My 2p. Its a tricky one isnt it. I read somehwere that you can achieve the targets through just installing an ASHP, I have my reservations. Im an architectural designer and about to build a new house for my family, we are very likely to opt for the MBC timber frame passivhaus envelope. We wont be seeking Passivhaus certification due to the sites orientation and setting within a valley. I have considered traditonal masonry builds and increasing the cavity width, Denby Dale passivhaus (Green building Store) shows the potential of cavity wall systems that could be adopted by traditonal builders in the future. The attention to detail required to ensure airtightness of the envelope will be the biggest hurdle for builders, this is probably more important than the thermal resistance of the insulation. MBC guarantee the airtightness to passivhaus standard which is one of the biggest benefits. Just need to make sure the builders that will finish it dont puncture the airtight barrier.
The costs of using the passiv system is maybe 8 to 10% above traditional masonry. Our budget is very tight so simplifying the internal fit out to invest money in a low energy envelope.
In my opinion a highly insulated prefabricated timber frame will be the future. The costs should reduce as they become more popular and the general awareness of the benefit of building with airtight buildings with controlled ventilation shows reduced energy consumption and more importantly running costs.