Dunc

+1 Good luck finding any groundworkers with experience in insulated raft foundations up here; I couldn't. Econekt based in Glasgow were prepaired to travel to Inverness. They were very helpful and engaged but ultimately the cost was prohibitive for me. I've gone with strip footings and, with advice from @JohnMo and others here, I think I've achieved a pretty decent design.

Haven't used it but this looks like it might allow for >1 pipe/cable per hole and be re-penetrable https://www.filoform.co.uk/filoseal-re-enterable-duct-sealing-system

Thanks for the thoughts, everyone. Very helpful! Clarifications: @ReedRichardsTypo (I can't edit the orignal post). Companies I've requested designs from all use flow 45C and return 40C, deltaT 5C. @Chanmenie U values should be correct, MBC Timberframe "pasive" frame with blown cellulose fill...so yes, we should be on the way to passive levels for both insulation and airtightness. Form factor is reasonable but not perfect, 1.5 storey but with a lean-too entry/utility, and thermal bridging not perfect. @BeelbeebubYes, we are far north: Inverness-shire. We can see snow on Ben Wyvis this morning 🙂. Great point on future proofing the installation site! I'm really curious about the design temps. Only one company will share the details prior to me coughing up significant cash. This one calculated transmission heat loss of 2.373 kW and ventillaiton loss of 1.506 kW for a total of 3.879 kW (26 W/m2, 9 W/m3). I asked them to recalculate performance for flow 35C / return 30C and they sent the data below...seems to suggest that with a 26C temperature differential, a flow of 35C is not sufficient?

@JohnMo are you suggesting that a 4kW pump would be fine because the heat load is being over estimated, and my 1.7kW figure is correct. Or that even if heat load is ~3.9-4.8kW a 4kW pump is sufficient? @Beelbeebub not built yet but rather late to make a subtantial change in the overall plan!

Apologies for yet another "how big does my ASHP need to be" question; I've read a lot but understand very little. ASHP to drive UFH for a 1.5 storey house, well insulated (U=0.1/0.12/0.1 W/m2K roof/wall/floor) and airtight (<0.6ACH) with MVHR. JHarris spreadsheet heating demand 1.7 kW (-5C out/21C in, to match UFH design quotes). Various quotes from ASHP/UFH designers suggest heat losses and pumps sizes: 4.8 kW loss/8.5kW pump, 4.7 kW loss/5kW pump, 3.9kW loss/5kW pump. All seem to be using -5C out/21C internal temps, flow temp 45C, return temp 5C SAP calc indicates 4.45 kWh/m²/yr, if that's relevant. 3 key questions: 1. Have I done something wrong in the JH spredsheet? 2. Can the ASHP be too big? i.e. supplier recommending 8.5kW pump for a load of 4.8 kW (or 1.7 if I'm right!)? 3. If the 4.7 or 3.9 kW load is correct and paired with a 5kW pump, is there enough overhead for DHW duty? Bonus Q: One supplier suggested that fitting a buffer will "deal" with an ASHP that is oversized. Does that sound right?

How much effort/time do you want to put in? If you have plenty of these, pretty much any standard construction technique can be made passive. Are you sure that you/your contractor can do the airtightness work well? If you want it wind & watertight & near-passive as fast as possible the MBC twinwall seems a no-brainer. I don't really see how either the KTS or the MBC HP "systems" are different from any other 140mm stud timber frame; they're just variations on the same theme you could achieve using a stick build on site or purchase from a variety of other suppliers and they will all need significant design detailing and on-site airtightness work. I would expect the floor area as drawn in the initial design reflects the INTERNAL wall (plasterboard). When your frame supplier draws up their plans they build outwards from that so your usable floor area stays the same, but your foundation footprint gets bigger for the MBC twinwall.

@Dave Jones that diagram appears to be for a single skin (e.g. 140mm timberframe) where the only horizontal insulation is 25mm PIR + the marmox block. In the case under discussion the build appears to be 2 skin blockwork with cavity insulation. If the cavity is insulated, the thermal bridge of interest is downwards to the foundation blockwork. @JohnMo's maths shows 440mm deep aircrete blocks will do just as well (if not better) than the marmox block.

@SBMS the architectural drawing appears different to your initial coloured plan: the insulation seems to be UNDER the slab on the architectural drawing?

Yes: radon membrane under slab, continues horizontally under the aircrete the turns up inside the cavity join the horizontal DPM throgh the wall. I'm at the design stage too and haven't specified the insulation for the cavity, so reading this with interest to see what options are available. Mine is twin wall timber frame with cellulose fill, so my above ground cavity insulation is a separate consideration. Because of this, I'll probably run the radon membrane under my foundation cavity insulation then up the inner face of the external block to keep both insulation and aircrete block dry. General question for you & others: I'm not sure on the value of 2x300mm aircrete blocks? With insulation above the slab the top block is pretty much continuous with this and limits both the horizontal and vertical bridges. What does another lower insulating block add?

The way I looked at it very crudely [hope I don't embarrass myself here!]: Thermal conductivity (lambda, k) of a Marmox block is 0.05 W/mK (https://www.marmox.co.uk/products/thermoblock). Maximum thickness of marmox is 0.1m (100mm) and you can't stack them. So thermal resistance (R) = 0.1/0.05 = 2 m2K/W. The U value (1/R) is 0.5 W/m2K. Thermal conductivity of an aircrete 7 or similar is about 0.18 W/mK (https://www.forterra.co.uk/product/thermalite-aircrete-hi-strength-7/). Assume you've got a single 300mm block R = 0.3/0.18 = 1.67 m2K/W. The U value (1/R) is 0.6 W/m2K. So not a lot in it as long as the aircrete block is kept dry by appropriate routing of the DPC? I've gone with a standard thermal block because round my way, no groundworker was likely to bother reading up how to install the Marmox blocks correctly.

Good info. I'm currently contracted with MBC...it's been generally good but a couple of points which anyone building in Scotland may find helpful: 1. They can't supply Rationel/Velfac windows in Scotland (someone else has the contract for that aparantly). This isn't made clear on the website. 2. The groundworks team don't want to travel to NE scotalnd to install a raft, even though the timberframe side of it is fine up here.

I haven't got a spade in the ground yet so not best placed to advise on future economies. But I have been through the "what can we afford" question very recently. What do you want to achieve in three key areas: Style: Architecturally interesting or simply functional? Performance: a bog standard building regs house? A passive house? Build: will you do any practical or management work, or do you want a project manager or main contractor to do it all? Talk to architects/designers. Some we spoke with gave us expected costings in the £3000-£4000/sqm. These were architects whos portfolios clearly show "flair" (balconies, ovehangs, fully glazed walls...i.e. expensive things to engineer) and/or who work most often to passive standards. Others suggested lower costs per square meter and clearly work more with self builders who have a tighter budget and want a nice, but fairly simple house. Even these latter types started at >£2000/sqm. I think unless you're very hands-on, the lower end of the range is not going to be achievable. YMMV. Cost per area doesn't usually include the plot itself. Or the professional fees associated with that...I'm rapidly finding that I didn't budget for all sorts of things that do add up. £180 just to register the address, for example. Fees for servitude to get services to site. Site surveys of various sorts. Insurances... I think self build is a bit like the stock market; it's an investment that contains an element of risk.

thank you @Dave Jones. Not sure I can use Marmox thermoblock as these require the sole plate to be at lest the same width as the block (100mm seems to be the narrowest availalbe). The MBC frame has 89mm verticals.

Do you just need to cross a road, or is the main under a road? Could you use a moling company to go under the road?

I suspect this is a typo from the architect; it's already on my list of things to check, but thanks for pointing it out!