CategoriesInsulation

Insulation Calculators

Energy efficiency calculators for all insulation types including spray foam, cellulose, and fiberglass. Calculate R-values and potential energy savings.

8 Calculators

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Insulation is specified by R-value, sold by coverage, and limited by cavity depth.

Every insulation decision is a target R-value divided by the R-per-inch of the material, which gives the thickness you need. Fibreglass batts run about R-3.1 per inch, blown cellulose around R-3.7, open-cell spray foam about R-3.7 and closed-cell about R-6.5. That last figure is why closed-cell wins where depth is limited and loses where it is not, because it costs several times more per R.

The constraint that decides the material is usually the cavity. A 2 by 4 wall gives you 3.5 inches, so R-13 to R-15 is the practical ceiling with batts, and reaching R-21 there needs foam. An attic has no depth limit, which is why blown material at R-49 to R-60 is the standard answer up there.

Settle this before you calculate

Target R-value for your climate zone: Attics range from R-30 in the warmest zones to R-60 in the coldest. Walls and floors have their own targets. Get the zone first.
Cavity depth, actual: A 2 by 4 is 3.5 inches and a 2 by 6 is 5.5. That is the hard limit on any batt or blown fill in a wall.
What is already there: Adding to existing attic insulation means you only need the difference. Measure the current depth and work out its R-value before ordering.

Where the estimate goes wrong

Compressing batts to fit: A compressed batt loses R-value in proportion to the squeeze. Cut it to fit the cavity rather than stuffing an oversized one in.
Insulating over soffit vents: Blocking eave ventilation traps moisture in the roof deck. Fit baffles before blowing, and keep the insulation clear of the vent path.
Buying R-value without air sealing: Air leaks bypass insulation entirely. Seal top plates, penetrations and the attic hatch first, or a good R-value underperforms badly.