I'm renovating a room, and I've got the drywall and old batts removed, and trying to decide between standard fiberglass batt insulation and foam board insulation. The room has two exterior walls and one wall facing an attached (but unheated) garage, so in effect, the room has three exterior walls. It's also the coldest/hottest room in the house during winter/summer. I'm located in Long Island, NY, which is at the northern end of climate zone 4 (mixed-humid). From the outside to the inside, the wall assembly is brick, exterior gypsum (apparently this was popular in the 50's and 60's in this part of the country), 2x4 framing, insulation, drywall.

Due to some repairs, I have a lot of sistered up studs, so the stud bays are narrower than standard 14-1/2" width. So I'll have to do a lot of cutting, regardless of whether I go with fiberglass batts or rigid foam board.

Given the air permeability of batts and how this room is the coldest/hottest in the winter/summer (with batt insulation), I'm thinking foam board might be a better choice. The Super Tuff-R available at Home Depot is 2" thick and is rated R-13. I'd glue a 1/2" shim of OSB to the 4 corners of the stud bay to create a little air pocket between the exterior gypsum siding and the foam board (so any moisture that gets behind the exterior gypsum can dry out more easily). I'd cut the foam board stuff to leave 1/4" gap between the board and stud bay and seal the gap using Great Stuff Window and Door spray foam. My understanding is that the Window and Door version will better flex with expansion/contraction of the studs.

As far as R-value, at R-13, the rigid foam is nominally the same as the R-13 batts, but the difference is (1) air sealing, (2) the batts require perfect installation, and with all the cutting I'd have to do because of the non-standard width stud bays, the installation is likely to be far from perfect, and (3) the batts R-value drops a lot at very cold temps. Plus 2" foam board in a 3-1/2" depth stud bay leaves 1-1/2" of air (1/2" between foam and exterior gypsum, and 1" between foam and interior drywall), which helps boost overall R value a little bit (not sure how to quantify the R-value of the air space). So I'm thinking real-life performance of the rigid foam will be far superior to fiberglass batts.

Going with foam board is a little more expensive than FG batts plus the cost of spray foam cans, but it's only one room, so not a big deal. If it makes this room warmer in winter and cooler in summer, it will be worth it.

I suppose I could air seal the stud bays and then install batts. I used PL Premium construction adhesive to seal the junction of the doubled up top and bottom plates. So I could use the same product to seal around the inside of all the stud bays and along the seams of the exterior gypsum. That's a lot of linear feet, and applying this stuff is more time consuming than applying Great Stuff spray foam out of a spray gun. Plus, you still have the R-value reduction that FG batts experience in very cold temps.

I've got ~150 sq. ft. of stud bays, so while cutting the rigid foam will be time consuming, it's one small room. I looked at two-part spray foam kits, and seems they come in packages of ~200 and ~620 board feet. Since I have ~150 sq. ft of stud bays and would want 2" depth, I'd need to buy two of the 200 board feet kits or one of the larger kits, and it would be ~$700 either way. Apparently once you open up the tanks, you have to use the stuff within 30 days, so I'd end up wasting material (or trying to sell it on Craigslist). I'd opt for a kit if it came with ~400 board feet and cost ~$500, but doesn't seem to be an option. Professional spray foam installers tend to have job size minimums to roll a truck and crew, so that's not cost effective for one small room.

What do you guys think of this cut and cobble approach vs FG batts? Or is there another approach I should consider?

I would use Rockwool Insulation batts. Much denser material than fiberglass batts. 3.5" batts = R15. Plus you get the full R value of the 3.5" stud and thus less thermal bridging than your approach with only 2" rigid foam depth. Plus, Rockwool is fireproof and a good sound damper.

Thanks, I'll go with Rockwool.

Cut'n'cobbled foam between framing is in most cases a waste of good foam, and reduces the drying capacity of the assembly. Don't do it. The ability to adequately air seal the foam to the framing that will stand the test of time is highly limited. The thermal bridging of the studs pretty much robs high R/inch goods of their potential too. The math on that lives here:

https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste

https://s3.amazonaws.com/finehomebuilding.s3.tauntoncloud.com/app/uploads/2017/07/05111418/cell-foam-chart-1200x1020.jpg

R13 polyiso will DRAMATICALLY underperform R13 fiberglass (or even R11 fiberglass) in an air-tight 2x4 wall, due to the
Blowing cellulose behind mesh is pretty easy as a DIY, and even with a 1-stage rental blower you should be able to achieve 2.8-3.2 lbs per cubic feet density. Unlike fiberglass or rock wool, the carbon footprint of cellulose is negative (it becomes sequestered carbon). The thermal mass characteristics of cellulose also delivers a lower thermal diffusivity than fiberglass or rock wool, and it will outperform them R for R during the shoulder seasons. For small projects landscaping fabric is good enough as a blowing mesh. Or, if you don't mind patching holes in wallboard, blowing it after the wallboard is up can work. That is sufficient density to mitigate against settling issues in a US climate zone 4 climate such as L.I..

The 2-part spray foam kits are pretty easy to use, but they're the opposite of "green building", primarily due to the HFC245fa blowing agent (now contraband in countries that signed on to the Kigali Amendment to the Montreal Protocol), but also due to the high polymer per R.

https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

If you can give up 0.75-1" of interior space, installing 0.75" - 1" polyiso over just the framing edges would more than make up for the added on thermal bridging of the sistered-on studs, cutting the thermal bridging losses by half. An unfaced R19 batt compressed to 4.5" would perform at about R15.5-R16, and is cheaper than a high density R15. At 2.8lbs density cellulose would deliver about R17 @ 4.5".

No matter what cavity fill, it's important to air-seal the framing to the sheathing the full perimeter inside each stud bay. Polyurethane caulk sticks well to old wood, and goes up fast if applied with a powered caulking gun.

Posted By Dana1 on 24 Oct 2019 09:38 PM
Cut'n'cobbled foam between framing is in most cases a waste of good foam, and reduces the drying capacity of the assembly. Don't do it.

Between the time and effort involved and the point re: increased thermal bridging via reduced stud R-value, I'm sold on not going this route, thanks.

Posted By Dana1 on 24 Oct 2019 09:38 PM
If you can give up 0.75-1" of interior space, installing 0.75" - 1" polyiso over just the framing edges would more than make up for the added on thermal bridging of the sistered-on studs, cutting the thermal bridging losses by half. An unfaced R19 batt compressed to 4.5" would perform at about R15.5-R16, and is cheaper than a high density R15. At 2.8lbs density cellulose would deliver about R17 @ 4.5".

I was thinking of going with 1/2" polyiso board across the entire interior of the wall. Easier than cutting it to cover just the studs, though pairing strips of polyiso over studs with slightly compressed R19 FG batts is an interesting idea!

Posted By Dana1 on 24 Oct 2019 09:38 PM
No matter what cavity fill, it's important to air-seal the framing to the sheathing the full perimeter inside each stud bay. Polyurethane caulk sticks well to old wood, and goes up fast if applied with a powered caulking gun.

Yep, definitely going to air seal, and I'm using polyurethane caulk. Might have to look into getting a powered caulk gun! Almost afraid to watch a video of it in action as I'm sure it will make using a regular caulk gun seem unbearably slow.

Maybe I can get your thoughts on this issue. This pic shows how the exterior sheathing (black) doesn't reach the top of the stud bay - it stops about 5" short of the top plate.

[img]https://i.imgur.com/ssCiHZy.jpg[/img]

If embedded link doesn't work, here's a direct link:
https://imgur.com/ssCiHZy

To air seal this area, I was thinking I'd cut some 1/2" foam board 6" tall and the width of the stud bay minus 1/2" (to leave 1/4" gap on either side), then seal that to the upper portion of the stud bay using Great Stuff.

Decent approach? Or is there a more effective and/or cost/labor efficient way to seal this area?

For the amount you're going to have to buy of the Great Stuff?  Just to patch and fix stuff in place?
Not to mention the potential for human error and farting up on the sealing?

It'd probably just be cheaper to get a spray foam kit.

Cut & cobble is okay for small areas of maybe a dozen bays.  Not three full walls.

$340, 200 sq feet of coverage.  That's a sealing pass, a filling pass and a bit left over for goofs/imperfect coverage/another small project (like spraying the underside of creaky stairs to lock them together)/etc
.
https://www.homedepot.com/p/Touch-n-Foam-200-ft-Board-Polyurethane-2-Component-Spray-Foam-Kit-4006022200/204962748

The price is fairly stable.  Ace/Lowes/etc in your area.

Only use the canned stuff (low expansion, NOT the "big crack filler") around window bucks.

As mentioned in my prior post, this thread has convinced me to drop the cut and cobble idea.

Re: spray foam kit, that's 200sqft at 1". I've got ~180sqft and to get 2" (which I understand I'd need in this area), I'd need two of those kits, so ~$700. Out of my budget for this one small room.

First, "...I've got ~180sqft and to get 2" (which I understand I'd need in this area)...", no, you don't need ANY foam insulation in your area for dew point control in wall assemblies.

If you're looking for a current code minimum R20 you could get there with 3-3/8" of foam in a 3.5" deep stud bay. It would meet code, but it would be an expensive waste. But if you were really going there (I wouldn't) 600 board foot kits run about $700, about the same as a pair of 200 board foot kits.

Second, a continuous layer of half-inch foil faced polyiso becomes a true vapor barrier, reducing the drying capacity toward the interior to near-zero. That reduces resilience compared to the framing-edges only approach. It might be "safe enough" depending on the exterior side stackup of sheathing & siding, as well as the roof overhangs and window flashing details, but in a zone 4A climate installing a true vapor barrier anywhere in the stackup is more likely to create problems than solve them.

Cutting 1.5" & 3" wide strips of sub-1" foil faced polyiso can be done very quickly & cleanly using a 4-5" steel wallboard taping knife that has been sharpened on the edges. See:

https://www.finehomebuilding.com/2009/01/29/theres-a-better-way-cutting-rigid-insulation

Finally, the non-continuous sheathing and several other aspects in that picture are problematic prompting a number of questions that may bear on how to air seal and insulate those stud bays.

*Is the sheathing asphalted fiberboard?

*What layers are on the exterior of the sheathing? (#15 felt tarpaper, board & batten siding? Brick? Cedar shingles?)

*It looks like there are electrical runs on the exterior side of the studs(?)


[edited to add]

I re-read the original post, which answered some of those questions. "From the outside to the inside, the wall assembly is brick, exterior gypsum (apparently this was popular in the 50's and 60's in this part of the country), 2x4 framing, insulation, drywall."

Gypsum board with asphalted fiberglass facers is highly moisture tolerant, but it's also highly vapor permeable compared to OSB or plywood (or even plank) sheathing. With brick cladding that ABSOLUTELY rules out using a continuous layer of foil faced foam on the interior, or you WILL get summertime condensation issues collecting moisture in the stud bays if the room is air conditioned!

That prompts another set of questions:

*Are there weep holes in mortar of the bottom course of brick, and corresponding vents to the outdoors at the top of the brick veneer?

*Is the gap between the gypsum & brick vented into the attic?

*How deep is the air gap between the gypsum & brick?

Dana, figured I'd number my responses to make it easier for you to respond since chopping up quotes can be cumbersome.

First, "...I've got ~180sqft and to get 2" (which I understand I'd need in this area)...", no, you don't need ANY foam insulation in your area for dew point control in wall assemblies.

1. Not sure I follow. Without insulation in the wall assembly, doesn't the dew point move to inside the wall assembly?

Second, a continuous layer of half-inch foil faced polyiso becomes a true vapor barrier, reducing the drying capacity toward the interior to near-zero.

2. I was thinking OC Foamular, which I thought was more vapor retarder than true vapor barrier.

https://www.homedepot.com/p/Owens-Corning-FOAMULAR-1-2-in-x-4-ft-x-8-ft-R-3-Square-Edge-Rigid-Foam-Board-Insulation-Sheathing-36L/100320356

Also, if the walls are air sealed (as is my plan), the amount of water vapor entering the walls should be small. Plus, can't it dry to the outside - as there is a clear path back out?

Posted By Dana1 on 25 Oct 2019 06:35 PM
Cutting 1.5" & 3" wide strips of sub-1" foil faced polyiso can be done very quickly & cleanly using a 4-5" steel wallboard taping knife that has been sharpened on the edges. See:

https://www.finehomebuilding.com/2009/01/29/theres-a-better-way-cutting-rigid-insulation

3. Definitely not a dealbreaker, and since you seem to think using full sheets of foam board on the interior is asking for condensation issues, I can modify the plan to using strips of foam board to cover only the studs.

Posted By Dana1 on 25 Oct 2019 06:35 PM
Finally, the non-continuous sheathing and several other aspects in that picture are problematic prompting a number of questions that may bear on how to air seal and insulate those stud bays.

*Is the sheathing asphalted fiberboard?

*What layers are on the exterior of the sheathing? (#15 felt tarpaper, board & batten siding? Brick? Cedar shingles?)

*It looks like there are electrical runs on the exterior side of the studs(?)


[edited to add]

I re-read the original post, which answered some of those questions. "From the outside to the inside, the wall assembly is brick, exterior gypsum (apparently this was popular in the 50's and 60's in this part of the country), 2x4 framing, insulation, drywall."

Gypsum board with asphalted fiberglass facers is highly moisture tolerant, but it's also highly vapor permeable compared to OSB or plywood (or even plank) sheathing. With brick cladding that ABSOLUTELY rules out using a continuous layer of foil faced foam on the interior, or you WILL get summertime condensation issues collecting moisture in the stud bays if the room is air conditioned!

That prompts another set of questions:

*Are there weep holes in mortar of the bottom course of brick, and corresponding vents to the outdoors at the top of the brick veneer?

*Is the gap between the gypsum & brick vented into the attic?

*How deep is the air gap between the gypsum & brick?

4. Trying to understand the summertime condensation issue with full sheets of foam board behind the drywall, assuming the stud bays are filled with mineral wool. So the room is air conditioned, and warm humid air wants to get inside the wall assembly. So I guess you're saying that to the extent the air sealing isn't perfect, the humid air will make it inside the cool stud bays and condense between the batts and interior foam board?

5. Yes, there are weep holes in the bottom course of brick. Not sure if there are vents at top of brick veneer, I'll have to check; but the brick veneer only extends up as high as the first floor. Some kind of aluminum(?) siding covering the second floor. There are vents in the soffit.

6. Hard to say for sure, but I think it's 1" to 1.5".

How about an unfaced foam board like this 1/2" unfaced XPS foam board?
https://www.lowes.com/pd/Kingspan-Insulation-Common-0-56-in-x-4-ft-x-8-ft-Actual-0-56-in-x-4-ft-x-8-ft-R-3-Unfaced-Polystyrene-Foam-Board-Insulation/1000235443
If link doesn't work, this is item # 511270 at Lowes.

Couple of ways I could go about using this:
1. cut patches to cover and air seal the open areas near the tops of the stud bays (sealed in place using canned foam). Then Rockwool, then place full sheet of this on the interior of the framing (instead of cutting strips to cover only the framing), and then drywall.
2. cut this XPS board to fill the entire stud bay (sealed in place using canned foam). Then Rockwool, which will stick 1/2" out past the framing. And strips of the XPS board to cover the framing to mitigate thermal bridging. And then drywall.

My understanding is that 1/2" unfaced XPS has a perm rating greater than 1 and would be considered a Class III vapor retarder. My very limited understanding of the vapor condensation issue makes me think in Long Island, NY (northern end of climate zone 4) neither of the two alternatives above would create a condensation problem. I like option 2 because it doesn't result in any compression of the Rockwool (in option 1, the Rockwool would be compressed where it presses against the patches of foam board). Would love to hear what you guys think, thanks.

So after much hemming and hawing and trying to understand the vapor diffusion issue, I've settled on the following:

1. use 1/2" XPS rigid foam board, which has a facing and a perm rating of 0.2, which makes it a Class 2 vapor retarder
2. cut this XPS board to fill the entire stud bay (sealed in place using canned foam)
3. then Rockwool, which will stick 1/2" out past the framing
4. then cut strips of the XPS board to cover the framing to mitigate thermal bridging (also creates spacing for 3.5" thick batts to fit without compression). And then drywall.

So as I understand it, this should do a pretty good job of air sealing as well as minimizing summer vapor drive into the wall, and any vapor that gets into the wall can easily dry out to the interior. And with an airtight drywall (I'll use polyurethane adhesive on the strips of foam board covering the wall framing to adhere the drywall to the foam) and take care to seal off electrical outlets, I should be able to eliminate air leaks, limiting winter vapor to that which diffuses through the drywall, which as I understand it, isn't much. And since the XPS is 0.2 perm, I'm guessing that vapor can dry to the exterior. Also, we keep it pretty cool in the house in the winter (high 60s day, mid 60s night).