Insulation R-Value Calculator — Assembly R-Value vs. Climate Zone

R-value measures resistance to heat flow, and resistances arranged one after another — like insulation layers stacked in an attic or a wall cavity — simply add together: the total assembly R-value is the sum of each layer's own R-value, and each layer's R-value is its published R-value-per-inch figure multiplied by how many inches thick it is. That's the entire arithmetic this calculator runs, but the material figures it multiplies by matter: fiberglass batt, blown fiberglass, cellulose, XPS and polyiso rigid foam boards, and open-cell or closed-cell spray foam all insulate at meaningfully different rates per inch, and this calculator uses a single commonly cited representative value for each — for example roughly 3.2 per inch for fiberglass batt versus roughly 6.2 per inch for polyiso foam, nearly double for the same thickness. A custom-value option is included for a product whose data sheet states a different exact figure than the representative one used here.

Knowing your total R-value only matters relative to a target, so this calculator compares it against a small reference table built from published US Department of Energy climate-zone guidance — the same 8-zone map (from Zone 1's hot, humid climates like Miami through Zone 8's subarctic conditions like Fairbanks, Alaska) that US building codes use to set recommended insulation levels for attics, walls and floors. DOE's own published guidance is expressed as ranges that shift further with construction details like whether framing is wood or steel, so this tool picks one representative recommended value per zone and application to give a clear meets-or-falls-below verdict, rather than pretending to reproduce every branch of the full guidance.

Two honest limits are worth stating plainly. First, the material R-per-inch figures used here are single representative numbers pulled from commonly cited published ranges — real products vary by manufacturer and density, so a product's own data sheet is the authority when it disagrees with the figure used here. Second, and more importantly, a real wall or ceiling assembly loses some of its calculated R-value to thermal bridging: heat moves more easily through solid wood or steel studs and rafters than through the insulation between them, so a wall's true effective R-value is somewhat lower than a simple sum of its insulation layers alone — engineers call this the difference between 'cavity R-value' and 'whole-wall R-value'. This calculator computes the straightforward cavity/layer sum, which is still the right first step for sizing an insulation job, but it is not a substitute for a full thermal-bridging calculation on a critical energy-modeling project. Everything here runs locally in your browser — nothing you enter is uploaded or stored.