Semidetached dwelling

Figure 8.1 is a plan view of the ground floor and first floor of a semi-detached dwelling. The dwelling lies approximately on a north-south axis. The windows have wood frames, with 11.8m2 on the southern face, 9.2m2 on the northern face, and 1.2 m2 to the side. It is proposed to fit a mains gas fired boiler with a SEDBUK rating of 81%. The areas and U-values of the elements of the dwelling are given in Table 8.1.

The total area of openings is 26.00 m2 and the total floor area is 94.08 m2. The openings are therefore 27.6% of the floor area. Therefore the dwelling does not meet the requirements of the elemental method, but can be assessed using the target U-value method.

Fig. 8.1 Semi-detached dwelling.
Table 8.1 Semi-detached dwelling.




Heat loss per degree












Ground floor








Doors (2 No.)





At = 191.08

ZAU = 99.60

Step 1 The initial target U-value Ui

47.04 47.04 94.08

191.08 191.08 Step 2 Adjust for boiler efficiency


0.482 W/m2K

SEDBUK of proposed boiler = 81%

SEDBUK of reference boiler = 78% (from Table 2.2)

81 78


U2=1.04 x 0.482 = 0.501 W/m2K Step 3 Allowance for additional solar gain due to window frame material The window frames are wood, and so there is no change:

U3 = U2 = 0.501 W/m2K Step 4 Allowance for additional solar gain due to orientation of windows

South facing area North facing area Total window area

Solar adjustment factor AS = 0.04 Step 5 Convert to final target U-value



Step 6 Find Uavg

For compliance, UAVG must be less than or equal to UT . However in this case UAVG is greater than UT, and so the dwelling does not comply by the target U-value method. There are several ways, applied either singly or in combination, in which the design of the dwelling could be altered to improve the possibility of compliance. These may either increase the target U-value, or reduce UAVG, or affect both. The following are examples.

(1) Raise boiler efficiency - and hence raise Ut

A boiler with a higher SEDBUK rating, say 85%, could be used. The boiler efficiency factor is then 85/78 = 1.09, and this would alter UT as follows:

This has resulted in UAVG being less than the target U-value, and so the dwelling now complies.

(2) Reduce the U-values of some of the elements - and hence reduce Uavg This may be attempted by trial and error by choosing lower U-values and repeating the calculation of UAVG. Alternatively, the required reduction can be estimated by setting UAVG equal to UT (which would satisfy the target U-value requirement) and calculating a new value for EAU.

Reduced EAU = ATUAVG = 191.08 x 0.506 = 96.69 Required reduction = 99.60 — 96.69 = 2.91

This reduction may be obtained from any one of the exposed elements, or from a combination of several. If only one element is altered, the effect on its U-value would be:

New U-value = 0.35 - 0.04 = 0.31 W/m2K Roof Reduction = 2.91/47.04 = 0.06

New U-value = 0.20 - 0.06 = 0.14W/m2K Ground floor Reduction = 2.91/47.04 = 0.06

New U-value = 0.25 - 0.06 = 0.19 W/m2K Windows Reduction = 2.91/22.20 = 0.13

Most of these reduced U-values may be difficult to achieve, with the exception of the doors. An insulated door construction providing a U-value of 2.23 W/m2K or less is feasible, and would be sufficient to ensure compliance. Otherwise, if the doors cannot be altered, it may be necessary to reduce the U-value of more than one element.

(3) Reduce the window area - and hence reduce UAvg

From (2) above, the required reduction in EAU is 2.91. However, if the window area is reduced, the wall area is increased by the same amount. Consequently the gain from reducing the window area is offset by a corresponding increase in wall area. If 8A is the reduction in window area, then we may write the following equation:

2.91 = 1.90 x 8A - 0.35 x 8A = 1.55 x 8A 8A = 2.91/1.55 = 1.88m2


New window area = 22.20 - 1.88 = 20.32 m2 New wall area = 71.00 + 1.88 = 72.88 m2

In this case, adopting a window area of 20.32 m2 or less would be sufficient to reduce UAVG to 0.506 W/m2K, and hence ensure compliance.

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