Inputs
Results
0.00 in
Required Thickness
(0.0 mm)
R 0.00
Total R After Upgrade
U 0.000
0
Heat Loss Rate
Results History
| # | Units | Area | ΔT | Existing R | Target R | Material | Required Thickness | Total R | U | Heat Loss | Est. Cost |
|---|
Example Data
| Space | Area | ΔT | Existing R | Target R | Material | Action |
|---|---|---|---|---|---|---|
| Attic | 800 ft² | 40 °F | R 11 | R 49 | Fiberglass batt | |
| Exterior wall | 320 ft² | 35 °F | R 13 | R 21 | Mineral wool | |
| Basement rim joist | 120 ft² | 45 °F | R 3 | R 10 | Closed‑cell spray foam | |
| Garage ceiling | 450 ft² | 30 °F | R 0 | R 38 | Cellulose loose fill | |
| Floor over crawlspace | 300 ft² | 28 °F | R 5 | R 30 | XPS board |
Formula Used
R_total = R_existing + R_addedR_added = thickness_in × (R per inch)(imperial)RSI_added = thickness_mm × (RSI per mm)(metric)U = 1 / R_total(U in BTU/h·ft²·°F for imperial, W/m²·K in metric)Heat loss rate = Area × ΔT ÷ R_total- Convert:
1 R = 0.17611 RSI,1 in = 25.4 mm,1 W = 3.41214 BTU/h - Required thickness:
max(0, (R_target − R_existing) ÷ performance_per_thickness)
These are steady‑state conductive heat flow approximations and exclude air leakage and thermal bridges.
How to Use
- Select your unit system.
- Enter the surface area and typical temperature difference.
- Provide your existing total R (or RSI) and the desired target total R.
- Pick a material; the performance and a default cost rate will populate.
- Click Calculate to see required thickness, U‑value, and heat loss.
- Optionally click Add to History, then export CSV or PDF.
- Use the example table to quickly try realistic scenarios.
FAQs
R is imperial thermal resistance in h·ft²·°F/BTU. RSI is metric in m²·K/W. They convert by 1 R = 0.17611 RSI.
They are typical catalog values. Real performance depends on density moisture installation quality age and temperature. Always verify with manufacturer datasheets and codes.
Codes energy programs or comfort goals specify recommended overall thermal resistance. The calculator sizes added insulation so that existing plus added meets your target.
U is overall heat transfer coefficient. Lower U means less heat flow. It is the inverse of total R or RSI.
It reports steady state heat loss for your ΔT. Annual savings depend on climate heating degree days equipment efficiency and usage which are outside this simple scope.
Cost uses your rate multiplied by area and required thickness. Include labor waste and accessories in your rate for better estimates.
No. Fasteners framing and gaps reduce real world performance. Use continuous exterior insulation and good air sealing to mitigate bridges and leakage.