After my last ask, we've negotiated with the neighbours to redirect their boiler flue so we can put the MVHR in the most logical place: the garage. I've also compromised by putting upstairs ducting in the cold loft. This has simplified things:   A few notes (or skip to the calculations below) Upstairs I've compromised by putting ducts in the cold loft outside the thermal envelope, as my wife is 100% against any boxing in The loft is well ventilated at all eaves (