Glazing Cavity resistance

[DS40]

Hi,
The glazing cavity resistance used in APcdb for default glazing is shown as 0.3247 for a 12mm cavity. Where is this figure from? In CIBSE it shows 0.17 - 0.18?

I am trying to create glazing with U value of 2.2 including frame for double glazing (low e) 6mm/16mm airgap/6mm but not sure of best way to do this (Concept stage only so no frame info.) I have created the new glazing using the correct glazing and changed the default airgap to 16mm but even by playing with frame can't get 2.2 U value. Does the cavity resistance need to change too? I can easily change this to say 0.2684 to achieve exactly 2.2 but it seems like a 'cheat'? Would it impact on anything else by doing it this way?
Thanks

[PCully]

Hi,

I can't find a record of where this default value came from, it may have been arbitrary or matched some data we were using at the time.

Modifying this value is usually the fastest way to match the target U-value, try to match performance data form your manufacturers sheet as much as possible then this value can be edited to reach the final U-value. There is further guidance on this process in the Constructions Database User Guide that can be accessed from VE using the Help >> User Guides menu

Phil

[DS40]

Thanks for for your help Phil