Benpointer

So, I am thinking there will be a calculated u-value from, say, 100mm PIR between the joists with nothing within the joists (i.e. between the webs), then say 150mm PIR over the joists. Sure it will be less than full-fill 100+150 PIR but better than just the 150mm over the top. Presumably could be calculated for the whole roof by calculating the % area with PIR versus % of unisulated joist. For 97mm joists at 600mm centres that's 83% PIR at 0.22u and 17% 47mm joist at 2.55u = 0.6u... Then add the 11mm OSB and 150mm PIR on the top to get an overall u of 0.11. Still, it might be easier/cheaper to simply put 200mm PIR on top and forget insulating between the joists. I don't want to full-fill the easi-joists because I'd like to thread services including MVHR through them. I am mindful of making the roof look very clunky though.

Wall structure is as per this diagram but we're cladding in cedar / cement boards & render rather than having a masonry skin. Either 50mm additional PIR for u = 0.12 or 100mm for u = 0.09.

Oh, some pictures: Floorplan: Roof truss plan: Cross-sections:

Getting structure quotes in for our 180m2 single-storey self-build. We've got vaulted ceilings which is complicating things a bit. The quotes are varying between £400 and £800 per m2 to get to u-values of 0.12 walls, 0.1 roof. One quote I quite like is Turner Timber's Super Advanced, which seems like a good price, and uses 254mm Easi-joists with 11mm OSB over for the roof, so no purlins intruding into the ceilings and potentially lots of room to run MVHR ducting and other services through the Easi-joists. Question is: should we insulate Between and under the joist, which loses the service space benefit, or Go for a warm roof with all the insulation on top, or Between and over, say 100mm PUR between and 150mm over (is that still a warm roof)? The roof will be a standing seam zinc so if we go option 1, we'll need membrane, 50mm battens, 18mm OSB or ply on top. VMZinc say you don't need that for a warm roof so is that a saving? Of course, I'll be asking our architect, who I rate, but fear I could be paying him £££ per hour to research this out, so if I can steer him in the right direction... 😉 Any views?

This was answered by @Roundtuit back in April - sit the sole plate on a course of Marmox blocks. And/or increase the size of the PIR upstands. I note you say no screed but what are you going to put UFH in (presume you are having UFH)?

Although most houses have at least two sides, so if the other one faces south west, that'd be good for PV. Regarding the friends who 'regret their tiled floor because they hardly ever put their ASHP on', they are confusing things a bit. Presumably they had the tiled floor because they had underfloor heating and they had UFH because they had an ASHP. But none of those links are compulsory: you could put in a wooden floor over UFH and you could run UFH without an ASHP (though that would be an odd choice today). If you are only using the house for short periods the woodburner may be sufficient if the insulation levels are very good - your SAP calculation should help you decide that. But in any event at build time it would be sensible to put UFH pipes in when the floor is laid if there is any chance you might need more heating than the woodburner at some future stage.

Problem with the UFH mixing valve?

You mean a glue - you glue your stud wall sole plates to the screed?

Just on this point, the spreadsheet inputs can easily be adjusted to reflect that an insulated UFH slab is likely to be 4°C warmer than the room temperature - just set the under slab soil temperature (cells B4 and B44) to 4°C lower than you expect it, so 4°C rather than 8°C, or 6°C rather than 10°C.

Just on the original spreadhseet... If I get a worse case* heat loss rate of 2,602 Watts that tells me a 5kW HP should be plenty big enough, is that right? (* Worse case being when I set the outside temperature to -10°C. An even worser case would be if I set it to -20°C but the heat demand is still only 3300W - and for -20°C in Dorset, I suspect the gulf stream will have stopped so other issues may be relevant.)

Great, thanks everyone. (Although, I have to say @JohnMo that every single time I have tried to use hammer fixings it has ended in tears - wobbly fixings, bent screws or both.)

Ok thanks guys. Still not quite sure what we should do off of our floor which is planned to be block & beam + 200-300mm EPS + 70 screed with UFH. Is it ok to fix stud walls to 70mm of screed? If we leave the screed until after the stud walls are in what do we fit the base of the stud walls too? Not the EPS insulation obvs. Nor to the B&B itself. It'd be helpful if we can avoid building up from the B&B under every stud, with the cold bridging issues we would have to address.

I know SE input will be required at some point but I am just trying to get my head around this now. Planning just submitted (see pics), plans out for initial quotes with several TF / SIPS company (we'd prefer TF with cellulose fill if we can afford it). Also trying to get an indicative quote from my preferred groundworker for the foundations. Floor is going to be B&B + 200-300mm EPS + 70 screed with UFH. Question I have is can the non-structural internal stud walls be built off the screed or will we need to build block upstands under each, with the concrete trenches, insulation detail etc. that those will require? Here are some pics:

Our draft drawingsa specify the FFL in metres above see level (100.35 in our case). That is taken from our topographical survey. (We're currently debating with the architect whether we should raise the FFL up by 0.2m, given the amount of rain we've had this winter.)

Wouldn't it be better if the Marmox overlapped the PIR a bit more? At the moment there's a cold bridge in your diagram where there's only effective 25mm of Marmox between as warm screed and a cold concrete block. Ideally, surely the Marmox should extend down to the bottom of the PIR. Scrub that - you're not able to lay it in sufficient thickness according to Marmox's own specification: https://www.marmox.co.uk/file-uploads/Doors___Windows.pdf

We probably will put in RWH as part of our SuDS, but for garden watering, not toilet-filling and clothes washing; grey toilets really suck.

High ceilings reduce stuffiness; MVHR avoids it completely.

Comfort, the freshness throughout the house all the time. Go away for two weeks, come back and the house feels great. We've never had that in any other house. Our current rented accommodation is by many standards a lovely house - well above average house price value - but it manages to be drafty and stuffy, at the same time, we even have mould in places. So yes, MVHR remains no. 1 on our list. (I agree with you about fads though - 3D TVs say "Hi", and rain water harvesting for the toilets and washing machines will not be repeated by us again!)

Of all the 'eco' things we did in our last house (amajor renovation) - triple-glazing, MVHR, ASHP, UFH, PV, Solar HW, masses of insulation... Of all those things, MVHR was number 1 on the list for our forthcoming self-build. Not for payback but for comfort.

Would tool-hire plant and tools be covered by the tool-hire company's insurance?

This has got me thinking about a couple of points: For a block and beam floor, the conduits presumably need to come up between beams. So do you cut a hole in a block, leave a gap between blocks where the conduit i, or what? Do the conduits get sealed in with expanding foam where they come through the B&B? Same for the insulation layer above the B&B? How are the conduits mad airtight once the cables, pipes etc are drawn through them - is expanding foam acceptable? Thanks

How thick is your screed JohnMo?

Quick question, and apols for taking this away from Botus's original crisis, but with UFH why wouldn't you just leave it on at a comfortable temperature 24x7? As I understand it Botus is doing a well-insulated new build.

We went down to see Norrsken last week and were quite impressed, so that's good to hear. However, I do quite like the look of Internorm's HV 450 Alu-clad triple-glazed + blind + outer-pane units (so effectively quad-glazed). I suspect we would only be able to afford one window though and would have to board up the rest of the openings... which may be sub-optimal.

These rendered images are great, what tool did you use to create it @Conor and @Great_scot_selfbuild? PS +1 for the hob on the island unit, close to the oven - casseroles, roasts etc often need to be transferred between the oven and the hob.