looking for new construction roof framing/insulation/ventilation advice
I’m having an ICF house built in MA and there is a difference of opinion between the architect and the builder on how to frame the low side of the saltbox roof. I plan to use cellulose insulation and my target R-value is R-49. The attic is going to be outside of the thermal envelope so this will work fine for the 2nd floor ceiling. The trouble is with the north side sloped ceiling/roof.
The architect designed the house with pre-fab wood raised-heel roof trusses for the peak and 2×12 @ 24″ o.c. rafters, that end above the main load-bearing wall, for the sloped ceilings.
The ICF builder, who is revising the drawings to change the walls from 2×6 to ICFs, has changed the framing to all stick-built. The sloped ceiling is now drawn as 2×10 @ 16″ o.c. He said he did this because it will cost less.
But now the rafter cavities are not deep enough to get enough insulation in them and the amount of thermal bridging has increased from (20) 2x12s to (33) 2x10s and the 22″W roof window no longer fits between the rafters.
Does what the builder is doing make sense or should I insist he change the framing back to my original plan? I do not want to change from cellulose to spray foam insulation. Will adding rigid foam board to the roof for added insulation and to reduce thermal bridging be an option, or will that not work because of the venting baffles below the sheathing? If I insist that the builder undo the changes he made to the drawings and go back to the truss and 2×12 rafter design, and also add a layer of rigid foam board to the roof, what would be the best way to raise the roof above the unconditioned attic space, that will not need a layer of rigid foam, to the height of the sloped ceiling roof?
Any advice would be greatly appreciated.
Replies
although there are plenty of times that and architect designs are not very field friendly, most times there is a reason why the design is spec'ed out the way it is.
This could or could not be the case but I would assume from your post that the builder was not aware that by decreasing the 2x12's you would loose your targeted r value.
Is it cheaper to use 2x10 vs 2x12 on the roof, sure it is but it will cost you more in the long run because you are now outside the planned insulation design and continually batteling this issue.
First, did the builder get a change order or approval from you prior to making the change?
Either way, I would, if it were me, make him get back to the original design. It will either be at your expense (if you approved the change) or at his expense (if he just made the change)
Not sure of the prices in your area, but my local prices have 2x12 about 25% more than 2x10.
Using 33 pc of 2x10 will be about 32% more expensive that 20 pc of 2x12.
Did your builder say how much it would save?
Don't think adding inuslation above a vented area makes any sense. It will not add any insulation value.
If you use trusses for the attic area, any difference the the roof deck height due to insulation can be accounted for in the truss design itself.
Adding insulation above a vented attic can help reduce sun load in the summer.
Of course, whether the difference is enough to justify the cost and effort is a big question.