Example calculation

Part of the top floor of a college building is used as a studio for student art classes. The room is 14m x 7m on plan, with a floor to ceiling height of 3 m. One of the long walls is an external wall facing south-west with four windows, each 2.5 m wide by 1.5 m high. The window frames have a framing ratio of 20% of the window area. Daylight along the long internal wall is provided by a line of six horizontal rooflights, each 1m square, with a framing ratio of 25%. The centre line of the rooflights is 1 m from the internal wall, as shown in Figure 13.1. Both the windows and the rooflights are double glazed with clear glass and an internal blind. The room can be divided into two zones. A perimeter zone can be formed by the external wall, the two end walls, and a line drawn 6 m from and parallel to the external wall. The remaining space between the perimeter zone and the inner long wall is an interior zone. Exactly half of the rooflight area falls within the perimeter zone, and the other half within the interior zone. Each zone must be considered separately.

Perimeter Zone And Inner Zone
Fig. 13.1 Solar gain to a college art studio.

Perimeter zone

Area of zone

Area of windows

Solar load, south-west window

Glazing/blind correction factor

Framing ratio for windows

Az = 14 x 6 = 84m2 Ag = 4 x 2.5 x 1.5 = 15m2 qSw = 198 W/m2 fc = 0.95

1 2257 2

Solar load from windows Qslw = — (15 x 198 x 0.95 x[1 - 0.201) = '

w 84v 84

Solar load, rooflights qsr = 327 W/m2

Glazing/blind correction factor fc = 0.95

Framing ratio for rooflights frr = 0.25

1 699

Solar load from rooflights Qslr = — (3 x 327 x 0.95 x[1 - 0.25]) = -—

84 84

Qslr = 8.3 W/m2 Total solar load for perimeter zone = 26.9 + 8.3 = 35.2 W/m2

Interior zone

Solar load, rooflights qsr = 327 W/m2

Glazing/blind correction factor fc = 0.95

Framing ratio for rooflights frr = 0.25

1 699 0

Solar load from rooflights Qslr = — (3 x 327 x 0.95 x[1 - 0.25]) = —¡4-

Qslr = 49.9 W/m2 Total solar load for interior zone = 49.9 W/m2

The results for both zones exceed the limiting value of 25 W/m2, and so the window and rooflight design must be reconsidered. There are several possible courses of action, but in view of the room's function as an art studio, it may be inappropriate to use tinted or reflecting glass. One possible solution would be to specify, for both windows and rooflights, a double glazing system using clear glass, but with the blind fitted between the panes, for which fc is 0.69. This, however, would not give a sufficient reduction to the result for the interior zone. Therefore, in addition, the line of rooflights could be moved an extra 250 mm away from the inner wall towards the window wall, so that three-quarters of their area is in the perimeter zone, and only one quarter in the interior zone. The revised calculations are as follows.

Perimeter zone

Area of zone

Area of windows

Solar load, south-west window

Glazing/blind correction factor

Framing ratio for windows

Area of rooflights

Az = 14 x 6 = 84 m2 Ag = 4 x 2.5 x 1.5 = 15m2 qsw = 198 W/m2 fc = 0.69

Solar load, rooflights qsr = 327 W/m2

Glazing/blind correction factor fc = 0.69 Framing ratio for rooflights frr = 0.25

1 1639 4

Solar load from windows Qslw = — (15 x 198 x 0.69 x[1 - 0.20]) =

84 84

Solar load from rooflights Qslr = 84 (4.5 x 327 x 0.69 x[1 - 0.25]) = T^p

Qslr = 9.1 W/m2 Total solar load for perimeter zone = 19.5 + 9.1 = 28.6 W/m2

Interior zone

Solar load, rooflights qsr = 327 W/m2

Glazing/blind correction factor fc = 0.69

Framing ratio for rooflights frr = 0.25

1 253.8

Solar load from rooflights Qslr = — (1.5 x 327 x 0.69 x[1 - 0.25]) = -4-

Qslr = 18.1 W/m2 Total solar load for interior zone = 18.1 W/m2

The interior zone is now well within the limiting value, but the result for the perimeter zone is still too high. This suggests a variation on the above solution. If the same clear-blind-clear double glazing units are used, but the rooflights are left in their original position with their centre line 1 m from the inner wall, it is possible to manipulate the calculation by drawing the boundary of the perimeter zone 5 m from the window wall instead of the maximum permissible 6 m. The interior zone then becomes 14 m x 2 m and the rooflights fall entirely within it. The calculation becomes as follows.

Perimeter zone

Solar load, south-west window qsw = 198 W/m2

Glazing/blind correction factor fc = 0.69

Framing ratio for windows frw = 0.20

1 1639 4

Solar load from windows Qslw = — (15 x 198 x 0.69 x[1 - 0.20]) = ^

Qslw = 23.4 W/m2 Solar load from rooflights Qslr = 0 W/m2

Total solar load for perimeter zone = 23.4 + 0 = 23.4 W/m2

Interior zone

Solar load, rooflights qsr = 327 W/m2

Glazing/blind correction factor fc = 0.69 Framing ratio for rooflights frr = 0.25

Solar load from rooflights Qslr = ^ (6 x 327 x 0.69 x[1 - 0.25]) = 1015,3

28 28

Qslr = 36.2 W/m2 Total solar load for interior zone = 36.2 W/m2

The perimeter zone is now just within the limiting value, but the interior zone is too high. However, it can be seen that the interior zone can be brought within target by reducing the number of rooflights. The required reduction in the solar load is approximately two thirds, and so four rooflights instead of six should have the desired result. Thus for the interior zone:

Solar load from rooflights Qslr = ^ (4 x 327 x 0.69 x[1 - 0.25]) = 6769

28 28

Qslr = 24.2 W/m2 Both zones are now acceptable.

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