Insulation R-value calculator
A batt labeled R-21 can produce a wall far below R-21 once studs join the heat-flow path. This insulation R-value calculator shows nominal R, framing-corrected R, U-factor, metric RSI, and conductive heat loss.
R-value slows heat flow; U-factor describes what gets through
Higher R is better. Lower U is better. They are reciprocals when the units and assembly match, which makes U convenient for parallel paths. The calculator converts each path to conductance, area-weights them, and turns the result back into a whole-assembly R.
5.5 in fiberglass
R-17.6 nominal
R-3.2 per inch before drywall, sheathing, or framing correction.
R-20 assembly
U-0.050
The reciprocal in U.S. customary thermal units.
R-20 in metric
RSI 3.52
Metric U is about 0.284 W/m2-K.
The whole-wall correction uses parallel paths
A wood-framed wall has insulated cavities and a framing path through studs, plates, headers, and blocking. With a 23 percent framing fraction, nearly a quarter of the wall follows the weaker wood path. Continuous insulation helps because it covers both paths.
Framing fraction is project-specific. Advanced framing can lower it; small walls crowded with openings can run higher. The default is a planning value, and assembly test data or a detailed takeoff deserves priority.
Cavity path
Insulation plus continuous layers
Framing path
Wood plus continuous layers
Use heat loss as a comparison, not an HVAC size
Conductive heat flow is U multiplied by area and indoor-outdoor temperature difference. Change only the assembly R-value to compare retrofit options under the same weather. A drop from 1,500 to 900 Btu per hour across that surface is meaningful, though it says nothing about the air leaking around the window beside it.
Heating and cooling equipment is sized from a full load calculation with windows, infiltration, orientation, ducts, internal gains, and local design temperatures. Keep this result in its lane.
Size the batt purchase for a framed wallQuestions people ask before buying
How do I calculate insulation R-value?
Multiply the tested R-value per inch by installed thickness, then add the R-values of other continuous layers. Six inches of a product rated R-3.5 per inch contributes R-21. Real assemblies can perform lower when framing, gaps, compression, or thermal bridges create easier heat-flow paths.
How do I convert R-value to U-value?
For the same area and unit system, U is the reciprocal of R. An R-20 assembly has a U-factor of 0.05 Btu per hour per square foot per degree Fahrenheit. Use the effective whole-assembly R-value rather than the cavity label when estimating heat flow.
What is RSI and how does it compare with U.S. R-value?
RSI is thermal resistance in square meter kelvins per watt. Multiply U.S. R-value by 0.1761102 to convert to RSI. R-20 is about RSI 3.52. Metric U-value is the reciprocal of RSI and is reported in watts per square meter kelvin.
What R-value per inch should I use?
Use the value in the exact product data or label. Planning values often sit near R-3.2 for fiberglass batts, R-3.7 for cellulose, R-3.6 for open-cell foam, and R-6.5 for closed-cell foam. Density, temperature, moisture, age, and test method can move a product away from a generic number.
Why does framing reduce wall R-value?
Wood studs usually resist heat less than an insulated cavity. Heat moves through both paths at once, so averaging their R-values is incorrect. The calculator averages conductance across the framing and cavity fractions, then converts that combined U-factor back to effective R-value.
Does the heat-loss result include air leakage?
No. It estimates steady-state conduction using U multiplied by area and temperature difference. Infiltration, duct leakage, solar gain, thermal mass, wind, ground coupling, and moisture are outside this calculation. HVAC sizing needs a room-by-room load method rather than this one result.
Method, sources and limits
Insulation R equals rated R per inch multiplied by thickness. Other continuous layer R-values are added. When framing correction is on, the calculator combines cavity and framing U-factors by their area fractions. Heat flow equals effective U times area times temperature difference.
NIST factors convert U.S. R to RSI by multiplying by 0.1761102 and U.S. U-factor to metric U by multiplying by 5.678263. Rounded display values can differ slightly from calculations made with the unrounded result.
- NIST Guide to the SI, thermal conversion factors (National Institute of Standards and Technology)
- Insulation materials and typical performance (U.S. Department of Energy)
- Whole-wall thermal performance guidance (Oak Ridge National Laboratory)