WVDV

Hi, Im new to the site: Stumbled across it when trying to resolve my querry. Does anyone know how to account for low emissivity cavities in U-Value calculators? I'm using the BRE calculator, trying to calculate yhe U-Value of the following timber frame closed panel build-up: 12.5mm Plasterboard 35x50mm vertical battens to create a service cavity Reflective AVCL layer 25mm PIR Insulation 38x89mm timberframe studs at 600mm centres, with 100mm lambda 0.044 insulation between the studs 9mm OSB Reflective breather membrane 50mm Cavity 102.5mm brick I come up with the calculation below, but it seems to me that the reflective membranes on the inner and outer cavities have a far to large impact on the calculation? Walter V