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Building Regulations and Standards: EWI over DPC

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  1.  
    • CommentAuthori_DaveJ
    • CommentTimeMay 29th 2020
     
    Is there an authoritative position on installing EWI over the damp proof course? I plan to install rendered EWI down to the foundation level and will be talking to BC soon and wondered if a particulate argument satisfies any concerns they may have. Are there proven details?
    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.
  2.  
    Authoritative? Probably no. Some system providers (whose 'blessing' is strictly required to get B.C. approval) have specific stipulations where EWI is to continue below dpc (or 'notional dpc' (!) where there isn't one!). These tend to include a different adhesive/base-coat *and a base-track and physical stop to the 'above dpc' EWI, before starting again below (and in my experience it is done like that - EWI above dpc on starter track, then grovel about on the floor trying to get a tight fit with the below-dpc stuff. Gut feeling says to me that things may be 'tighter' when the 'plinth' (below-dpc layer) is done first.

    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.
    •  
      CommentAuthorfostertom
    • CommentTimeMay 30th 2020
     
    In other words, run the EPS down unbroken to as deep below as you like. The finish also run down to just below finished GL, provided you actually don't mind risk of a bit of splashback/staining as 'natural' or traditional. Below that, some protection but not the full expensive render finish.
    • CommentAuthori_DaveJ
    • CommentTimeMay 30th 2020
     
    Thanks Nick and Tom,
    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?
    •  
      CommentAuthordjh
    • CommentTimeMay 30th 2020
     
    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.
  6.  
    I'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.
    • CommentAuthori_DaveJ
    • CommentTimeMay 31st 2020 edited
     
    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.
  8.  
    To echo what was said above, run the EPS down to the foundation, no base track needed, (actually start at the foundation and work upwards) ignore the DPC and go straight over it, (expect some ''discussion'' with BC as they probably want their backsides covered), IMO EPS is better than XPS below ground and I have had no problems with EPS below ground. Below ground (sight level) the thin coat render is not needed but I would put on the adhesive and mesh render coat to help keep out the mice etc.
  9.  
    I_DaveJ, Yes, come out of the ground (French drain, preferably) level with EPS and take your level from that first course. P-in-H, agreed re 'some ''discussion'' with BC. BCOs do not have to accept anything as doing what it says on the tin unless it has 3rd party certification and is done strictly in accordance with that certification. This is frequently ignored, but some may adhere to it. It means, for example, that the 'eclectic common-sense' coming out of discussions like this may well be Best Practice, but not actually formally acceptable to BC. Mostly it goes OK, but occasionally I have followed the system provider's approved model (rather than my own preference) where I feel that BC might quibble.

    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...
  10.  
    Someone (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.
    • CommentAuthori_DaveJ
    • CommentTimeMay 31st 2020
     
    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...
  12.  
    If 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.
    • CommentAuthorRobL
    • CommentTimeMay 31st 2020
     
    I used 100mm XPS down to foundations, and covered it with fake tiles (cement fibre board tiles). I still used a starter track, and 150mm EPS above DPC. I think the tiles are ok for XPS, but not EPS - that needs a vapour open covering I think.
    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.
    •  
      CommentAuthordjh
    • CommentTimeMay 31st 2020 edited
     
    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.
    • CommentAuthori_DaveJ
    • CommentTimeJun 1st 2020
     
    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.
    • CommentAuthorwookey
    • CommentTimeJun 5th 2020
     
    I used 160 or 200mm XPS, starting from the foundation, well above the DPC, no track or step, then continue with woodfibre above that. Plastic bobble drainage sheet in front of the XPS below grade, and fibre-cement (fake slate) tiles as spade protection to ~10cm above ground level.

    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.

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