Deep energy retrofit of a 100-year-old house – need advice

A homeowner in southeast Alabama posted an online forum question about renovating a 2,200-square-foot, 100-year-old house and asked other builders for feedback on the plan. The original part of the house has old wood framing, a vented crawl space and attic, older fiberglass insulation, and worn siding. A newer addition has different framing, a plastic vapor barrier (Visqueen), and foam board insulation. The planned work includes stripping the old siding, replacing all windows, sealing air leaks, treating the framing against rot and pests, reinstalling insulation without its paper facing, adding new structural sheathing (ZIP system), continuous rigid foam insulation on the outside, a rainscreen gap behind the siding, and new fiber-cement or vinyl siding.

Two people who work in building science responded. One pointed out that swapping in "new construction" style windows will tear up interior trim, while "replacement" windows that reuse old frames are hard to seal tightly against the new sheathing — a tradeoff worth understanding before choosing either option. He also warned that on a house this old, deep renovations often cost far more than the finished house will be worth, and cautioned that walls in century-old homes usually contain lead paint, plus outdated wiring and plumbing that may need replacing too. The second commenter, focused on building science, agreed the sealing and insulation plan sounds sound for Alabama's hot, humid climate, and said the plastic vapor barrier in the addition's walls and ceiling should come out, since it can trap moisture in this climate — though he'd want more detail before advising on the floor.

For any homeowner in a similar house, the exchange is a reminder that gutting and re-insulating old walls raises both moisture and cost questions that go well beyond the visible renovation work.