Hi.
I’m building a NetZero house in coastal Massachusetts (5A) using the recently implemented 2021/2023 Mass IBC and IECC codes. All was well until the structural engineer nixed the I-joist roofing and insisted on 2x12s. This resulted in the HERS rater wanting 4″ of polyiso above the sheathing to then another layer of sheathing to create a layer of continuous exterior insulation.
How do you build an R-60 CEI roof without using I-joists in 2023?
Many thanks
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Replies
Utilize rafters instead of I-joists: Since the structural engineer has recommended 2x12s, you can use these as rafters for your roof construction. The 2x12s will provide the necessary structural support for the roof.
Install rigid insulation above the sheathing: To achieve the desired R-60 CEI roof, you can incorporate rigid insulation above the sheathing. The HERS rater has suggested using 4" of polyiso insulation.
Add an additional layer of sheathing: After installing the rigid insulation, add another layer of sheathing on top. This layer will act as an additional barrier and create the necessary continuous exterior insulation.
Choose appropriate roofing materials: Select roofing materials that contribute to the overall energy efficiency of your NetZero house. Opt for options that have high insulation values and are durable enough to withstand coastal Massachusetts conditions (such as wind and moisture).
Ensure proper ventilation: Proper roof ventilation is crucial to prevent moisture buildup and maintain the longevity of the roof.
Hi delam90
Good advice, thank you. I have a couple of questions:
1. How does the inside layer of sheathing dry? Through the 2x12 rafters? I can't envisage a way of it drying unless there is an air gap above it, which defeats the value of the polyiso
2. If I have 2" of polyiso board above the rafters (no sheathing between the rafters and the polyiso) then have sheathing above the polyiso, nailed to the rafters, do I need a ventilation gap and is only one sheathing layer adequate
Many thanks. Stay cool.
Before you reduce the amount of foam board insulation, look at the resulting temperature profile. You want to use internal and external temperature expectations, and then consider the expected dew point of air.
What you do not want is to create a situation where the inside surface of the foam board is below the dew point.
As you suspect, with foam on top, any moisture at the sheathing below can only escape into the structure.
So a suitable film would be a good idea in your zone for interior vapor control.
Another way to get higher effective R value is to make the roof cavity deeper, perhaps by use of a suspended ceiling below.
Why does the reply from delam look like a computer-generated one?
(to me)
Ha! That's what I thought too!