Correct way to calculate benefit of a thermal break
I am working on a school building that will be cooled by thermal mass and night-time ventilation.
The wall construction is 2×6 steel stud, 16″ OC. Thermal mass takes the form of a heavy plaster layer on the inside surface of the walls, plus a heavy slab.
Nominal cavity insulation is R-19, but this is reduced by thermal bridging, which is what concerns me. In our mild climate, no one would normally spare a thought for this in a mechanically cooled building. However, since we have no mechanical cooling and are relying on thermal mass storage to carry us through the warm afternoons, I am concerned that the heat gain from thermal bridging which is normally “in the noise” may become a significant issue as it slowly discharges our thermal mass from the outside in.
My questions are:
1) How to best calculate the impact of this bridging effect? Various different sources estimate a range of R-5 to R-10 for this wall. That’s a pretty big spread
and
2) Will a relatively thin thermal break significant mitigate the problem?
Most of the methods I have seen will treat a thermal break as another serial R-value to be added to the area-weighted R-value of the stud-and-batt assembly. Whether this is R-5 or R-10 (see previous question), a small amount of insulation (say R-0.5) doesn’t contribute very much.
However, I think this method underestimates the value of the thermal break. In reality, the weighted R-value of the stud-and-batt is really two conduction paths in parallel. One has a high R-value, while the other has a very low R-value. It seems to me that if I put even a fairly minor amount of insulation into the low-R-value path, it should add to the R-value of that path. A little goes a long way in that context.
e.g. If the area-weighted result of R-19 batt and R-0.01 steel is R-5, then the result of R-19 batt and R-0.51 steel-plus-thermal-break should be much, much better. I’ve improved the R-value of the short-circuit path by more than an order of magnitude. However, I’ve had trouble confirming that this is a valid approach.
So: Does this makes sense? If so, how do I best calculate the benefit of the thermal break to the overall wall assembly?
Thanks folks!
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Replies
Brent,
You are correct to be concerned
Have you seen this?
http://www.buildingscience.com/documents/insights/bsi-005-a-bridge-too-far/?topic=/doctypes/building-science-insights
Do you have a heating season?
Brent,
Oak Ridge National Laboratory has tested lots of steel-framed walls and measured their whole-wall R-values. These results may provide you some clues.
They provide a whole-wall R-value calculator here:
http://www.ornl.gov/sci/roofs+walls/AWT/home.htm
See also:
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/99/991110.html
http://www.ornl.gov/sci/roofs+walls/articles/wallratings/index.html
http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/steel_frame/index.html
The project site is the SF Bay Area in California -- a very mild Mediterranean climate. The upshot is that we have a mild but longish heating season. However, heating energy use is not my primary concern. I am more concerned about the ability to pre-charge the thermal mass during the cool summer nights, and then preserve that charge throughout the early part of the day.
I'm sure that thermal bridging is a marginal effect at best in our climate. However, because the cooling strategy is passive, I am concerned about even marginal impacts of this sort.
The main point of my question is to try to understand whether a very modest thermal break, which could be incorporated into the current wall design, will in fact restore most of the wall's lost R-value.
Brent,
I am bumping your question .... so others can see it ..who may have a numerical answer....
My understanding is that Steel will have a much greater 3 dimensional effect than wood framing.
If you have a heating load.... why not go for more than marginal or modest improvement in the thermal bridge?
It would be a two-fer... more comfort during cooling season....less energy during heating.
While a 0.5R improvement in the steel stud path is an order of magnitude greater, the thermal bridge is still more than an order of magnitude greater in conductivity than the cavity insulation, so it makes little difference.
It's correct that a thermal break merely increases the "serial R-value", as you say, of the area-weighted framing path. The variation you've seen (R5 to R10) for such a wall assembly is likely the difference between clear wall R-value and whole-wall R-value, which includes typical headers, doors and windows.
In order to realize a significant improvement in thermal performance, you'll have to incorporate a thermal break equivalent to at least 1" of foam sheathing. This would best be placed on the outside where it will reduce heat gain to the metal framing.
Brent,
I recently performed a HERS rating on a steel framed residential project here in Boulder, CO. I was having trouble reconciling the parallel path values with the REM/Rate calculated U-values - so I called around the corner to Architectural Energy Corporation, the software designers. I was urged to compare the REM/Rate results with the output of this calculator from ORNL and found my answers there.
http://www.ornl.gov/sci/roofs+walls/calculators/modzone/index.html
I hope that this will help you as well.