Hello, I am zone 6, Halifax Nova Scotia. House built in 1938. It’s  pretty airtight now (blower test confirms) and have a Lifebreath HRV. Winter RH inside is between 30 and 35%. My issue is a little main floor addition under a low slope, single pitch roof.  It’s effectively an attic but not vented and absolutely needs proper insulation. The top of the roof is right below a second story window. According to NAIMA, N.S. will adopt the 2020 NBC effective April 2025. 

So, if I wanted to make it a conditioned, unvented attic, I would need R (effective) 26.5 but could only put it on the underside of the roof decking. The effective r-value is dependent on the building assembly so I think I’d go with something closer to R 35 (nominal) given the home’s age, etc…? This is probably the preferred option but financially it’s not really feasible to have a contractor come in and do the spray foam. Could I install rigid xps or eps to the underside of rafters? Like about 10″? If that’s feasible, how do I deal with soffit area? It’s cramped where rafters connect to top plate. Install as much rigid foam as I can fit and then seal nooks and crannies with canned spray foam?

If yes to unvented and conditioned space, how do I deal with gable ends – same approach? New code here will be effective R 17 so maybe bump to 25 (nominal)? All seams taped/foamed etc.

If unvented conditioned option isn’t workable, could convert to vented unconditioned. My roofer says vents installed at gable ends would be ok but not sure that conforms to code. Having said that, because it’s a low slope and short gable ends, the  vents would end up being pretty close to the top of insulation anyway. Would lack of soffit vents be a terrible idea/contrary to code? I can be extremely meticulous (touch of OCD) – air sealing between conditioned and unconditional would be well done. Class II vapour barrier (Benjamin Moore vapour retarder primer) on conditioned side of drywall. Lots of rock wool on ceiling floor to take it to around r60. Do I need a vapour retarder (1″ eps?) over the rock wool so moisture doesn’t seep into the top plate or other places where rock wool could touch wood framing?

There are 3/4 inch nailing strips nailed to thr underside of ceiling joists that the drywall is screwed to. Is it a problematic to have a 3/4 gap between the drywall and insulation? If yes, could I install 3/4″ unfaced eps to fill the gap? The rock wool on top of that. I’m planning on taking down pretty much all the drywall anyway… no access hatch, too confined to move around in blahblah so I’ll have to work from below, come up between ceiling joists. If this sounds weird, it is! Former homeowners did some wacky stuff. If this approach is feasible, I will need an air barrier over the rockwool to combat wind washing from gable vents. Could I use 1/2″ unfaced eps? Spray foam / tape seams? Unfaced eps is permeable enough that if I have rockwool sandwiched between 3/4″ eps on the floor (fill gap between rockwool and drywall) and 1/2″ as air barrier on top, I won’t trap moisture?

On to the attic kneewall.. I can’t access stud bays, has been covered with 1″ plywood. Can I install  unfaced eps? Code in 2 days will be r17 (effective) for exterior walls. Put in about r 22 – 25 nominal? Seams taped, edges foamed, etc. Should keep plywood sheathing warm enough to avoid moisture issues? 

Many thanks for any advice/thoughts you might have.
– Kirstin