Gable Vents with Insulated Roof deck in Old House – GreenBuildingAdvisor

Hi,

We have a 200 year old house in upstate new york that is 1400 sq ft between 2 floors, with a simple gable roof. We are redoing the roof, and have gotten quotes from two companies. We are thinking through the pros and cons of insulating at the attic plane vs insulating at the roof plane (vented vs unvented attic). 

We think we want to insulate the roof deck and have the attic be conditioned so that when we run line sets for mini splits in the attic, they will stay at a good temperature, and remain serviceable i.e. we don’t want to have buried ducts. 

One of the guys who came to quote the roof said that the roof needs “to breathe” and that he doesn’t recommend insulating the roof at all.

The other guy is pushing us to insulate at the roof plane. He wants to basically put new roof rafters 24″ OC and cut pieces of 4″ foam to fit between the new rafters, tape these seams, then nail on a layer of plywood, and do a standing seam roof on top of that. Between the plywood and metal roof he said they would put a waterproof membrane.

I am leaning towards this plan, but two things give me pause. One, the way that the foam is installed between framing, rather than continuously on the outside, means that there will be some thermal bridging between the attic and outside of the roof. 

Second, he is saying that we wants to keep using two gable vents that are at either end of the attic (our current roof does not have ridge and soffit vents). I was under the impression that if you insulate at the roof plane, the attic should be unventilated, but that the actual surface of the roof needs to be ventilated.

What do you think about insulating the roof and ventilating the attic? Doesn’t it defeat the whole purpose?