We need to insulate the ground floor, but to take up the existing slab would cost over £2k and be very disruptive - so do we do it? After much debate, the answer is no - we have decided to apply the insulation (130mm phenolic plus screed) on top of the existing slab and expose the first floor joists to compensate for the reduced floor to ceiling height. The PHPP hasn't said no despite the perimeter condition remaining a significant cold bridge.
We are working with Green Tomato Energy (now Enhabit) to develop the strategy as Russel hasn't started his Passivhaus Designer course yet. GTE have done a first run using PHPP (Passive House Planning Package) and provided options for achieving certifcation to meet the two refurbishment standards - EnerPHit and AECB Silver. Subject to budget constraints, we are aiming for EnerPHit - the tougher of the two standards.
We have debated whether to include the extension in the Passivhaus envelope, but the PHPP has shown that it doesn't make too much difference, partly because even though the house is quite compact, it's 'small', which directly affects the relatively poor form factor (3.8, or 3.6 with the extension).
The PHPP model shows that the existing building requires 423kWh/m2 per year of energy to provide space heating. To achieve EnerPHit this needs to come down to less than 25 kWh/m2 per year! Gulp.
Russel and Anna Hayden