I am a first time home builder.
Building a home entirely with my own two hands.
Loving the learning process and having a great time expanding my skill set.

I have backed myself into a bit of a corner on my roof insulation system and want to get on a good plan for finishing it before I go any further.

This building is in climate zone 6B, very long winter with snow on the roof for more than half the year. Roof has good solar exposure when not under snow.

This is a flat roof, 1/4″ rise for each foot of roof, so just enough to drain water.

Roof is supported by parallel chord 2x wood trusses, creating a cavity that is 27.5″ deep at the high end and 24.5″ deep at the low end.

I had a very strong desire to avoid foam on this building and did not know about rockwool TopRock when I did the initial roof installation, so here is where I currently stand:

Outside/sky
EPDM self adhered roofing membrane
3/4 TG CDX roof deck/sheathing
Trusses
R23 Rockwool comfort batts directly against the underside of roof sheathing
Cardboard baffles to keep the rockwool in place

My hope had been to use the rockwool on the underside of the roof sheathing to avoid having any air gap, and prevent the underside of the roof sheathing from becoming a condensation surface.

I had planned to then fill the rest of the roof cavity with loose fill blow in cellulose which would give me something like R80 overall, a little bit less once the cellulose settles.

The other hope, was that the gap between the cellulose and the cardboard would create a temperature differential that would make the cardboard the recipient of any condensation, rather than having vapor diffuse further up through the rockwool and onto the underside of the sheathing.

Here are my concerns:

1. The rockwool did not press as tightly against the underside of the sheathing as well as I had hoped. Some places it is perfect, others it sagged a bit and left a 1/8″ air gap.

2. I wanted the rockwool up there because it is not susceptible to mold, but perhaps its ability to allow vapor to diffuse through it will just mean more moisture gathering on my roof sheathing and rotting my roof out in a couple years.

Here are the options I am contemplating:

1. Sky
EPDM self adhered roofing membrane
3/4 TG CDX roof deck/sheathing
Trusses
R23 Rockwool comfort batts directly against the underside of roof sheathing
Cardboard baffles to keep the rockwool in place
Loose fill cellulose all the way to underside of cardboard, knowing it will settle.
Intello Plus smart vapor barrier
3/4″ stringers to support vapor barrier and
Drywall ceiling
Clay plaster surface coat
Inside air/dwelling space

2. Retrofit exterior insulation and new decking and membrane for something like this:

Sky
EPDM self adhered roofing membrane
CDX, denso deck, or some sort of walkable rigid roof sheathing
R30 of reclaimed foam (not poly-iso for my climate) or Rockwool Top Rock
Original epdm layer that is already installed
3/4 TG CDX roof deck/sheathing
Trusses
R23 Rockwool comfort batts directly against the underside of roof sheathing
Cardboard baffles to keep the rockwool in place
6″ air gap
Loose fill cellulose, 13″ or so.
3/4″ stringers to make drywall easier to screw in place
Drywall ceiling
Clay plaster surface coat
Inside air/dwelling space

From all the reading I have done on this matter it seems pretty clear that attempting to ventilate this ceiling cavity passively will definitely not help and using active ventilation also seems unlikely to be a good choice.

It seems clear that getting a really tight installation of my drywall ceiling is critical, making sure there are no air gaps to allow moist air to rush up into the cavity, so that the only moisture intrusion is through vapor permeability of the drywall and the possible smart vapor barrier.

I feel confident in my ability to make a perfectly tight ceiling as I am able to take as much time as needed.

I still have very little interest in taking down the rockwool and blowing spray foam on the underside of the roof sheathing, though that would seem to be a very functional solution. I want to build a house that will not only last a long time, I want it to be able to be deconstructed and recycled, wood chipped, or not just be foam covered garbage when the building is finally done.

I wonder about some sort of house ventilation system to help prevent humidity issues and reduce the amount of moist air that could get up into the ceiling cavity.

I wonder about the true effectiveness of smart vapor barriers and if they will still be effective 30, 50 or 100 years from now.

I know that if I do exterior insulation, I should have at least 51% of my R value on the outside of the roof sheathing, yet I can’t help but think that the more cellulose the moisture has to rise up through, the cooler it will be by the time it reaches my rockwool, the less temperature differential there will be, the less likely there will be condensation. But perhaps the thought is that any insulation sitting on the top side of my ceiling, will basically achieve indoor temperature at some point and therefore warm moist air diffusing up through that cellulose will still be warm and moist by the time it hits my cardboard/rockwool and roof sheathing.

I wonder if I would be better off just having cellulose all the way to the roof sheathing as it seems to be better at holding moisture, whereas the rockwool may simply act like a wet sponge and keep wicking moisture into the wood?

I did talk to the folks over at 4 7 5 (I think that is their business name) they said that the following layering would be fine:

Sky
EPDM self adhered roofing membrane
3/4 TG CDX roof deck/sheathing
Trusses
Dense Pack cellulose all the way to underside of roof sheathing allowing no air gaps
Intello Plus smart vapor barrier
3/4″ stringers to support vapor barrier and
Drywall ceiling
Clay plaster surface coat
Inside air/dwelling space

I am not a huge fan of dense packing cellulose as it uses so much more cellulose for not much insulation gain, but perhaps in this case it would be more cost effective than retrofitting insulation on the exterior and having to buy more epdm and plywood given current materials costs.

I am also not a huge fan of the idea of ripping out the rockwool currently in place, but would vastly rather do that, if needed, than rip apart my entire roof in 3 years because it is rotten!

I know this is a long question/post, but I would really appreciate any input. I have been researching this and read many, many great articles about unvented roof assemblies, but haven’t quite found anything that addresses my specific scenario.

Thanks so much GBA and this community for being such great resources!