Re-roofing a Greek Revival to fix ice dams
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
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Replies
Am I the only one wants to quit reading the question after the poster says they do not have the budget to do the job correctly and mitigate the asbestos?
The way I see the problem is that warm air is leaking from the interior of the house and escaping thru the attic along the way it warms parts of the roof enough to melt the snow on the roof making the liquid that becomes the ice dam. Without an affective air barrier, I think the ice dams are going to continue.
If you have a full written plan for the roof and ceilings and this years goal is the exterior part of the plan and next year the interior gets done that is one thing. Seems silly to do something to the exterior without even considering the interior.
Really the new roof is temporary until you can afford to deal with the asbestos?
Have you have the insulation lab tested for asbestos or is the an assumption?
Today’s codes are generally based in logic and pretty much require the R value that is likely to recover its cost over the next 20 years or so. The only reason to skimp on the R value is if you are flipping the house and not going to pay the energy bills.
Walta
I think you missed the part about me being an architect which explains the inability to afford removing the asbestos. Funny because architects make less money than most professions and are in enormous student loan debt. LOL.
It has not been tested but paper bags from the manufacturer remain in the attic and my research suggests it's 100% guaranteed there is asbestos. I'm not suggesting it won't be mitigate eventually but when you have a roof that is actively leaking and limited funds, you have to pick and choose your battles.
Btw, not flipping this house - just trying to save it. Zero insulation in the house for 165 years. Seems like any insulation for the next 165 years is saving more energy than an average home built today that won't last more than 30 years.
The question is not about energy bills. It's about the science of how to properly vent the roof to control moisture and add to the durability of the assembly while not destroying the aesthetic value.
Even if I do remove the asbestos I'm still not sure what is the best solution given that the sloping ceiling is only 6" and even less if I introduce vent baffles under the existing deck.
Noah,
Option 1 makes the most sense to me. If the vent space has a good continuous path from eaves to ridge then 3/4" should work alright, both in providing a path to remove moisture, and a cooler space under the sheathing to reduce the chance of ice dams.
Option 2 is risky. In Climate Zone 6 you need a ratio of at least 50/50 foam to permeable insulation. See assembly #3 in this link: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
I think Option 3 precludes too many things in the future by not having the vent channels above the existing sheathing.
Good luck!
A common variation of option 3 in my area is to do standing seam for the first ~ 2 feet of roof line and then transition back to shingles for the rest of the roof. It does nothing to address the air-sealing and insulation issues but it does help the ice dams slide off, especially on steeper roofs. It is an idea if you think you will tackle things from the inside eventually.
strausjw,
Interesting - I’ve never seen that. How is the transition managed when the roof has to rise over the ribs?
Hammer the ribs flat, and preferably hem the entire top edge over to create a water check. Here is a picture from google with slate, but its the same idea with asphalt.
strausjw,
Thanks!
Malcolm,
I've seen this a few times - very similar but without the standing seams hammered flat. I've always thought of hammering the seams flat at ridges, etc as a Northeast / New England detail, though I don't know if that's actually true.
In the photos, you can see the standing seam going up and meeting a higher-pitched roof. Not exactly the same scenario, but basically there's a piece of turned-down flashing that covers the seams and gets slit so it can turn down between them. If this was all one roof plane, the standing seam gets accounted for in the framing - similar to how you can account for exposed soffit boards and exposed rafters, where you drop the applied rafter down 3/4" (or rip the top of the actual rafter down 3/4") to account for the height difference.
strausjw,
I love the look of that hammered-down copper and the slate. Slick detail!
Paul,
Some local architects use a similar step to add exterior insulation over the living space. I like the look.
I would go for option 2 slightly modified and unvented. Bump up the rigid to 3" which gets you in the ballpark for condensation control. The facia will be slightly taller but from two stories up you won't notice. You can even keep the facia as is and bend up a custom drip edge for the metal roof that hangs lower to cover the gap.
This now gets you two things. A solid air barrier to reduce air leaks and extra insulation, both of which will greatly reduce chance of ice issues. Since it is about the same labor and only a bit more material cost, I would call that a win. If you ever open up the ceilings, you can always insulate properly and have an even better performing roof.
It would also be good to seal the soffit area above the wall plates to the roof deck. You can pull up the bottom row of roof deck and install rigid foam sealed in place between the rafters for this. I would install a layer of peel and stick over the original roof deck to form your main air barrier. This has the benefit of getting the place weather tight until the rigid and actual roof goes on.
Rigid+zip over this, use a screw pattern from say a Hunter panel for securing to the existing roof deck, no need to attach to rafters. Metal roof over that.
The only modification is if you are in heavy snow country (above 60lb snow load and lowish slope roof), the roof should be vented. This means installing strapping first over the rigid and the zip on top of that.
Thanks everyone for your input. I'm still contemplating the best solution but this input is all very helpful.
Here's where I'm currently leaning:
Rip off asphalt, install peel and stick over the whole existing board decking which gets me dried in quickly. Then install 3/4" furring strips at 24" o.c. running parallel to rafters to maximize ventilation while minimizing thickness, 5/8" zip sheathing, then metal roofing. That's the thought for the outside.
For the inside, I get the asbestos removed and I fill the 6" rafter cavity with closed cell spray foam (R42) from the outside wall all the way to the ridge leaving an unvented attic. Then I add an additional layer of 2" rigid insulation just to the sloped ceiling (leaving the plaster and lath in place) and cover with new GWB.
Sound ok?
Thanks!
Noah
Maybe I am not understanding your buildup, but I don't know what the furring strips and ventilation channel is getting you. Nothing is going to diffuse through CC foam, zip and peel and stick (I assume you mean a low perm product like Grace Ice and Water shield.)
Because it's hard to ensure a perfect install of the closed cell given the desire to keep interior finishes in tact I was think the ventilation would help mitigate any moisture that might sneak through. Maybe that's not necessary as you point out.
though probably not necessary for moisture it should help ice damming as you will now have a cold roof. vent at your fascia and ridge.
I think that is a decent plan. You can save a bit of cost by only spray foaming the bottom section of the roof up and including the sloped ceiling and keep the top vented.
You also don't need full rafter fill with spray foam on the lower section. I would go about 2.5" of SPF with 3.5" mineral wool batts. The batts now also become the barrier for the spray foam so if you put ducting there, you won't need an intumescent coating. Make sure the spray foam installer carries the spray foam all the way down to the wall plates on the outside, you want a continuous air barrier.
I would not bother with the interior iso unless you are already renovating.
For the top attic area, top up the insulation as much as you can fit and add either gable or ridge vent.