ALF calculations and solar gains.
The solar heat gain coefficient (SHGC) is an index describing the proportion of light that is let through a window. The larger the SHGC, the more light that can enter through the window. SHGCs range from approximately 0.70 for single clear glass to 0.31 for low emissivity glass (toned or coated glass). For coloured glass the SHGC has to be estimated but will generally be close to 0.3.
ALF determines solar gains by calculating the effective solar opening area of all the windows facing in each of the eight primary orientations. After considering 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 metre of window with a SHGC of 1 (no light reduction through the glass) and no shading. 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, 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.
Windows can have obstructions nearby that block direct sunlight for at least part of the time. In such cases designers can choose an appropriate shading based on the time the windows are shaded. 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. This figure illustrates how the position of external shading and the path of the sun affect the shading.
A diagram illustrating how shade should be calculated
Conservatories can have a big influence on the thermal performance of the house. Two types of conservatories have to be distinguished:
- those with large openings to the rest of the building that cannot be closed;
- those with 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. In other words, 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.
Two figures illustrate the different treatment.
Illustration of how conservatories should be treated
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.
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.