Building performance
Envelope heat-loss comparison
Compare two envelope schemes by steady-state heat loss, annual degree-day energy and heating cost.
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What the envelope heat-loss comparison calculates
An envelope heat-loss calculator estimates how fast heat conducts out through walls, windows and roofs: for each element, heat loss = U-value × area × indoor–outdoor temperature difference. This tool runs that sum for two schemes side by side, so you can see how much an insulation or glazing upgrade changes the total.
The sum of U × A across the envelope is called UA, or the conductance H. Multiply it by the temperature difference (ΔT) and you get steady-state heat loss in Btu/h or watts. The tool reports both schemes, the difference and the percentage reduction, with a table by element.
It is a comparison tool, not a heating load calculation. It covers conduction only and leaves out air leakage, thermal bridges, solar gains and ground contact, so it is best used to compare options drawn on the same boundary. Enter heating degree-days, a system efficiency and an energy price and it also turns each scheme's UA into an annual conduction estimate in kWh, therms and MMBtu, and a heating cost per year.
How to use the envelope heat-loss comparison
- Choose Imperial or Metric. Areas are in ft² or m², U-values in Btu/h·ft²·°F or W/m²·K, results in Btu/h or W and degree-days in °F·days or K·days.
- Enter the indoor minus outdoor temperature difference. The imperial example uses 36 °F and the metric example 20 K.
- Pick a heating degree-day preset for a typical city, or keep Custom and type your own annual degree-days. Set the heating system efficiency and an energy price per kWh, therm or MMBtu (0 skips the cost).
- Fill in one row per envelope element: a name, then the area and U-value for Scheme A and for Scheme B. Use net wall area, with windows and doors as their own rows, or switch the window-to-wall ratio split on, enter the gross wall area and a ratio for each scheme, and the tool fills the Net walls and Glazing areas for you.
- Read Scheme A and Scheme B heat loss, A − B, the reduction relative to A and each scheme's UA, then the annual conduction loss, fuel energy and heating cost, the bar chart by element and the table. Download a PDF, CSV or SVG, copy the result, or save your inputs to reopen later.
Worked example
Imperial: 1,000 ft² at U = 0.05 Btu/(h·ft²·°F) and a 36 °F difference loses 1,800 Btu/h; with 6,000 °F·days, 90% efficiency and 1.50 per therm, the whole example Scheme A costs about 420 a year.
Metric: 100 m² at U = 0.3 W/(m²·K) and a 20 K difference loses 600 W.
How to calculate heat loss through walls, windows and roofs
Conduction through each element is Q = U × A × ΔT. In US units, U is in Btu/h·ft²·°F, A in ft² and ΔT in °F, so Q comes out in Btu/h. In SI units, U in W/m²·K, A in m² and ΔT in K (the same size step as 1 °C) give watts.
If you have an R-value, U = 1 ÷ R. Use the whole-assembly R, including framing, where you can; the R-value to U-value converter handles the units. To convert results, 1 Btu/h = 0.29307 W and 1 W = 3.412 Btu/h.
Summing U × A over every element gives UA. Because heat loss is directly proportional to ΔT, UA is the single number to compare: a scheme with a 20% lower UA loses 20% less heat by conduction at any temperature difference.
Worked comparison: two envelope schemes for a house
A single-story house has 1,400 ft² of net wall, 250 ft² of windows, 40 ft² of doors and a 1,500 ft² ceiling. Scheme A is a baseline specification. Scheme B adds continuous wall insulation, better windows and more attic insulation. All U-values are illustrative, and the temperature difference is 70 °F (70 °F inside, 0 °F outside).
Scheme A has a UA of 206 Btu/h·°F and loses 14,420 Btu/h, about 4,226 W. Scheme B has a UA of 156 Btu/h·°F and loses 10,920 Btu/h, about 3,200 W. That is 3,500 Btu/h less, a 24.3% reduction.
The windows are 7.8% of the envelope area but 36.4% of Scheme A's heat loss. The wall upgrade (UA down 21.0) and the window upgrade (UA down 20.0) save about the same, while the extra attic insulation saves less than half as much (UA down 9.0), because the ceiling was already well insulated.
| Element | Area (ft²) | U, Scheme A | U, Scheme B | Heat loss A (Btu/h) | Heat loss B (Btu/h) |
|---|---|---|---|---|---|
| Net walls | 1,400 | 0.060 | 0.045 | 5,880 | 4,410 |
| Windows | 250 | 0.30 | 0.22 | 5,250 | 3,850 |
| Doors | 40 | 0.20 | 0.20 | 560 | 560 |
| Ceiling | 1,500 | 0.026 | 0.020 | 2,730 | 2,100 |
| Total | 3,190 | UA 206.0 | UA 156.0 | 14,420 | 10,920 |
Heat loss at different temperature differences
Because conduction scales linearly with ΔT, the percentage saving is the same at any temperature difference; only the absolute numbers change. The table uses the two schemes above. For a design-day ΔT, subtract the winter design temperature for your location, from ASHRAE climate data or ACCA Manual J, from your indoor set point.
For a rough annual comparison, conduction energy ≈ UA × 24 × heating degree days (°F·days). With an illustrative 5,000 heating degree days, Scheme A's UA of 206 gives about 24.7 million Btu a year and Scheme B about 18.7 million Btu. The tool's degree-day estimate does this arithmetic for you, divides by the heating system efficiency to get fuel energy, and multiplies by your energy price. The city presets are typical annual values (base 65 °F / 18 °C) rounded from NOAA climate normals and national climate summaries, and the estimate still ignores air leakage, solar and internal gains.
| Temperature difference | Scheme A (Btu/h) | Scheme B (Btu/h) | A − B (Btu/h) |
|---|---|---|---|
| 20 °F | 4,120 | 3,120 | 1,000 |
| 36 °F | 7,416 | 5,616 | 1,800 |
| 50 °F | 10,300 | 7,800 | 2,500 |
| 70 °F | 14,420 | 10,920 | 3,500 |
| 90 °F | 18,540 | 14,040 | 4,500 |
What a conduction-only comparison leaves out
Steady-state conduction is a sound way to rank envelope options, but it is not a heating load or an energy model. These effects are not included unless you build them into your U-values:
- Air leakage and ventilation, which can be a large share of heat loss, especially in older or leaky buildings.
- Thermal bridges at slab edges, balconies, window perimeters and structural connections.
- Solar and internal gains, which offset heat loss during the day and make glazing orientation matter.
- Ground coupling through slabs and basement walls, which does not follow a simple indoor–outdoor ΔT.
- Thermal mass and changing conditions. Real temperatures swing through the day; this is one steady condition.
Who it is for and when to use it
Comparing insulation upgrades
See how much adding continuous insulation to walls or more insulation to a roof reduces conduction compared with the baseline.
Choosing between window specifications
Enter two window U-factors over the same glazed area to see the difference in heat loss, and how it compares with a wall upgrade.
Testing glazing ratios in early design
Switch on the window-to-wall ratio split, enter the gross wall area and try a different ratio for each scheme to see how a larger or smaller glazing ratio changes UA and annual cost before running a full energy model.
Explaining envelope choices to clients
Export a PDF showing heat loss by element for both schemes, so the case for an upgrade is visible line by line.
Method, formulas & assumptions
Conductance UA = sum(area × assembly U-value), W/K. Steady-state heat loss = UA × indoor–outdoor temperature difference. Annual conduction loss = UA × heating degree-days × 24 h; fuel energy = annual loss ÷ system efficiency; cost = fuel energy × energy price (per kWh, therm or MMBtu). The optional window-to-wall split divides a gross wall area into the net wall and glazing rows. A bar chart compares element contributions.
Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.
Common mistakes to avoid
- Entering gross wall area and then entering windows separately, which counts the window area twice. Use net wall area, or switch on the window-to-wall split so the tool divides the gross area for you.
- Using insulation R-values instead of whole-assembly U-values. Framing and thermal bridges raise the real U-value above the insulation-only figure.
- Changing the areas between schemes by accident. A lower result may then come from a smaller envelope, not a better one.
- Mixing units, such as a U-value in W/m²·K with an area in ft². Pick one system and convert values first.
- Using the result to size a furnace or heat pump. Equipment sizing needs a full load calculation that includes infiltration, ventilation and duct losses.
- Comparing the result with a utility bill. Bills include air leakage, hot water, appliances and weather that this tool does not model.
Scope & limits
Conduction only. Enter net wall area excluding glazing unless the window-to-wall split is on. Degree-day presets are typical annual values (base 65 °F / 18 °C), not design data. Does not model air leakage, thermal bridges, solar or internal gains, ground coupling, cooling or compliance.
Frequently asked questions
How do you calculate heat loss through a wall?
Multiply the wall's U-value by its area and by the indoor–outdoor temperature difference. A 1,000 ft² wall at U-0.05 with a 36 °F difference loses 1,000 × 0.05 × 36 = 1,800 Btu/h.
What is UA in heat loss calculations?
UA is the sum of U-value × area for every envelope element, in Btu/h·°F or W/K. Multiplying UA by the temperature difference gives total conduction heat loss.
How do I convert Btu/h to watts?
Multiply by 0.29307. 10,000 Btu/h is about 2,931 W, and 1 W is about 3.412 Btu/h.
What temperature difference should I use?
For peak heat loss, use your indoor set point minus the local winter design temperature from ASHRAE or ACCA Manual J data. For comparing two schemes, any consistent ΔT works because the percentage difference does not change.
Why do windows account for so much heat loss?
Windows usually have U-factors several times higher than insulated walls. At U-0.30 versus U-0.05, a square foot of window loses six times as much heat as a square foot of wall.
Does this calculator size heating equipment?
No. It calculates steady-state conduction only and leaves out air leakage, ventilation, thermal bridges, solar gains and ground losses, all of which a heating load calculation needs.
How does the annual energy and cost estimate work?
Annual conduction loss = UA × heating degree-days × 24 hours, converted to kWh. Dividing by the heating system efficiency gives fuel energy, reported in kWh with therms and MMBtu in the notes, and multiplying by your price per kWh, therm or MMBtu gives the yearly cost. The city presets are typical degree-day values; enter your own for a specific site.
Can I compare more than three elements?
Yes. Add a row for each wall type, window type, roof, door or exposed floor, and remove any rows you do not need.
Do I need an account to download the result?
No. All calculations, local file processing and exports on this page are free, without an account.
Where are my inputs stored?
The tool keeps inputs in this browser tab. It does not upload them or create a cloud copy. Save or download anything you want to keep before leaving the page. Standard site analytics may operate independently of the tool.
Can I work in metric instead of imperial?
Yes. Use the Imperial / Metric switch above the inputs. Imperial (feet, inches, pounds, gallons) is the default; switching converts every dimensional field and result, and your choice is remembered across the free tools in this browser.
Further reading
- Understanding R-value and U-factor
- Typical exterior wall assemblies
- IECC 2018 to 2021 changes
- All building performance tools
- All free architecture & construction tools
- Harth resource library
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