GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Feedback on Roof Retrofit Plans

cgmorgan | Posted in Energy Efficiency and Durability on

Hi all,

So after pouring through the wealth of information on this site I’ve made some large changes to plan to redo my second floor. If you could point out any glaring errors it would be appreciated, it’s hard to keep it all in line sometimes.

Current situation: gutted second floor, Cape Cod, 5″ true rafters, Zone 6 Montreal. Soffits are closed but there are 2 roof vents (not ridge).


1. Drill 2 x 2 1/8″ hole per rafter bay in soffits, staple insect screening overtop, vinyl soffit covering
– attic is around 300 sq ft, with 40 rafter bays I get about 144 sq inch of soffit ventilation

I’ll redo the soffits in the upcoming years and will install a continuous soffit vent, but for now I need a quick and easy way to open them up.

2. Insulate rafter bays, soffit to attic. Outside to inside design (roof is staying as is unfortunately, so no wiggle room there)
– roof
– 1 inch air gap
– 1 inch polyiso rigid foam for baffle (sealed in the soffit above top plate with rigid foam to stop wind washing)
– 3 inch dense blown cellulose
– 2 inch polyiso foam directly over rafters
– 1×4 furring strip
– drywall

I attain a fairly miserable R29 on this section, but I’m not sure how else to improve upon this without redoing my roof. Would the ideal for the dense blown cellulose be to install the rigid foam first, and the installer shoot behind it, or would it be easier if they install netting and then I install the foam?

My ducting runs fairly close to the roof behind the knee wall. Should I consider wrapping in fiberglass even though it’s in the conditioned space, as the insulation isn’t great?

3. Insulate the attic
– 18 inches of loose blown cellulose

I attain around R60 here.

When I did the demo on the second floor there was 6 mil poly all along the ceiling. From what I’ve been reading here I shouldn’t worry about re-installing it, but simply ensuring I have solid air sealing. Does this go for the bathroom as well, which has a fan, but will still see a lot more moisture?

4. Insulate the exterior walls, outside to inside
– 4 inches of dense blown cellulose
– 1 inch of polyiso rigid foam
– drywall
I attain an R20 on these walls. Do I need to add furring strips overtop of the foam, or can I attach the drywall directly into the studs? Worried about space on an already small floor.

From what I’ve been reading this seems like the usual recommended plan for insulating a vented gable roof. Ensure there’s ample soffit ventilation (I know 1″ is the minimum, but again there’s not a lot of wiggle room here), rigid foam over rafters to stop thermal bridging, focus on air sealing above all else … I’m wondering if there’s anything I’m missing, or anything I can do to improve the design. I’m staying away from spray foams for a number of reasons which obviously lessens my attainable R value, but it seems to out weigh the risks for me.

Does this system allow me, down the road, to redo my roof to reach a higher R value, or if I do that will I have to remove everything I’m currently doing and start from scratch?

Thanks a lot as always for all of the valuable information provided on the site.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. plumb_bob | | #1

    If you have 2 roof vents for all of those rafter bays, I think something is wrong. Also, 1" gap for ventilation is not enough.

    To properly vent that roof you will need proper sizing of open area at the soffit, and depending on your rafter length and spacing I do not think 2x2" holes will be enough. Then your ventilation channel should be 2", and it should terminate at a ridge vent. A ridge vent is not hard to install after the fact.

    1. cgmorgan | | #2

      Thanks for the response.

      Would you recommend increasing the air gap to 1.5" or 2" and losing that depth of cellulose? Is the trade off for the R value worthwhile in that circumstance?

      For the venting, please correct my ignorance if I'm off, but I believe I read you're supposed to have 1 square foot of venting for every 300 sq feet. With 2 holes per rafter bay we're looking at 80 vents, and if the goal is 144 sq inches we'd need a diameter of about 1.5" to attain that. Am I misunderstanding something, or it's also related to the length of rafters which I'm not taking into consideration?

      Am I overcomplicating it with this, and it would simply be easier for me to install a continuous soffit vent right away?

  2. Expert Member


    - Practically I think your plan sounds fine. From a code perspective you may have trouble as it requires an interior vapour-barrier (hence the ubiquitous poly in Canadian houses).

    - I think plumb_bob was referring to the two roof top vents as perhaps being inadequate, not the ones at the soffits. Do they provide enough vent area?

    - You can fasten the drywall through the foam without adding furring.

    - Insulating ducts is always beneficial.

    - Rather than staple bug screen over the new vent holes I'd suggest something like these plastic vent covers, which are available at most big box stores.

    1. cgmorgan | | #4

      Thanks for the response Malcolm.

      - solid point on the vapour barrier, however I’m worried about the summer humidity and questionable 1950s craftsmanship. I might just go with EPS for baffles, then cellulose, then EPS, then wrap the rooms in Intello. More labour intensive but seems more forgiving, and keeps it to code.

      Edit: as Intello is good for dense pack, would there be an issue if I applied it before the EPS? My installer recommended the netting, but if I could kill two birds with one stone it would be ideal. I just don’t want there to be an issue with a vapour retarder behind EPS, even though it doesn’t count as a vapour barrier.

      - ah, perhaps you’re right. The two roof vents are 11” square each, so 1.62 square ft of attic ventilation. If I remember correctly the rule is 40/60, so should I look for a 3-3.5” vent for each of the 40 rafters? I saw the vents you posted, however my concern is they’ll protrude from the soffit such that I won’t be able to reattach the vinyl soffit back on top of the plywood. Am I incorrect and they’ll lie flush enough?

      Thanks again,


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |