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Building Science

Exterior Insulation on 2×4 Walls Versus 2×6 Walls With Cavity Insulation Only

A look at some modeling results may help you decide

What is the benefit of going beyond a 2x4 wall in a hot or mixed climate? And if you decide to do so, should you do continuous exterior insulation or a 2x6 wall?
Image Credit: Energy Vanguard

UPDATED on December 18, 2017 with a corrected energy savings table.

If you live in the world of 2×4 walls, as I do, you may have wondered about the savings you’d get by going to a more robust wall assembly. The typical house in southern climes has 2×4 walls with R-13 insulation in the cavities. The two ways to beef that up would be to add continuous exterior insulation or to go to a thicker wall. But which saves more energy? And how do they compare to the plain old 2×4 wall?

This question comes up fairly often in our HVAC design work. Clients want to know not only about how much energy they’ll save but also if they’ll be able to downsize the HVAC system. The former saves on operating cost, the latter on first cost.

Total R-value

A wall assembly is a combination of materials that allows heat to flow in both series and parallel. For this article, I’m going to show you the results I got for four different wall assemblies. First, I’ve set up a spreadsheet that calculates the total R-value of an assembly and then used that tool to calculate the R-values for these four walls. Here’s a summary of them:

The first one is your standard 2×4 wall with plywood or OSB sheathing. The second one is the base wall plus half-inch, R-3 continuous exterior insulation. The third one is the base wall plus two-inch, R-10 continuous exterior insulation. The last one is a 2×6 wall with R-19 cavity insulation and no continuous exterior insulation.

The best R-value is the one with 2 inches of exterior insulation. The 2×6 wall is roughly equivalent to a 2×4 wall with a half-inch of exterior insulation.

HVAC design loads

To see what effect these different R-values have on the size of…

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20 Comments

  1. User avater
    Stephen Sheehy | | #1

    Installed cost
    How does ZipR-12 compare with osb and 2" of exterior foam plus wrb on an installed basis?

  2. Antonio Oliver | | #2

    Question on Comment 1
    Is the WRB needed with 2" of exterior foam?

  3. User avater GBA Editor
    Martin Holladay | | #3

    Response to Antonio Oliver
    Antonio,
    A water-resistive barrier (WRB) is required by code. In some cases, rigid foam can be used as a WRB, but there are limitations on that type of installation.

    For more information, see Using Rigid Foam As a Water-Resistive Barrier.

  4. P Ditty | | #4

    Rigid foam bridging
    Martin,

    I'm wondering about the impacts of the rigid foam not being in complete (100%) contact with the underlying sheathing. My contractor is using 1x4 PT strapping fastened with screws to gusset the foam against the sheathing. In reviewing his work my experience has been that the center (unsupported) regions of foam bow out slightly say 1/16" - 3/16" of an inch. Does this gap present a problem in terms of the dew point the wall will be exposed to throughout the heating and cooling season? Of course, I'm worried about the wall surface (behind the foam and WRB becoming wetted should the dew point temperature be achieved. The wall in question is a old balloon framed 2x4 wall with blown in cellulose in the cavities. The exterior has upgraded with a vap-perm WRB and 2" of XPS.

    Thanks for your feedback and thoughts.

    Regards,
    Patrick

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Patrick Netreba
    Patrick,
    The "bowing rigid foam" problem you describe is rare, but I don't doubt your observation. The key to good performance is to make sure that there are no air leaks between the rigid foam and the sheathing. This can be accomplished by installing each rectangle of rigid foam with a bead of caulk at the perimeter, between the rigid foam and the sheathing.

    By the way: Furring strips say dry, so you don't need to use pressure-treated lumber for furring strips. Ordinary spruce, fir, or pine is just fine.

  6. P Ditty | | #6

    Rigid Foam Bridging
    Martin...thanks much. Appreciate it.

  7. Rob Hunter | | #7

    One more comparison, please?
    Awesome article, really sums up the options in an understandable way. And it focuses on warm climate building!

    The tables would be even better IMHO if they included the typical (for California, anyway) 2x6 with 1" of hi-R foam - which meets the Calgreen 2020 R value requirements and eliminates thermal bridging. Any chance you could add that configuration, Mr. Bailes?

  8. Rick Evans | | #8

    Roof and Air-tightness
    Great article as always- Allison!

    It seems that walls are often the most complex and expensive place to achieve lower u factors and therefore lower energy use.

    As you pointed out, these Btus would drop further if some slab edge and under slab insulation was applied.

    Roof/ceiling insulation can be cheap. An extra $1,500 of cellulose would likely bring the attic insulation to R-60 or more. Raised Heel trusses would add only slightly more. This would further reduce btus/hr.

    Finally, taping the sheathing seams and adding gaskets would bring these number down further. I suspect you could get the heat loads down to around 18,000 btus/hour using the 2x4+R10 wall assembly in GA. Now you can use just two small mini splits on each floor for heating and cooling. Money saved!

    Except not really because you'll need an ERV system :-)

  9. User avater
    Dana Dorsett | | #9

    orders of magnitude
    "a hundred" x $0.13 = $13.00, not $1.30

    I expect the intended meaning was a hundred dollars in annual savings, not a hundred kwh.

    However the annual loads seem to be off by two orders of magnitude, but probably not three. (The annual heating wall losses of 2400 square feet of 2x4 /R12 wall in Decatur GA will add up many therms, not half a therm! It might be ~50 therms, not ~500 therms or ~5 therms. )

  10. Stephen Cook | | #10

    Wall Heat Loss
    Q = U x A x (HDD +CDD)

    I think the number in that chart show the result of that formula, but to get BTU/hr we need to multiply that by X 24 .... doing that will get you in the $300 savings from the standard to a +R10 wall.

  11. Stephen Cook | | #11

    Confused by KWH Savings
    The article states

    "The bad news is that the BTU is a pretty small unit of heat. If I had shown how many kilowatt-hours those BTU savings add up to, the number would be only about a hundred"

    This is in the articles section of annual BTU usage for the wall assembly. Does this really mean at a rate of .13 per KwH its a $13 in annual savings from going form R13 2x4 to the best wall in the article, or am I missing a k for K somewhere?

  12. User avater GBA Editor
    Martin Holladay | | #12

    Updated tables coming soon
    I've been in contact with Allison Bailes by email on these questions. Allison acknowledges an error, and promises corrections soon.

  13. User avater GBA Editor
    Allison A. Bailes III, PhD | | #13

    Reply to Stephen Cook & Dana Dorsett
    When I first set up the spreadsheet to do the annual load calculations, I was surprised to find the biggest reduction to be 112 kWh/yr for the home in Phoenix. And yes, at a rate of $0.13/kWh, the annual cost would be $13. But that assumes you're heating and cooling with equipment that has a coefficient of performance (COP) of one. If you use a heat pump with a COP of 3, your savings would be only a third of that. (The carbon reduction, however, would be based on about the original number since you give that factor of three back because of power plant and distribution losses.)

    If you're heating with natural gas, the cost savings is even worse. At best, you save about 3 therms of gas. Here in Georgia, we pay less than 60 cents per therm so going from the standard 2x4 wall to a 2x4 wall with R-10 exterior insulation might save you two bucks.

    But Dana's intuition is correct. I couldn't find a mistake in my calculations when I first put the spreadsheet together last week, even though I checked and rechecked, not comfortable with how low the numbers were. But I couldn't find it.

    Having gone back and taken another look, I found my mistake. I forgot to multiply by 24 in that calculation with HDD and CDD. (That puts the results off by one order of magnitude, not two.) I'll get the tables corrected and reposted today.

    Thanks for calling me on this, Stephen and Dana. I was hoping someone would help me find the problem.

  14. Lee Harper | | #14

    Vapor barrier for ceiling?
    Hi, I live in northern Minnesota and I want to build a house with 2by6 walls and 2-2” Layers of exterior rigid foam. I was wondering if there was somewhere I could find the cost savings for my area. Also could someone point me in the right direction as far as the vapor barrier goes? I’m thinking the Sheetrock with paint would be all needed for walls, but what about the ceiling barrier? I am new to posting here, any help would be appreciated.
    Thanks,
    Lee

  15. User avater GBA Editor
    Martin Holladay | | #15

    Response to Lee Harper
    To calculate your annual energy savings, you'll need energy software like REMrate. If you don't have access to the software, you might want to hire an energy consultant or an energy rater.

    You can also calculate the difference in heat flow through each square foot of wall, and use that information (along with the number of square feet of wall for your home and heating degree days) to calculate the annual energy savings. But if all of this is new to you, you probably need some help from a consultant.

    You don't need a ceiling vapor barrier. What you need on your ceiling is an air barrier -- usually the drywall.

  16. Hugo Viens | | #16

    What would happen for our region, Montréal Canada?
    Because it would complicate the interpretation of the results, let’s not consider water vapour migration. Considering only the energy transfer aspect, it would be tremendously instructive for the industry up here to be faced with the results of your same exact models (which are all frequent wall assemblies seen around) for our zone 6 Montréal and Ottawa regions.
    I bet that would spark many debates around here!

  17. User avater GBA Editor
    Martin Holladay | | #17

    Response to Hugo Viens
    Hugo,
    Allison Bailes has provided the whole-wall R-values of a variety of wall assemblies. You can choose whatever wall assembly you want, depending on your R-value goals and your budget.

    If you are designing a house, you should be familiar with energy software that allows you to calculate how different wall R-values affect your energy bills. If not, you should talk to an energy consultant or an energy rater. This is "Design 101."

  18. Bruce John Stracke | | #18

    Controlling humidity in our mixed humid climate
    Allison...

    Great article as always, and thank you for addressing cooling rather than heating issues. Here in central Texas could it be a more efficient use of resources to address humidity directly, rather than reducing heat transfer?

    1— Many areas of the south have issues with termites and ants, both love to nest between sheathing and exterior foam.
    2— Managing humidity increases the comfortable temperature range, especially in cooling environments where the mean radiant temperature is more forgiving ‘while walking about naked.’
    3— Repurposing condensate is a resource that may be useful to maintain soil moisture for plants and foundations.
    4— It may be more efficient when considering costs and energy use over time to install a dehumidifier in mixed and hot humid zones than increasing insulation.
    5— Reducing heat transfer through the roof by adding passive cooling techniques like insulating at the rafters to create a conditioned attic and adding a second roof deck on 2x sleepers (which also reduces leaking and rot like a rain screen wall assembly) to act as a radiator may passively reduce heat transfer.

    Though sometimes I over think these things; how do we combine the best overall strategies to our conservation methods?

    Your comments, especially the pointing to the flaws of my thoughts, are most appreciated—

  19. Rob Hunter | | #19

    Can we infer anything from these numbers?
    Specifically, if a 2x4 wall with R-5 is 32% more BTU-efficient than a stock 2x4 wall in all 3 climates, is it likely that a 2x6 wall with R-5 would be 57% more efficient? Or, more importantly - is a 2x6 wall with R5 likely to be 32% more efficient than a naked 2x6 wall?

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to Rob Hunter
    Rob,
    We can't compare the "efficiency" of different wall options, because the word "efficiency" applies to machines or processes that involve work. Efficiency is the ratio of the useful work performed by a machine or in a process to the total energy expended or heat expended.

    We can, however, compare the R-value of different wall assemblies.

    Every time we double the R-value, we cut the rate of heat flow in half. Needless to say, it's easier to justify the cost of upgrading from an R-13 wall to an R-26 wall than it is to justify the cost of upgrading from an R-26 wall to an R-52 wall -- because the annual energy savings from the second example are less than the annual energy savings from the first example.

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