QUESTION:
I don't understand what is meant by "Alt. solar gain" Heavy and Light ? In my project (existing building), if I modify these values, the interior temperature of the room strongly increases (entirely glazed passage).
ANSWER:
"Alt. solar gain" actually refers to alternating solar gain, a value used for windows in the CIBSE Admittance Method. I have included an extract from CIBSE Guide A as the original source explains it best:
5.6.3.3 Parameters associated with solar gain.
The thermal load on a space due to solar irradiation depends upon the radiation transmitted by and absorbed within the glazing system. The amount of radiation transmitted and absorbed is a function of the intensity of the incident radiation and the angle of incidence between the solar beam and the glazing.
Therefore, for an accurate determination of this load, it is necessary to calculate the gain occurring at various times of the day. A computer-based method is required to achieve this cost effectively.
However, if the objective is to determine either the maximum space temperature or the maximum cooling load, the transmission and absorption characteristics calculated at the time of peak solar irradiation are often the most significant. The Simple (dynamic) Model is based on these assumptions.
...
Typical values of the Solar Gain factors for various glazing configurations are given in Table 5.7. The transmission (T), absorption (A) and reflection (R) components (for thermal short-wave radiation) and emissivities (for thermal long-wave radiation) for generic glass and blind types used in calculating the solar gain factors are given in Appendix 5.A4, Table 5.44.
Glazing manufacturers do not usually provide values of these solar factors for their products. More commonly, one or more of the following are provided:
- properties at normal incidence (see Appendix 5.A4)
- total solar energy transmittance ("G-value").
Only the first of these can be used to calculate solar gain factors. However, shading coefficients and G-values can be derived from fundamental properties and so it is possible to obtain the corresponding solar gain factors by a process of iteration. Shading coefficients and G-values are defined in 5.2.2.
CIBSE Guide A (1999)