How the U-Value and Energy Calculator Works
This calculator estimates heat transfer through the outside wall envelope, including opaque wall area, doors, and windows. It calculates wall R-value, wall U-value, weighted envelope U-value, heat loss rate, energy loss, and estimated energy cost.
Wall Area Method
The most important correction from the old version is the wall area calculation. Floor area is not the same as external wall area. This calculator uses:
If you leave external wall perimeter blank, the calculator estimates it from a square building footprint:
Wall R-Value and U-Value
For each material layer:
The simplified wall U-value is:
A combined inside/outside surface resistance of 0.17 m²·K/W is included as a simplified default.
Weighted Envelope U-Value
Doors and windows are included with typical U-values. The weighted U-value is:
Heat Loss and Energy
Heat loss rate is estimated from:
Energy loss over a duration is:
Material Data Used
| Wall / insulation material | Thermal conductivity, W/(m·K) |
|---|---|
| Brick | 0.70 |
| Concrete, non-insulated | 1.70 |
| Concrete, insulated | 0.80 |
| Wood | 0.15 |
| Stone | 2.00 |
| Steel | 47.00 |
| Fiberglass insulation | 0.04 |
| Foam insulation | 0.03 |
| Wool insulation | 0.04 |
| Door / window type | Typical U-value, W/(m²·K) |
|---|---|
| Wood solid door | 1.50 |
| Wood insulated door | 0.70 |
| Metal insulated door | 1.20 |
| Metal non-insulated door | 2.50 |
| Fiberglass door | 1.00 |
| Plastic door | 1.30 |
| Single glazed window | 4.50 |
| Double glazed window | 2.00 |
| Triple glazed window | 1.00 |
Frequently Asked Questions
What is a good U-value?
Lower is better. A low U-value means less heat is transferred through the building element for the same temperature difference.
Why did you add external wall perimeter?
External wall area is based on perimeter and height, not floor area and height. The optional perimeter input makes the estimate more realistic.
Does this include roof and floor heat loss?
No. This version focuses on the wall envelope, doors, and windows. A full building heat-loss calculation should also include roof, floor, ventilation, infiltration, and thermal bridges.
Can I use this for building-code compliance?
No. Use it for education and rough comparison only. Building-code compliance requires approved methods, accurate product data, local regulations, and professional review.