Zone 6 vapour retarder questions – GreenBuildingAdvisor

Hi all! We live just outside Halifax, NS, Canada, climate Zone 6. We are putting up a new building, and I have a question about vapour retarders.

Our wall buildout (interior–>exterior) on the ground level is: drywall, 2×6 framed cavities with Rockwool R24 ComfortBatt insulation, sheathing, VaproShield barrier (orange frogskin), Rockwool ComfortBoard 80 R6 exterior insulation, strapping/air cavity, wood siding.

The wall buildout upstairs (loft spaces, all open to the interior of the building) are the same, except the gable ends, knee walls, and rim joists are done in closed cell foam instead of R24 Rockwool ComfortBatt.

We have two heat pumps going into the building, primarily for Winter heat but also for some limited air conditioning in the Summer. The slab is insulated.

I have read multiple articles (some on GBA) and used a calculator that looks at interior vs. exterior insulation, overall wall buildout, and climate zone to generate a recommendation for what (if any) vapour retarder to use in our situation… I am seeing recommendations for Class I behind the drywall, Class II behind the drywall, and none whatsoever.

Our house, adjacent to the new building and completed in 2015/16, has the same wall buildout as described above but with open cell spray foam in the main floor wall cavities (instead of Rockwool ComfortBatt) and uses a Class I vapour retarder under the drywall. However, we do not have a heat pump/do not air condition the interior space at any point of the year. We have fairly dry Summers and Falls here with a lot of air movement throughout, and just opening and closing windows controls our interior climate just fine in the house. The new building is a workshop space in which climate control is more of a concern (storing a lot of large sheets of paper, for example), but we still expect the cooling/air conditioning needs to be light/low/infrequent.

I am just about done stuffing the wall cavities with Rockwool and have to figure out how to proceed with/without/what-type of vapour retarder. I am tempted to do what we did in the house, because although we will be doing some air conditioning, it will be pretty low-intensity… but I would be happy to hear plastic sheeting might cause more harm than good, and either move to a Class II or go without entirely.

Any insights?