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Table G1 to Part J Appendix G: Demonstrating compliance with the Target U-value standards
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STEP |
ACTION |
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1 Estimate if the SAP energy rating of the proposed dwelling is likely to be 60 or less. 60 or less, the Target U-value is lower. |
The examples in Appendix C, Annex D will help in estimating the SAP energy rating. If it is likely to be |
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2 Calculate the surface area and U-value for - |
Exclude semi-exposed elements from the calculations. Tabulate
the results to determine - |
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3 Check that any semi-exposed elements have a to U-value of 0.6 or less. |
It is not permissible for semi-exposed elements have a U-value worse than 0.6. |
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4 Calculate the average U-value. |
Use results of step 2 to calculate the ratio - total rate of heat loss per degree This is the average U-value for the dwelling. |
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5 Calculate the Target U-value. |
Use the total floor area and the total exposed surface area from step 2 to calculate the Target U-value as given in J2.3. |
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6 Compare the average U-value with the Target U-value |
If the average U-value (step 4) does not exceed the Target U-value (step 5) the proposals meet the requirements. If the average U-value is more, the proposals should be modified by taking account of solar gains or providing a more efficient heating system as indicated in steps 7 and 8 below and/or by altering the construction of the elements. An example is included in the following pages. |
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7 If desired, adjust the average U-value to take account of solar gains. |
The window area from step 2 can be reduced if the area of glazing facing south (± 30°) exceeds that facing north (± 30°). |
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8 If desired, adjust the Target U-value to take account of a more efficient heating system. |
The Target U-value can be increased by up to 10% if, for example, a condensing boiler is used. |
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9 Check that the proposals will achieve a SAP energy |
Complete the worksheet in Appendix C. rating of more than 60 if this was the original basis for the calculations. |
EXAMPLES ILLUSTRATING THE USE OF THE TARGET U-VALUE METHOD FOR DWELLINGS
Example 1 - A detached dwelling
G1 Consider the example in Diagram G1 which has details as given in Table G2. It is proposed to adopt the Target U-value method with U-values for the walls and roof a little higher (worse) than would otherwise be required in the Elemental method. The SAP energy rating is to be more than 60. The cavity walls are to have dry-linings and the windows and doors are to have metal frames with thermal breaks and sealed double-glazing with 12 mm air gaps.
Diagram G1 Plans of the detached dwelling
Table G2 to Part J Appendix G: Data for the detached dwelling
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Exposed |
Exposed |
U-value |
Rate of heat loss |
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(m2) |
(W/K) |
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Floor |
56.2 |
0.45 |
25.29 |
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Windows |
24.8 |
3.3 |
81.84 |
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Doors |
3.8 |
3.3 |
12.54 |
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Walls |
121.4 |
0.5 |
60.7 |
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Roof |
56.2 |
0.3 |
16.86 |
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Totals |
262.4 |
197.23 |
The Target
G2 From J2.3 the Target U-value for dwellings with SAP energy ratings of more than 60 is given by:
Therefore, the Target U-value is: 
The Average U-value
G3 The average U-value is given by the ratio of the two values: 
Using the values from Table G2 above, the average U-value is therefore:
G4 The proposed design does not meet the requirements and modifications must be explored. Possibilities include improving the thermal resistance of the exposed walls and the windows and doors and taking account of the benefits of solar gain and more efficient space heating systems (as described in J2.3). For illustration purposes they are all considered as follows.
Taking account of solar gain
G5 The total window area in the example is 24.8 m2 of which 15.0 m2 faces south + 30 ° and 9.3 m2 faces north ±30 ° (the remaining 0.5 m2 faces east). In accordance with J2.3e, therefore, the area of window and hence the total area used in the calculation of the average U-value can be reduced by 40% of (15.0 - 9.3) = 2.28 m2.
Improving the thermal resistance of the windows
G6 Table 2 to J2.2 gives indicative figures for the U-value of various types of windows and doors although manufacturers' information should be used in preference if available. For the purposes of this example it is proposed to alter the window and door specifications to obtain U-values of 2.9 W/m2K and 3.0 W/m2K respectively.
Improving the thermal resistance of the walls
G7 Appendix E can be used to develop wall designs to achieve a range of U-values. For the purposes of this exercise it is proposed to alter the wall specification to obtain a U-value of 0.45 W/m2K.
Determination of the revised average U-value
G8 The revised average U-value is calculated in the same way as before. The revised table of data follows with the alterations highlighted.
Table G3 to Part J Appendix G: Revised data for the detached dwelling
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Exposed |
Exposed |
U-value |
Rate of heat loss |
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Floor |
56.2 |
0.45 |
25.29 |
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Windows |
22.52 |
2.9 |
65.31 |
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Doors |
3.8 |
3.0 |
11.4 |
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Walls |
121.4 |
0.45 |
54.63 |
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Roof |
56.2 |
0.3 |
16.86 |
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Totals |
260.12 |
173.49 |
For the revised proposals, therefore, the average U-value is:
Selecting a higher performance heating system
G9 In accordance with J2.3f a hot water central heating system incorporating a condensing boiler could be specified in return for relaxing the Target U-value by 10%. The target in this case would therefore be increased from 0.67 W/m2K to 0.74 W/m2K.
Compliance
G10 The detached dwelling as described in Diagram G1 and Table G2 could be made to comply with the requirements by, for example:
either taking solar gain into account and increasing the performance of the windows, doors and walls,
or increasing the performance of one of the elements, such as the windows, and installing a hot water central heating system incorporating a condensing boiler.
Example 2 - A semi-detached dwelling
G11 Consider the example in Diagram G2 which has details as given in Table G4. It is proposed to adopt the Target U-value approach with the walls having a U-value of 0.55 W/m2K. To compensate for this the windows and doors are to have an average U-value of 3.0 W/m2K. The SAP energy rating is to be more than 60.
G12 The party wall and the semi-exposed wall at the garage (which has a U-value of 0.6 W/m2K) are not included in the average U-value or Target U-value calculations.
Diagram G2 Plans of the semi-detached dwelling
The Target
G13 From J2.3b the Target U-value for dwellings with SAP energy ratings of more than 60 is given by:
In this case the Target U-value is: 
Table G4 to Part J Appendix G: Data for the semi-detached dwelling
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Exposed |
Exposed |
U-value |
Rate of heat loss |
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Floor |
40.0 |
0.45 |
18.0 |
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Windows |
14.2 |
3.00 |
42.6 |
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Doors |
3.8 |
3.00 |
11.4 |
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Walls |
59.0 |
0.55 |
32.45 |
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Roof |
40.0 |
0.25 |
10.0 |
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Totals |
157.0 |
114.45 |
The Average U-value
G14 The average U-value is given by the ratio of the two values:
Using the values from Table G3, the average U-value is therefore: 
The proposed design therefore meets the requirements of the Technical Standards.
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