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U-Value and SHGC Weighting

Use area-weighted averages for mixed window and door types.

11 minFenestration Fundamentals
Diagram for U-Value and SHGC Weighting
Fenestration Fundamentals U-Value and SHGC Weighting
How to use the diagram Read the visual first, then connect each label to the lesson text and your calculator inputs. The aim is to make every assumption visible before you calculate.

Learning Objective

By the end of this lesson you should be able to calculate weighted U-value and SHGC values for a storey or orientation group.

Weight by area

When multiple fenestration types are used, the average performance must reflect their areas. A large sliding door affects the result more than a small bathroom window, even if both have valid supplier values.

Use the same grouping as the rule

Calculate U-value averages per storey for the relevant vertical fenestration set. For SHGC, keep the orientation groups separate where the standard applies different limits.

The basic method

Multiply each element area by its performance value, add those products, then divide by the total area in that assessment group. Repeat separately for U-value and SHGC.

Practice Task

  • Element A: 3.0 m2 at U 2.8 and SHGC 0.32.
  • Element B: 6.0 m2 at U 1.9 and SHGC 0.24.
  • Calculate the weighted U-value and weighted SHGC for the group.

Calculator Tip

Use the result cards to compare weighted values against the applicable table row. A single non-compliant unit may still pass in a weighted group, but only if the total weighted result complies.

Worked Example: First-Pass Fenestration Check

Scenario

A single storey has 52 m2 nett floor area and three glazed elements: 1.8 m2 at U 2.8 / SHGC 0.32, 5.0 m2 at U 2.2 / SHGC 0.25, and 1.1 m2 at U 3.0 / SHGC 0.35.

Calculator Entry

Enter each glazed element with its area, orientation, U-value and SHGC. Use the result cards to check glazing percentage and weighted performance.

Step-by-step method
  1. 1Add the three glazed areas to get total fenestration area.
  2. 2Divide total fenestration area by nett floor area to get glazing percentage.
  3. 3Calculate weighted U-value and SHGC using area x value products.
  4. 4Compare the results to the applicable SANS 10400-XA:2021 route and document supplier evidence.
Expected conclusion

The important decision is not only whether the numbers pass, but whether every value can be traced to drawings and supplier data.

Common Mistakes

  • Using glass-only U-values or SHGC values instead of whole-window values.
  • Mixing solar and non-solar orientation groups in one weighted average.
  • Calculating glazing percentage across the whole building instead of per storey.

Quick Knowledge Check

1. What should you confirm before applying this lesson to a project?

Were area x value products used?

2. Which piece of evidence should support the main input in this lesson?

Element A: 3.0 m2 at U 2.8 and SHGC 0.32.

3. What is the safest action if the information is incomplete?

Flag the missing evidence, use a conservative assumption where appropriate, and avoid claiming compliance until the information is confirmed.

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