Guides
Plain-English background to the numbers behind the calculators — how much insulation you need, what R-value to aim for, how to read an assembly’s R-value, blown-in bags vs depth, spray-foam board-feet and kits, attic and spray-foam cost, wall and basement thermal insulation, rigid foam, material comparisons, and whether more insulation pays off.
How much insulation do I need?Measure the area, set a target R-value, pick a material, then convert to batt bundles, blown-in bags, spray-foam board-feet or rigid sheets. Worked example.What R-value do I need? Attic, wall & floor by climate zoneYour IECC climate zone and the assembly set the target: attics R-30 to R-60, walls R-13 to R-21 cavity, floors R-13 to R-30. The bands and how to read them.How to calculate the R-value of a wall or ceilingR-values add in series: R-assembly = sum of each layer's thickness times R-per-inch, plus air films. Convert to U with U = 1 / R. Worked 2x6 wall example.How many bags of blown-in insulation do I need?Two methods: bags = area / coverage-per-bag-at-R off the printed chart, or bags = area x depth / bag yield. Worked 1,000 ft2 attic example inside.Batt insulation coverage & how many bundlesBundles = ceil(area / coverage-per-bundle), and coverage falls as R rises. Worked wall example, the coverage-by-R table, and the gotchas on width and waste.Spray foam board-feet, kits & sets (open vs closed cell)Board-feet = area x thickness; sets = ceil(board-feet / kit yield). Open-cell needs more depth per R, closed-cell less. Worked 500 ft2 example inside.Attic insulation cost & how much you needAttic cost = area x your $/ft2 x (1 + contingency); quantity by bags or bundles to a target R. Worked 1,200 ft2 example and a blown-in vs batt comparison.Spray foam insulation cost guide (attic, walls, board-feet)Foam cost = board-feet x your $/board-foot + labor, x (1 + contingency), or area x $/ft2. Worked board-feet and by-area examples, plus open vs closed cell.Wall insulation cost & quantity (cavity R, batts vs blown)Net area = length x height minus openings; bundles = ceil(net / coverage); cost = area x your $/ft2. Worked example, cavity-R limits, and batts vs blown-in.Basement, crawlspace & rim-joist thermal insulationThermal insulation of below-grade walls, crawlspace floors and rim joists: area x your $/ft2 for cost, plus rigid pieces or board-feet for the rim joist.Rigid foam board: EPS vs XPS vs polyiso (R-value & sheets)R = thickness x R-per-inch; sheets = ceil(area / 32) for 4x8 boards. EPS ~4.0, XPS ~5.0, polyiso ~6.0 R/inch. Worked example plus where each foam wins.Fiberglass vs cellulose vs mineral wool (R/inch, cost, use)Depth = target R / R-per-inch, cost = area x your $/ft2, compared across the three. Worked R-49 attic example plus where each material earns its place.Will more insulation pay off? A rough payback from your billEstimate a relative heat-loss cut from U = 1/R, apply it to the share of your heating bill through that assembly, for a rough payback. Worked R-11 to R-38 case.