Q: Our 100-year-old walls are very drafty–quite icy to the touch in winter. In multiple home energy audits, we’ve been told it is impossible to blow insulation into them. If we replaced the siding, would that allow us to add insulation outside the existing walls. Likewise, we definitely don’t have 10 inches to spare in the attic. If we replace the roof, is it possible to add a layer of insulation?

A: If you replace the roof, you can add a layer (or two) of ISO board to the roof on the outside of the plywood sheathing. This is what we did on our flat roof. The DOE recommends that roofs be insulated to at least R49 in MA. This would require 5 layers of ISO board. In my opinion this is spending too much money given how much you can save – insulation has diminishing returns to investment. After about R35-R40 the savings from the next layer of insulation rarely pay for the extra cost. Spend the money you save on a good air-sealing job.