Best venting/insulation option for 2/12 monoslope roof assembly
Zone 4A. I am building a 36×42 ICF house with a 2/12 mono-slope roof that will face North. My initial plan was to use 14′ TJIs for the rafters to span halfway to a load bearing wall running the length of the house. It would be an unvented assembly topped with sheathing, roofing membrane, 1″ vertical furring strips and 1″ horizontal furring strips to attach a standing seam metal roof. The interior would have drywall and paint to allow vapor diffusion.
After learning more of vapor control, I am considering 24″ trusses that are able to span without the need for the load bearing wall. It would be filled with 18″ of Rockwool and have 6″ of vented area bef0re the roof sheathing and standing seam roof. It looks like a smart vapor retarder secured by furring strips for electrical would be needed prior to drywall and paint. The draw back is the height this assembly adds to the wall.
My main goal is durability and a minimum of R49
1. Which is a more forgiving structure for vapor/condensation?
2. Would I need to vent under the Standing Seam with the truss assembly?
3. Would a 2/12 roof vent enough with top and bottom eave vents?
4. Any recommendations for the assembly?
Thanks in advance,
Robert
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Replies
As long as you do a good job of making the ceiling airtight, I think the vented assembly will be more durable (no matter what insulation you were planning for the unvented alternative). If a minor roof leak gets through to the sheathing, it will be able to dry out through the bottom side.
You would not need to vent between the metal and the roof sheathing. Most metal roofing manufacturers recommend installing it on continuous sheathing.
With a 6" vent space, I think it will vent fine. Making the ceiling as airtight as you can will greatly reduce the amount of moisture entering the roof, so there will be much less to remove anyway.
My recommendation would be to look at cellulose. Your roof seems ideal for it, it will likely be a lot less expensive than Rockwool, and it will save you from having to cut and fit insulation between all the truss webs.
Robert,
I agree with Frank. A 6” vent space helps mitigate the low slope. You end up with a much cheaper, more resilient roof assembly by going with the trusses and permeable insulation.
Figure out how you want to transition your air-barrier from the walls the ceiling before you start framing.
My plan was drywall straight on the ICF foam for the walls. Vapor barrier on the ceiling that I could tuck between the top of the drywall and foam. Thanks for the reply.
For the extra roof height, you don't need to stop the siding at the bottom of the i-joist. Look at detail 10k here, you only need to extend the top chord and a bit of the web to support a standard 1x6 soffit.
https://www.starktruss.com/wp-content/uploads/2014/02/APA-D710.pdf
Thanks, Akos. I am leaning toward the 24in truss option now, but trying to keep the total 1st floor wall height under 16ft. ICF will be 14’6” on the tall end with the truss having a 2x6 for the overhang is right at 16’.