# Using rigid foam insulation to acheive R-19 in true 2x4 exterior walls?



## degnem (Dec 8, 2009)

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.


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## user1007 (Sep 23, 2009)

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?


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## Scuba_Dave (Jan 16, 2009)

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


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## gma2rjc (Nov 21, 2008)

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.


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## Daniel Holzman (Mar 10, 2009)

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.


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## gma2rjc (Nov 21, 2008)

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


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## pyper (Jul 1, 2009)

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.


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## ahmed (Dec 8, 2009)

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


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## Chucky Jesus (Jul 21, 2009)

*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:


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## Maintenance 6 (Feb 26, 2008)

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.


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## degnem (Dec 8, 2009)

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?


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## pyper (Jul 1, 2009)

degnem said:


> 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.


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## Gary in WA (Mar 11, 2009)

"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: http://www.buildingscience.com/docu...rol-for-new-residential-buildings?full_view=1


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). http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11620
http://www.panhandleinsulation.com/buildingmaterials.html

Find your climate here: http://www.google.com/imgres?imgurl...mage_result&resnum=1&ct=image&ved=0CAoQ9QEwAA
Be safe, Gary


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## degnem (Dec 8, 2009)

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?


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## Gary in WA (Mar 11, 2009)

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. http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810 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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## degnem (Dec 8, 2009)

I'm still confused. Your post seems to contradict itself.

"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..."

"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"

Living in Minnesota, with somewhere around 8000 heating degree days, a vapor barrier is necessary on the warm (inside) of the wall. Thus, it would dry to the exterior. I'm not sure what you are getting at with your post. Are you saying that foam (interior) would not be wise in my climate? Or just that IF foam is used (on the interior), an additional poly VB is not necessary?


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## ccarlisle (Jul 2, 2008)

I am confused too - not by the science of insulation but by those cut-and-paste posts that Gary seems to provide as typically they are taken out of their context and a link is commonly lost. I respect Gary's input, it's always helpful, but sometimes a bit extra is needed. Let's see if I can help:

_"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..."
_
A 'building assembly' is basically the combination of exterior cladding, building paper, walls, studs, drywall or any combination of such elements. In cold climates, you install vapour barriers or vapour retarders on the warm side of the building assembly i.e indoors. Now, in cold climates usually the heating system causes a positive air pressure on the inside of your home which causes the warm moist air outwards. So we say the assembly 'dries to the outside'. That's for cold climates....

_"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"
_
Now in some climates, "vapour barriers" are replaced by "vapour retarders" and can sometiimes be placed elsewhere from what we do in cold climates. 

Finally, if you spray-foam the inside of your walls, there is no need for a vapour barrier because spray-foam (closed) is a vapour barrier in itself. If you use any other type of insulation, then there would be a need for one...

Hope this helps...


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## Energyrater (Dec 11, 2009)

Hello,

I am assuming from your post that your building inspector is referring to compliance with the latest International Energy Conservation Code which would be 2006 or 2009 depending on what your state has adopted. Without knowing where in Minnesota you are, I can say the Minnesota straddles climate zone 6 and 7, 6 being the southern and 7 being the northern.

Your building inspector is likely requiring compliance with the prescriptive chart in chapter four which says that minimum wall insulation for zone 6 has to be EITHER R19 cavity OR R13cavity +R5 continous. The first being a typical R19 Batt or blow and the second referring to an R13 cavity batt or blow + an R5 insulated sheathing on the outside. Without knowing how extensive your remodel was, I assume that you cant do th R5 sheathing on the outside. So, you need to make your R19 on the cavity alone. You might have to spray an inch of foam (or rigid) and then do the remaining area with an R13 batt (some compression though). Depending on the foam type, you may end up with an R19 cavity. If you strip the exterior sheathing (or boards) as part of your gut, then just go with the R13 cavity and put up the R5 insulated sheathing on the outside of the studs.

Hope that helps or if nothing else provides a bit of education as to what the inspector expects.


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## degnem (Dec 8, 2009)

I appreciate all the replies. I love to learn as much as possible. But, I guess I should be more blunt about what I would specifically like to know.
*
What is the cheapest way to achieve R-19 in a TRUE 2x4 exterior wall, without removing or covering exterior lap siding?*

Currently there is no insulation of any kind. And, all stud walls will be completely exposed from the interior.


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## ccarlisle (Jul 2, 2008)

IMO, foil-backed polyisocyanurate sheets, at R7 per inch, is the only way you'll get R19 in a 4" deep cavity - and still have some space to spare. Three sheets stacked on top of the other...

Two 2" XPS sheets will reach R20 but at _exactly_ 4" thick - but then you might have trouble putting the gyproc on.

I don't know what materials cost in your town, so can't say. But if those are the only criteria, then you don't have much choice in getting R19 in 4" deep. That's why they're now doing to 2x6 walls, where getting R19 is a lot cheaper to reach...


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## pyper (Jul 1, 2009)

If you could give up 3/4 inch you could fur out the wall and have icynene foam sprayed in (they have a foam that's R4 per inch). I don't know how much that option would cost in comparison to the rigid sheets. But it would sure be a lot faster.


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