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Not for me. Reasons to support that system? IF the ground doesn't need to be excavated and tipped to get to that level IF there are no experienced builders available or it's diy Reasons against? The holistic cost is likely to be high, A local builder may be suspicious of some new trendy system A local builder can do traditional build much more readily. 300mm of eps on the middle areas where it isn't so important but 200 at the critical perimeter is weird. you have great ground conditions so don't waste it on a floating slab. I have an inherent dislike for a heavy structural beam to the perimeter ( compare it to a big floor beam holding an upper car park floor?) sitting on top of 200m of eps. trench footings and masonry to Floor level masonry or timber above. 100-150 mm concrete slab with crack control only 150mm pir or 250mm eps then a screed. that will likely save you half your cost to floor level.

Thank you for posting with the outcome.

Hi we originally were not having as much space on the second floor it was just going to be one room and storage but with a redesign of the roof structure and trusses we decided to capitalise on the space hence the first design the games room was almost a bolt on so more our own design. the second design is with the architect input to meet the concerns flagged on the first design so my query isn’t whether my architect is competent it’s whether the design is now meeting regs or any input on if it can be better utilised.

Without seeing the outside, it’s impossible to say but do you mean survey as in inspect?

Very poor vs 300mm blown frame. Been in both offerings and it's quite a significant downturn. Do you really think you'll miss 6" of GIA? Home for life? Factor in all the hidden benefits with MBC, turnkey, SE included, slab & frame fully coordinated and project managed, with guarantees, guaranteed airtightness, fully service-battened interior, cheaper downstream 1st and 2nd fixes, list goes on. I had this very conversation with a clint who had almost instantly dismissed MBC as "too pricey", and they were the most expensive at a glance. After I broke down all the other quoted they'd had from the other big hitters, and realised the opportunities and hidden cost of others / reduced spec / poorer airtightness unless you want to buy some more bells & whistles, and so on, and on, and on. Needles to say, MBC ended up with the job and it's going very well, very VERY quickly, and is likely only a slight bit cheaper than the best of the others (who then didn't make grade). Concrete poured end of March '26, and I am pushing the project forward with some very good people so they can have xmas dinner there this year (less landscaping and 2 of the 4 bathrooms tiled etc). Needs an impartial perspective to show you who is cheaper, and for what (sometimes hidden or 'not immediately obvious) reason(s).

Doesn't you architect understand the rules? That's a big red flag, you are paying them for a compliant design. Even if you came up with your own design and then went to an architectural technician they should still be fully aware of the rules and be able to advise you of any issues/propose fixes.

Hi, building regs have advised my loft (second floor) habitable rooms are not fire safety complaint due to all rooms not having a protected route (direct access) to the stair case. I have had this redesigned by architect and would like an opinion on whether this now meets the regs or how it can be improved? first image is the original design, games room was subdivided into a sep room, second image the doors removed so it becomes part of the space. The cinema, study and server now all have direct access to the stairwell.

In a nutshell. A lot of mis-information and simply "bad" ideas here, with a significant price tag to boot. 400mm of EPS means a load more unnecessary excavation and muckaway......... Buildlite, erm, no thanks, I'll pass.

It's 'doable', but leaves very little headroom to stuff free or v cheap electricity into the UVC as an 'energy balloon'. The more free juice you can store = the longer before you need to lean on the heat source to replenish it.

You have basically taken a passive slab and killed all its benefits. You have no continuity of insulation. Take a look below, you can follow the insulation in the floor and up the walls. The slave takes the structural loads the block work sits outside the insulation. Your Marmox block works but is an expensive bodge.

Have to say, I'm having the same issue as @crispy_wafer, doesn't seem to be a way to stop it leaking attaching to a plastic cistern feed with the flat washer as per a flexi pipe. Takes a brass stop end with the tapered washer with zero issues, but flat washer and cistern, nope. Starting to suspect I might have loosened something in the cistern messing about with it but that'll be a full disassemble of the unit to confirm.

I'm sorry if I misread your comment where you say 'manufacturers of insulation/blocks/TF etc (as competent persons) the psi-values are acceptable'. I read this as comparing SAP10.2 and 10.3 as following the same path with respect to what source of values are acceptable. For sure the ethos is the same as building physics is building physics. However, SAP10.3 does not allow the use of manufacturer details. This was a specific change as is was found the manufacturers were not necessarily giving accurate psi-values as it could give a competitive disadvantage if the real values are poor but the stated values are not. With respect to I-Joists BR443 recognises that the modelling of these is poor which is why Section 4.4.2 three layer method exists for I-beams (I-Joists). Even using this method the result is still higher than can be achieved with a FEM, although close enough for most. However not applying section 4.4.2 at all really does negatively impact the U-value. For the property that started this thread I just wondered if the timber bridging had not been applied originally but had then been applied by the SAP assessor . If so it could also use the wrong ratio (I think 11% or 15% is used as a default) while a typical 600mm insulated bay using 45mm timber would actually be about 7.5%.

Agree with comments re the OP. If the FEM Uvalue of an I-joist wall is so much better than that from the BS EN 6946 method I would suggest one result is incorrect. The combined method gives accurate results provided the I joists are modelled correctly. Therm output is not very intuitive!! I’ve used both methods on a wide range of wall details and they tend to agree. A quick read of SAP 10.2 and 10.3 appears to have identical guidance on dealing with linear thermal bridges. Where do you see any differences?

Paddocks532 story is hard to understand. The 'As Designed' SAP assessment should have picked this up way before you cut ground. For sure the assessor should be able to say what failed not just 'it failed' and if they can't give you more then it may be worth talking to Stroma or whoever that are 'accredited' by. One thought I did have is that timber frames under BR497 have to take into account the thermal path though the insulation as well as the thermal path through the timber frame. You must use the combined method or a finite element model (FEM). Both of these will more accurately calculate the wall true U-value, with FEM giving the true value. The defaults SAP uses for combined method are not very helpful to getting the U-value lowered. I am using I-Joists in my wall. I used a FEM to calculate the U-value and it was 97% better than the combined method as I-Joists are hard to model without FEM.

Also a fair point. I'm pretty good at grunt work

I've done a bit more research and so of course have even more questions. MBC and a few others have a twin wall cellulose offering. I feel confident that this approach would be a quality option, but there are a few issues: - It requires really thick walls and roofs, approaching 400mm before cladding. - It's not exactly cheap - It feels like overkill for what I want to achieve thermally. I'd prefer thinner walls and a bit more space. ...but what are my other options? I'm finding that timber frame manufacturers standardise on 140mm studs, which doesn't seem enough for anything but PIR. - 225mm mineral wool would achieve what I need thermally (if my calcs are correct), but how would I make that work with a 140mm timber frame kit? - If I went with 140mm mineral wool + PIR board, what would the acoustic performance be like? Now I'm also wondering about the relative importance of wall insulation relative to glazing, especially with large spans of glazing? Money better spent on the insulation or glazing?

SAP10.3 is very different to SAP10.2 with regard to thermal bridges and is a stepping stone to HEM. You cannot use manufacturer tables or values. With regard to the competent person requirement; there is no register for such people and the main concern that the SAP assessor will have is that the calculations have been done to ISO standards and BR497. BR497 requires certain boundary conditions for the report and getting these right can be horrible. I tried using THERM (which is free) but came badly unstuck with respect to BR497. Also psiclops is fully ISO verified and BR497 compliant, which is what is required for SAP and BR. It's low cost so if it's not producing acceptable output then I will not have lost much compared to going to a thermal engineer. Also, the SAP assessor has to tell me why the output is not acceptable. I do not yet have building control. Getting all of the paperwork together that I need has been a big task and I decided to wait until February to cut ground. While I could have done my work under SAP10.2 the design of the house is such that my total heat loss is circa 2.5kW and SAP10.2 doesn't do this justice and I have to use defaults or dig through manufacturer data to get a psi value. Using software like http://psiclops.io just made this all easy. It also has an architect verification piece if you think you need that reassurance. Make no mistake - SAP10.3 and HEM are really forcing calculations for pretty much all of your junctions. The defaults will potentially fail your building as you can see from table K1 in the SAP10.3 documentation.

I Have fitted normal flat windows/front/back doors/french doors and sliding doors so I'm fairly confident i can fit a pair of bay windows. The kind i have are the ones that just stick out with no brickwork below or above, I think some people call them bow windows. The current window bays are a bodge and have some big trims around the edges. so i think they got their measurements wrong so im not going to copy them. When i first moved in i peeled the internal trims off and there was some nice big gaps filled with scrunched up newspaper so i dug that out and foamed them. Is there a way i can survey them from the outside? like ive done all the other windows/doors ive done.

Do you already have Building Reg approval for this house? SAP 10.3 is in support of the next Appr Doc L1 (2026) effective from March next year. Using both SAP 10.2 and SAP 10.3 is just extra work for no real benefit. As I understand it both versions deal with linear thermal bridges in the same manner and manufacturers of insulation/blocks/TF etc (as competent persons) the psi-values are acceptable. Not much background info on who or what is behind psiclops - tread carefully!

Expanding on my thoughts. If you have ground that suitable for this solution and are happy with the performance then it's a valid option if it is cheaper/quicker/easier to install. But I suspect you can get the same performance from a traditional foundation with somewhat lower cost (can get any groundworker to do it vs a specialist) and the industry is just much more comfortable with the engineering of traditional slabs which may make other things (insurance, future sale, warranty, etc) easier.

I missed that before, sorry. Still not sure it really changes my view. Marmox blocks are good but they are still much lossier than the other mentioned solutions. There is also nothing stopping using marmox blocks with a traditional foundation and save all the custom eps/engineering costs (use it on the inner leaf and ensure the non-load bearing floor slab is thermally broken from the foundation).

@-rick- Thanks for your comment. Just checking, did you see the whole image? For some reason, after I uploaded it, I only see the whole image if I click on it and open it. Here it is uploaded a different way. Does that answer your question? They have a toe on the raft which the bricks sit on, but with a course of Marmox blocks.

That looks cool. Just need to find a suitable competent person willing to use it and sign which might be a problem. Having said that, in time this looks like a solution that will work once the competent people get familiar with it.

While SAP10.2 allows manufacturer data and fallback to default values that are 'tolerable', SAP10.3 and it's follow on Home Energy Model (HEM) essentially force the calculation of thermal bridge PSI values. Using the defaults may fail your whole property and manufacturer data is no longer allowed. Failing SAP10.3 would mean you do not get you completion certificate and not be able to occupy your house. I have deliberately chosen SAP10.3 as it certifies a higher thermal performance which to me translates to increased value in the house. However, getting the psi-values done has been a cost and professional involvement nightmare with one wanting between £5-8k for 12 junctions. Recently I came across a real alternative that would give you back control. It's easy to use and does not cost a fortune. The site is https://psiclops.io with plenty of verification links, the ability to use DXF drawings as well as layer definitions and it validates against BR497 and gives you a full report.

Should we consider BS 6297 as just a guide when applicable to England and Wales? There are conflicts - for example undisturbed ground between trenches. BS 6297 says 'equal to or greater than 1 metre'. Part H says 2 metres. That's a huge difference when it comes to designing drainage fields!