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I'd discounted the idea of going for full passive certification for the exact reasons you have outlined but am very interested in doing something close to it. The biggest issue that I can see with the plans I have bubbling away in my head is that I want to retain all but one wall of the original structure then build on top of that with a glue lam timber structure. Where the join is will be difficult to make airtight.
I've talked to you in the past about MHVR and I'm sold! Done some more reading around it and I honestly can't see many downsides, the idea of being able to purge the air in the house I like, my wife already calls it daddy had a curry last night mode and we haven't even submitted planning drawings yet.
You say you don't have a boiler, does that mean that your hot water comes from an immersion heater or do you have hot water solar panels?
Bobbo-
What's the construction for the rest of the house? We had some odd details to work through, but the timber frame people we used were very good at working out the most cost effective (and air tight) solution... thermally breaking everything can be more difficult than ensuring a good seal.
Yeah, we get all our hot water from the SunAmp PV unit which is usually all we need, we sell some PV back to the grid in Summer and we top it up in Winter on an overnight tariff. We went with an unconventional DHW solution to minimise any standing losses from a thermal store (as our main risk was over-heating in Summer, not being too cold in Winter). It's well worth looking into PV-powered battery technology (for DHW)... it's a great, simple solution that isn't well understood: https://www.youtube.com/watch?v=D3_VgxsOxoE
I'll have to dig out some more exterior/build shots, but the gable end (with in/out vents) and interior is here:
https://photos.google.com/share/AF1QipNUbVX-N-y_tTU7dU9LQfQ-F92SvjusMuvL_Zkw7ren6ZYSbYVISKccITxmw9duNg?key=UzlDM0t6ejM0UTY1NzNTV3lrYzRteUZPYkw3X2pn
JonoMarshall
What do you need to know? There's no reason to pay for actual PassiveHaus status (an expensive plaque that won't increase the performance/value of your home), but we've 'achieved' the standard with a new build in Devon (~100 sqm 3 bed, so actually no mean feat given the space-to-walls ratio).
Outgoings:
MVHR, vents, design, fitting and setup was ~£10k
Ultra high-spec windows/doors/skylights were ~£12k
PV array and SunAmp PV Unit (DHW) was ~£7k
PassiveSlab foundation was ~£1k (excludes £4k piles cost)
Timber frame (installed to 0.6 airtightness or better) for ~52k
Cost savings that helped to offset the above:
No boiler
No thermal store
Bare minimum for plumbing
Almost no heating (3 small electric towel rails)
Benefits:
Very quiet
Very fresh air (always)
Much lower bills (~10% of the average)
Much better for the environment
Cooler in summer
Warmer in winter
I met with a few RetroFit companies when looking for a PH timber frame supplier, the retrofit stuff seems more complicated, but it's certainly possible.
Being able to plan/design/manage the freshness/humidity/temperature of your home is amazing. So many great (glazed) architectural builds feel somewhat stuffy by comparison!