Walls and windows

ALF calculations and walls and windows.

Walls information

Wall orientation

The wall orientation (north, north-east, east etc.) affects only the solar gains through the window on the particular wall being considered. It does not affect the building losses.

Wall length, height and area

ALF calculations only use wall area, not the actual length or height of a wall. It is possible to combine walls with the same orientation and construction type. This may be useful for complicated multi-storey buildings with more than ten individual walls.

The wall net area is the total wall area of a particular wall excluding the window area.

If the building includes spaces that are never heated and are generally thermally separated from the rest of the building (attached garages are a common example), do not include these spaces in the building area. In this case, increase the R-values of the walls separating the heated and unheated spaces by the R-value of the external walls of the unheated spaces. This rough adjustment, although useful, does not account for gains, thermal mass or the area of external walls in the unheated areas. 

Windows information

Window area

ALF only uses the window area in the calculations, not the window dimensions. Because of this, it is possible to combine windows on each wall if they have the same glazing and frame type and the shading is the same. This may be useful for buildings with large numbers of windows. Note that windows include all vertical glazing and the glazing in any doors.

Solar Heat Gain Coefficient (SHGC)

The Solar Heat Gain Coefficient (SHGC) depends on the glazing and the frame because the area of the opening which is covered by the window frame depends on the frame type. Since it describes the solar transmission through the windows, SHGC is not affected by curtains. ALF assumes that curtains are only drawn during hours of darkness when solar gain does not apply.


Shading describes the external shading of the window by eaves, plants or other obstacles. Shading of 'none', 'a little', 'some' or 'a lot' means that all, 70%, 50%, or 30% respectively of the available sunlight reaches the window. 

The calculations

Average r-value

The average R-value is the area-weighted average R-value of all windows in a wall/roof. It is calculated using the formula:

\begin{equation} R_{Average} = \frac{\displaystyle\sum_{all Windows} \frac{R_i}{A_i}}{\displaystyle\sum_{all Windows} A_i} \end{equation}

Average shading

The average shading takes account of the SHGCs and the shading of all windows in a wall/roof. It is calculated using the formula:


\begin{equation} Shading_{Average} = 100\% - \frac{\displaystyle\sum{all Windows} SHGC_i \times Shading_i \times A_i}{\displaystyle\sum{all Windows} A_i} \end{equation}


Opaque doors should be treated as double-glazed, timber-framed windows with 100% shading if their R-value is not known. If the R-value is known, model the door as part of the wall using the 'custom' construction type. This also applies to garage doors. Glazed portions of doors should be treated as windows.