Green Building Forum - EWI over DPC Wed, 22 Apr 2026 10:38:17 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280939#Comment_280939 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280939#Comment_280939 Fri, 29 May 2020 23:09:33 +0100 i_DaveJ Details: Foundations are about 300m below GL and DPC about 200mm above GL. EWI is EPS though will consider XPS below ground if EPS is not recommended. There will be a French drain at foundation level with gravel up to GL. I'd prefer an unbroken finish from soffit to ground i.e. without beading at DPC but could live with that if necessary.
There are a few posts about this but wondered if thinking had matured and a consensus of opinion had triumphed.]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280944#Comment_280944 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280944#Comment_280944 Sat, 30 May 2020 09:35:55 +0100 Nick Parsons
Lots of previous discussion on here and in other places re EPS vs XPS below dpc. I have used XPS as stipulated by system providers (and if the system provider stipulates then arguably BCO can refuse to sign off if you don't do it) (to be fair, more often than not they don't even ask), but I am quite sold on the view expressed on this forum - can't remember who; possibly F-T?) that XPS is perhaps more likely to 'lock in' moisture below dpc, while EPS, even below dpc, can do more to 'manage' the issue. Most system providers also specify a 'step back' - say to 80mm where the main body of the EWI is 100mm. Discussion on here opines that that 'step' is more likely to be a site for moisture ingress.]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280945#Comment_280945 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280945#Comment_280945 Sat, 30 May 2020 10:57:54 +0100 fostertom EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280962#Comment_280962 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280962#Comment_280962 Sat, 30 May 2020 21:26:35 +0100 i_DaveJ My inclination would be to run the EPS down to the foundation - I guess if the supplier is ok with that BC will be too. I admit I'm not familiar with BC's steer from suppliers. Is it a backside covering thing?
Tom, I hear you about splashback - I'd be comfortable managing that and see it as a minor issue relative to the aesthetic achieved.
I'm planning a French drain with a perforated drain at the bottom of a trench of gravel all encased with a textile. Would this be sufficient a protection for the EPS underground? Maybe a bitumen based paint? I'll take the render just below ground.
I'm not too keen on the step back either - I also see it as a path for runoff ingress at a point where it is most unwanted. And to be honest, nor am I keen on any line. If I keep the thickness above and below the DPC the same, would I need an e.g. silicon bead or just render over the base track so that I maintain a continuous, monolithic look?]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280963#Comment_280963 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280963#Comment_280963 Sat, 30 May 2020 22:02:36 +0100 djh Posted By: i_DaveJWould this be sufficient a protection for the EPS underground? Maybe a bitumen based paint?
EPS doesn't need any protection underground. It can sit underground quite happily by itself. Various railway embankments would fail otherwise. I'd be worried about a vapour impermeable paint such as bitumen.]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280967#Comment_280967 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280967#Comment_280967 Sat, 30 May 2020 23:03:18 +0100 Nick Parsons EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280974#Comment_280974 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280974#Comment_280974 Sun, 31 May 2020 09:19:52 +0100 i_DaveJ Posted By: Nick ParsonsI'd say don't use a base track. Do it the 'right way up' (not as I described above) and you don't need one.
i.e. essentially use the foundation as the 'base track'? And just run the EPS over the PDC without any break? That would be my preference so I guess it's off to talk to the supplier and BC...
Any further gems welcomed.
Thanks again everyone.]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280975#Comment_280975 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280975#Comment_280975 Sun, 31 May 2020 09:55:27 +0100 Peter_in_Hungary EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280977#Comment_280977 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280977#Comment_280977 Sun, 31 May 2020 10:15:40 +0100 Nick Parsons
On a side note, I don't think many would deny the benefit of a French Drain in such circumstances, but it's surprising (or not?) how many such drains do not have a formal 'out'. French drain good; 'moat' arguably less so...]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280980#Comment_280980 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280980#Comment_280980 Sun, 31 May 2020 13:00:27 +0100 WillInAberdeen EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280981#Comment_280981 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280981#Comment_280981 Sun, 31 May 2020 15:51:18 +0100 i_DaveJ Posted By: WillInAberdeenSomeone (Tony?) posted pictures of what happened when mice got into their underground polystyrene insulation. Others have suggested laying paving slabs on-edge to protect the underground section of the polystyrene, extending a few inches above ground level.
I think the French drain which will comprise gravel down to the base of the footing all wrapped in a geotextile nestled against the EPS should keep the little critters out. I'll render down a few inches below GL.
Still interested to see the photos if anyone has them...]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280982#Comment_280982 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280982#Comment_280982 Sun, 31 May 2020 17:05:32 +0100 Peter_in_Hungary EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280983#Comment_280983 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280983#Comment_280983 Sun, 31 May 2020 17:33:05 +0100 RobL You can see the joins, so it's not as neat as the render above, but I think it'll be ok against an accidental spade attack.]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280985#Comment_280985 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280985#Comment_280985 Sun, 31 May 2020 20:07:44 +0100 djh Posted By: WillInAberdeenOthers have suggested laying paving slabs on-edge to protect the underground section of the polystyrene, extending a few inches above ground level.
I have paving slabs on edge against my EPS to an above ground height of 150 mm or so, with a French drain filled with sharp stone against that as other have mentioned. We haven't had any problems with it, but I don't know what that says. Plants try to grow in the French drain but I haven't seen any sign of wildlife invading it. Cats don't like walking across the sharp stone but will do it if they need to.]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280988#Comment_280988 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=280988#Comment_280988 Mon, 01 Jun 2020 09:27:47 +0100 i_DaveJ Posted By: Peter_in_HungaryIf you are going to use gravel as the defence against the little critters I would suggest using crushed stone rather than gravel. Gravel being round moves around easily whereas crushed stone being sharp and angular will compact together and be much more difficult to burrow through.
I've always thought of gravel as being coarse but yes, I agree with the essence of what you are saying (no pea gravel!)
Posted By: djh
Posted By: WillInAberdeenOthers have suggested laying paving slabs on-edge to protect the underground section of the polystyrene, extending a few inches above ground level.

I have paving slabs on edge against my EPS to an above ground height of 150 mm or so, with a French drain filled with sharp stone against that as other have mentioned. We haven't had any problems with it, but I don't know what that says. Plants try to grow in the French drain but I haven't seen any sign of wildlife invading it. Cats don't like walking across the sharp stone but will do it if they need to.

I like the idea of something like cement board. It's in 900mm widths so I can cut it into 3 lots of 300mm widths which is the depth to the foundation. The French drain can sit against this.]]> EWI over DPC https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=281102#Comment_281102 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16579&Focus=281102#Comment_281102 Fri, 05 Jun 2020 02:36:03 +0100 wookey
If you get an argumentative BCO, just put in a plastic starter track at DPC level and whatever insulation above and below it so there is a pointless 1mm of PVC to keep them happy about their precious DPCs. So long as there is no actual gap letting cold air in it's fine.]]>