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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Heat loss through a surface is area divided by R-value, times the temperature difference. Enter your assembly to see the BTU per hour and what it costs you each year.
With these inputs:
| Result | Value |
|---|---|
| Heat loss now | 4,154 BTU/hr |
| Heat loss after upgrade | 1,102 BTU/hr |
| Reduction | 73% |
| Annual fuel now | 161 therms — $233 |
| Annual fuel after | 43 therms — $62 |
| Annual saving | $171 |
Heat loss (BTU/hr) = area × temperature difference ÷ R-value · Annual load ≈ area × heating degree days × 24 ÷ R ÷ 100,000 · Saving = fuel before − fuel after, at your energy price
Because the attic is only part of the envelope. Going R-13 to R-49 cuts that surface's loss by about 73%, but walls, windows, doors and air leakage keep on losing heat.
Design conditions: your indoor set point minus the coldest typical outdoor temperature. Forty to sixty degrees is a realistic range for most of the US.
A measure of how cold a place is over a season: for each day, the number of degrees the average temperature falls below a base of 65°F, summed across the year. It lets you compare climates on one number, and it is what turns an instantaneous heat loss rate into an annual fuel figure.
Because a conduction calculation assumes the house sits at a constant indoor temperature with no free heat. In reality the sun, your appliances, your cooking and the people in the house all contribute, and most people set back the thermostat overnight. Treat the figure as an upper bound for comparing before and after, rather than a bill prediction.
No — it models conduction through the assembly only. In a draughty house infiltration can be a large share of total loss, and it is not something R-value fixes. If the house is leaky, air sealing usually returns more per pound spent than adding insulation does, and it should come first.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.