"Did the Thermoblock and Celcon cause you any issues at all during the cutting/laying, and did you suffer any Celcon cracking when the frame was bolted through to them?"

Had no issues laying the Thermoblocks - same method as any block. Just a lot lighter to move around which made a nice change! As for cutting them, you won't be able to cut them if you hit one of the cement cylinders within the actual block. What I did, was to get a nail to pierce through the EPS to locate the columns to make sure I was not about to blunt my saw!

I'd probabaly go for an extra wide Thermoblock so that you have extra tolerances once your sole plates go on top. I had to drive to Belgium to get them though as Jewsons screwed me around. I had to courses of celcon blocks on their side (rather than upright) with the extra wide TB on top.

No issues with chemical fixings - just pre-drill the holes all the way through into the celcon blocks.

I don’t know your roof shape, but if you allowed the ceilings to take up some of the loft space, could you give yourself the option of having the floor a little above ground?

Also I can’t help thinking that a “passive foundation” with the EPS sitting on top of your slab (or block and beams) may be the best option.

Shame we can’t hang the walls from the roof and support the roof with internal posts on the insulated floor, and therefore just have EPS below the walls……. (Your Structural Engineer would LOVE to do the calcs for this; most builders would hate building it.)

Something that's weird about this “air holding water” myth continuing to be repeated is that it's actually more complicated than a less inaccurate and misleading explanation: water accumulates on a surface when the surface is sufficiently cool that evaporation is less than condensation.

No need to invoke tiddly layers of cooled air. No need for quantum physics [¹]. Still works when it's not a flat surface of pure liquid water because it's ice, a salt solution, a concave meniscus in a capillary or a convex surface of a rain drop - just the rate of evaporation is modified by those conditions so the equilibrium vapour pressure is a bit different.

As to thermal bridges: why would the bridge be warmer than the surrounding materials? Because it's in better thermal contact with the indoor air [²]? But mightn't it also be in better thermal contact with the outdoor air so shouldn't it be cooler, too?

[¹] First published by John Dalton in 1801 without, not surprisingly, the use of quantum physics though I don't doubt quantum physics comes into it if you dig a bit deeper.

[²] Assuming we're talking about the case where indoors is warmer.

Yes, I realise the surface resistances are likely to be the same for the bridge and the insulation. I did wonder whether to go into the different thermal resistance of the inside (still air) surface and the outdoor (windy) surface but decided it would be a distraction. It would, though, tend to make the bridge colder than the insulation absent other considerations.

However, my point was that the middle of a bridge such as a joist, stud or sole plate is both in better thermal contact with the indoor air (which CJard assumes makes it necessarily warmer) and outdoor air (which he doesn't seem to think would make it cooler) than the middle of the insulation between the bits of wood or whatever.

I didn't suggest any notion of a bridge having a single temperature.

The actual surface layers (first few mm) are likely to be reasonably well ventilated so water's not likely to accumulate there long term; it'll likely dry out every few days. Inboard of that the bridge material could easily be a bit cooler than the adjacent insulation depending on the construction. That's likely if there's any insulation across the whole wall on the inside.

We're clearly talking at cross purposes, but I don't understand what you're trying to say well enough to see whether it makes sense.

The rules inside timber are completely different. We are talking about surface temperatures.

Posted By: Ed DaviesModelling the timber and mineral wool as 245 mm and 5 mm to get an “instrumentation point” I calculate the temperature 5 mm inboard in the mineral wool is 0.34722… °C whereas in the timber it's a fraction cooler at 0.270270… °C.

I don't think you've allowed for the surface resistances. I make the outside surface temp of the timber warmer than the rockwool.

Incidentally, if you're bothering with PUR on the inside for some reason, then what about the plasterboard, VCL, OSB, breather etc etc? Why 245 mm, the frame is 140 mm?

5 mm inside a solid capillary structure like timber it's pointless just computing temperatures. It's more helpful to compute surface temperatures and drying/wetting periods. Those are what the limited data use to categorize the risk of decay.

Clearly any additional insulation either interior to or exterior to the timber is going to change the profile. But it is well known that exterior insulation is beneficial whilst interior insulation increases risk, so I'm still not clear what point you are trying to make. In the neutral (no extra insulation layers) case, then the temperature in the centre will be equal (offset slightly because of the different surface resistances). Outboard of the centre, the bridge will be warmer, inboard it will be colder.

@zgreentouch, Probably best to delete your question here and just keep it in your New build planning &heating options thread, or better yet delete it there as well and start a new topic with a more informative title. Asking the same question in multiple places just confuses things.

You can 'delete' your post by editing it and replacing the text with a full stop or with the word 'deleted' for example.

Thank you, djh, you're absolutely right. I read the comment here, and I posted my question here by mistake. I'm sending my question as a new post under "Housing-New Build". Thank you.

Peter-Green, thank you very much for your quick response. I'll post this comment anew, and if you don't mind I will also send you some questions privately.
Thank you again for calling my attention to this, and for the answer.