Sources & formulas

Every calculator on InsulationCalcs rests on established building physics plus stable insulation conventions. Unlike topics with a time dependency, “verification” here is mathematical: each formula is tested against known values and is therefore correct for good. Here are the technical foundations by area.

Identities & physics

  • Assembly R-value: R of a layer = thickness × R/inch; assembly R = the sum of the layers, plus interior (0.68) and exterior (0.17) air films where relevant.
  • U-value: U = 1 ÷ R and R = 1 ÷ U — the electrical-thermal analogy (conductance = 1/resistance).
  • Required thickness: thickness = target R ÷ R/inch.
  • Quantities: batt bundles = ceil(area ÷ coverage); blown-in bags = ceil(area ÷ coverage-at-R) or ceil(area × depth ÷ bag yield); board-feet = area × thickness; spray-foam sets = ceil(board-feet ÷ yield); rigid sheets = ceil(area ÷ 32). 1 board-foot = 1 ft² at 1 inch; a 4×8 sheet = 32 ft².

Convention typicals (labeled)

  • R/inch by material: fiberglass batt ~3.1–3.4, blown fiberglass ~2.2–2.7, cellulose ~3.2–3.8, mineral wool ~3.0–3.3, open-cell foam ~3.5–3.7, closed-cell foam ~6.0–7.0, EPS ~3.6–4.2, XPS ~4.5–5.0, polyiso ~5.6–6.5 — labeled, confirm on the data sheet.
  • Climate-zone target R: IECC/ENERGY STAR attic R30–R60, wall cavity R13–R21, floor R13–R30 — labeled, confirm current code.
  • Coverage: blown coverage-per-bag falls as R rises (e.g. R-38 ~37 ft²/bag, R-49 ~28 ft²/bag); batt coverage per bundle likewise (e.g. R-13 ~88 ft², R-38 ~40 ft²) — labeled, read the bag.
  • Spray-foam yields: DIY kits ~200 and ~600 board-feet; larger pro sets yield more — labeled, confirm on the kit.
  • Waste / overage: ~5–10% for framing, gaps and settling — labeled default.

Cost tools

  • Every cost tool uses the prices you enter ($/ft², $/board-foot, $/bag, $/bundle, labor $, removal $, $/kWh, $/therm) — no material or labor price is stored, so the site needs no maintenance. Cost bands are a labeled sanity guide.
  • Cost: total = (quantity × your $/unit + labor + add-ons(air-sealing, removal, access) − discount) ×(1 + contingency).
  • Simple payback: a RELATIVE before/after heat-loss index (area × U × ΔT) × your heating cost → annual saving → payback — never an HVAC size or an absolute load.

Material and labor prices, product data-sheet values, code and local rules vary by place and change over time — always confirm coverage-per-bag, R/inch and set yield against the exact product, order a little extra (~5–10%) for framing, gaps and settling, follow the product data sheet and your local energy code, defer fire/vapor detailing and energy audits to a certified professional, and get itemized written quotes from a licensed, insured insulation contractor before you commit.