Hello,

I’ve got a 1914, 1-1/2 story house in zone 6. The roof is 2×4 rafters, currently insulated with dense pack fiberglass. (No ventilation from soffit.) Half the roof is over the conditioned space, the other half is over the unconditioned attic. See photos for clarity.

I’m getting a new roof and see it as my opportunity to improve the insulation from the outside: pull up decking, closed cell spray foam against the back of the lathe & plaster, and I’ll double my R-value. (Yes, I realize it will still only be R-20 at best, and yes I also realize that the optimal solution is actually to install rigid foam over the roof decking. For reasons I won’t get into here, that won’t be happening.)

My spray foam guy said they don’t fill the rafter cavities to the max, which at first I saw as a boon to get airflow to the attic. However, I’m wondering if I’m only opening the door for my decking to rot since:

a) The insulation is still insufficient, inviting moisture to condense on the decking? 

b) I’m switching  from a permeable insulation to an impermeable one (trapping moisture between roofing and spray foam?)

c) The “air gap” will be irregular/inconsistent, so possibly ineffective at drying anything out.

Would I be better off just leaving things alone rather than possibly creating a bigger problem?

The biggest issue, ice dams, is going to be solved by the new roofing being smooth metal. Snow should slide off before it can dam. Aside from that, my heating costs are surprisingly okay, so I really could just turn a blind eye to the heat loss and leave it all alone.  

Thanks!
Suz