How to use the insulation r-value calculator
- List each layer of the wall, roof or floor with a name.
- For each layer enter its R-value, or its thickness and R per inch if only those are known.
- Optionally enter a target R-value, an area and a temperature difference.
- Read the total R-value, the U-factor, how each layer contributes and the heat flow.
Formula
Total R = R1 + R2 + R3 + ... for layers one behind another
Layer R = thickness x R per inch, when the R-value of the layer is not given
U-factor = 1 / total R
Heat flow in BTU per hour = area x temperature difference / total R
Adding R-values works only for layers in series. Heat that travels around the insulation through framing is not included.
R-13 batt plus R-5 foam board
A fiberglass batt of R-13 and a foam board of R-5 give a total of R-18. The batt supplies 72.2% of it and the foam 27.8%. The U-factor is 1 / 18 = 0.0556.
Across 1,000 square feet with a 40 degree temperature difference, the heat flow is 1,000 x 40 / 18 = 2,222.2 BTU per hour. If your goal was R-30, the assembly falls short by 12. Entering a layer as 3.5 inches at R 3.7 per inch gives R-12.95 for that layer, and the example values are not recommendations.
Series and parallel paths
Insulation in a wall does not cover everything. Studs and joists let more heat through than the cavity does, so the whole wall performs worse than the cavity R-value alone suggests. This tool adds layers in series only, so treat the result as an upper bound for a framed assembly.
To find how much material a target needs, use the insulation calculator. To see what heat loss costs in energy, the electricity cost calculator is a next step.
What the U-factor is
The U-factor is the reverse of R: how much heat passes per hour per square foot per degree of temperature difference. Windows are usually rated by U-factor and walls by R-value, so converting helps compare them. A lower U-factor means a better barrier.
The heat flow shown is a steady state estimate for a constant temperature difference. Real buildings also gain and lose heat through air leaks, sunlight and ventilation. Read about those costs in the solar payback calculator.
Assumptions and limits
- Every R-value, thickness and R per inch is whatever you enter. Nothing is built in.
- Layers are in series and the whole area has the same layers.
- Air films, air gaps, moisture, compression and air leakage are not modeled.
- This is not an energy code or engineering calculation.
Frequently asked questions
How do I add R-values?
Add the R-values of each layer in the path of the heat. R-13 plus R-5 is R-18.
How do I find the R-value from thickness?
Multiply the thickness in inches by the R-value per inch for that material. A layer that is 3.5 inches at 3.7 per inch is 12.95.
What is the U-factor?
It is one divided by the total R-value. It measures how fast heat passes through, so a lower number is better.
Why is a real wall worse than the total here?
Framing carries heat around the insulation, so the average over the whole wall is lower than the R-value of the cavity alone.