BACK PAGE BASICS: Thermal Bridging

Beginner
Double-stud 2-by-4 advanced framing reduces thermal bridging with little increase in lumber.
Thermal bridging can readily occur through this 2-by-6 window framing.

Thermal bridges might seem small, but they can be a source of significant energy loss. A superinsulated home is only as good as its resistance to heat flow, and that means addressing thermal bridging during the home’s design and construction. You can pack your home’s walls with insulation, but thermal bridging—places in which the insulation is thinner or nonexistent, such as across a sill or through a wood or steel stud—will cut into your home’s energy efficiency. As the 2013 California Energy Code states, “R-value is used to describe insulation effectiveness, but R-value does not describe the overall performance of the complete assembly,” i.e., the complete wall.

These thermal “short-circuits” primarily occur at the wall framing, where materials with low insulative value, such as wood or metal, help heat escape through the envelope. In conventional stick-frame construction, up to 25% of the wall area can be made up of framing members. The result? Suddenly, the R-value of your “well-insulated” stud-framed wall starts to slide. Add in details like windows and doors, and the whole-wall R-value commonly takes a nose-dive. (You can estimate a building assembly’s whole-wall R-value through Oak Ridge National Laboratory’s interactive calculator at web.ornl.gov/sci/roofs+walls/AWT/home.htm.)

The result is reduced energy efficiency, decreased comfort, and, on cold surfaces, the potential for condensate to develop. So how can you reduce thermal bridges in a home you’re building or one you’d like to retrofit?

  • Use advanced framing techniques, which reduce the amount of wood by increasing the spacing between framing members. For example, 16 inches on center becomes 24 inches on center for a stud-framed wall. A thermally broken double-stud wall can also be used with advanced framing.
  • Consider adding a continuous layer of exterior insulation, such as rigid foam or rock-wool (mineral fiber) board, over the wall before sheathing it.
  • Use an alternative wall system. For example, the wood I-joist splines in structural insulated panels are thinner than most studs, and panels are usually 48 inches on center (or more), which further reduces thermal bridging.
  • Implement proper insulation and thermal breaks around the foundation/slab. A well-insulated slab may mean two pours: one for the foundation wall and one for the slab, so you can provide a layer of rigid foam between the two components.
  • “Appendix JA4—U-factor, C-factor, and Thermal Mass”—of the 2013 California Energy Code provides detailed data and other ways to reduce thermal bridging for a variety of building-assembly types.

Comments (5)

Arthur Perrea's picture

Arthur Perrea,
I built my 1200sg ft. home slab on grade. I built 10.5 inch stagger double walls 24 on center with 7/16 OSB on the inside and outside and 1 in and 1/2 high r sheeting stagger on the outside so no seams , put 3in. blue foam on sill plate and fitted blue foam under windows , 6 mill moisture barrier on inside on at top of my 10 ft walls I built a 12in. box on the inside outer wall, so I could blow cellolose insulation in from my attic so I can add later if need be I also made cat walks though out. I put 24in in ceiling . My slab has two 2in layer of blue foam stagger so no seams . I use a geo spring HWHP on a open radiant floor system to heat last winter ,I live in upstate NY by Canada border . my heating bill was $ 89 a month using the regular hot water heater elemets , This year I plan to add more hot water storage so I can use just the heat pump and get my heating bill down to about $40 a month

Pondhockey's picture

(Neophyte here) how important is the wall to the integrity of the structure? Is it only certain posts/studs that are structurally important? In other words how do we get away with moving from 16 inch centers to 24 inch centers without compromising the structure?

Robert Pollock_2's picture

I'm a contractor of 30 years or so, with a dozen houses to my credit and many other projects too. Mostly in Canada, I moved to Alabama and built a house in 2002, and since worked in Virginia, Los Angeles, and now I'm in Palm Springs, in the desert.
The Canadian building code(s) are roughly 25 years ahead of the US codes I ran into, especially Alabama, which is still building houses according to the codes Canada used, in the '80s. The National Research Council (NRC) in Ottawa, ON, CA has all the information you could want regarding stick frame houses, super-insulation systems, and vapor and air barriers etc.. They've already made the mistakes the US is about to.
In 1982 or so, NRC decided to hire a 'dream team' of architects, University professors and other scientists to build 3 super insulated houses in the middle of Saskatchewan, where wind chill and winter are as severe as Moscow. They used Passive solar design which combined with the insulation levels, resulted in houses that needed no heat so keep from freezing. Great. Then they decided to put a family into one, to see what living in them was like and the whole project fell apart. No one had thought about how many times the dog wants out, nor what to do with all the stale air inside, generated by inhabitants. So they invented the HRV. (Heat Recovery Vent) which is basically two radiators back to back to exchange all the air in the house every hour, but not the heat.
Here in the desert we have a million (not exaggerating) stick-frame, slab on grade houses with stucco on the outside. Between LEEDs and articles like this one, everyone is stuffing fiberglass, blowing in cellulose, and caulking the heck out of every nook and cranny. That works, and is a terrific improvement but ultimately it can go only so far.
In my view, we should revert to passive design basics first. Make sure the house takes maximum advantage of it's location. Make windows bigger or smaller, the original architects didn't consider this. Analyze how air moves in your house and then supplement with good design those aspects that work in your favor, and close out those that don't. And add Thermal Mass or it's Phase Change Material equivalent. In hot climates keep that out of the sun, in cold climates put it in the sun. Ditto for foam insulation. In a hot climate the ground is our friend, keep it cool with insulation. Wrap all the stucco siding with 3" of Polyisocyanurate and then protect it from weather and the sun with a light weight metal siding, that reflects radiant heat. The Polyiso should be applied in overlapping layers, to give you the ultimate 'envelope' for these aging designs. Each window and door will need a 'build-out' to bring all vertical surfaces flush, both sides, but should be 'foamed' in place, which is easy. That keeps the air/vap barrier integral (complete) from the inside Finished Frame of the window, to the outside ambient air mass. No thermal bridging, anywhere.
The attic is another consideration and will have one or two passive 'compromises', trying to maintain the integrity of the air/vap membrane as it carries from the wall to the attic insulation but it can be done acceptably. Remove all the HVAC equipment from the attic. You'll be installed solar powered vents because that's next. Attics need 1/250 th of the floor area as open venting letting air in, down as low as possible (the eaves, or soffits) and again, another 1/250th of the floor area as open venting (discount the screens etc., the information is printed on the vents) letting air out either from the Gable Ends, or the Ridge. Essentially the roof is like an umbrella.
Around 1978, Canada reassessed how stick framing was done, and managed to eliminate roughly 25% of the framing pieces, (corner studs, framing 24" O.C rather than 16" O.C., eliminating headers over openings in non-load bearing walls, in favor of adding insulation to these places. 20th Century framing codes are mostly obsolete. Don't get me going on LEEDs, it's right up there with Kyoto of the '90s, or the European Union, as an example of top heavy bureaucracies that have good intentions, but are unworkable, impractical, or of no use, the the average home owner.
Now 75% of my designs use concrete or CMUs for load bearing walls, and that also contributes to the enveloped Thermal Mass. I've replaced about 30 lineal feet of 2x4 interior walls in my own house, with CMU's, which is messy but easy and cheap. I call them 'Heavy Houses".
By the way, I used Home Designer Pro for the house in 'bama. It was terrific back then. SketchUp is the bomb now.

Nelson Buck's picture

If you use 2x6 studs spaced 24 inches apart, the wall has about the same amount of wood in the studs as one with 2x4 studs spaced 16 inches apart.

Paul K. Hearsey's picture

Pondhockey, most homes are terribly overbuilt structurally. You can easily take your typical 1 or 2 story home and build it with studs on 24 inch centers. And don't forget that the sheathing, OSB or plywood, adds tremendously to the strength of the wall assembly.

I built a small home with double-stud 2 x 4 walls (inner wall, outer wall, 5 inches in between for a total of 12 inches) and would never ever go back to conventional building. These R48 walls, and R67 ceiling, resulted in a house that takes very little energy to heat. In the summers, when my neighbors cannot sleep because of the heat, our home is as cool as if it were air-conditioned.

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