
I know in hindsight exterior roof insulation should have been planned but I’d like to work around that if possible. Can I still use rockwool batts and just buy extra to cut and fit where needed, is there a better option fiberglass batt?
Is closed cell foam the only route to go at this point? If so are there viable DIY options or no?
I am located in MD so colder winters and humid summers to factor in.
I appreciate any insight
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u/cagernist 1d ago
If I read your OP correct, you say not filling the full depth of rafter causes moisture. That is incorrect. It is the lack of air impermeable (foam) insulation in your climate for an unvented rafter assembly. You could have a 108" deep rafter, and as long as foam is against the sheathing, and you achieve your total R value, a 101" air gap below the insulation makes no difference.
But anyway, Maryland is Zone 4, 5, or 4 Marine. R49 total value and either R10, R15, or R20 foam against the sheathing (depends if 4A/B or C, and 5).
You can accomplish that in an unvented rafter assembly, using a process of elimination to what fits in your unstated I-joist depth, with say:
- Full 7" ccSPF;
- Flash-n-batt of 1.5" ccSPF+R38 of any batt (meeting the R10);
- Cut-n-cobble 2" XPS with spray can gaps foamed+R38 of any batt (meeting the R10).
Or you can make a vented rafter assembly with:
- Low/high vents+baffles+the thickness of any insulation type that fits in the I-joist depth yet still meets R49.
See IRC R806.5 and N1102 and verify any local amendments, it is an installation manual with the science of vapor already figured out for you.
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u/ecg86 1d ago
Ah ok so I was mistaken on how the moisture could happen. That’s a bit of a relief. Thank you for the detailed options
I am zone 4A. The goal is to stay unvented and if I need to control humidity when the doors and windows are closed beyond what the mini split can do, I’ll add a stand alone unit. It’s really seeming like closed cell spray foam is the logical move even if it may cost more.
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u/Fasterandfaster-2000 1d ago
What is the spacing? What is the depth? Which CZ?
If it is 12” oc (looks like it), you can order 24“ oc Rockwool batts and split to have less waste. I have typically bought special order from Lowe’s. My insulation supplier seems to have a harder time getting it but your results may be different on the East Coast. Have to buy a pallet at a time.
It‘s not hard to make a cutting table jig to get an even cut. Festool, Mafell and Milwaukee all make long jigsaw blades for insulation. I‘ve had my guys do it multiple times.
What is the joist depth? You may need to think about a closed cell spray foam layer and then Rockwool batts. ‘Flash and Batt‘ strategy.
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u/ecg86 1d ago
Yes to 12 OC, I am unsure what you mean by CZ?
Depth is 11 3/4
I’ll check with the local Lowe’s and see if they have access here. Thank you!
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u/Fasterandfaster-2000 18h ago
CZ is Climate Zone. MD should be CZ 4 which means you need R-49 in the ceiling. If you are in the northern part of MD you will be close to CZ 5 (but not in) which requires R-60 generally.
Rockwool will get you to R-30 in 7 1/4 inches. An additional layer of R-15 Rockwool would fit and get you to R-45 but likely isn’t going to fix moisture issues because your bays are not ventilated.
You definitely need a layer of closed cell spray foam up there to avoid moisture issues. CCSF is R-6 to 7 per inch so a single 3 in lift will get you where you need to be and leave enough room to where the bottom of the batts can sit on top of the I-Joist bottom flange.
You also need to block the joist bays at the wall line above the door.If you want to avoid spray foam you could cut and install 2 layers of 2” R-10 XPS foam board and tape the seams and then Rockwool batts but it’s a bit of a pain in the butt.
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u/Fasterandfaster-2000 18h ago
Just glanced at my Rockwool catalog and it does list a R-38 ComfortBatt 24“ oc that is 9 1/2” thick but I know I asked a supplier about it a few years ago and was basically told I couldn’t get it without ordering a full truck load. Again though, you being on the East Coast and closer to the plant you may be able to get it.
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u/mikkoala1 1d ago
Unvented assembly? Several inches of closed cell under the deck followed by dense pack cellulose held up by 2 layers of insulweb or similar. Different ratios of R values for cc vs dense pack depending on your climate zone.
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u/RocknrollClown09 1d ago
Just cut rock wool to size with a dry wall knife, though any serrated blade, including a bread knife, will work fine. It's hydrophobic, air permeable, and very mold and pest resistant, so it's the best product for a damp location. Leave an air gap against the roof deck though. I'd also recommend a full face respirator, gloves, and Tyvek suit though because the dust and debris is like razor dust.
I wouldn't recommend closed cell with wood because you're literally encapsulating your entire ceiling in an impervious membrane. As soon as you get a roof leak, which you eventually will, the water will have nowhere to go, so A) you may go a really long time before even discovering the leak, and B) the roof deck or joists won't be able to dry out, accelerating the rot, then C) it'll be a nightmare to do any structural repairs compared to just pulling down a bat. Also, D) it encapsulates any electrical or plumbing.
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u/ecg86 1d ago
I am trying my best to prevent any areas of failure for the roof but that is definitely my worry with the foam. If this were like the conditioned attic in my house I’d have less stress about the foam.
The roof is standing seam metal, fire and ice underlay on entire roof deck, zip sheathing with taped seams and I used lexel to seal every nail penetration prior to underlayment install.
The appeal to rockwool is everything you outlined and I can DIY install.
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u/Jumpin_Joeronimo 1d ago
Aside from the insulation type, it looks like the photo shows a large exterior door opening, correct? And that header goes to the underside of the TJI framing. If you are insulating those bays, also make sure you are blocking and sealing each bay above that header. From header to underside of roof deck. You need to seal off that ceiling area from the exterior area on the other side of the header.