Doing the pricing calcs for my barn conversion and the heating system - pellet stove, rads, thermal store, etc is hitting the budget hard. If I divert some of that money into hyperinsulating the roof, and use a bit more hemcrete on the walls, so as to achieve something in the passivehaus region of insulation, will I get away with relatively little solar gain (small roof lights as its a listed building) and some form of low cost electric heating - through the wall heatpump or underfloor matting - any opinions or experience?

Wet UFH - too much labour costs as we'd have to take up the floor - fit it - the relay the floor, and on the ground floor we'd have to dig out the slab and replace it. wood burning stove may be a possibility as that is what we heat the main house with, but the chimney doesn't help the overall u-value of the building much!

secondary question - is it cheaper to have an immersion heater, thermal store / water tank using E7, or just use an electric instant heat shower system, when we would only be using the shower in the barn a couple of times in the week?

I'm hoping that the hemcrete will seal the walls, we only then have to seal floor above garage, roof, and two doors... I'm hoping to budget in heat recovery ventilation, if only to get rid of the computer heat in the summer!

Hemcrete will be added to 500mm stone and rubble walls... not yet asked building regs people if they like it, but I'm hoping to present enough evidence that they can't refuse. Floor is a concrete slab of unknown construction, but not very old so we are assuming it has some insulation value at least. As 2/3rds of the ground floor is garage and will be outside the insulation envelope, heat loss through the floor isn't too worrisome.
I'm hoping that the high levels of insulation (I'm currently able to get an overall u-value of 0.21) and incidental heat (computers!) will provide background heat, and then we may look to electricity for top up heat, as its cheap to install, and renewable (from right source).