Re-roofing a Greek Revival to fix ice dams – GreenBuildingAdvisor

Hey there GBA. I’m an architect and I have a difficult roof situation (on my own house) and could use some advice/reassurance on how to deal with it. This is an existing 1860’s Greek Revival home in Climate Zone 6 that needs a new roof and suffers from really bad ice damming. It has a pretty typical sloping ceiling (8:12 pitch) on the second floor that transitions to a flat portion – straight gable with no dormers. The rafters and walls are both 6″ deep. The walls are insulated to about R21 and the sloping portion of the ceiling is about the same. Unfortunately the roof/ceiling is stuffed with vermiculite (asbestos). Obviously in an ideal world that would all be removed but it’s not in the budget at the moment and the roof desperately needs to be repaired. I am also not too concerned about meeting current energy code since this is an existing building and anything I do will be better than what is there now. My primary concern is about mitigating the ice dams and moisture within the assembly/attic.

The exterior trim details are really nice and make this home what it is so the last thing I want to do is add a super thick layer of exterior foam on top and totally destroy the historic character of this home. I’m ok adding some thickness but not what it would take to make this meet code or ideal insulation levels for this climate zone.

This home is also fully occupied with lath and plaster on the ceiling that is in really good condition so we don’t have the opportunity to take down the ceilings (not to mention it would spread asbestos everywhere!). I have read suggestions of adding rigid insulation to the interior of the sloping ceilings and then drywall over that. That seems ok but it does nothing for the partition walls that intersect with the sloped ceiling leaving a good amount of thermal bridging so I’m not sure that method will work all that well.

Here are three options I’m considering and I would love some feedback and reassurance that one of these is a relatively good/safe solution given all the above constraints.

Option 1

1. Tear off the existing asphalt shingles down to the board decking.

2. Install a vapor permeable membrane over the whole thing. Open to suggestions on products (Intello, Siga, etc.)

3. Install 1×3 strapping on top of each rafter running in the same direction and coravent at the eaves giving me 3/4″ of airflow.

4. Install 5/8″ zip sheathing w/ ice and water shield at the eaves/valleys.

5. Ideally standing seam metal roof with continuous ridge vent if the project can afford it but otherwise asphalt shingle.

I know that 1-1/2″ is preferred for ventilation but I’m wondering if the 1×3 is a reasonable alternative given that I’m trying to minimize how much thickness I’m adding on top of the roof. Isn’t something better than nothing??

Option 2

Another idea I had was to install 1-1/2″ of repurposed polyiso or GPS on top of the existing roof deck for roughly R7-10 continuous and then add the zip sheathing on top of that with screws to the rafters. This gives me roughly 1/3 insulation continuous and 2/3 cavity. In this scenario would it be ok to leave the roof unvented or should I add soffit and ridge vents? Also, do I need to add a vapor barrier over the board decking before installing the rigid insulation and/or should I tape the insulation before installing the zip sheathing?

Option 3

A third idea is to just re-roof over the existing deck for now with good ice and water shield over the whole roof and metal roofing w/ ridge vent which allows me to keep all the historic details in tact. Then at a later date I can come back to address the insulation from the interior and ideally remove all the asbestos and fill the sloped ceiling rafters with dense packed cellulose which transition to loose cellulose at the flat portion. It will be tricky to make this a truly vented but I can try my best. I could even add the continuous layer of rigid to the underside of the sloping ceiling with new drywall finish. My concern with this option is that it might be a few more winters before we can address the asbestos and insulation issue from the interior side but at least we will have a dry roof in the meantime.

Which of these options feels safest/smartest? Would you recommend anything else?

Thanks!

Noah