Whole-House Heat Loss

Sets your design temperatures from the CIBSE weather zone for your area. Enter your postcode (the area letters are read — e.g. CO, M, EH). Covers all UK postcode areas.

99th suits most UK homes. 99.6th is colder and more cautious — used for exposed locations, or where MCS heat-pump compliance is needed.

Sets how many walls are exposed to outside. A mid-terrace shares two walls with neighbours (little loss there); a detached house loses through all four.

When it was built — sets the likely wall type and insulation era. Finer resolution for modern builds, where heat-pump retrofits happen.

Look at the brick bond: all long faces = cavity; a pattern with short ends showing = solid. Drill-hole patches in the mortar mean the cavity's been filled. Not sure? Pick the safest (unfilled). For homes built 2007 onwards, building regs set the wall performance regardless of type — so all three options give the same result. (Photo examples coming to the guide.)

Retrofit insulation added since the walls were built. Often you can't tell by eye — check the plans or EPC, ask the client, or look at the window reveals: external insulation deepens the outside reveal and usually means newer render; internal insulation brings the inside face forward, leaving deep internal reveals and a deeper windowsill board. A borescope confirms cavity fill. Leave as None unless you're confident. Internal and external give the same U-value in RdSAP — depth is what matters.

Measure the depth against a tape, using the joist depth as a guide. If it falls between options, pick the lower one — that's the safer (higher heat loss) choice. Under 50mm counts as None.