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Community and Q&A

R-13 in 2×4 walls

user-470028 | Posted in Building Code Questions on

I have a project in north Georgia (climate zone 4), where the minimum code-prescribed R-value is R-13 for walls. I’m stumped on a couple of issues:

1. Several fiberglass insulation manufacturers have products that claim an R-13 or R-15 in a 3-1/2″ wood stud wall. How do they achieve a R-15 in the same depth as they would an R-13? Do they just jam more stuff in the same volume? It seems that if it’s the same basic product in either case, it’s almost counterproductive to jam more stuff in the same volume, as the dead air is really doing the insulation work.

2. If I go with an open cell spray foam, it looks like I can only assume an R per inch of 3.5 or 3.6. At R-3.6 per inch, a 3-1/2″ studwall only get me up to R-12.6. The code minimum for my building is R-13. I realize I could always add a rigid insulation board to the exterior of the wall sheathing to make up the difference, but that’s not necessarily common practice in this area. Does this essentially rule out open cell spray insulation as a feasible option in this case? Do people just call R-12.6 good enough and move on?

3. At least in the residential portion of the IECC, the code prohibits computed R-values from including contributions from “other building materials or air films” (IECC 402.1.2). It only permits calculated R-value to come from what might be described as “manufactured insulation” products. Air films, plywood sheathing, etc don’t have a lot of insulative contribution, but they could theoretically get me over the hump from R-12.6 to R-13. But I’m not permitted to do that. Of course, by the time I account for the periodic wood studs in the cavity, I’m not even really getting the full R-12.6.

I feel like open cell spray foam is inherently a better product than fiberglass. I realize there’s plenty of debate out there regarding the relative “healthiness” of foam, fiberglass, cellulose, etc. I’m just having a hard time justifying the insulation values I can expect.

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  1. Expert Member

    My first suggestion is to find out how other builders in North Georgia are meeting this requirement, perhaps by approaching the local building inspector. That may save you a lot of legwork.

    Barring some other more elegant solution - while FG batts do lose insulating value by packing them beyond their suggested width, they do nontheless gain R value per inch. So that while a 6" batt in a 3 1/2" cavity does not give you the full R-20, it does give you more than R-15

  2. wjrobinson | | #2

    1- Somewhat denser glass batts do slow air movement (convection heat transfer).
    2-Spray foam slows air movement much more than batts of glass. 12.6=13 (rounding, remember mathematics.) And since much less air convects, spray foam will do a much better ACTUAL insulation job. But so will other methods such as spraying Spyder glass or dense pack cellulose or dense rock wool.... or gasketing via spray products or actual gaskets or tapes, or paints... to stop air movement within cavities that are insulated and between framing members such as sill plates, etc.
    3- 12.6=13 when rounded to a hole number. Remembering 7th grade math.

    And your last statement is spot on. I have a spray company I trust. That is a must if spraying foam. Also if anyone is sensitive to anything chemical forget spray foam. Also best to use spray foam in new unoccupied builds that have had time for the frame to lose moisture (slower build) IMO. Cellulose, rock wool, and Spyder may be good alternatives. Moving to a nice Hawaiian climate IMO may be the best of all. No need for insulation or even windows.

  3. Expert Member
    ARMANDO COBO | | #3

    If your intent is to build a minimum code house, R13 batt or OC foam is approved by the IECC code, and as long as your local code enforcement is 2009 IECC, then your are fine, and your local building inspector should answer that same way. Demilec OC fam at 3.5" is R13; Cellulose insulation at 3.5" is R13.3, and it should be your best insulation for your climate. See:
    If your intent is to build a good wall system, you should install R5 minimum outsulation, or rigid foam, on the outside of the sheathing; that will take care of thermal bridging and condensation in CZ4, especially in a humid climate.

  4. GBA Editor
    Martin Holladay | | #4

    If providing more air and less fiberglass always gave you more R-value, then you could just install a single fiber, like a length of thread, in each stud bay. But that doesn't work.

    Actually, more fibers provide a higher R-value -- up to a point. So, yes, R-15 batts are denser than R-13 batts, and they have a higher R-value, more fibers, and better performance than R-13 batts.

  5. user-1137156 | | #5

    Roxul brand mineral wool is r15 in 3 1/2" bats & almost as cheap as even fiberglass.

  6. user-1087436 | | #6

    To Jerry Liebler (or anybody else):
    Is that really true about Roxul being almost as cheap as fiberglass? I would love to use rock wool batts in the house I'm planning, but from the estimates I've made (admittedly very, very rough) the Roxul looks to be about 4 times the price per sf of the same size fiberglass batts. I'm in the Pacific NW, zone 4c. I'd love to find out that I'm wrong.

  7. user-1137156 | | #7

    Gordon you are probably not wrong but should use rock wool anyway. The lowest price is probably not the best value! Mineral wool is far greener and fire resistant as well as performing better and aging more gracefully. I priced Roxul at Lowes. It came out at $0.042/r sqft, about 2x their really cheap UN-faced fiberglass but slightly cheaper than their cellulose material only and less than half their cheapest foam. I've also gotten quotes from a distributor on 8 # density Roxul boards at about $.06/r sqft. very competitive with XPS or even EPS If you want "outsulation'.

  8. user-1087436 | | #8

    To Jerry: Great info! Thank you!

  9. user-470028 | | #9

    To clarify, the intent of my original question was to determine whether open cell foam alone can meet the prescriptive R-13 requirement in 2009 IECC, in wood framed walls in Z4. Of course there are improvements that could (and perhaps should) be done, like adding outsulation, etc., and I’m not discounting the value of limiting infiltration or dealing with thermal bridging. I interpret section 402.1.2 to say that “other materials” other than the manufactured insulation (I’d argue that perhaps the wood stud is one of these) can’t count toward R-value calculation. Does that make logical sense? No. Is that what the code says? Yes. Sure, 3.5” of R-3.5 per inch is 12.6, which some people may call good enough to be 13, if there are no studs in the assembly. But by the time you account for the stud itself, you have to reduce the net R-value. Studs at 16” o/c are going to account for about 9% of the wall composition. Even if you account for the slight insulating value of the wood stud itself (which again, I don’t think 402.1.2 says you can do), you end up with about R-11.7 net, which clearly doesn’t meet the minimum R-13. The Demilec OC is a good find (they claim R-3.81 per inch instead of others I’ve found at 3.5 or 3.6), but it still can’t get you to R-13 after you account for studs.

    So to me, the question is not about what’s good practice, whether company A or company B is better, or what makes the local official happy, but simply can generic open cell foam meet the prescriptive R-13 requirement in a common stick-framed wood wall. I’m all for understanding the actual science and behavior behind what’s happening, but on a very basic level I think you have to answer whether you’re meeting the basic code requirement.

  10. GBA Editor
    Martin Holladay | | #10

    Q. "Can generic open cell foam meet the prescriptive R-13 requirement in a common stick-framed wood wall?"

    A. Yes, because the prescriptive code requirement for wall R-values does not require builders or code officials to compute whole-wall R-values that discount the effect of the framing lumber. The reasoning makes sense: it's a way to keep the code simple.

    Of course R-13 insulation between the studs does not make an R-13 wall. However, it complies with the code.

  11. user-470028 | | #11

    Martin, that totally makes sense, and that's very helpful. Just to beat a dead horse, can you point me toward where the IECC says that?

  12. GBA Editor
    Martin Holladay | | #12

    It's the same section you quoted -- IECC 401.1.2: "Insulation material used in layers, such as framing cavity insulation and insulating sheathing, shall be summed to compute the component R-value. The manufacturer's settled R-value shall be used for blown insulation. Computed R-values shall not include an R-value for other building materials or air films."

    Also, see footnote c to Table 402.1.1: "The first R-value applies to continuous insulation, the second to framing cavity insulation..."

  13. wjrobinson | | #13


  14. wjrobinson | | #14

    Jamie, my bet is you are a doctor....

    Spray foam beats the pants off glass batts in actual use. Forget R especially code R.

    A better than code wall includes continuous R and airtight less air movement through any insulation choice.

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