Is the thermal bridging of LVLs over roof too risky?
We’re building in SE Wisconsin (zone 5) using monopoly framing and continuous exterior insulation. One part of the house has very large overhangs on the front and the back over a porch and over a patio so the structural engineer has specified 8″ LVLs on edge nailed to each truss (through the roof sheathing) almost the entire roof from front to back once you have the required length for each overhang. My original plan was to use a continuous layer of 8″ of Rockwool comfortboard (along with R30 comfortbatt inside between the trusses) and then when I realized how expensive and difficult it is to source I planned on 7″ of GPS foam. Now since we need those LVLs I’m thinking we could put Rockwool comfortbatt in between the LVLs and then the second layer of sheathing on top. My question is should I be concerned about the thermal bridging at those LVLs since it’s no longer continuous insulation? I’m planning an interior vapor retarder (Intello) sealed well for an air barrier as well as an exterior air barrier with a self-adhered WRB (Proclima Adhero) wrapping around the walls and roof sheathing under this insulation so I’m hoping that reduces the risk. It’s a 1:12 monoslope roof so we could put some vent space in between the insulation and the standing seam metal roof but I’m not sure how effective it would be with the low slope. Thanks so much for your help!
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Those lvls above the roof deck make things complicated, cut and cobble is a waste of time.
The issue with any fluffy above the roof deck (comfort board is still fluffy), is you need to vent it. Our code allows for venting low slope roof but also requires 2.5" of vent space. Venting low slope doesn't work as well as a pitched roof but still works if you don't have a leaky ceiling.
Since you'll have a solid air barrier at the roof deck, there is much lower risk with reduced venting, so maybe go with 5.5" batts in the 7"LVL.
The one issue you might have is your monoslope roof is probably very long which further reduces the effectiveness of the vent channel. You can also strap out the LVLs with 2x2 to get a bit more space if you want a piece of mind.
The one change that is easy now is bump up your roof slope to 2:12. At 1:12 water will wick uphill so any roof details are critical to get perfect. The steeper slope also gets you out of the seamed/gasketed roof panels so it will be a simpler install. Bonus, it will also improve the venting.
Thanks Akos, I'm not sure what you mean cut and cobble is a waste of time? Do you mean if I tried to piece together GPS foam to fit in between the LVLs?
It's too late in the process to change the roof slope mostly because one roof section starts above where a perpendicular roof section ends so the whole thing would end up much much higher. I like your idea of 2x2 strapping to add some ventilation space so I guess that's the decision - whether to do that to add ventilation or hope that there are enough other precautions to eliminate the need for ventilation.
Thanks so much for your help!
I've used a similar approach on a renovation I designed and it worked well. I ran 2x4s across the roof first, in part to create a thermal break.
Framing lumber is around R-1.2/in and LVLs are probably similar, so in larger dimensions it's not a terrible thermal bridge. In most cases, what really matters is how the entire assembly (or home) will perform. Sometimes condensation due to thermal bridging is an issue, but unlikely in your situation.
Thanks Michael, do you mean you added 2x4s perpendicular to the trusses before you added the overhang lumber parallel to the trusses? I guess if that's structurally ok then I could add a layer of insulation in between the 2x4s and then the rest of the insulation in between the LVLs? Very interesting idea.
But you're giving me hope that maybe the condensation won't be an issue anyway as long as everything is executed well.
Thank you!
Here's a snapshot, with more photos and description here: https://www.instagram.com/p/C1-J8berpk3/?img_index=1. In climate zone 5, if you have at least 40% of the total R-value on the exterior, you should be safe with only painted drywall as a vapor retarder on the interior.
I see, thanks for the details. Your instagram description reminded me of the other question I have about this assembly and that is do I need the venting to help prevent ice damming even if I decide not to worry about venting to prevent condensation? I also wonder if any of those mesh kind of materials like Benjamin Obdyke's Cedar Breather ventilated underlayment would help with any of this or if the gap is too small to accomplish anything.
If your assembly is airtight and has enough R-value, you should have little risk of ice dams relating to heat loss, but there are exceptions: https://www.greenbuildingadvisor.com/article/ice-dams-causes-and-fixes. Spun-bonded rain screen products like Cedar Breather provide a capillary break and drainage plane but don't really have enough depth to allow for much air flow.
Your low pitch makes a difference; this is an important article: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs.