Underfloor insulation wins
A recent underfloor DIY thread documented layered insulation, sealing foundation gaps with foam board and targeted spray foam — a repeatable approach for cold‑floor problems (x.com). The thread also emphasizes sealing gaps at the foundation perimeter before adding insulation to stop drafts at the source (x.com).
Independent guidance stresses air‑sealing foundation and rim‑joist gaps before adding thermal layers, and ENERGY STAR categorizes basement/crawlspace sealing and insulating as a recommended but moderate‑to‑difficult DIY project. (energystar.gov) Rigid‑foam options differ materially: polyisocyanurate commonly delivers about R‑6 per inch (with cold‑weather derating), XPS about R‑4.5–5.0 per inch, and EPS roughly R‑3.8–4.4 per inch, so board type and thickness change the effective R‑value under a floor. (insulationrvalues.com) Closed‑cell spray foam used to seal sill plates and rim joists typically provides about R‑6–R‑7 per inch while acting as an air‑ and moisture‑resistant barrier, and retrofit guides commonly specify applying roughly 1–2 inches in tight perimeter cavities. (r-valueassociates.com) Guides and calculators put the payoff in plain numbers: insulating floors and crawlspaces can cut whole‑house heating use by about 10% in many homes, and industry estimates say unsealed rim joists can account for roughly 15–25% of thermal leakage around the building envelope. (greenyplace.com) Building Science research warns that interior foam applied at the foundation perimeter needs protection from physical damage and pests and that some jurisdictions require a removable inspection strip to preserve termite/perimeter defenses. (buildingscience.com) Material and retrofit pricing varies: common rigid foam panels typically cost in the neighborhood of $0.75–$1.25 per square foot at retail, while trade guides quote targeted spray‑foam rim‑joist work at roughly $0.75–$4.00 per linear foot depending on kit vs. contractor application. (oneanddoneprep.com)
