What this calculator does
Insulation is sold in R-values and paid for in dollars, and the connection between them is not obvious. The useful fact is that heat loss depends on 1 divided by R, so the first few units of R do nearly all the work and everything after that is a long tail.
Going from R1.5 to R3 halves the heat lost through that surface. Going from R3 to R4.5 only removes another sixth of the original. The second upgrade costs about as much as the first and saves a third as much, which is why there is a sensible stopping point.
The formula
The U-value is 1 ÷ R, in watts per square metre per degree. Multiplying by area, by heating degree days and by 24 hours gives watt hours lost in a year, and dividing by 1,000 gives kilowatt hours. Doing that for both R-values and subtracting gives the heat saved. Dividing by the heating system efficiency converts heat into the energy you actually buy, and multiplying by the price gives the saving.
| Term | Meaning |
|---|---|
| R-value | Thermal resistance in m²K/W. Higher is better, and the relationship to heat loss is reciprocal rather than proportional. |
| U-value | 1 divided by R. Watts lost per square metre per degree of temperature difference. |
| Heating degree days | A measure of how cold a year is, summing each day's shortfall below a base temperature. Published for most regions. |
| Efficiency or COP | How much heat you get per unit of energy bought. Around 0.85 for a gas boiler, 1 for resistance heating, 3 or more for a heat pump. |
The inputs explained
| Field | What to enter |
|---|---|
| Area being insulated (m²) | The area of the one element being upgraded, such as the ceiling. This is not a whole-house calculation. |
| R-value now (m²K/W) | The R-value now. An uninsulated timber ceiling is often around 0.5, and older batts settle well below their original rating. |
| R-value after the upgrade (m²K/W) | The R-value after the work, which is the total assembly value rather than just the new batt if you are adding to existing insulation. |
| Heating degree days per year (°C·days) | Heating degree days for your location, published by meteorological services. A mild temperate climate is around 1,500 to 2,000. |
| Heating efficiency or COP | Enter a COP for a heat pump, or a decimal efficiency for combustion. A gas boiler at 85 per cent is 0.85. |
| Energy price ($/kWh) | Price per kilowatt hour of whatever energy the heating system uses. |
| Cost of the upgrade ($) | Installed cost of the upgrade, materials and labour. |
When to use it
Deciding how far to go on R-value
Step the target R-value up and watch the saving flatten. Going from R1.5 to R4 saves $203 a year; going on to R8 adds only another $60. The payback gets worse with every step past the obvious one.
Comparing across heating systems
The same insulation saves nearly four times as much money for a gas household as for a heat pump household, because the heat pump buys each unit of heat so much more cheaply. Efficient heating makes insulation pay back more slowly, which is counterintuitive and true.
Checking an uninsulated ceiling
Set the current R-value near 0.5. The saving is dramatic because the first increment of R removes most of the loss, and this is almost always the best value energy work available on an older house.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How far is it worth raising the R-value?
Only the target R-value changes.
| R-value after | Saving per year | Heat lost after | Simple payback |
|---|---|---|---|
| R2 | $81.00 | 2,592 kWh | 30.9 years |
| R3 | $162.00 | 1,728 kWh | 15.4 years |
| R4 | $202.50 | 1,296 kWh | 12.3 years |
| R6 | $243.00 | 864 kWh | 10.3 years |
| R8 | $263.25 | 648 kWh | 9.5 years |
| R10 | $275.40 | 518 kWh | 9.1 years |
Does the heating system change the payback?
The fabric work is identical and only the heating system changes.
| Efficiency or COP | Saving per year | Simple payback |
|---|---|---|
| 0.7 | $1,110.86 | 2.3 years |
| 0.85 | $914.82 | 2.7 years |
| 1 | $777.60 | 3.2 years |
| 2.5 | $311.04 | 8.0 years |
| 3.2 | $243.00 | 10.3 years |
| 4.5 | $172.80 | 14.5 years |
Questions
Is this a whole-house calculation?
No. It covers one element, such as a ceiling or a wall area, and ignores every other surface, windows, ventilation and air leakage. For whole-house heating capacity see the furnace size calculator.
Where do I get heating degree days?
National meteorological services publish them by region, usually to a base of 15 or 18 degrees. Use the base your source quotes and do not mix sources.
Why does better heating make the payback worse?
Because a heat pump buys heat at a third of the cost, so the same heat saved is worth a third as much money. It is a real effect and it is why fabric work is usually recommended before changing the heating system.
Should I count the whole new R-value or just the batt?
The whole assembly. Adding R4 batts on top of a settled R1.5 gives roughly R5.5 in total, not R4, and entering just the batt rating understates the result.
Does insulation help in summer too?
Yes, and this does not count it. Where cooling is a significant load the real saving is larger than the figure shown, so treat the payback as conservative.
For thickness and materials rather than economics, see insulation thickness and coverage. The heating side is on furnace size and heat pump running cost.