DonnaP123

Totally right - I meant 2 combi boilers! But UFH is okay to run from a combo boiler isn’t it? I’m not sure what you mean by needing a buffer?

Totally right - I meant 2 combi boilers!

So we currently have a decent condensing boiler which we picked to have extra ‘welly’ in case we ever added a second bathroom. Well we’ve gone a bit further than that and built a large L-shaped wrap around ICF extension. The current boiler can easily service 2 bathrooms that will be above it, without moving it or adding much extra pipe work. However, our en-suite, the kitchen and the manifold for the UFH are at the side of the extension, about as far away as you can get from the existing pipe work. we are wondering if it wouldn’t be simpler to add a second condensing boiler in the new kitchen for the en-suite and UFH. This would take up way less space than a system boiler, and would be relatively easy to get the gas to it and much simpler pipe work. I think the cost would even out with the labour bill being lower. We will have a highly insulated and airtight house when done, so the demand for space heating should be low. We’ve ruled out an air source heat pump for now (and realise we are possibly ruling one out forever if we don’t put the pipe work in for a single central point now when everything is easy to get to). Just after views as to whether this is a ridiculous idea or not?

Excellent advice! And I guess I WILL have to be extremely fastidious with the VCL and just make sure we get it right! We will also have MVHR which should (I guess) also help to mitigate excess moisture. Or at least it can’t hurt.

Thank you - you are correct, the roof is built with the sarking above the rafters, tiles etc on top. We couldn’t increase the ridge height to have a full warm roof so have compromised with the hybrid version. Counter-battening would work and is a good idea, thank you.

Thanks everyone! Food for thought….dew point analysis and seeing if we can find a way to make wood fibre alone work.

Advice needed! For our woodcrete extension we have built a warm pitched roof with 80mm wood fibre sarking boards over the rafters. We planned to put approx 190mm flexible wood fibre bats between the rafters, but now realise many of the rafters are not deep enough to accommodate this. So my question is, is it okay to put PIR between the rafters (as would need less depth than wood fibre for the equivalent U value), and how much would we need to reach a U value of 0.15 overall, including the wood fibre sarking already there? I can’t find an online calculator that allows me to mix wood fibre with PIR! We would lose the breathability of the wood fibre but should still hopefully benefit from not overheating in the summer (which was the main reason for using it) given we have 80mm wood fibre above the rafters, so I don’t think there is an inherent problem with mixing the two, but could be missing something?