If you're anywhere near Birmingham, Ecclestons had the best EPS prices when I did a thorough price comparison for myself a couple years ago. https://ecclestons.com/
Once including delivery a lot of the online merchants prices came closer.

I’m in Suffolk, near Norwich so a little way from both those locations, though thank you for the suggestions.

I was hoping to to do the ewi fixing myself and then having a render company come in to silicon render up. Has anyone bonded brick slips to eps directly or do you think a backboard would be required first?

''I’m in Suffolk, near Norwich so a little way from both those locations, though thank you for the suggestions.''

I suspect that wherever you get your EWI stuff from you will have a pallet delivery (and it won't necessarily be coming from the merchant, but perhaps their supplier whose location you don't even know), so it becomes difficult to *know* that you are saving 'load miles'.

I am 90+% certain that both of the brick-slip jobs I am aware of (aware as in having seen them at various stages in the process) are 'straight-onto-the-EPS'.

Well you didn't say exactly what you wanted (and still haven't!), nor why, so I don't think there's much chance of anybody suggesting any detailed links. Did you consider the likes of Porotherm blocks?

Apologies if I haven’t expressed this quite as I intended — Though I am intended in to do a lot of the work myself I do have a builder as well. I spoke with my builder who I am using who is also a friend, and he wasn’t keen on using Porotherm, partly because his supplier doesn’t stock them and I guess he is stuck in his ways. I also suspect that, since this is only an extension, the relatively small quantities needed would be harder to source compared to standard off-the-shelf blocks from the local merchant. It’s a shame, as Porotherm looks like a great product, but it seems better suited to a full house build.

Proposed build-up:

Front & Rear Extensions
100 mm Lignacite block (? 0.27)
70 mm Grey EPS beads (if available) (? 0.032)
100 mm Lignacite block (? 0.27)
200 mm Grey EPS (? 0.032)
Silicone render + brick slips

Existing House
100 mm block
70 mm cavity refill with Grey EPS beads
102 mm brick
200 mm Grey EPS
Silicone render

EPS to be both mechanical attached as well as cementitious rather than CT 84 or equivalent, can any explain why this is preferred?

In both new and existing areas, EPS will run down to the strip footings.
For the new front & rear extensions, I’m considering using Marmox blocks at the base for a thermal break.
New windows would be partially recessed between the EWI and the external cavity wall, with the outer frame wrapped in EPS.
Existing uninsulated concrete floors will be removed and replaced with 300 mm EPS, then 100 mm concrete with underfloor heating pipes embedded at 100 centres. I need to get the UFH layout designed.

The roof requires re-roofing. I haven’t given this a huge amount of thought yet, though suspect a warm roof design connecting to the EWI is best?

Goals:

We are working with a ‘limited’ budget and intended to complete all external works including, windows, doors and the replacement concrete ground floor first. The niceties like new bathrooms ect will come as and when.

We specifically want to eliminate the existing mould, cold drafts, and hot/cold spots in winter, as well as improve damp control and indoor air quality using MVHR. Using the Change Plan tool, the calculated U-values are around 0.17 W/m²K for the proposed wall build-up, or 0.11 W/m²K if aerated concrete blocks such as Ytong or Celcon are used instead. That difference doesn’t seem huge, so I’m weighing whether the switch is worth it and cost. Our aim is to achieve a low-energy home that’s comfortable year-round, with low running costs.

Moreover, I suppose I was simply looking to discuss what we’re doing and to get some reassurance that we’re on the right trajectory, especially as we’re not employing a specific Passivhaus or similar consultant.

First thought was lay block on flat, which seems too obvious, so you presumably have a reason not to do that?

Your Uvalues for 200mm plus 70mm EPS (if that what you're referring to) seem a bit conservative. A quick calc gives me 0.12W/m2K, assuming graphite EPS, but 0.13 for regular EPS.

If you did 200mm GEPS onto 215 blocks on flat, even regular dense conc blocks with lambda of 1.3W/mK, you'd get a Uvalue of 0.15W/m2K. Got to be easier than building 2 skins and filling a cavity. Block on flat is easy. Batten inside and plasterboard, giving a service void rather than chasing into blocks.

GreenPaddy and I must have been typing at the same time. I had not seen his comments when I posted.

Posted By: englishjohnaced retrofit standard

Did you mean AECB Carbonlite or something else?

Djh, yes it was the AECB CarbonLite Retrofit, potentially.

Greendaddy, all new pitched roofs.

https://drive.google.com/file/d/1qIJTL1QYystEb9lr8NXnpkOMIMNcCc66/view?usp=drivesdk
https://drive.google.com/file/d/1XLA72aERb0v-mWVDkXItQQQZRvQp-nJg/view?usp=drivesdk
https://drive.google.com/file/d/1ZVN9ESEgnBN_Nsv3bYET8HmBiVtSQSU9/view?usp=drivesdk

I see the website First in Architecture, for example, has a large amount of detailing including Passivhaus, so I will give that a read.

I’m going to use the standard Lincrete blocks, as Jewsons has loads of them, so they’re readily available and well priced—unless someone persuades me to use aircrete blocks, like the Celcon Standard aircrete. The Lincrete blocks appear to offer a good key for wet plaster and hopefully a good key for the external EWI adhesive.

The neighbour to the north, where the garage is, is refusing to allow me access to his property to undertake work. That side was supposed to be rendered, so he will just have to look at unfinished blocks. This, though, has disrupted my continuous EWI in this location, so I will now construct a larger cavity wall and no EWI here.

My question is about connecting the EWI to the cavity wall. I was thinking something like this:

https://drive.google.com/file/d/1UFhbrfGgje2mJ9v-T8r_lToNy_fYoj_h/view?usp=drivesdk

Try this:

https://postimg.cc/gallery/hKGQCRx

Foundations are all strip and have been improved agreed with BC. I’m now think to achiever a value close to the rest of the walls I could use something like Thermaclass Cavity Wall 21 140mm in this small section.

re. roof design.

Maybe consider timber I joists for the roof? Full fill a 300mm joist with a 0.032 W/mK lambda glass wool, and get a Uvalue of 0.11W/m2K. This is my default for projects, as BRegs (at least here in Scotland) for roofs is 0.12W/m2K.
There are manuf details for how to land on ridge beams, wall heads, etc (try googling "JJI technical manual")
Cut the Joist vertically flush with the block wall outer face. Add timber rafter sprockets to carry the roof, facia soffit. EWI between those rafter sprockets, over the end of the I joists, to the underside of the sarking/tiles. That gives almost 100% continuous wrap, and it's a simple detail for the timber installers and the EWI installers.

I noted the warm roof ref. which is what I was describing. Insulation in the rafter voids gives a warm roof. Maybe the term "Joists" for the I joists caused confusion. The I joist can be utilised as a floor joist, roof joist (flat), roof rafter, wall. In this case I was referring to rafters, hence my comment about "using I joists for the roof" and "landing on ridge beam and wall head". Drawings are much less confusing than words.

Using I joists sound a like a good idea I will speak to the roofing supplier and see what they can offer, I know the initial design was with metal webs. Though this was quite a time ago when I had initial conversations.

I am planing to use Marmox Thermoblock to remove the thermal bridge at the floor. I am trying to decided if we should use it on both the internal and external walls, though this would be twice as expensive. Would using it only on the internal leaf be ‘enough’? I have done 4 drawing, two of the wall leafs and two with the doors. I’ve used other peoples drawings and copied pasted sections to make my own layout.

https://postimg.cc/gallery/Z9nB8T8

So after going round in circles it has been decided that we are going to carry on and match the existing cavity wall with the existing house and open up the end of the cavities making it a continuous cavity. This will also allow the cavities to be filled prior to the winter just in case we don’t get the ewi and render finished and matches up internally and externally for the wet plaster and the EWI.

Green paddy, thank you for your thoughts. Detail3/4 is my preference.
since you're going with cavity, I prefer detail 4

- leave out some/all of the inner leaf in detail 4 at the opening, and continue the floor out to the door

Was unsure if that could be done, but i see no reason why now I have re-looked at it after your suggestion.

- assume the VCL on the floor is a liquid membrane to retard moisture from the concrete during the drying out process to let you lay the floor finish sooner?

That’s a good point, final floor finish will probably some time away, but I guess 100mm concrete will take some time to dry so that may be a better idea to use a liquid.

- I would break the EWI at the DPC level, so not a contiguous layer from below ground, and set back the EWI by say 25mm (or more) for the layer below the DPC, giving a drip line on the upper wall surface. This also allows the lower EWI portion to take a smooth render finish, so that the splashing at ground level cleans off more easily, and doesn't build up in what might be a less smooth render finish to the rest of the wall.

That’s is sound advice, will spec maybe 150mm ewi at that area under the ewi started trays providing that drip line.