Appendix E Avoidance of Solar Overheating
E1 This Appendix provides the detail for the procedure referred to in paragraph 220.127.116.11 (a).
E2 When estimating the solar load, the space being considered should be split into perimeter and interior zones. “Perimeter zones” are those defined by a boundary drawn a maximum of 6 m away from the window wall(s). Interior
zones are defined by the space between this perimeter boundary and the non window walls or the perimeter boundary of another perimeter zone.
When calculating the average solar cooling load, the contribution from all windows within that zone should be included, plus the contribution from any rooflight (or part rooflight) that is within the zone boundary.
For interior zones, the contribution from all rooflights (or part rooflights) that is within its zone boundary should be included.
For each zone within the space, the total average solar cooling load per unit floor area should be no greater than 25 W/m².
The total average solar cooling load per unit floor area (W/m²) is calculated as follows:
- The average solar cooling load associated with each glazed area is calculated by multiplying the area of glazing by the solar load for the appropriate orientation (see Table 7) and by a correction factor applicable to the
relevant glazing/blind combination (see Paragraph E3 and Table E1);
- The average solar cooling loads thus calculated are added together and the sum divided by the zone floor area to give a total average solar cooling load per unit floor area (W/m²).
Where the actual glazed area is not known, it can be assumed to equate to the opening area reduced by an allowance for framing. The default reduction should be taken as 10% for windows and 30% for rooflights.
E3 Standard correction factors for intermittent shading using various glass/blind combinations are given in Table E1.
Table E1 Correction factors for intermittent shading using various glass-blind combinations
Where available shading coefficient data for a particular device should be used to calculate the correction factor, in preference to using the figures given in Table E1. The correction factor is calculated as follows:
(a) For fixed shading (including units with absorbing or reflecting glass), the correction factor (fc) is given by:
fc = Sc/0.7
(b) For moveable shading, the correction factor is given by:
fc = 0.5(1 + (Sc/0.7))
where Sc is the shading coefficient for the glazing/shading device combination, i.e. the ratio of the instantaneous heat gain at normal incidence by the glazing/shading combination relative to the instantaneous heat gain by a sheet of 4 mm clear glass.
(c) Where there is a combination of fixed and moveable shading, the correction factor is given by:
fc = (Scf + Sctot)/1.4
where Scf is the shading coefficient of the fixed shading (with glazing) and Sctot is the shading coefficient of the combination of glazing and fixed and moveable shading.
E4 A school classroom is 9 m long by 6 m deep, with a floor to ceiling height of 2.9 m. There is glazing on one wall, with rooflights along the internal wall opposite the window wall. The windows are 1200 mm wide by 1000
mm high, and there are six such windows in the external wall, which faces SE. The windows are clear double glazed, with mid-pane blinds, of wooden frames with a framing percentage of 25%. There are three 0.9 m² horizontal rooflights, with an internal
blind and low-e glass on the inner pane of the double pane unit. Is there likely to be a solar overheating problem?
(a) As the room is not more than 6 m deep, it should be considered as a single “perimeter zone” – there is no “interior zone”
(b) The calculation of the average solar cooling load (W) is set out in the following Table
(c) the total average solar cooling load per unit floor area (W/m²) is derived by dividing the total average solar load by the zone floor area. In this case the floor area is 54 m² and the total average solar cooling load per unit floor area is 24.20
(W/m²). As this is less than 25 W/m², there is not likely to be an overheating problem.
E5 An office building has a floor to ceiling height of 2.8 m and curtain walling construction with a glazing ratio of 0.6. The long side of the office faces south and the short side faces west. On each floor, the main office
area is open plan, but there is a 5 m by 3 m corner office, with the 5 m side facing South. It is proposed to use double glazing with the internal pane low-e glazing and the external pane absorbing glass, and with an internal blind.
For the open plan areas, the perimeter zone is defined by the 6 m depth rule, but for the corner office, it is defined by the partitions. The glazed area is taken as the nominal area less 10% for framing. Three different situations must be considered
- the south facing open plan area;
- the west facing open plan area; and
- the corner office.
Open plan area
From Table 7, it can be seen that the solar loading for a West orientation (205 W/m²) exceeds that for a South orientation (156 W/m²). Thus, on the assumption that the same construction would be used on West and South facades, it is sufficient to check
the West orientation for the open plan offices.
For a typical 5 m length of West facing office, the floor area of the perimeter zone is 30 m², and the area of glazing is 7.56 m², i.e. width (5 m) x height (2.8 m) x glazing ratio (0.6) x framing correction (0.9). The glazing/blind correction factor
is 0.62 and the solar loading is 205 W/m². Thus the total average solar cooling load per unit floor area (W/m²) is
(7.56 x .62 x 205)/30 = 32W/m².
As this is greater than the threshold of 25 W/m², it is necessary to decrease the glazing ratio or provide alternative or additional shading devices, e.g. a reduction in glazing ratio to 0.47 or provision of fixed shading devices which would provide a
shading coefficient of 0.34 (giving a correction factor of 0.43), or a combination of these measures would reduce the risk of solar overheating to acceptable levels.
For the purpose of this example, it is assumed that it has been decided to reduce the glazing ratio of the building to 0.47. On this basis the average solar load for this office can be calculated as set out in the following table.
Example - Corner Office
The office floor area is 15 m² and the total average solar cooling load per unit floor area (W/m²) is 68 W/m².
To achieve a total average solar cooling load per unit floor area (W/m²) of 25 W/m² would require a reduction in the total average solar load to 375 W. This can be achieved by a further reduction of glazing area, e.g. through the use of opaque panels
so as to reduce the glazing ratio for the corner office to 17%. An alternative would be to use external shading devices to give a correction factor of 0.22. This implies fixed shading with a shading coefficient of 0.16. Such a shading coefficient
is quite demanding to achieve in practice. Alternatively a more detailed calculation could be undertaken.
If the corner office was not partitioned from a general open floor area, it’s solar load could be considered as part of the load of one of the facades it shares.