# Foam board insulation instead of fiberglass batts for stud bays?



## Windows on Wash (Aug 30, 2011)

Air seal the walls and insulate with high density FG. At a certainly point, you aren't loosing that much more there an R-13/19. 

Air seal the attic too.


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## Yodaman (Mar 9, 2015)

I wouldn't bother with the rigid foam unless you want to take advantage of the extra space and double up those 2" sheets.

FG cuts and installs real easy. When I have odd size cavities, I separate the paper from the FG, cut and install the FG. Then staple the uncut paper over it. Does a little better job sealing. 


BTW your link did not lead to a product. Typically 2" high density is R10, I was curious if you actually found a R13.


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## jbrah (May 29, 2018)

Yodaman said:


> I wouldn't bother with the rigid foam unless you want to take advantage of the extra space and double up those 2" sheets.
> 
> FG cuts and installs real easy. When I have odd size cavities, I separate the paper from the FG, cut and install the FG. Then staple the uncut paper over it. Does a little better job sealing.
> 
> ...


Not sure why link broke. But here it is:
https://www.homedepot.com/p/Super-T...-R-13-Insulating-Sheathing-99060464/300528092

Edit: this link isn't working either. Maybe a forum issue?

If you go to Home Depot site and search "2 in. x 4 ft. x 8 ft. R-13 Insulating Sheathing", it will pop up. And yes, it's R-13. They do have R-10 for a bit less money.

I could do the FG batts, but that's what was in the walls before (though some of it had sagged). If I use FG, I'll get pretty much the same result - i.e., it will still be the coldest/hottest (winter/summer) room in the house.


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## Bud9051 (Nov 11, 2015)

The polyiso is r-6.5 per inch.
You mentioned some sistered studs which could use a layer of rigid over the inside to reduce the thermal bridging.

As for a cavity fill, as mentioned picture frame each cavity with some construction adhesive (large tubes less expensive) for air sealing and then fit Roxul mineral wool for r-15. You can skip the plastic vapor barrier as long as inspectors are ok. I have links that say it isn't needed or desirable. But it won't kill the wall either.

There are hidden seams in top and bottom plates under studs sogo crazy with the caulking. The importance of stopping all air flow through those walls can't be over stated.

One in of rigid over the entire wall before the drywall would add r-5 for an r-20 wall, pretty good.

If there is a basement or crawlspace some caulking around the perimeter will warm the floors. Then check the attic.

Bud


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## jbrah (May 29, 2018)

Bud9051 said:


> The polyiso is r-6.5 per inch.
> You mentioned some sistered studs which could use a layer of rigid over the inside to reduce the thermal bridging.


Yeah, that's the plan - 1/2" of rigid foam across the interior of the framing to act as a thermal break.



Bud9051 said:


> As for a cavity fill, as mentioned picture frame each cavity with some construction adhesive (large tubes less expensive) for air sealing and then fit Roxul mineral wool for r-15.
> 
> There are hidden seams in top and bottom plates under studs sogo crazy with the caulking. The importance of stopping all air flow through those walls can't be over stated.


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.

Since I'll have to cut the FG anyway, why not just cut rigid foam instead and then seal using Great Stuff using a spray gun? Seems like it wouldn't be much more work, and I'd have > R-13 in the stud bays (R-13 of the 2" foam + R-value of 1-1/2" air pocket) even in the dead of winter (where FG would be less than it's nominal R-13 due to very low temps, and even less than that because getting R-13 requires perfect installation).

Or put another way, is there any reason to prefer R-13 FG over R-13 rigid foam (+ additional R-value from 1-1/2" air pocket)?


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## carpdad (Oct 11, 2010)

If you take your time cutting and air seal the exterior sheet, all are good choices. 

That room being most hot/cold, maybe there were other problems. HVAC, ceiling above, windows, siding itself as well as weather movement outside. I used to live in a house where there was strong west to east winds, often. Which way the sun travels.



About that empty space, it will not give any insulating value. Air will not stay still, even if it is hot/cold spots creating currents. As long as the cost is not a problem, I would use 3 layers of xps for r15 or something like it. Use the whole cavity for most r value. Or 1/2" xps against the sheathing for air seal and batts or such. I'm not sure about r6.5 polyiso sheets. I thought they smell/offgas more. Also if ok, use 1" sheets. 2" sheets will be harder to cut. Coping saw should help. 

About electrical boxes, i assume you're moving them? I thought maybe there was 1" rings for them but just guessing.


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## Yodaman (Mar 9, 2015)

jbrah said:


> Yeah, that's the plan - 1/2" of rigid foam across the interior of the framing to act as a thermal break.
> 
> 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.
> 
> ...




You seem to have made up your mind so this may be of little value.

First, no need to use PL construction adhesive for air sealing at $5 per tube. A simple acrylic latex with silicone at $3/ per tube is adequate.

Next, leaving voids in cavities is not a good idea. Cold or hot air will collect there contributing to a cold or hot surface on the drywall. The problem is that it will be cold air in the winter and hot air in the summer. 

May even cause condensation if cold enough.

The FB R-value reduction you speak of, relates to blown in insulation, not FB bats. https://www.askthebuilder.com/insulation---fiberglass-and-cellulose/

And last, cutting FB is super simple. All you need is a utility knife and stapler. 

Cutting foam. can be done, but will be far more time consuming than you realize. Long rips can be done with a table saw. (makes a ton of dust)
With the foam board there will be a fare amount of waste. 

Using FB bats there will be little to none.

If money is not a concern, than consider having the cavities spayed foamed with high density foam. Doing this will air-seal and insulate.


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## jbrah (May 29, 2018)

Yodaman said:


> You seem to have made up your mind so this may be of little value.


It's not so much that I've made up my mind, but rather I don't see the compelling case for FG batts over rigid foam. My experience with FG batts has me looking for alternatives, though admittedly, the cold/hot issue in this room could have been all driven by lack of air sealing, rather than the batts that were here (of course the fact they had fallen a bit, leaving the tops of the stud bays uninsulated surely contributed).

I'm just afraid of replacing old batts with new batts and finding the room's insulation performance hasn't improved. Again, air sealing may make all the difference, so maybe I need to stop associating the room's poor insulation performance with the batts.



Yodaman said:


> First, no need to use PL construction adhesive for air sealing at $5 per tube. A simple acrylic latex with silicone at $3/ per tube is adequate.


I'd seen different products mentioned in this context, but the one I saw most frequently mentioned was PL Premium construction adhesive, which is apparently supposed to flex a bit with the framing's contraction/expansion. For a couple of bucks a tube, it's not worth spending more time researching this. And it comes in the large tubes, which works out much cheaper than buying it in the small tubes.



Yodaman said:


> Next, leaving voids in cavities is not a good idea. Cold or hot air will collect there contributing to a cold or hot surface on the drywall. The problem is that it will be cold air in the winter and hot air in the summer. May even cause condensation if cold enough.


I had seen the recommendation for an air pocket between the sheathing and the rigid foam in a few places, including Green Building Advisor, though I wasn't clear on why.

Thinking it through, in the winter, that pocket between sheathing and rigid foam will get cold. Not an issue as it relates to condensation, as far as I can tell, though I guess it would suck more heat out of the framing than if the rigid foam was pressed right against the sheathing. And since I'm considering 2" rigid foam, there will be an air pocket between the foam and the drywall that will have warm air. That air could condense, but with R-13 foam, my understanding is the interior side of the foam won't be cold enough for condensation.

In the summer, the situation reverses, and again, the insulation prevents the exterior face of the insulation from being cold enough for the warm air to condense on it.

All that said, I don't understand why some are recommending the air gap between rigid foam and sheathing. If the foam has enough R-value to prevent it from being a condensing surface in the summer, then I don't see the benefit of separating it from the sheathing.




Yodaman said:


> The FB R-value reduction you speak of, relates to blown in insulation, not FB bats. https://www.askthebuilder.com/insulation---fiberglass-and-cellulose/


Thanks, that's helpful. So ok, that's not an issue with batts. Interestingly, the Super Tuff-R rigid foam I was considering is polyiso, which performs WORSE in cold weather. So even though it has a higher nominal R-value than XPS rigid foam, the XPS has a higher R-value at low temps.

https://www.greenbuildingadvisor.com/article/in-cold-climates-r-5-foam-beats-r-6



Yodaman said:


> And last, cutting FB is super simple. All you need is a utility knife and stapler.
> 
> Cutting foam. can be done, but will be far more time consuming than you realize. Long rips can be done with a table saw. (makes a ton of dust)
> With the foam board there will be a fare amount of waste.
> ...


Points taken.

What about rock wool/mineral wool? Seems like it installs as easily as FG but better performance if it gets wet. And it's R-15 in a 2x4 cavity.

And of course with this, just as with FG, I'd seal the perimeter of each stud bay (as well as the seams between sheathing that are exposed between the studs) to try to get it air sealed.



Yodaman said:


> If money is not a concern, than consider having the cavities spayed foamed with high density foam. Doing this will air-seal and insulate.


Yeah, this is the best performance option, but the problem is everyone wants a minimum job size to roll out a truck and crew. So for my small job (they can spray these walls in 30 minutes), I'd end up paying a lot. If I had a lot of exterior walls open, I'd definitely go with spray foam.


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## Yodaman (Mar 9, 2015)

Cutting Roxul is almost as easy as FG. But cuts best with you favorite serrated knife.


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## Bud9051 (Nov 11, 2015)

Be sure any cuts in the mineral wool provide for a tight fit, say an extra 1/4 to 1/2 inch. IMO, once you start using something like Roxul, you won't go back to the fiberglass batts.

Sorry for not keeping up, threads like this get too long for me, but wanted to add. A typical r-13 cavity wall is only about r-9 dues to framing. Add in common voids and modest attention to neatness and r-8 or less is appropriate. Then account for air leakage which further degrades both short term and long term insulation performance and we are lucky to end up with half of that cavity rating. This points out the benefits of adding a continuous layer over either inside or outside as it adds its full value to the entire wall.

If you do the detailed air sealing mentioned and a very neat install on the insulation, plus a better insulation like mineral wool, you can more than double the performance of those walls and it is affordable.

Bud


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## CanadianRockies (Nov 24, 2019)

Yodaman said:


> Cutting foam. can be done, but will be far more time consuming than you realize. Long rips can be done with a table saw. (makes a ton of dust)


I've read that an electric carving knive also works well for rigid foam insulation. Haven't tried it, though.


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## Old Thomas (Nov 28, 2019)

My house is old and has studs that are 4” deep. When I was gutting it out, I had access to unlimited 1/2” and 3/4” Dow Styrofoam in half sheets. I cut it to fit between the studs and caulked it in place. Then I still had room left for batts. It is a very tight and nearly sound proof house.


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