VOICES | SUSIE DIAMOND a bespoke calculation, but assumes top-hung windows where the AD assumes side-hung, which adds to the confusion. Also note that it is the equivalent area values that must be used from ClassCool, not the free-area values, as might easily be assumed. It will be important to complete a Part O assessment pre-planning, while window dimensions can still be readily altered, but the calculation requires more detail opening mechanisms and free areas, frame dimensions, and so on than is generally known at this design stage. In high-risk locations, glazing limits are generally the same or lower (except, oddly, in west-facing homes with cross-ventilation), and you have to apply shading. In moderate-risk locations, you dont have to provide shading, but it would add flexibility if this option were factored in. The dynamic thermal modelling route This essentially follows CIBSE TM592, but with some important changes: Internal blinds are not to be included in the model: this will make it more challenging to gain compliance an odd move, as most occupiers will put up some form of blinds or curtains in most windows for privacy, as well as shading/glare control. The rate at which windows are assumed to open is explicitly set, starting at 22C and with maximum opening not reached until inside temperatures exceed 26C. This is slower than many modellers have previously assumed and, again, will make compliance slightly more challenging. Bedroom windows (if they are secure) are required to be left fully open all night if bedroom temperatures exceed 23C at 11pm, and closed all night if not. This is not a feature that most modelling tools can support, but there are scripts and workarounds to make this possible, and the software companies have promised updates soon. Its not clear what impact this change will have on results, but our initial tests at Inkling suggest this might lower the overheating risk predicted compared with results using the TM59 protocol, which would close windows below internal temperatures of 22C. There is no advice given on how to select a sample set of units to include in a modelling assessment. The Department for Levelling Up, Housing and Communities says this is up to building control. There is no rationale given for deviating from TM59, as it was published by CIBSE, and it could lead to confusion, with the modelling methodology split between two documents. This route requires dynamic thermal modelling software and an experienced modeller, so will incur higher assessment costs than the simplified route, but may, in some cases, allow more cost-effective design options. Which of the two compliance routes you follow is up to you, unless your project features any of the following, in which case you must use the dynamic thermal modelling route: A communal heating or hot water system with significant amounts of horizontal distribution routes A mechanical cooling system 74 May 2022 www.cibsejournal.com CIBSE May 22 pp73-75 Overheating.indd 74 22/04/2022 16:13