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Area-Weighted
U-Value

BETA

Calculate the area-weighted average U-value across a set of building elements.

Results from brickdims.com tools are indicative only. Using this site does not create a professional relationship or duty of care. Not a substitute for professional advice or formal compliance sign-off.

Elements

Element typeDescriptionArea (m²)U-value (W/m²K)Heat loss (W/K)
Description
Area
m²
U-value
W/m²K
Heat loss
9.00 W/K
Description
Area
m²
U-value
W/m²K
Heat loss
5.20 W/K
Description
Area
m²
U-value
W/m²K
Heat loss
6.60 W/K
Totals / weighted average
Area120.0 m²
U-value0.173 W/m²K
Heat loss20.80 W/K

Results

Area-weighted U-value
0.173
W/m²K, area-weighted average
Total heat loss
20.80
W/K, sum of area × U-value
Total area
120.0 m²
3 elements
Summary by element type
ElementCountArea (m²)Avg U-value (W/m²K)Heat loss (W/K)
External Wall150.00.189.00
Roof140.00.135.20
Floor130.00.226.60

This method checks fabric performance only. It does not account for thermal bridging, air permeability, or services efficiency. This tool is for early-stage design reference and does not constitute a SAP, SBEM or full building regulations submission. Always verify outputs with a qualified energy assessor.

FAQs

What is an area-weighted U-value?

It is the average U-value of several elements, weighted by how much area each one covers. Multiply each element's U-value by its area, add the results, and divide by the total area: Σ(U × A) ÷ ΣA.

Can you show a worked example?

50m² of wall at 0.18, 40m² of roof at 0.13 and 30m² of floor at 0.22 give (9 + 5.2 + 6.6) ÷ 120 = 0.173 W/m²K.

How do I estimate heat loss from U-values?

Fabric heat loss is U × A × ΔT for each element. Σ(U × A) gives the heat loss in watts per degree of temperature difference, which you multiply by the design temperature difference. This ignores thermal bridging and ventilation, so treat it as an estimate.

Why does a small area of glazing affect the average so much?

Windows typically have U-values five to ten times higher than an insulated wall, so even a modest area pulls the weighted average up sharply.

Related toolsU-value·Glazing percentage