ALF help

Design help

On the Design pages you enter information about the particular house design and location to perform an ALF analysis on it.

Solar Gains


The SHGC allows for the reduction in light transmission through a window system. A high SHGC represents a large light transmission through the window. For colored glass the SHGC has to be estimated. It will generally be close to 0.3.

Solar gains are determined by calculating the effective solar opening area of all the windows facing in each of the eight primary orientations. After consideration of the shading factors and shading of the window areas the solar gains are calculated by multiplying the areas with the Annual Gain Factors.

The total seasonal solar heat gain through each set of windows is given by:

where AGF is the Annual Gain Factor. The values of AGF represent the solar heat gain through a square meter of window with a SHGC = 1 (no light reduction through the glass) and shading = 0. It is given for each window orientation as a function of the climate region and the heating schedule. The Annual Gain Factors are calculated for the whole winter heating season, which is defined by the severity of the climate. This leads to Annual Gain Factors being higher in colder locations than in warmer ones. These higher solar gains are offset against the higher Annual Loss Factors which are also given on the basis of the longer heating season.

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Windows can have obstructions nearby which for part of the time block direct sunlight. In such cases an appropriate shading can be chosen depending upon the time the windows are shaded from the sun. If for example a set of north windows have trees blocking direct sunlight 30% of the time, then the shading is 30%.

Take account of the path of the winter sun when selecting the shading. The following figure illustrates how the position of external shading and the path of the sun affect the shading.

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Conservatories can have a big influence on the thermal performance of the building. Two types of conservatories have to be distinguished:

1. Conservatories that have large openings to the rest of the building which can not be closed.

2. Those which have doors, walls and windows to close them off from the rest of the building.

They should be treated differently:

If the conservatory cannot be closed off from the rest of the building it should be treated like large windows. Both solar gains and window losses should be calculated as for normal windows.

If it can be closed off it should only be dealt with as increasing the insulation value of the parts of the building closing it off, i.e. the R-value of the separating walls, windows and doors should be increased by the conservatory glass R-values. The SHGC and the shading of the internal windows should not be changed. This approach generally underestimates the solar gains because not all the heat trapped behind the glazing is accounted for, but in most cases it also underestimates the losses because adding the glass R-value to the separating components ignores the size of the glazing area.

The following two figures illustrate the different treatment.

Type 1, conservatory can't be closed off the rest of the building.

Type 2, conservatory can be closed off from the rest of the building

Conservatories cause large inward and outward heat flows. The energy transfers are very complex and the net effect is difficult to estimate. In both cases the ALF results are likely to be less accurate than in houses without conservatories.

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The area of skylights used in the heat loss calculation is their actual area. For heat gain calculations only their horizontal projection should be used. For very highly pitched skylights (>45°) it is more accurate to treat them as vertical windows. Some judgement is required in these cases.

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 Contact and Feedback

Contact us at Branz for further information about the ALF 3.2: Annual Loss Factor.