Insulation R-Value Calculator
Insulation R, RSI, and U from thickness and material.
What it does
R-value rates resistance to heat flow: thicker and better foam raise it, while U is its inverse for whole assembly math. This page multiplies thickness by the material rate per inch, then shows RSI and U beside US R.
The default is 6 in of fiberglass at 3.5 per inch. That is R-21 with U 0.048. Metric defaults convert the same batt to 15.2 cm and give RSI 3.69 with U 0.27, a touch below the exact 15.24 cm conversion by rounding.
Rates are lab labels, not field promises. Compressed batts lose loft, blown cellulose settles, and every stud is a thermal bridge around the cavity. Air sealing and continuous exterior board often beat simply stuffing deeper between joists.
Use this for layer comparison. Stack layers by adding R in the same units, then convert once at the end rather than mixing US and RSI mid chain.
How to use
- Pick imperial or metric first.
- Enter insulation thickness.
- Pick the material closest to stock.
- Read R first, then U and thickness.
- Derate for gaps and framing on site.
Formula
- thickness
- Insulation depth in the active units.
- rate
- Label R per inch for the material.
- RSI
- Metric resistance (R-US over 5.678).
Rates are label averages per inch at settled lab conditions. Compression lowers batt R, blown fills settle, and studs bridge heat past cavities, so field R sits below center of cavity math. Thickness converts at 2.54 cm per inch.
Worked example
Example: 6 in fiberglass
6 in fiberglass (metric 15.2 cm).
- Rate: fiberglass 3.5 per inch.
- US: 6 x 3.5 = R-21.
- U: 1 / 21 = 0.048.
- Metric: 15.2 cm is 5.98 in, R 20.9, RSI 3.69.
- Tool shows R-21 imperial and RSI 3.69 metric.
Reference table
| Material | Per inch | 6 in gives |
|---|---|---|
| Fiberglass batt | 3.5 | R-21 |
| Cellulose | 3.7 | R-22.2 |
| Mineral wool | 4.2 | R-25.2 |
| XPS rigid | 5.0 | R-30 |
| Closed spray foam | 6.5 | R-39 |
Limits
- Center of cavity only. Framing bridges heat.
- Label rates. Age and moisture lower them.
- One layer per pass. Stack by adding.
- Ignores air leaks, the larger loss.
Common mistakes
- Mixing US R with RSI mid chain.
- Compressing batts and keeping label R.
- Leaving gaps at edges and wires.
- Adding U values instead of R.
- Trusting thickness without the rate.
Try it yourself
Quick checks. Answers sit tucked under each question.
10 in fiberglass?
R-35 (10 x 3.5), U 0.029 (1 / 35).
3.5 in closed spray foam?
R-22.8 (3.5 x 6.5), RSI 4.01.
Questions
How is R figured?
Thickness times rate per inch. Here 6 at 3.5 is R-21.
What is RSI?
US R over 5.678. Here 21 is RSI 3.70 (3.69 at 15.2 cm).
What is U?
One over R. Here 0.048 US and 0.27 metric.
What rates are used?
3.5 batt, 3.7 cellulose, 4.2 mineral wool, 5.0 XPS, 6.5 spray foam.
Does it work in metric?
Yes. Enter cm for RSI and metric U.
Is this installed performance?
No. Gaps, compression, and framing cut real performance.