Venting and insulating 6″ rafter bays
Hello. First post here. Have been living in and working on a 1950’s cape in MA for about 3 years now. I’m planning on redoing the venting and insulation of the roof and wanted to ask a few questions.
The upstairs is finished: stairs leading up, 2 rooms on the ends, half-bath, unfinished storage space behind the knee walls. Plain roof, no dormers, no skylights. The insulation is pretty bad. Assorted paper-backed fiberglass batts in the rafter bays throughout that storage area, with poly over it in places. Ceiling space below that storage area has spotty blown-in cellulose. The space above the slanted ceiling of the upstairs rooms has effectively no insulation. They slid 1″ rigid foam up there but it’s not sealed. It’s cold and we’ve gotten ice-dams. Above-ceiling space was vented with gable and roof vents but new roof has a vented ridge.
The rafters are nominally 2×6 but cut so that some of them are actually almost 5″. They’re also not spaced 16″ OC, more like 17″. I would rather avoid foam: there are a lot of carpenter ants here and it seems too complicated.
The plan:
– Rip out all the wall/ceiling board on the 2nd floor to get full access to rafter bays
– Form 1-1/2″ vents by nailing up planks to sides of rafters and attaching 1/4″ plywood (or tempered hardboard) to them from below. Go from soffit to above ceiling so that storage area is interior space.
– Fill rafter bays with mineral wool batts. Get additional insulation thickness by spacing out rafters with 2-by’s ripped to 2 inches attached perpendicularly to rafters (Mooney wall). This requires cutting the batts to shape them around these cross-pieces but I’m patient.
– Cover all this and the gable wall insulation with membrain smart vapor barrier (or similar).
– Everything covered with drywall. All electrical goes on interior walls. No overhead lights. Put some well-sealed hatches in the flat ceiling portion. Top ceiling can get 7-1/4″ of insulation.
This should get me a decently vented R-23 roof with thermal bridging mitigation. R23’s not ideal in zone-5 winters but we’ve been living with worse and there’s not that much space. I like mineral wool: it seems like bulletproof stuff and easy to work with. The rigid vents prevent wind-washing. Priority is robustness so I’m ok if it’s not as warm as it could be but it needs to be a safe design that doesn’t rot.
The questions: Does this seem like a reasonable approach or am I missing some flaw? Would tempered hardboard be good for forming the vents or will it deteriorate from moisture being drawn from outside? Do I need to spring for the 1/4″ CDX plywood? I would like it to survive any leaks that may happen down the line.
Replies
My first home had a similar situation. If your home is like what I had increasing rafter depth for high r value basically means you loose all your head height and function, so was impossible to meet current code requirements for insulation w/out rebuilding entire roof structure. For myself I found it easier and less complicated to just have closed cell sprayed and create a ventless roof. I did have a new roof and roof sheathing, so wasn’t concerned with needing to replace the roof sheathing for a very long time if ever. If I were to have chosen a vented assembly I would do what you are describing. I would utilize a ridge vent and make sure soffit ventilation is properly sized as well. I would not use the hardboard. It’s not great for moisture resistance and acts as a sponge, so takes longer to dry out. I also had a ant issue. You’ll need to take care of that before you do anything. The ant issue will continue no matter how great you are with insulating and all that. Luckily my best man has a pest control company. I do recommend pros for ant removal. The main thing for ants is to replace any wet wood and stop all water intrusion as they love the wet wood. You’re on the right path as you are describing, just constantly remind yourself as your doing it to make sure every rafter space can breathe. If going unvented make sure to use closed cell and proper depth for your region.
Dumb question but I get hung up on fasteners. 1-by's attached to rafters with 4d galvanized nails, 1/4" plywood stapled to 3/4" edge of 1-by's with 18-gauge staples? How close do the staples need to be? Is it safe to fasten that close to the edge of the plywood or should I use 2x2's instead? It's not a load bearing structure but I don't want the plywood falling down and compressing the insulation either.
You're worrying more about the fasteners than you need to. You'll be fine with 1x furring and 4d nails, or just 1 5/8 in. screws. The ply is a good idea, as it prevents wind washing of the insulation. I've done a similar detail just using Tyvek and battens. In the apartment over my garage, I ran 2 in. polyiso foam across the bottom of the rafters, then screwed 1/2 in. OSB below that as a base for the drywall.
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Having second thoughts about the plywood. Could I use tyvek or similar? Stretch it across the rafter bay attached to the sides of rafters with narrow battens or just stapled through some cardboard drywall shims then slide in the mineral wool batt beneath it? Andy, you mentioned something that sounds similar, have any pictures?
Can I just glue some roofing felt to the top of the batt with spray adhesive?
I don't have photos. I used 1 in. battens up against the sheathing and screwed to the rafters, then stapled the Tyvek tight across the bottom.
Andy, I'm still planning this. Did you use tyvek instead of something rigid like plywood for cost reasons and ease of assembly or some other reason?
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