Effect on thermal bridging through metal studs when coupled with exterior insulation
Jory_H
| Posted in General Questions on
I’m interested in building a home with metal stud walls. I realize that any insulation in the stud cavity will have reduced performance due to thermal bridging. What I can’t seem to find is the effect that a layer of continuous exterior insulation will have on this. For example: If a metal stud wall with no exterior insulation has a framing factor of 50%, what will the framing factor be if the same wall has a layer of 3” exterior rock wool? Does the reduction in r-value of the cavity insulation change or am I thinking about it the wrong way?
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Replies
The cavity insulation and continuous insulation are considered independently. For a 2x4 steel stud wall, effective R-value is half (or less, as the better insulation, the more it's reduced) the rated value. Note, this is not the same as a 50% framing factor, it's worse. Eg, R-19 becomes R-7. But you take that value and add the R-value of the continuous insulation to get the total wall r-value.
>"If a metal stud wall with no exterior insulation has a framing factor of 50%, what will the framing factor be if the same wall has a layer of 3” exterior rock wool? "
I'm not familiar with the term "framing factor". What means that in 'merican dialect?
>" Does the reduction in r-value of the cavity insulation change..."
Nope, not a bit.
The R value of the cavity insulation is the same (not reduced) whether the studs are steel or wood, or whether there is additional insulation on the exterior or interior. It is what it is.
The R-value of the framing FRACTION (the percentage of the whole wall area represented by the area of the stud edges) goes up by the R value of the exterior insulation, as does the cavity insulation area fraction.
It feels like what you're asking about is "whole wall R", the average performance of the wall after factoring in all the thermal bridging and the thermal performance of the non-insulative layers:
https://web.ornl.gov/sci/buildings/docs/Thermal-Performance-and-Wall-Ratings.pdf
Jory,
There are metal studs that have better thermal performance, still nowhere near as good as wood:
http://www.steelform.ca/portfolio/delta-stud/
They were pretty straight forward to use. The extra holes make wiring and pluming much easier.
Hi Jory -
Building codes have separate insulation requirements for steel framing because the thermal conduction is so high.
There is a pretty robust discussion on steel framing in this past Q&A Spotlight: https://www.greenbuildingadvisor.com/article/building-with-steel-framing.
Peter
Thanks for all of the thoughtful replies. I’ve read over that ORNL document before but didn’t feel like it quite answered my question. I also found the GBA discussion about building with steel framing quite informative. That being said I am still left with some uncertainty. I understand whole wall r-value, thermal bridging of steel vs wood studs etc. What I am lacking a clear understanding of is whether the layer of continuous insulation over the studs reduces the severity of the thermal bridging in any way. On one hand I can clearly see that the continuous insulation has a given r-value no matter where it is placed in the assembly and the same goes for the whole wall r-value of the metal stud wall with cavity insulation. But on the other hand I find myself pondering(no doubt due to my ignorance of thermodynamics) if the layer of continuous insulation slows down the transfer of heat to the metal studs beneath, does that in turn have any reduction on the negative effects of the thermal bridging since it would seem there would be less heat that reaches them in the first place. Not sure I am making any sense but my curiosity leads to my need for fully understanding this scenario. After all, I wouldn’t even be pondering such a thing had I not happened upon GBA some years ago and started down this building performance worm hole. It’s been an amazing journey though and I am grateful to all of the wonderful contributors to this community.
I understand what you're asking. The answer is no, the thermal bridging is not ameliorated by the presence of the external insulation. If the effective R-value of the studs and cavity insulation is R-8, it remains R-8 whether you have zero inches, 2 inches or 6 inches of exterior insulation. Only the whole wall R-value changes. There is a pretty widely held belief that when you "break" the thermal bridge, something special happens, but it's not really true. There may be some localized effect on condensation where the the bridging is happening, but when it comes to total heat loss there is no difference between an R-20 wall with only cavity insulation and an R-20 wall with a thermal break in the form of continuous external insulation.
Thanks for the clarification.
Many commercial building are built with steel studs generally they do not fill the cavity and put all the insulation on the outside.
Joseph Lstiburek quote
“commercial wall” (Figure 8). It has a conductive structure—metal studs. All of the insulation should—and must be located on the outside.
You may enjoy this article.
https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall
Walta
Akos,
The design of those studs is very cool. If I do decide to go the metal stud route I will be utililtzing a local company that forms their own studs and sells the whole house as a kit so I wouldn’t be able to utilize them. Very neat though. Thanks.
Jory,
I've gone down the road with a prefab light gauge steel. Spent a lot of money on engineering, drawings just to get a price that was silly.
The idea sounds great, till you actually start doing it. Thermal performance of steel studs suck (delta studs are only slightly better). Everything needs a special fastener. The only reason I went with the steel studs is building code required it, even then the prefab part was way more then just building it on-site.
One plus is getting all the studs shipped exactly to the length which reduces waste.
If you want a quick build, look at SIP. Or even better, build a platform in the backyard and prefab the sections. That is what I ended up doing in the end.
Walter,
I’m very familiar with BSC and the perfect wall approach. That is the route I will take if I do go with metal studs. I’m just pondering ways that may get me to my r-value goals without having to deal with the challenges that are presented when dealing with 5+ inches of exterior insulation both from a buildability standpoiont and a financial one. That being said, I appreciate your input.
-Jory