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degnem 12-07-2009 08:34 PM

Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls?
I am in the process of completely remodeling the interior of my 1896 Minnesota home. I will be gutting down to studs. Minnesota requires R-19 in exterior walls. I would rather not furr out my studs to 2x6. Can I (according to code) acheive R-19 by stacking two, 2 inch, R-10 panels in the stud cavities? Would this work?

This seems like it would work logically. But I can't find a single similar example on the internet.

user1007 12-07-2009 09:19 PM

Check your fire codes! I would not even think of putting in the walls I work on even if could drywall over it under cover of darkness. Spray foam instead?

Scuba_Dave 12-07-2009 09:49 PM

It's used in basements all the time against the concrete, then the wall studs
There are different types, not sure which would be the one to use
Cutting into 16" strips would be a pain
Then you need to make sure it is sealed correctly around all edges
Do you have 4" ?
Newer 2x4 walls are only 3.5" deep

gma2rjc 12-07-2009 10:33 PM

I'm not questioning the accuracy of your answer Sdsester. I'm just curious as to how spray foam would be any different than using 2" extruded foam boards and maybe sealing around it with Great Stuff. Maybe it would be cheaper and faster to spray by the time the foam board and Great Stuff is purchased?

As stated, cutting through the 2" is a pain. I bought an electric foam cutting knife that cuts (melts) through it. But it works VERY slowly and can't be hurried. Tried a serrated edge knife first, but that didn't work well at all.

Daniel Holzman 12-07-2009 10:50 PM

Foam cuts beautifully on a table saw, I finished my basement using extruded pink styrofoam, cut it all on the saw with a carbide blade, no problems.

gma2rjc 12-07-2009 10:59 PM

That's good to know. I have to cut two squares of it for my mom's attic hatch. Thanks!

pyper 12-08-2009 12:25 AM

R-Matte makes foam that's R-8 per inch. It's rated to install in your walls. So you use one inch of R-Matte and R-11 batts, or 2 1/2 inches of R-matte.

ahmed 12-08-2009 09:40 AM

rigid foam is a pain to cut and fit. I used a table saw which made it a little better.

Chucky Jesus 12-08-2009 11:02 AM

Cutting rigid foam board...
I used 1.5" board to insulate my garage door. Scored it with an X-acto blade and snapped it, just like drywall. Sealed with great stuff. :thumbsup:

Maintenance 6 12-08-2009 12:25 PM

Use a layer of really good foam and fill the rest with fiberglass. It's all concealed in the stud cavity, so it's not a hazard. I always cut it on a table saw. Stick it fast to the back of the siding/sub-siding with some adhesive or cut it snug for a friction fit. Try not to get it so tight that it bows in. You really want to "avoid" having "a void" :laughing: behind it for air to travel.

degnem 12-08-2009 03:25 PM

Thanks for the replies. These are TRUE 2x4 studs, not 3 1/2. So, it sounds like it would pass code and acheive R-19 (at least technically)? Also, don't the sheets come prescored for easy snapping?

pyper 12-08-2009 10:06 PM


Originally Posted by degnem (Post 363550)
Thanks for the replies. These are TRUE 2x4 studs, not 3 1/2. So, it sounds like it would pass code and acheive R-19 (at least technically)? Also, don't the sheets come prescored for easy snapping?

I have true 2x4's also. I put the R-matte 1/2 inch product in and screw it in place. I'm worried that over time it will warp or twist, so I use the screws (with washers) to prevent that.

R-matte is not scored.

My studs weren't particularly plumb, so I measured the top and the bottom and then cut the foam with a utility knife and an 8' straightedge (Home Depot sells it for cutting sheet materials with a circular saw). I also found that my studs weren't particularly straight, so usually they either didn't fit tight or they wouldn't go in. I either trimmed them or stuffed more material in. Sometimes I used Great Stuff foam -- like where there were gaps or angles or other funky things.

If you do this job in the winter, it's just amazing how much better the room feels after you get the rigid insulation up. And then how much better again it feels when you add the fiberglass batts.

Gary in WA 12-09-2009 01:52 PM

"Reservoir claddings on the exterior of buildings can be a problem. Reservoir claddings are materials that can store rainwater—sponges that get wet when it rains. Once the reservoirs get wet, the stored water can migrate elsewhere and cause problems. Common reservoirs are brick veneers, stuccos, wood siding, wood trim and fiber cement cladding".

"Avoidance of the installation of vapor barriers on both sides of assemblies—i.e. “double vapor barriers” in order to facilitate assembly drying in at least one direction."

"Vapor diffusion from the interior can be a concern in cold climates and is typically a concern in very cold climates. Vapor diffusion retarders, when specified in cold climates and very cold climates, are located towards the interior of the thermal insulation. When vapor retarders are used, walls and other building assemblies are designed and built to dry to the exterior, should they get wet or start out wet." From:

Find the permeability rating of the foam board as it may be 1.0 per inch x 4 = a vapor barrier inside the wall, not on the warm side. (As you double the inches, you 1/2 the rating).

Find your climate here:
Be safe, Gary

degnem 12-10-2009 04:15 PM

I'm not sure I completely understand what you are trying to say in that last post. I don't see how 3 inches of polyiso or 4 inches of EPS would be any different, vapor barrier wise, than 3 1/2 inches of spray foam. All should act as a vapor barrier on the warm (inside) of the wall. Right?

Gary in WA 12-10-2009 08:55 PM

If your B.D. requires a vapor barrier on the inside, then it wouldn't matter much what kind of foam you used.
This from the last site I gave: "In cold climates, air barriers and vapor retarders are installed on the interior of the building assemblies. And building assemblies are designed to dry to the exterior by installing permeable sheathings and building paper or housewraps toward the exterior (Lstiburek 2004). Unless specifically required by local building code, a polyethylene vapor retarder (between the framing and the drywall) is not recommended because it limits a wall's ability to dry to the inside. In a wood frame wall with carefully installed batt insulation, the kraft facing on the batt would provide the necessary protection against vapor diffusion from interior sources for this climate. In the case of other exterior wall assemblies, drywall painted with latex paint suffices (Broniek, 2003)." If you don't need a v.b. according to the map and it will dry to the inside without one. But not with certain foams: Spray closed-cell = 1.8 perms per inch....3-1/2" = .51 perms Tuff R rigid foam = 2" - .015 perms Foamular 150- 2" - 5.6 perms. So it depends on if you need a v.b. or not as to which way the wall will dry- outward or inward. The drywall is your air barrier, what does your building department require for vapor barrier or vapor retarder. The foam you choose can be a vapor retarder or a barrier. Notice the different classes of vapor retarders in the first post from Building Science.
Be safe, Gary

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