My two storey brick house has a Mansard roof on the entire front facade of the second floor . The roof area measures approximately 16′ wide and 11′ high. The main roof of the house is ?flat? and slopes down from this Mansard front towards the back of the house. There is what amounts to a dormer window in the centre of the Mansard portion that has a gable roof over it. The Mansard portion is covered in what appears to be vinyl siding installed vertically! There is a ventilated soffit along the eaves with no other apparent venting at the Mansard – flat roof intersection. As part of a third storey addition project (which will be set 8′ back from the front of the house so shouldn’t?t impact these second storey issues) this material will be replaced with slate or shingles.

Looking at things from the inside, the dormer is entirely occupied by a window seat, which I can get access to the underneath. One side of the inside wall is occupied by a closet. I cut a hole through the front wall of the house, via this closet, and basically found a 3′ deep by 10′ high attic space. However, this space has been stuff full of FG batts. In order to see this I had to cut through drywall, then lath. There is no vapour barrier. Since the lath is still there I am thinking this FG must have come through the front, so the sheathing on the Mansard front was likely replaced at the time the vinyl went up (approx. 8 years ago).

From reading similar threads here, and other places, my take on roof ventilation, vapour barrier etc. is this. The back side of the roof deck (remember it is a Mansard, so almost vertical) needs to be vented to keep the shingles etc. cooler. This venting causes the roof deck to be a cold surface in winter (Southern Ontario, Canada). This is an issue because warm moist air leaks into the attic space and condenses on the cold surface. The way to prevent this is completely seal the living space from the attic space.

So what is the most practical way to do all this is? Would using sprayed in place foam on the backside of the roof decking prevent this warm moist air from reaching any cold surfaces? This seems more practical than trying to completely seal the area in a 100 year old house using vapour barrier etc.

Then I could just fur out the front of the decking to address the shingle temperature concerns. But then, what about exit venting at the top. If the soffit is vented, as it appears to be, then surely I need an outlet at the top so the air can flow through the space not just get into it.

Is there such a thing as a ‘ridge’ vent for the top edge of a Mansard roof where it meets a flat roof? The flat roof seems very old but the soffits around the perimeter do have vents -I just don’t know how effective these are or how the vent space is configured. What with all the framing members etc. I can’t see how the top of the Mansard could be vented into the flat roof air space.

I have only been here one season but so far no icicles, or signs of water damage inside but I think alot could go on inside that space before it manifested itself on the visible interiors. If I am doing all the third storey work, plus changing the front of the existing second storey to slate/shingles,I would like to address all the issues at the same time.

Thanks for any input/advice you can give.

Edited 9/17/2006 11:23 am ET by riverdale