Is it OK to use manually controllable normal internal window blinds as a method of reducing design solar load?
A. Yes. In some cases only. Paragraph 25 on page 19 of ADL2B states that: If the area of windows, roof windows (but excluding display windows) within the areas served exceeds 40% of the façade area or the area of roof-lights exceeds 20% of the area of the roof and the design solar load exceeds 25W/m2, then the solar provisions should be upgraded such that at least one of the three following criteria is met: the design solar load is no greater than 25W/m2 the design solar load is reduced by at least 20% the effective g-value is no worse that 0.3. This will reduce the solar gain and hence the space cooling demand. The calculation of the effective g-value is explained in CIBSE TM37. It would therefore suggest that if the triggering criteria are met then it would be acceptable to use internal blinds if they meet any one of these requirements. It will naturally depend on the size and orientation of the windows and the type of the blinds specified.
