Advice on new house with (damp) sealed attic
Hello GBA community,
I’ve just bought my first home, a 1911 Foursquare in Seattle. Overall, it’s in great shape, but the roof and attic need immediate attention.
The 30-year-old roof is failing and will be replaced right away. The sealed attic has fiberglass batts with clear plastic sheeting over the rafters. Inspection revealed significant moisture and mold in the insulation and on the rafters beneath the plastic. This system clearly failed.
My plan is to remove all the old insulation, and get a mold abatement crew to come in and clean the roof framing.
After this, I want to keep the attic as a sealed system, but make whatever changes necessary to the design to keep my attic dry. My main concern is moisture management, especially since the rest of the house is leaky (original single-pane windows that I plan to keep).
There are a few decisions I’m having a hard time making:
What are the best insulation options, given my moisture concerns, preference to avoid spray foam off-gassing, and Seattle’s climate? Rigid foam boards above the roof decking sound ideal, but seems like it might throw off the whole look of this house. Like the added roof thickness would look super wonky on a house with a hip roof with a large 36 inch overhangs and exposed rafter tails.
What will I need besides better attic insulation to keep my sealed attic (including roof framing and decking) dry and mold free all year? I’ve heard folks mention the need to direct more HVAC up to the attic to get fresh airflow. I only have a gas furnace at the moment, and I’d like to put off the expense of a heat pump for a while. Will a dedicated dehumidifier up there be necessary?
Thanks in advance, really appreciate any thoughts on this. Also, if anyone wants to refer me to a trusted expert in the Seattle area who could help me figure this out, that would be super helpful as well.
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Replies
Three options:
1- Rigid foam insulation ABOVE the roof sheathing. All the concerns you mentioned with architectural details are an issue here. If you do go this route, use a few layers and STAGGER THE SEAMS.
2- Rebuild the roof as a vented assembly. This means putting in vent channels from soffit to roof, or using roof vents at transition points if you can't keep a vent channel continuous. You can build vent channels using 1x2 furring strips against the inside edges of the rafters up tight to the underside of the sheathing, then put 1/4" waferboard over those. This will create a 1.5" deep vent channel (code minimum is 1" depth). You can use regular batt insulation after this, with various "upgrades" to get more as needed (lots of info on this in the GBA forums, BTW).
3- Closed cell spray foam directly against the underside of the roof sheathing. This is the ONLY safe way to do an unvented roof assembly IF you can't put the insulation above the roof sheathing. The risks are really overstated online, and nearly always the result of inexperienced installation crews. If you have an exerperienced crew, it's very unlikely you'll have any issues, and a few days of ventillation will get rid of the smell. My guess is that ccSPF is probably your best option with the roof you're describing.
BTW, there is NO safe way to seal moisture out of a roof assembly using any type of membrane (polyethylene, etc.). That's why you have mold/rot problems now. You need to either put the insulation above the roof deck so that you won't have the same thermal gradient induced moisture drive issues, provide a vent channel to carry moisture away, or use a fully adhered insulating material which means spray foam.
Bill
This article explains how to do it:
https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work
If you encapsulate the attic, then you need to add some form of ventilation to remove water vapor that will build up over time. I added a dessicant dehumidifier and vented moisture out of the house with a 4 inch vent hood once used by an exhaust fan. I set the dehumidstat to 40% RH . So far so good.
This is not a code-compliant solution.
Thanks for the replies folks.
The ventilation channels under insulation was sounding like a good idea, but then I realized this probably won’t work for a hip roof like mine. It’s not a true pyramid that comes to a point, but there is only about a 10 foot long ridge on a 32 foot long house. So most of the roof couldn’t have channels running from soffit to ridge.
That leaves me with the options of spray foam below or rigid foam above. I’m leaning towards spray foam because it doesn’t require reworking the aesthetics of the roof. It still seems like the rigid board is a superior solution in other ways. I couldn’t find much info online about retrofitting old homes with rigid foam roof insulation, and addressing the added thickness aesthetically. Who would one talk to about that, an architect specializing in remodeling old homes? Sounds like that could get super expensive…..I am becoming more open minded towards spray foam
An architect should be able to come up with details to deal with the increased thickness of the roof. That extra thickness tends to mess up the soffit areas, and sometimes also complicates trim details in other places. Those complications are probably the main reason why you don't often see people add insulation in this way to existing homes.
Bill
What would it look like if I wanted to move to a vented attic? This is a hip roof with a 10 foot ridge, so a ridge vent won't cut it. What if I rip out the old rafter insulation and polyethylene, add insulation between the joists, cut new holes for soffit vents, and add a powered attic fan near the top of the roof?
kithymel,
If you don't need the attic, that's what I'd do. Air-seal and insulate the attic floor, add soffit vents, and either spot vents near the ridge, or a ridge vent. Ten feet should be plenty. Calculate the net vented area you need for the size of your roof (1:300), and locate 60% at the eaves.