I am building a cabin with 12/12/ pitch roof. I want exposed rafters and I can use polystyrene insulation over the roof decking,but how do I ventilate a shingled roof?
Would it be better to use metal roofing and let the corregations of the metal roof serve as the ventilation?
Thanks
pntcont

Probably the best bet is to build a conventional roof and install false rafters under it for appearance. Insulating a residential roof with huge thicknesses of foam requires some difficult and careful detailing. and fastening anything, be it shingles or metal roof, through a pile of foam is a tricky business.

quote:

Originally posted by pntcont:
I am building a cabin with 12/12/ pitch roof. I want exposed rafters and I can use polystyrene insulation over the roof decking,but how do I ventilate a shingled roof?
pntcont

In your case, NO VENTILATION is required whatsoever.

Why?

Because Builing Codes only require ventilation where an attic would be present or where condensation occuring in the framed ceiling assembly could cause damage.

If you insulate ABOVE the roof sheathing by using sheets of polystyrene or other foam insulation material...

You eliminate the "attic" and extended the thermal envelope outside the sturcture...so that condensation CANNOT occur up against the roof deck from the inside.

Therefore no ventilation would be required at all.

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You get to have exposed rafters while having the ceiling properly insulated above.

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Bottom line:

Insulate ABOVE the roof deck to at least R-38 to R-49 depending on your location in Michigan, and you will need to do NOTHING more to have your exposed rafters.