Insulation R-Value Calculator
Pick your climate zone and what you are insulating, and this works out the R-value you should be aiming for, the inches of material that takes, the quantity and cost, and how many winters it takes to pay for itself.
What you are insulating
Material
Energy savings assumptions
What this estimate assumes
How to use this calculator
1. Measure the area, not the volume
Insulation is sold and specified by surface area. For an attic, that is the floor footprint of the space you are covering, not the roof surface. For walls, take the outside perimeter times the wall height and subtract the windows and doors. For a crawlspace you are usually insulating either the floor above it or the perimeter stem walls — pick one and measure that surface.
2. Find your existing R-value
Measure the depth of what is already there and multiply by the material's R per inch. Loose grey-brown fluff is cellulose at roughly R-3.5 per inch; yellow or pink batts are fiberglass at about R-3.1; white or grey pellets are old vermiculite at roughly R-2.7 and may contain asbestos, so leave it alone and call a pro. A bare cavity is 0.
3. Check the depth against the space you have
The headline number on this page is inches, and that is deliberate. A 2x6 wall cavity gives you 5½ inches, so R-21 is realistic and R-38 is not without adding exterior foam. Attics are the opposite — depth is nearly free up there, which is why attics get the highest recommendations and the fastest payback.
4. Sanity-check the payback
The savings figure only counts heating. If you run air conditioning for a long season, real savings are higher than shown. On the other hand, material cost is not the whole job cost — rental blowers, vent baffles, air sealing, and labour all sit on top. Treat the payback as a ranking tool between options rather than a promise.
Insulation R-value FAQs
What R-value do I need for my climate zone?
The US Department of Energy publishes a recommended band per climate zone and per part of the house. Attics run about R-30 to R-49 in the hot southern zones 1 and 2 and about R-49 to R-60 in the cold zones 6 through 8. Walls are far shallower because the cavity is only a few inches deep, typically R-13 to R-21. This calculator loads the band for the zone and location you pick and uses the low end of the band as the default target, which you can edit.
How many inches of insulation is R-49?
Divide the R-value by the material's R per inch. R-49 of blown cellulose at R-3.5 per inch is 14 inches. The same R-49 in closed-cell spray foam at R-6.5 per inch is 7.5 inches, and in blown fiberglass at R-2.5 per inch it is 19.6 inches. Depth is the number that decides whether the insulation actually fits in your joist bay or wall cavity.
Does insulation R-value add up when I layer it?
Yes. R-values of layers in the same assembly add. If your attic already has R-19 and you blow another R-30 on top, the assembly is roughly R-49. That is why this calculator subtracts your existing R-value from the target and only prices the difference. The catch is compression: squashing a batt into a shallower cavity loses R-value, so never stuff an R-30 batt into a 2x6 bay.
How is the annual energy saving estimated?
It uses the standard degree-day heat loss model: annual heat loss in BTU equals area ÷ R-value × heating degree days × 24. The saving is the difference between the old and new assemblies, divided by your heating system efficiency, then priced at your fuel rate. The bare assembly is credited with the R-value it has before any insulation — roughly R-2.5 for a ceiling, R-4 for a frame wall, R-3 for a floor or crawlspace wall — otherwise a starting R-value of 0 would divide by zero and promise infinite savings.
Is more insulation always worth it?
No. Savings follow 1/R, so the first R-10 saves far more than the tenth R-10. Going from R-11 to R-30 cuts heat loss through that surface by about 63%, but going from R-30 to R-49 only trims another 14% of the original. Use the slider on this page to push past the recommended target and watch the payback period get longer while the extra dollars saved shrink.
Do air sealing and moisture control matter more than R-value?
Often, yes. Air leaking through gaps around can lights, top plates, and attic hatches bypasses the insulation entirely, and insulation laid over a leaky ceiling underperforms its label badly. Seal first, then insulate. In crawlspaces and cathedral ceilings, vapor control and ventilation details decide whether the assembly stays dry, which is why closed-cell foam is common there despite the higher price per inch.