🧊 Insulation R-Value Calculator
Enter your area, target R-value and insulation type to find the thickness needed and approximate volume.
How thickness comes out of an R-value
Every material has an R-value per inch, and R-value is linear with depth, so the thickness is just the target divided by that figure. The volume is that thickness spread over your area.
thickness(in) = target R / R per inch
cubic feet = area x (thickness / 12)The defaults - 1,000 sq ft at R-49 in fiberglass batt at R-3.2 per inch - need 15.3 in, or about 1,276 cubic feet of material. The same R-49 in blown cellulose is 13.2 in, and in closed-cell spray foam 7.5 in.
What the number does not include
- Depth may be fixed before you start. A 2x4 wall gives 3.5 in and a 2x6 gives 5.5 in, so R-49 does not go in either. Squashing a thick batt into a shallow cavity loses most of the extra R - the loft is what insulates, not the fibre.
- Blown cellulose settles. It has to be installed deeper than the finished depth, and bag labels give coverage at a settled depth for exactly that reason. Follow the bag's sq ft per bag at your depth, and add 10% for the material that ends up on the attic floor and in the hose.
- Batts get trimmed. Around wiring, pipes, blocking and at the ends of a run, so allow 10% over the gross area, and more in an attic full of trusses.
The finished assembly also measures below the cavity figure. Wood framing runs about R-1.25 per inch, so a stud is a low-R path straight through the wall - fill it with R-21 and the whole-wall value typically lands 15 to 25% lower once framing, plates and headers are counted. Layered insulation is the opposite case and does add up: R-19 laid over an existing R-30 really does give R-49, provided the second layer runs crosswise and covers the joists.
Frequently asked questions
How thick is R-49 insulation?
About 15.3 in as fiberglass batt, 13.2 in blown as cellulose, 17.5 in as blown fiberglass, or 7.5 in in closed-cell spray foam. The depth is what varies between products - the R-value is the same.
Can I put new insulation on top of old?
Yes for attics, and the R-values add together. Lay the new layer at right angles to the joists so it covers the wood as well as the bays, and leave it unfaced - a second vapour barrier partway through the stack traps moisture between the layers.
What R-value fits in a 2x6 wall?
R-19 to R-21, because the cavity is 5.5 in deep and the batt has to fill it without being compressed. Getting past that in the same wall depth means closed-cell foam in the cavity or a layer of rigid board over the studs.