Unlike the UK’s National Calculation Methods (NCMs), such as SAP and SBEM, where each rough opening can be considered as one window, PHPP needs accurate window entries, which count each casement separately. Entering two adjacent window casements as one means that the frame losses are underestimated, and the solar gains overestimated. Also, opening windows nearly always have thicker frames than fixed, so it is good practice to assume that all windows are opening ones early on in a design; this should avoid underestimating frame heat losses.
Using an installation psi-value of 0.04W/m2K is a good conservative estimate for early design, and can easily be improved upon. This typically represents a window installed into a standard timber frame.