Thorfun

I did ask if EPS 300 really was required as @Bitpipe has EPS200 and the answer was a resounding yes from the SE so we have done as designed. some form of render board might be a possible solution, also some on here have used metal of sorts (iirc) but as that detail is a long way off it's quite a way down in my list of things to research! it is a very long list.... ? our design doesn't have that stuff shown outside on the MBC twin wall system as we're not having a twin-wall system. simple open panel timber frame for us but still sitting on an insulated slab.

the groundworks company we're using have subcontracted the basement build out to a construction company that specialise in basements and they've brought their own formwork to site. it's all under a fixed cost so I'm very hands off on this part of the build.

dense under the slab as designed by the structural engineer and normal outside the walls as they're not taking any compressive loads, again as designed by the structural engineer. I assume you mean the small amount that is above ground? if so, I'm undecided on that one as yet but I believe there are many ways to do it and I'm pretty sure it comes down to personal taste (or planning requirements if they've mandated something).

200mm EPS300 for me. after calculating the U-value when taking the depth below ground in to consideration I decided that 200mm was enough. we also only 250mm thick walls. it's interesting how similar builds can have such different structural calculations! exactly what we're doing except we've got EPS100 on the walls. don't know why not EPS70 though before you ask. ? the membrane covering our flat EPS300 is part of the total waterproofing system which wraps the basement underneath and up the walls.

didn't realise you had the twin-wall system. also, you have timber joists between the basement and ground floor so the airtightness fabric wouldn't take as much of a beating as it might with block and beam so would be less likely to be damaged during the joist installation.

yep. just the spacers at, I presume, 600mm centres. so very much reduced cold bridges. I'm also interested as to what the stuff outside the envelope is for! no worries about the basement, I'm very happy to have people challenge/question what we're doing as it's good to have extra eyes checking for stuff. ?

I had a quote from Rhys when I was considering ICF. went to a site to see his work and it was impressive and the quote was pretty good as well. in the end I decided on RC rather than ICF though.

could save a lot of money doing it yourself! I thought about it, hiring a big digger and digging the hole but when I saw what 4m deep dig actually is I'm glad I left it to the professionals. I'd say digging that deep is definitely not for the faint hearted. and then I didn't want to be responsible for the main structure holding up my house so, again, decided that this part of the build should be left to those that know what they're doing and that's before I consider the time it would've taken me and the wages I would've lost as I'm self-employed so, all-in-all, I'm very glad I'm not doing it myself.

it's actually tough to separate the basement from what the foundations would've cost without it. but total cost for all foundations and basement is £180k. I actually think that without the basement the foundations would've been about £40k - £50k as we'd have to dig down 2m to get to bedrock anyway so I think the basement bit is coming in at about £1k/m2.

assuming you're talking about the MBC twin wall system? if so the inner sole plate takes all the load from the house and the outer sole plate is there for the outside of the wall to hold all the cellulose insulation. quite a few on here have this design (and not all from MBC) and it does work. There isn't a cold bridge at the sole plate either as the entire space between the outer and inner sole plates is filled with blow cellulose. personally, if I had the extra money I'd have gone for this over the open-panel system with me fitting the insulation in a heart beat but I just couldn't afford it. yep, lots of waterproofing. 2 types in fact. Type A external membrane and Type B waterproof concrete. There's also a land drain around the perimeter which is working well and filling the sump we've dug constantly. here's a photo of it working. also a tonne, or rather 9.5 tonnes, of steel in the RC slab and walls. it is on the drawing but I removed the text detailing it as it wasn't pertinent to the question I was asking and didn't want to confuse matters!

thanks for this. I'll definitely investigate it further never heard of Wufi before but a quick Google has told me what it is. Every day is a learning day! ?

thanks and glad it's not just me! I could potentially bring the insulation up past the sole plate but it'll have to somehow have a cutout for the osb. I'll have a think about it and what it might impact and, maybe, speak to the architects. but maybe it's just a detail too far. ? standard external detail tbh. frame, osb, breathable membrane, battens, cladding.

thanks and it's starting to make more sense and I certainly wouldn't want decay on the sole plate. but I'm confused as I'm sure I've seen lots of sole plate details on here and, iirc, they're all the same as the one in our drawings. even the MBC passive slab and twin wall detail has the outside sole plate in the same way as mine. so, what is the solution here and are loads of timber frames soon to be the victim of decaying sole plates?

can you supply a bit more detail about this please as I don't fully understand what you mean?

there is a land drain around the basement , I just excluded it from the drawing when I took a screenshot! the external EPS goes all the way to the sole plate. are you suggesting that the sole plate should sit partially on the EPS? thank you. I will research rubberised VCLs

I'm researching the next detail for our self-build and in about 3 or 4 weeks the basement walls will be finished and the block and beam floor (was originally going to be hollow-core concrete slabs) will be laid to create the ceiling/ground floor. I want to continue the airtight membrane/VCL from the above ground levels down in to the basement. Here's the detailing the architects and structural engineers have for the basement ceiling and I've added a blue line showing a VCL running around the block and beam/hollow-core which can then be taped to the VCL above ground and also continued to the basement floor level. I think that above ground this is called a 'Tony tray'? my questions on this subject are: 1. is this overkill when considering that the basement is one big concrete box? 2. with the ground floor being block and beam will any VCL just get ripped to shreds with the concrete beams placed on it? 3. if the answers to 1 and 2 are 'no' then can anyone recommend a VCL specific for this scenario or are they all the same? @Bitpipe what did you do in your basement? I don't remember reading any specific detail from you around this.

it's an interesting idea but with the pour on Tuesday it's all a little late in the day! I'm happy with our choice and there are always low level ufh options later down the line if required. especially as we won't be fitting out the basement for a couple of years due to budget constraints so we'll have time to live in the house and can then decide the best course of action.

I'll save my UFH pipe laying for the ground floor when I will have lots of time. thanks all for the advice and I'm 99.99999999% sure we've made the right decision. ?

anyway, I've made the decision now. we're not having UFH in the basement for now. we'll see how it all goes and can always add something later if at all required but we don't think it will be going on the levels of insulation we have and other member's experiences. means my original Saturday plans are now back on! woo-hoo! ?

I probably could get a refund on the UFH pipe from Wunda but as we're going to install wet UFH on the ground floor I might as well just keep it for when that happens. when I discussed this with the wife she also said that most of the rooms in the basement are active rooms so when we're playing music or in the gym then we'll be generating heat, the cinema projector will chuck out enough heat as well and so it'll just be the games room area which will probably end up being a den for the kids and their mates so that might be the only potential area that needs heating and if there are a few of them then I'm sure it'll be fine. We don't have much natural light. we were thinking of light wells but it was an expense and complication that we decided not to bother with as the activities that the rooms have been earmarked for don't really need natural light and there should be enough coming through the sunken courtyard sliding doors to make the games room area light enough for teenagers. ?

we'll have between 30 - 40mm between top of mesh and top of slab. working on 30mm with the 16mm pipe it leaves only 14mm between top of pipe and top of slab. I sent this information to the structural engineers and they said that isn't enough. not sure 12mm pipe would help much tbh and, it's probably too late as the pour is on Tuesday.

@joe90 and @PeterW I've taken another look and lifting up the top mat is never going to happen. here's a couple of photos of the mesh and chairs. if you both think it'll be a complete nightmare to run the UFH pipe as per the Wunda plan... then I think we're leaning towards just scrapping the UFH in the basement. I know that @Bitpipe doesn't have heating in his basement and says it's a comfortable temperature all year round so it is a possibility just to scrap it. I guess this is a dilemma all first time self-builders go through as I have never lived in a well insulated and air tight house so trying to get my head around the concept of not needing heating is a challenge!

I'm pretty sure the basement guys have already tied all the steels together so would be a nightmare to undo it all and then re-tie it. (I'll just pop out and take a look) bugger.

it's too late for that @PeterW. top mesh has already been laid so it'll be a case of threading pipe under the top mesh or just forgoing UFH in the basement altogether! I'm aware it'll be very time consuming but it's the only option I have now.

anyone have an opinion on this before I make the assumption that it's fine to do this? I've just been told by the SE that I have to attach my pipes to the underside of the top mat of rebar as there's not enough space above the top mat to do so (it would leave only 14mm between top of pipe to top of slab) and so anything I can do to make threading the pipe through the top mat easier the better!