Constructing an Overroof
My family purchased a rehab house in western New York (Zone 5) that we are currently gutting. We’re not flippers – just want to create a nice, affordable, energy-efficient home). My question is related to getting the roof up to energy code (R-49) at least, hopefully more.
A brief description: The roof of the 1900 home is constructed with 2 x 4 rafters 24″ O.C. The upper story has a knee wall with a sloping ceiling following the roof up to the 8′ ceiling joists. The two one-story rooms on either side of the center house have low-sloping roofs and uncomfortably low ceilings. If I remove the ceilings from the side rooms, then it seems that creating an unvented hot roof makes the most sense for this roof configuration.
Please open the attached (virus-free) pdf sketch for clarification.
The 2 x 4 rafters do not provide enough depth for insulation to meet code even if I use closed-cell spray foam, which is really expensive anyway. I like the idea of putting rigid insulation on top of the roof deck, but I would need 8 inches of rigid insulation to reach code minimums, and that would also be quite expensive.
I’ve read about Larsen trusses, but every article I’ve seen talks about adding Larsen trusses to exterior walls. Can Larsen trusses be added to the roof? I could site-build Larsen trusses out of 2×2’s and osb, and then fill the cavities with inexpensive fiberglass or cellulose. If I fur down the existing rafters a couple of inches, the Larsen trusses would need to be 9″ depth to give me a total depth of 14 inches, enough for low-cost R-3.5/inch insulation to reach code minimum R-49. The top of the Larsen trusses could be topped with a waterproof smart vapor barrier membrane, 1 x 3 purlins, and through-fastened metal roofing. Alternatively, I may top the Larsen trusses with tapped Zip-system sheathing to create a solid deck.
I do understand that the Larsen trusses on the roof will become structural, unlike when applied to the exterior wall. I may consider purchasing I-joists or simply using dimensional lumber. One advantage to creating a structural overroof, is that the slopes of the lower roofs can be improved and the overall roof structure will become stronger.
I have no idea what my code official will say about this scheme, but I thought I would ask here first.
What am I missing?
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