R-Value and Heat Loss Calculator

How do you calculate heat loss from an R-value?

Multiply the area by the temperature difference across it and divide by the total R-value, the sum of the R-values of every layer the heat crosses: Q = A × ΔT / R. Area in ft², the difference in °F and R in ft²·°F·h/Btu give heat loss in Btu/h; m², K and RSI in m²·K/W give watts.

What an R-value measures

An R-value is a thermal resistance: the temperature difference it takes to drive a unit of heat flow through a unit of area. R-19 means a difference of 19 °F pushes 1 Btu/h through each square foot. The heat loss is proportional to 1/R, so each added inch of insulation saves less than the one before it. On 1,000 ft² at a 40 °F difference, R-19 lets through 2,105 Btu/h and R-38 half that, 1,053 Btu/h, while the next R-11, to R-49, saves only 236 Btu/h more.

The calculation assumes steady conduction through flat layers, the case Fourier’s law of heat conduction describes. Enter each layer’s R-value as installed, not a figure per inch: a batt’s label gives its R-value at its full thickness, and loose-fill insulation reaches a given R-value only at the depth its bag lists for it.

R-value, RSI and U-factor

A US R-value is in ft²·°F·h/Btu. The SI value, often written RSI, is in m²·K/W, and it is the one Australia and New Zealand print on insulation and call R, so an Australian R2.5 batt is RSI 2.5, about R-14.2 in US units. Each US R is 0.1761102 m²·K/W, the factor NIST SP 811 lists, so multiply a US R-value by 0.1761102 for RSI and an RSI value by 5.678263 for US R. Choose the unit on each layer and the calculator does the conversion.

The U-factor is the reciprocal, U = 1/R: the heat flow per square foot for each degree of difference, so lower is better. US window labels from the NFRC give a U-factor rather than an R-value, and in the UK and Europe walls and roofs are usually specified by U-value in W/(m²·K). A window with a U-factor of 0.30 is R-3.3, so each square foot of it conducts almost four times the heat of a square foot of R-13 wall cavity.

Adding layers in series

Heat leaving through an attic floor or a wall crosses each layer in turn, and at steady state the same heat flow passes through all of them. Each layer takes a share of the temperature drop in proportion to its R-value, the drops add up to the whole difference, and so the R-values add, the way resistors in series do in the resistor network calculator. Insulation laid over insulation, or foam board over a stud wall, is a second layer.

The thin film of air at each surface resists heat flow as well. The ASHRAE Handbook, Fundamentals gives R-0.68 for still air at the inside face of a wall and R-0.17 for the outside face in a 15 mph wind, and ISO 6946 uses 0.13 and 0.04 m²·K/W for the same two faces. Enter them as a layer of their own to count them.

R-values add only along one path. Where heat has routes side by side, through the batts between the studs and through the studs themselves, or through a window set in a wall, each route is a separate area. Work out the heat loss of each one and add the heat losses. Wood framing conducts more heat than the insulation it interrupts, so a framed wall loses more than its cavity R-value alone would suggest.

Recommended R-values for attics, walls and floors

ENERGY STAR, run by the US Environmental Protection Agency and Department of Energy, publishes the insulation levels below for adding insulation to existing wood-framed homes. It bases them on Table R402.1.3 of the 2021 International Energy Conservation Code (IECC), whose climate zones run from 1, the warmest, to 8, the coldest.

For walls above ground, ENERGY STAR’s advice applies whenever the exterior siding comes off. An uninsulated wood-frame wall should have insulation blown into the empty cavity, with R-5 insulative wall sheathing added beneath the new siding in zone 3 and R-5 to R-10 in zones 4 to 8. An insulated 2x4 wall in zones 4 to 8 gets R-10 sheathing. For basement and crawlspace walls it gives R-5 sheathing or an R-13 batt in zone 3, R-10 sheathing or an R-13 batt in zones 4A and 4B, and R-15 sheathing or an R-19 batt in zones 4C and 5 to 8.

These are levels for improving a home that already stands. New construction follows the energy code adopted where it is built. To see what a recommendation is worth, enter the insulation already in place as layer 1 and the insulation to be added as layer 2, then compare the heat loss with layer 2 set to 0.

From heat loss to a heating bill

Heat loss is a rate, so it becomes energy only over time. An electric resistance heater turns each watt it draws into a watt of heat, so the heat loss in watts is the power it needs to keep up with this surface alone. Enter that power in the electricity cost calculator with the hours it runs to price it. A heat pump delivers more heat than the electricity it uses, so divide the heat loss by its coefficient of performance first.

The 24-hour readout holds the temperature difference fixed for a whole day, which a real day does not. For a season, energy auditors use heating degree days, which NOAA counts from a base of 65 °F, in place of one fixed difference. And conduction is only part of a building’s heat loss: air leaking through gaps carries heat out without crossing the insulation at all.

Common mistakes

• Adding U-factors instead of R-values. Layers in series add their R-values. Adding the U-factors of R-19 and R-30 layers gives 0.0860, which is R-11.6, less than either layer on its own, when the pair is R-49.
• Entering a temperature as the difference. With 70 °F inside and 30 °F outside, the difference is 40 °F. In SI that difference is 22.2 K; 277.6 K is what 40 °F would be as a temperature.
• Reading RSI as a US R-value. An RSI 3.5 batt is R-19.9. Entered as R-3.5, it makes the heat loss 5.7 times too high. Pick m²·K/W on the layer instead.
• Taking the cavity R-value for the whole wall. The R-13 on a batt applies between the studs. The studs, the windows and the doors are other paths with their own heat loss.

Converting units first? Use the temperature difference, area and power conversion tables.