Thermal bridging of I-joist walls
Chrisroche
| Posted in Energy Efficiency and Durability on
Has anyone seen any efficiency modeling data that shows the thermal performance differences due to thermal bridging in a I-joist, wall versus either a double wall or a Larsen truss? I am leaning towards building with I-joists over site built wall trusses due to simplicity of construction but I can’t seem to find any data that shows what the impact of having a full thin web in an I-joist will have on thermal bridging. In my case I’m considering building a standard 2×4 wall with Rockwool, then adding 11 7/8 I-joists to the exterior with dense pack cellulose in climate zone 6.
Thanks!
Chris Roche
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Replies
Chris,
There are a lot of smart people on GBA that can probably provide actual heat loss figures for i joists. I'll share my $0.02 in the meantime :-)
The wall that you are proposing is sometimes referred to as the "Corson Wall"; named after Christian Corson of Ecocor. I think I read somewhere that he is a seeking a patent for the wall but many have used it with great success.
Anyway, Chris Corson/ Ecocor- as well as many others, have run THERM analyses and found that these walls have no thermal bridging- even at corners. This may be in part due to the fact that the OSB web of the i-joist doesn't actually line up with the interior load bearing stud. Rather, it's off center a bit, breaking the direct path of heat flow.
I Joists in Europe use wood fiber board as the web rather than OSB. These have slightly better insulating qualities than our American, load bering versions. Nevertheless, I think OSB has an r-value of around 1.3 to 1.5 per inch. So, even with an 8" or so web plus lumber, the i joist alone exceeds the performance of a typical 3.5" fiberglass batt.
Hi Rick,
Thank you for the detailed reply! I actually came close to considering having the walls fabricated by ecocor, but a combination of budget constraints and wanting more experience with I-joists for future energy retrofit projects has me leaning towards constructing them myself.
I joists typically have a 3/8" web, so if you look at a wall with 20% framing factor, with TJIs that drops down to 5%. The R value loss is only a couple of percent (dense packed 11 7/8 TJI is around R38 including the losses from the webs).
The question to ask is why do a new build with this construction? If you are dense packing, it is much cheaper to build a standard double stud wall and insulate it all in one shot.
The other option is to frame the walls with TJIs only, there is a German PDF out there from on of the I joist manufacturers showing the details. You would have to get an engineer to spec this though.
Hi Akos,
The reason I am planning to go with I-joists is because it provides a solution to the issue of thermal bridging at the slab perimeter.
With polished concrete floors and a double stud wall, the exterior wall can typically only be cantilevered with a 2x6 up to a few inches beyond the slab over the perimeter slab insulation. With this project my goal is to reach passivhaus energy performance so I am aiming to have 8 inches of perimeter insulation around the slab. With this is mind, with a double stud wall I would end up with a weak area around the slab that is both difficult to flash/trim and also an area that has poor thermal bridging characteristics. I am planning to go with a raft slab due to difficult sub-slab soil conditions. I will attach two images I created that showcase the differences in thermal bridging at perimeter. If you have any idea on how to construct a double stud wall that can achieve the same flush perimeter edge insulation details, I am all ears!
Thanks!!
Alex,
Design so that the your interior wall is load bearing. Over-hanging the exterior non load bearing wall on a wider top plate over the foam is not an issue with most light cladding. You can also always use an LVL as the bottom plate if you are worried about support.
Passive House has used this wall, TJI studs only I believe. The Smith House in Champaign-Urbana, IL. There may be some information on the performance. I was in there for a tour on a cold November day in 2007 and with about 40 people inside it was very warm, I think it is a wall that works. If I remember correctly the next Passive House built used the TJI's (14") with OSB sheathing on the warm side for the air barrier. Then they had a strapped wall inside of the OSB for a service chase. I do not know if the service chase was insulated, the home was under construction when I saw it. Fiberboard sheathing was used as exterior sheathing along with housewrap.
According to John Straube, around 25%. Can't find the link right now but it has to do about how dense TJI osb is. EDIT : Found it https://www.greenbuildingadvisor.com/article/two-views-of-double-stud-walls#comment-38047
Anyway, if you want to go with tij trusses, just offset them from the joists like in this video https://www.youtube.com/watch?v=_ANbY1dvUuA
I'm facing the same decision and the one thing I like about TJI is how straight they are. The thing I don't like about them ia that they are really expensive compare to custom made Larsen Truss (30$ CAD compare to 5 or 6$ for a Larsen truss without counting the time to make it) and that I can't choose the truss depth I want.
Hi Alex, thank you for sharing the video on how to offset the studs. Are you at all concerned about the structural strength of attaching the I-joists offset to only the top and bottom plates? The advantage of attaching directly to studs would be that you have multiple points along the vertical of the wall to attach the I-joists to, of course at the cost of thermal bridging. Maybe I am overthinking it.
I suppose in my case since I am going to a polished concrete floor, I will not have a rim joist to screw into. With only a single bottom plate to attach the I-joists to, it may not be enough wood to grab into. Maybe I need to consider going with a double bottom plate?
Not really unless you are putting on some heavy cladding. You need to use some structural screws though (6'' according to this video).
If you worry about it, you can always increase your sheating thickness so it doesn't matter if you hit a stud or not or talk to a structural engineer.
My house is made with factory built i-joist walls. I could show you the PHPP energy modelling if that will help.
Here is the worksheet for the wall. This is for 16" i-joists with 7/16" OSB web, installed 24" OC.
I’d check that TJIs can carry the same vertical load when acting as a column compared with conventional milled lumber. Load on a column is more dependent on total cross sectional area, so a TJI joist isn’t going to behave the same here when compared to dimensional lumber in the way it would when acting as a joist.
I do think you’ll see less thermal bridging due to the thinner web compared to a regular 2x4 or 2x6. I haven’t seen numbers to quantify exactly how much difference though. I’d probably just extrapolate a difference by working out the whole-wall R value with the reduced framing fraction presented by the thinner web. You might find corners to be a bit tricky to frame with TJIs, but you could always use regular dimensional lumber at the corners and TJIs everywhere else to still realize almost the same overall benefit in reduced thermal bridging.
Have you considered using exterior rigid foam here instead of TJIs? Exterior foam comes with a different set of problems, but it does solve your cantilever issues.
Bill
Hi Bill,
In the case of supporting the TJI's I was planning to cross strap all of the TJI which will help hold the whole assembly together, similar to how Ecocor makes their walls. Either way, it still seems like a grey area.
As far as using exterior rigid insulation goes, it gets tricky when you get to thicknesses at or above 6". I found one company called legalett out of Canada who sells an EPS wall panel system that comes with an internal steel track (for easily attaching panels to walls) that has a removable foam insert to allow access to the steel track. They offer an 8" panel. My main concern with this system is that it seems less vapor open at the sheathing and is also less environmentally friendly. But either way seems like a great system. http://www.legalett.ca/eps-foam-panel-wall-insulation-passive-house-zne-leed-nzeb-thermalwall-ph.html
If you apply an OSB interior sheathing, the whole assembly should be strong enough to hold up a three story building. You will need an engineer to sign off on it however.