My EWI is finally underway. I'm going to post updates and photos of various details as I progress, may be useful for others considering an EWI project.

first up, I'll kick off with the spec and a bit of background.

I have an existing full fill 50mm wool cavity walls on a large bungalow. I'm upgrading with an additional 100mm of graphite EPS which will be render finished. I also have two further walls of 1.5 storey extension in medium density blockwork with 125mm full fill wool all built last summer. I've decided to EWI these walls as well, I realised as I went through the project that should have gone for a wider cavity. Also, while I watched like a hawk, the block gang were sloppy, and I dont doubt they've left me some gaps in the insulation. Also we used standard cavity wall ties and catnic type lintels, all adding a degree of cold bridging. I decided worth the extra cost and time to add another 100mm of EPS over the outside of the new walls, fixes all the cold bridging, and almost doubles the amount of insulation. I reckon this must be the first new build hybrid cavity/EWI wall in the UK!

Why not thicker EPS? I could have done so, but was constrained by eaves overhang. Luckily we re-roofed, I was able to run new gable rafters to allow extra overhang clearance at the verge. I'm very happy with the prospective improvement the 100mm will offer.

The EPS I purchased is Kay-Cel Super+ EWI. This is the graphite enhanced stuff with the lower conductivity. The boards are cut from aged EPS blocks, seems this is considered important for dimensional stability.

The EWI system I'm using is made by Baumit. I've had a very good experience with their regional rep, and they have given me access to their field engineer to give what has so far been solid technical advice. They have a comprehensive EWI and render range, I'm using their StarContact white as an adhesive and basecoat, it's a cementitious product but different to many others in that it's lime enriched.

I am planning to go below DPC with a plinth EWI detail. In the interests of time, I've run a base track and will carry out the above DPC works first to completion. The below DPC will follow as I just didnt have the time to dig down and break up existing concrete paths. I'm on a race against the clock to be ready for rendering before it gets too cold. I've used a Wemico starter track. It's PVC and comes in two sections, the base sits to the wall and projects, there's then a beaded/mesh insert which clips in once the EWI is fixed. I will form a drip detail here, probably with a 20mm inset to the below DPC insulation.

that's enough for now, more to follow with various pics of some details so far.

Hi MarkyP - can you post a drawing with your DPC/plinth detail - are you using a different render below DPC.

For my build I have EPS formwork for the slab/beam and cant decide whether to try and make it flush with the wall ewi - or create a plinth detail at the formwork/EWI interface - which way does your plinth go - recessed or projected?

here's the first pic. Nothing very exicting happening here. But the hot knife cutter (about £50 on a popular auction site) was well worth the spend. Takes some practice but cuts well using a timber straight edge as a guide without filling the garden with hundreds of tiny EPS beads. Another tip, a hot knife with a temp adjuster helped to find the sweet spot temperature, too hot and it smokes like mad and the knife wanders and creates much wider cuts. See the level in the background, despite having purchased aged EPS, some of the boards were bowed. These were set to one side and used for smaller cuts. Slight bows could be pushed flat and the adhesive held them so. The whippet in the foreground is of no use at all for applying EWI.

Are you going for an adhesive only fixing option? - which adhesive are you using?

If I went for a recessed plinth and push in base bead I assume the horizontal bit would not be rendered. do you have a link to what you are describing?

We too have a dog that wishes to help - but is not very useful - I think it's the lack of opposable thumbs. You have the advantage in that your dog looks like it can get out the way fast.

Is your hot knife for sale after you're done?

Yes that looks perfect - just need to get the bco to agree that a dpm/dpc across the eps creates a dam preventing save passage of unintended leaks getting to the ground - wish me luck.

I fancied the spray foam with no mechanical fixings - I think that may be too brave.

Is that hot knife work around the rafters - if so very neat.

here's a pic of the base rail prior to EWI going on. I went out in the dark the night before and used a cross line laser to mark a datum all along each wall to use as a reference to set the base track level. We did the odd check with a spirit level but the reference line really helped speed things up. The base track is from Wemico, it comes in two parts. The base track is fixed to the wall, the meshed bead is inserted into the base track after the EWI is fixed. The render above the base track had to be chopped back as was previously a bellcast bead drip detail, it projected 10 - 15mm beyond the rest of the render, so we removed it to allow us to set the EWI. The walls were all covered in a spray applied rough cast. It came off with a long pole scraper, bloody hard work but the rough cast was all over the place in terms of flatness, under it we found a lovely flat sand and cement render layer, perfect to set the EWI on, so worth the effort of stripping back the rough cast.

It was well worth checking the walls for plumb before starting, we had a gable wall which was 10 - 15mm out of plumb, the top leaning out. Had we not known this we'd have fixed the base boards with 10mm adhesive and then as we set each course of the boards plumb we'd have got closer and closer to the wall, until eventually there would have been no room for adhesive and we'd have been trimming 10mm to make up 90mm thick sheets. As it was we knew to go a bit fat on the adhesive at the base and it thinned out as we went up the wall.

the cavity is fully filled, both existing and on the new extension. Given the EWI will be taken up into eaves, detailed around the rafters and up to the sarking, I couldnt see much need to do anything with the cavities at the top. BCO was not fussed about me using closers at the top given full fill (seems it's mainly a fire detail from their point of view, for partial fill situations), and the cavities are going to be on the warm side, and inside the air tightness envelope, so I'm leaving them as they are. The roof insulation will be taken down between rafters and tightly fitted to the inside of the EPS, and over the top of the cavity insulation. Note not warm roof, planning full fill mineral wool between rafters, then PIR under rafters. No ventilation or void planned behind the sarking, relying warm side VCL, and the fact that OSB is relatively vapour open.

what juntion would the AT tape serve in your proposa in my build up? Not sure I follow where you suggest it could be emplyed?

existing render on the older part of the house does serve as AT layer to an extent, but the EWI with new render skin will be the main AT layer to the walls.

Posted By: MarkyPwhat juntion would the AT tape serve in your proposa in my build up? Not sure I follow where you suggest it could be emplyed?

For me, I don't trust foam long term to be AT (or rather, not the cured foam itself, but the adhesion). So therefore, where you have foam, I would probably have foam plus tape. Furthermore, I wouldn't have the EWI being part of the AT layer - this would be an "internal" (to the EWI) lining, e.g. the existing render, and tape to that. I think you can see the render on EWI as wind tight.

But those are just my thoughts from research rather than actually doing and measuring.

Why did you go for a "system" rather than just making the system up yourself?

MarkyP, can I ask how long it has taken to fix the eps so far including all the preperation bits such as creating a level line etc. So far that work looks pretty tidy - well done. How would you rate your handyman/building experience?

Who is going to do the rendering?

Can I also ask what was your price from Baumit (whisper if you prefer).

Yes - please whisper the discount rate - Just getting to a ballpark discount rate is an achievement for me - my negotiation skill is pants - and my experience even lower.

the advice I've been given is to rasp the EPS prior to rendering, both to remove any high spots and also because UV exposure makes the surface go a bit "crispy" over time. You can get a rasp for about £15 though they are a bit brutal. I found holding a saw blade flat to the EPS and dragging it over the surface worked well as a alternative. The renderer who will be doing my job said they didnt bother rasping and he'd never had a problem. But I'm going to give it a bit of a rough up in any case.

next up some fixings. These were supplied by Baumit but they are manufactured by EJOT. They are the STR U-2G fixings and I've been impressed with them. First you drill an 8mm pilot hole to depth of the fixing plus 20mm. I chose a fixing which was taken 50mm into the underlying blockwork. The fixing is pushed in by hand until flat to the surface. Next up it is driven home using a special tool which cuts a clean recess and drives the fixing below the surface, the screw inside the fixing pulls the fixing into the wall and deforms the EPS forming a rebate, the tool has a guide which ensures the EPS deforms to form a cylindrical rebate. The screw head is well below the surface of the insulation at this point and the 20mm recess is finally filled with a ready made EPS cap which fits tightly. The fixing avoids cold briding and also helps keep the EPS surface flat, avoids the risk of "grinning through" the finished render had the fixings been on the surface of the EPS

That looks very professional, just like wood plugs to cover screws. Excellent.

Hi Mark would you mind sharing baumit rep details? I plan to use their render system on my house

Thanks

Thanks for your ongoing updates.

Can I ask: how did you create a window schedule? I'd like to know if it's sensible for an end user, with a hand in the design, to build this up from their own measurements (and also how a window schedule is supposed to be written).