What’s the best way to boost the energy efficiency of my 65-year-old frame house while I’m redoing exterior walls this season? My plan is to remove all exterior layers, including the original 1″ X 6″ sheathing boards, then fill the exposed stud wall cavities from the outside with spray foam, reinstall 5/8″ exterior-grade plywood sheathing on top, then 1″-thick sheet foam, building wrap and finally some factory-finished plank siding. Is there any way I can improve my plan? Our house is very drafty and expensive to heat, so I need to do something.
I think your plan is a good one, though you’ll want to count the costs beforehand. Besides being a lot of work, the spray foam component will be expensive. Get some hard numbers before you begin. You’ll also need to make sure that 
the foam you’re using is impervious to air infiltration. Not all spray foams are, and this is essential for your installation to resist internal condensation developing inside wall cavities. Polyurethane foam is one option that I know works well here. I’d also recommend creating drainage passages behind the siding before it goes down, to allow any leaked rainwater to trickle out harmlessly from the bottom of the wall. The best material I know for this job is DELTA-DRY rainscreen (905.563.3255). It’s a dimpled plastic sheet material that’s installed underneath the siding, creating a vertical space for drainage out of the bottom.
The good news is that your plan will result in enormous savings for heating and cooling. If your place is leaky right now, and if you upgrade all exterior walls and windows, I expect you’ll see at least a 60% to 80% reduction in overall energy consumption. Your house will also become draft-free and far less outdoor noises will be audible inside.
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