[Greenbuilding] insulating on sheathed roof

[RH Irving Co Inc.]

George

“but does it make sense to trash all of the fiberglass batts and existing
sheathing?”

I would say that it does as (especially if it is a very old house) there are
probably many air leaks in the insulation cavity which would short circuit
your foam.  Additionally, the roof surface is probably very uneven which
will make it difficult to get a good airtight seal with the foam.  If, on
the other hand you can assure yourself that the cavity is very well sealed
and decide to add the foam on top, then add a minimum of 4” – 6” or more,
sealing all the joints with spray foam and overlapping your seams.  You can
buy long drive screws (up to at least 12”) with which to fasten your
strapping to the rafters.  

 

There is also a product available in commercial roofing outlets similar to
stress skin panels but with sheathing only on one side which can be used
instead of the separate strapping and sheathing, but it does give you an
unventilated roof.

 

Bob Irving