Climate Zone 6A – A friend with a tight budget is getting new roof sheathing and shingles due to hail damage. The home is 60+ years old, drafty, with minimal attic insulation.  They have attic trusses, so there is a kneewall-to-cathedral-to-small overhead attic condition.

8-12″ of continuous exterior insulation (they would only do EPS) is likely out of their budget.

Would it be disastrously ill-advised for them to just do 4″ of HFO closed-cell spray foam the backside of their ceiling/walls during the short time that all their roof decking is removed?

I understand its not an idea R-value…but seems like it would be a substantial improvement for vapor, air leakage and R-value for what they have and not risk long term mold/moisture issues like just packing batts in would.