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Community and Q&A

Thermal bridging

GBA Editor | Posted in Green Building Techniques on

Fine Homebuilding this month says that a double stud wall with 12 inches of insulation gives you a r-40 wall, I saw no mention of how much benefit can be expected from eliminating thermal bridging. Can anyone tell me in layman’s terms how many “r”s you gain by using modified balloon framing, smart stud framing, and using a double stud wall to eliminate thermal bridging? If I’m shooting for twice the r value of energy code, does the wall really have to be twice as thick?

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Replies

  1. gaspr | | #1
  2. Robert Riversong | | #2

    You're quite right that eliminating most thermal bridging is a way to dramatically increase a wall's thermal resistance without adding excessive depth. Unfortunately, the way it's most commonly accomplished is by adding exterior foam board, which has it's own set of problems and unintended consequences.

    But most double-wall framing systems have significant thermal bridging at the floor and ceiling assemblies, areas that are problematic to insulate and air seal well.

    The framing system with the least thermal bridging is the Modified Larsen Truss (or Riversong Truss), a system that I've been using for more than twenty years after much experimenting with double wall techniques. You can see my system at http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm.

    Insulated with dense-pack cellulose, my 12" thick wall has a whole wall R-value of 43, and R-45 with interior and exterior skins.

  3. homedesign | | #3

    Robert,
    Your framing is a work of art.
    Your link is one of my favorites.
    Talk about reduced thermal bridging.

  4. Michael Chandler | | #4

    I'm not really sure what minimum code is these days but the way we frame is we order pre-cut and assembled wall panels and it's very fast to dry-in (two weeks typically) virtually all scrap is recycled and the cost is low. the house we have in the green home section here is 12" r-46 Spider micro-fiber fiberglass.

    If you are looking for a cost effective R-30 wall a similar assembly of 2x4 exterior walls with a 2x4 strap at mid-wall and window sills to link the walls structurally across the thermal break and a 2x4 interior wall netted and blown full of Spider or Cellulose depending on your climate would yield 8.5" at R-30.6 I don't know what the adjustment is for the thermal break but clearly it is a major advantage esp if you can off-set your studs using a blown-in insulation system. The added cost to go to a 9.5" wall if you wanted to use a 2x10 top plate would be negligible but the 3/4" OSB upper top plate seems like a decent solution.

    Adding the extra interior walls on our 2,500 sf project (with ten foot ceilings downstairs and a 2x12 upper top plate rather than the plywood upper top plates I see in a lot of these systems - we're in a hurricane zone) cost $2,400 and adding the extra insulation was about $1,800 on the 11.5" walls we used. So it was a $4,200 upgrade to seriously jump the R-value and the trades all loved having all that room for their wires and pipes in the walls with much less drilling.

    We got stuck by our inspector who wouldn't allow us to put a 2x4 load-bearing wall on top of an ICF because it was only bearing 2" on the edge of the concrete with the balance being foam and the interior wall being designed as a non-load bearing wall. The solution we came up with at the time was to use a 2x6 exterior wall and reduce our thermal break to 2.5" but next time I'll probably look for a way to shift the load bearing wall to the interior.

  5. Will Goodwin | | #5

    Robert, seems as if your saying there is only 6-7% benefit from designing a wall without any thermal bridging, which would only be equivalent to less than one extra inch of fiberglass. Is my math wrong?

    Will Goodwin

  6. Robert Riversong | | #6

    You cannot make any blanket assesment of the savings by reducing thermal bridging, since the ratio depends upon the baseline wall system and the improved system being compared.

    For instance, a standard 2x6 16" oc wall with fiberglass batts (ignoring interior and exterior finishes), with the typical 23% framing ratio, would have a clear wall R-value of 13.4 (assuming the batts were installed perfectly, which is never the case). That same wall with no framing (i.e. no thermal bridging) would have a clear wall R-value of 18.7 - nearly a 40% improvement.,

    That, of course, is a nonsensical theoretical limit to eliminating thermal bridging - the fiberglass equivalent to a SIPs wall.

    But taking that same wall to 24" oc framing gives a 4% improvement, and going to OVE construction yields almost 10% improvement over the standard wall.

    Looking at superinsulated walls, a 2x12 stud wall 24" oc filled with cellulose would have a clear wall R-30.4, and changing that to a double-stud wall would improve the R-value by 26% to R-38.4 (ignoring the thermal bridging at the floor & ceiling assemblies).

    Going from a 2x6 16" oc wall filled with EPS foam board to a 5½" SIPS EPS wall with no framing would yield a clear wall R-value improvement of 49%.

  7. homedesign | | #7

    Just remember... many SIP installations do have solid framing "Studs" between panels,
    A top plate, and 2! bottom plates.... as well as a concentration of solid lumber around openings.
    Also the thermal bridge weakness becomes more significant as the overall R-value increases.

  8. Robert Riversong | | #8

    John,

    You're right about SIPS having themal bridges, which I ignored for the sake of simplicity, though I haven't seen any requiring double bottom plates, and many use splines rather than solid studs to connect the panels.

    But if the insulation thickness and the thermal bridge (framing) thickness increase at the same rate, then the ratio of weighted U-value remains the same as does the percent of thermal degradation.

    For instance a 5½" core SIPS without framing will lose 13.6% of insulating value with three 2x6 wood plates and studs 48" oc. A 7¼" core SIPS will also lose 13.6% of R-value with 2x8 framing in the same locations.

    Where the relative percentage of themal degradation increases dramatically is when adding R-value without adding the same percentage of thermal break R-value. A common example might be an attic with an uninsulated hatch, because the thermal resistance of the ceiling might be doubled while the thermal resistance of the hatch remains the same.

    So a 600 sf R-19 ceiling with a 10 sf R-1 pull-down attic stairway would lose 29.7% of its insulating value (compared to an ininterrupted ceiling) with an effective R-value of 14.6, while increasing the insulation to R-38 would increase the thermal degradation to 61.4% with an effective R-value of only 23.5.

  9. Michael Chandler | | #9

    Doubling the wall R-value costs less than adding a solar water heater.

    Our wall panel quote on a recent 2,474 heated sf home (www.greenbuildingadvisor.com/homes/nc-home-grabs-five-green-ratings) was $12,810 total, $5 / square foot of heated floor area for double 2x4 construction (and $6.15 w/ 2x6 exterior walls needed with 2” of exterior foam on the foundation). The total cost for the JM Spider R-46 formaldehyde-free micro-filament fiberglass insulation was $1.30 per SF and this was done in a day and a half. So the wall cost for our 2,500 sf house was $16,000 with the 2x4 exterior walls (and $18,500 w/ 2x6 ext. walls). By comparison the panel quote for a similar house with energy framed 2x6 walls was $3.40 / sf and the 5 ½” R-23 spider insulation quote was $1.37 so the up-charge to go from 2x6 w/ R-23 to 12” dbl 2x4 walls w/ R-46 was from $12,000 to $16,000.

    At $4,000 this is less than the up-charge for a solar water heater!

  10. Jay Walsh | | #10

    Has any one had experience with, comments about, this new thermal bridge product ThermaBlok (www.thermablok.com). It is a flexible aerogel/fiber composite material, 1/4" x 1 1/2" wide x 8' long. It is installed directly to the edges of studs, plates and windows in the wall framing, before the installation of drywall. It has a peel and stick backing for quick and easy installation and is easily cut to size.
    I see it priced at about $1 a running foot and estimate an added cost of $60 per10 foot section of typical wall. While they don't make any claims as to the products R-value, they do suggest an improvement of 40% to the overall wall R-value (UA).
    One advantage I see is that it does not increase the overall thickness of the wall by much (1/4 inch).

  11. Robert Riversong | | #11

    The Thermoblock website does chart its thermal conductivity which, converted to R-value, amounts to an incredible R-12/inch (R-3 for a ¼" thickness), though on their downloadable literature they claim only R-10.3/inch (R-2.575 for a ¼" thickness).

    Their claims of a 40% improvement in clear wall R-values is absurd. Adding Thermoblock to a 2x4 16" oc wall with 25% framing factor and fiberglass batts will yield a 15.3% improvement. Doing so to a 2x6 wall will yield a 12.6% improvement.

    Not bad, but for half the cost you could add 2" of XPS to create a much better thermal break over both the framing and the cavitiy insulation, and yield improvements of 99% (2x4 wall) and 72% (2x6 wall).

  12. Anonymous | | #12

    I have 2x6 studs in place at 24 in. OC. and will sheetrock within a couple of weeks. The thermablock product is very impressive. However, I can't afford the $1 per ft. material. What do you think about using a material like Sill Seal stapled to the studs. Since thermal bridging is largely heat traveling by conduction, it seems like there would be some benefit to breaking the path even though the R value would be minimal.

  13. Robert Riversong | | #13

    Anonymous (how about sharing your name and location?),

    The benefit of a thermal break is directly proportionate to the R-value. Sill seal (or bubble foil) will do next to nothing. And any thermal break over the framing will offer substantially more benefit if it also covers the cavity insulation.

  14. homedesign | | #14

    I thought that....
    One of the best concepts of this forum was that anonymous comments and alias names are not allowed.
    Why has that changed?

  15. homedesign | | #15

    Michael,
    Nice project and I did vote for #20

    These are friendly comments and questions.....
    I agree with your quest to upgrade the walls.

    Are you really considering all of the costs for your wall upgrade?
    Does your wall panel quote include interior walls .. or is it Exterior walls only?
    I assume a crane was involved? Is that part of the wall panel price?
    Did the framers charge considerable less than if it were common stick frame?
    Did you factor in an extra 75 sf of foundation cost and roof cost?
    Why did the 5-1/2" of spider cost more than 11-1/4"?
    Did you consider SIPS?

    Are you doing airtight drywall?
    Are you "netting" your walls before you blow?
    If so is netting part of the spider cost?

    Do you pay any subs by the square foot?
    I am guessing a house that size would grow about 150 sf by adding 6 inches all around.
    Adding 150 square feet would also increase the tax appraised value by at least $15,000 where I live.
    At around 2%.... property tax would cost the homeowner $300 extra a year (in North Texas)

    It looks like you are thermal bridge free in the general field of the wall...
    But I still see a lot of thermal bridging around the windows....
    especially the ganged windows.
    I think we can all learn something about thermal bridge reduction by looking at Riversong's work.
    See the photo labeled Windows on this page
    http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm

    You must have concluded that you did not need spray foam for the walls...why did you decide to use spray foam for the roof?
    Since you like spider...why not blow it in the attic and go for more and cheaper R-value "at the ceiling"
    Is it because you are using the attic for ducts?

  16. Jesse Thompson | | #16

    Robert,

    They claim a 40% reduction in thermal bridging when used with steel studs, not wood. I'm not trying to back up that claim, they just happened to mail us their marketing package as well.

  17. Robert Riversong | | #17

    Jesse,

    And, as would be expected from a purveyor of a "miracle" product, their advertising is misleading.

    What their website claims is a tested 42% reduction in heat loss on a 2x4 (25% framing factor) metal stud wall with blown fiberglass, which is one of the the worst performing wall assemblies, and with 3/8" of the aerogel strip - even though it's available only in 1/4" (which offered only 30% improvement, according to their site).

  18. Michael Chandler | | #18

    I agree with your quest to upgrade the walls.
    Thanks, the thing is, where do we go after we get the “low hanging fruit” what’s relatively easy and what’s more money than it’s worth especially as we get to HERS 50 and lower, and what assumptions can we make about future energy cost trends?
    I remember when gas was twenty six cents a gallon, I was seventeen years old, and making $1.70 per hour slinging bales and mending fences up in Maine. When Martin talks about Solar Thermal having a fifty-eight year payback I have to wonder what number he is using for a cost of fuel from year twenty to year fifty-eight. Is he anticipating that gas will still be $3.00 a gallon in 2068?
    I notice that Warren Buffett is buying railroads and T. Boone Pickens is into wind & natural gas. I would love to see a huge extraction tax on coal from mountain top removal and an elimination on federal subsidies for oil with gas at $4.50/gal or more. But I don’t think that we need a crystal ball to foresee that oil and coal are in for some volatile pricing in the near future.
    Are you really considering all of the costs for your wall upgrade? Does your wall panel quote include interior walls... or is it Exterior walls only?
    In this case the jamb extensions were Sheetrock so it wasn’t much of a cost increase for them. Other than that it’s pretty inclusive. Had they been stain grade FSC pine it would have been slightly more.
    Yes it does include interior walls, but they don’t change between 2x6 and dbl wall.
    I assume a crane was involved? Is that part of the wall panel price?
    No the walls are light enough that they are just carried to position.
    Did the framers charge considerable less than if it were common stick frame?
    The price is an installed package, It works out to about the same as my old framer stick building and me supplying all materials except nails but it happens faster, with no material hasles and MUCH better nail patterns.
    Did you factor in an extra 75 sf of foundation cost and roof cost?
    No
    Why did the 5-1/2" of spider cost more than 11-1/4"?
    Oh holy cowabunga dude! I owe you a beer big time here. I’ve been paying $0.65 to $0.70/ htd foot for R-23 Spider and that price has crept up over the past few jobs to $1.30 and somehow I never questioned it even when I typed it out in the recent reply above. My insulator has some splainin to do!
    Assuming that the competitive price for 2x6 spider should be $0.70/ htd foot that pushes the cost differential to $5,750.
    My HERS rater predicts that the wall will save only $74 per year, so increasing R-value beyond R-23 looks like a bad investment assuming that gas will never go over $2.50 per gallon. He projects that $6,500 on a solar water heater will save only $122 per year so that’s also a “bad investment”. I’ve been promoting conservation and clean energy since Clamshell Alliance days in NH back in the mid-seventies so I look at it as more walking the talk than looking for a smart investment and I think my customers do too.
    Did you consider SIPS?
    No, I really find it annoying to have to carve up the exterior walls to install all the wiring and plumbing stuff we put in our houses. Lots of smart home wiring to keep away from line voltage.
    Are you doing airtight drywall?
    Some of that still carries on from the old days but generally not so much. We header-wrap & bury the top plates and band joists in spray foam and header-wrap, caulk, & sill seal the bottom plates and seal the house wrap, attic and crawl unless we’re on a slab.
    Are you "netting" your walls before you blow? If so is netting part of the spider cost?
    Yes and it’s included in the price.
    Do you pay any subs by the square foot?
    No, they’re all pretty unique so its bid and go.
    I am guessing a house that size would grow about 150 sf by adding 6 inches all around... property tax would cost the homeowner $300 extra a year (in North Texas)
    Not something I give a lot of thought to, taxes aren’t much of an issue here in rural NC. I’m more annoyed when I lose green points for the house being too big because I have thick walls.
    It looks like you are thermal bridge free in the general field of the wall...But I still see a lot of thermal bridging around the windows....especially the ganged windows.
    No plywood boxes, just the inner and outer jack studs 2.5” apart blocked solid at the sills w/ Sheetrock returns. I don't see bridging there. Too many big windows maybe.
    I think we can all learn something about thermal bridge reduction by looking at Riversong's work…
    Interesting approach, sort of labor intensive and slow for my taste.
    You must have concluded that you did not need spray foam for the walls...why did you decide to use spray foam for the roof? Since you like spider...why not blow it in the attic and go for more and cheaper R-value "at the ceiling" Is it because you are using the attic for ducts?
    I have return air vents up there and some supplies under the roof garden. I like burying the top of my exterior wall plates in foam for air sealing. I like doing away with soffit and roof vents.

  19. Robert Riversong | | #19

    Clamshell Alliance? I was media coordinator in 1978/79. I helped organize the 1978 occupation-turned-legal-rally-cum-safe-energy-fair for 12,000, which brought Amory Lovins out of the woodwork and many other superstars. Those were the days!

  20. GBA Editor
    Martin Holladay | | #20

    Michael,
    Thanks for your very complete answer to John Brooks' questions. All very interesting.

    Your HERS rater calculates that when you install a $6,500 solar hot water system, the homeowner saves $122 per year. The quick and dirty, back-of-the-envelope calculation yields a simple payback of 53 years. There are many, many limitations to such simple payback calculations, and I'm well aware of them. Any simple payback calculation should be presented with a list of caveats, but those of us who discuss energy issues sometimes resort to shorthand communication and we just say "simple payback calculation" without all the asterisks and explanations.

    The 53-year simple payback calculation that your HERS rater came up with is consistent with the calculations made by researcher Robb Aldrich, who came up with simple payback periods of 58 years and 76 years for two solar hot water systems he studied in depth.

    The following factors will shorten simple payback periods for solar hot water systems:
    * Rising energy costs
    * Increasing use of hot water (up to a point).

    The following factors will lengthen simple payback periods for solar hot water systems:
    * Dropping energy costs (unlikely, but it happened from 1979-1994).
    * Decreasing use of hot water (i.e., increased conservation, or less use when kids go to college).
    * Maintenance expenses like failed pumps, leaking tanks, or glycol replacement.
    * Failure of major components before 53 years are up -- in other words, the need to replace the entire system before the 53 year goal is reached.
    * The opportunity cost of having $6,500 of capital tied up in a low-yield investment.

  21. homedesign | | #21

    Michael,
    thanks for the answers.... sorry about the deluge
    I will pace my questions out next time.
    Concerning my Robert Riversong comment.
    His construction techniques may not be wholesale ready for prime time...
    It is the attention to detail that impresses me.

    The window is already a weakness and it is also where the concentration of framing usually occurs.
    Windows are also responsible for driving up the framing fraction....more windows=higher fraction.
    I think we should look for ways to reduce framing concentrations around windows.
    One cheap solution for non egress windows would be to choose windows with 22-1/2" Rough
    openings.
    Another strategy is to locate the load bearing wall on the inside as Robert has done.
    And IF you like outsulation .. the BSC method that is featured in the current GBA video looks good to me.
    High performance enclosures may not "pay back" at todays energy prices...
    But I am convinced that we need to start building them SOONER.
    I admire what you are doing.

  22. Michael Chandler | | #22

    Robert,

    Yes I was at that Clamshell event in '78. Scrawny kid with very long blond ponytail, I brought a nine foot carved wooden sculpture of a tern, "Seabrook Shorebird", that was on a pivoting stand like a wind sock. I planted it in the way of the bulldozers and left it there.

    I wasn't in charge of anything back then other than carving cool things out of wood. Generally large representations of fragile wildlife. Now I just build houses, pays better and I get to work with cool collaborators and experiment on different "what-ifs". Plus I get to preach the "gospel of green" to the NAHB crowd which includes global warming skeptics, panicked McMansion builders, and aggrieved real estate appraisers etc.

    Martin,

    Sorry to pick on you there about the simple payback. I know that you are just as passionate about resource conservation as I am. But I have to deal with the sales process and choosing between energy conservation and upgraded cabinets and plumbing fixtures or a bigger house. So my customers aren't trying to choose between a high yield investment and energy conservation, they are choosing between adding more living space or upgrading finishes and energy conservation. And they are asking for a return on investment calculation on increased living space or fixtures, though they do anticipate a higher appraisal and better re-sale.

    So I feel pained when discussing simple pay-back calculations simply because I never get asked for the payback on cherry cabs and granite counter tops. I want to sell and build a better house and that doesn't mean more granite to me. (though I've been cutting and setting granite and travertine in a really cool bath room over the last couple of weeks so I'm not opposed at all to the alchemy of working stone, just with the energy conservation rather instead of it.)

    Payback is a sore spot for me. I'm betting on increasing energy costs and pushing very hard for permission to build smaller, more energy efficient homes.

    John,

    Thank you as always for the thoughtful and thought provoking questions and for your kind words. I really appreciate the fellowship and mutual support in figuring it all out that is present in these discussions.

  23. Michael Chandler | | #23

    My apologies about that rambling post, wish there was a way to go-back and edit, and a way to view more than five or six lines of text at a time when writing replys, and a way to go back to using real names, and no more aliases or anonymous posts. But I hear there is a new site design coming up, maybe these Christmas wishes can come true!

    Happy holidays to all of you. May your solar and radiant systems be trouble-free and your feet warm and children happy and spouses content this happy season.

  24. Anonymous | | #24

    I'm not a builder but Aspen Aerogel has a product that it claims to stop or dramatically reduce thermal bridging and can be compressed. It is applied between the studs and the sheathing.

    http://www.aerogel.com/markets/building-framing.html

  25. Michael Chandler | | #25

    Anonymous
    I think it's pretty clear that Aspen Aerogel is a top-secret formulation of Snake-Oil, Nano-Vapor technology, and R-equivalent greenwash marketing. Far more effective at reducing your bank account than your energy bill.

  26. bill jr | | #26

    i had put on a cold roof this in tales taking off the cider shakes to the space boards. then you need to take 2by3s and toe nail them in. start by taking the first one and nailing about 24" in from over hang then take a piece of 2"ridged foam then another 2by3 then all the other pieces of foam are 15 or 16 ". then on the other side of roof leave the last piece short of edge of over hang 24" and install the last piece of foam make sure their is a 2by3 on top of the edge and flush with the front of over hang. all you have to do is sheet the roof and install the shingles. i had to replace the face board any ways so i put on like a 2by10 cut a little short so it looks like you normally would install a normal face board. then i installed 3 layers of ridged foam in the inside of house caulking all the seams the installed the drywall. i put the first layer of foam then put wires in then the rest of foam. i know a place that sprays the foam and the problems that go with it. the problems that they have is that most places spray it all at once and that is wrong they are to wait 24hrs then the next coat wait 24hrs then the third and so on another is that the houses that are built so fast that the wood is still wet or damp that after awhile the foam pop from the studs!!! theirs your mold right their i use the dow blue color board R-10. the out side walls i use the 1" blue board then sided. the inside of the house i use the white batt "ins". i have not a drop of ice on mine house and heat 3000sq of it with 3 to 4 ton pellet stove and its warm as can be. the cold roof gets rid of the ice and the foam keeps it warm it is a story and a half from 1926 ballon framed and i also installed blocking between all the top plates

  27. bill jr | | #27

    one more thing that everyone should take note that the more you make your house less to heat or use or energy efficent the more you have to pay for that little amount of gas or "ele" its like the more you save the more you got to pay i should take all the "ins" and put old windows in and let the cold come in . the electricity and gas delivery charges go up the more we save they have to keep the same amount that they need every day or month so we have to pay more. so whats up with the rebates when you buy lets say a new tank less hot water system well the water department sees a drop in the usage and raise the bill. the rebate goes to them to make up the difference. thanks hope you read this one and share it with others it true buffalo new york

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