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Green Building Bible, Fourth Edition
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General: EWI System

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    I think I'm going to go for EWI on my cavity wall detached property.

    Primarily because we want to renew the facade anyway, the existing render is traditional roughcast.

    What is the best EWI system in terms of overall cost per r-value? From what I've seen all of the K-rend jobs in my area look awful with staining, algae and so on. What is the best finish for a seafront property that will be low maintenance?

    We may look to DIY the cladding and pay to have it rendered. Can it go on existing roughcast??

    And if we do install ne wiwndows in teh outer insulation layer - can the EWI be added next year or is this just asking for leaks?
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    Posted By: VictorianecoWhat is the best EWI system in terms of overall cost per r-value? From what I've seen all of the K-rend jobs in my area look awful with staining, algae and so on. What is the best finish for a seafront property that will be low maintenance?

    The silicon based top coat (decorative render) is a lot less prone to staining and algae growth than the cheaper varieties.

    Posted By: VictorianecoWe may look to DIY the cladding and pay to have it rendered. Can it go on existing roughcast??

    The EWI can go on the existing roughcast providing it is sound and passes the tap test. (If you tap it and it sounds hollow then it is coming way from the wall) Any area that fails the tap test has to be removed but re-rendering is not needed as you just use a slightly thicker bead of adhesive for that area.

    Posted By: VictorianecoAnd if we do install ne wiwndows in teh outer insulation layer - can the EWI be added next year or is this just asking for leaks?

    You can put the windows up in advance of the EWI (leaks minor or major of either / both air and water will happen) but the biggest concern I would have from renovating experience of several houses is that next year in the scheme of renovations inevitability slips to 2 -3 years and there is no such thing as temporary !!.
  3.  
    **''What is the best EWI system in terms of overall cost per r-value?''**
    Graphite EPS unless you like wood-fibre or cork (I do) and want to pay a lot more (I did!)


    **''From what I've seen all of the K-rend jobs in my area look awful with staining, algae and so on. What is the best finish for a seafront property that will be low maintenance?''**

    Render, seafront, tricky. Staining tends to arise from poorly-detailed 'protruberances'. Avoid them and you've gone part of the way. Ask the system provider to suggest something for your specific situation.

    ''**And if we do install ne wiwndows in teh outer insulation layer - can the EWI be added next year or is this just asking for leaks?''**

    Don't put the windows in the 'normal' position on a reveal to the brick. Do the window-in-plywood-box thing and stick them out. Tape DPM with air-tightness tape to the frames and back to a smooth-rendered strip which you make over the roughcast.

    Yes, you can probably do over roughcast, but for fixing use the full-perimeter and 'domino' method of fixing rather than full coverage. Definitely use mech fixings too, in my view. Remind me how wide your cavity is (IIRC you were going to fill that. If it's over 50mm and you are going for 100 or less EWI a condensation risk analysis is even more important. I'd get one done anyway, but the 'rule of thumb' two-thirds/one-third rule comes in. If, for example you had a 100 cavity, filled with gEPS, and you did 50EWI, the inside of the outer skin of masonry could get cold enough for condensation to occur. The other way round and it'll be more 'tucked up snug'.

    Yes, you can DIY, but I would advise going on someone's course. Go down below DPC level into a French drain.

    I have used EWI Store on a number of occasions, though many other suppliers exist. They do trg too, tho' not sure if in Wales.

    **Ah, was typing while P-in-H posted.
  4.  
    Posted By: Nick Parsons**''What is the best EWI system in terms of overall cost per r-value?''**
    Graphite EPS unless you like wood-fibre or cork (I do) and want to pay a lot more (I did!)

    I have never found that the graphite EPS worth the additional cost - if you can manage with the extra thickness of standard EPS to get the same U value. Of course that is with prices over here so YMMV.
  5.  
    Hi all, cavity is 80mm, so I was thinking of 100mm EPS at a minimum? there must be a point of diminishing returns to cost when EWI'ing?

    The one area (extension) has 80mm cavity batts
  6.  
    Posted By: Victorianecohe one area (extension) has 80mm cavity batts

    The cavity batts should be against the inner face with air flow outboard. Probably not possible to additionally fill this cavity nor stop the air flow so putting EWI on this probably won't add anything other than cost.

    Posted By: Victorianecothere must be a point of diminishing returns to cost when EWI'ing?

    There is but it is a calculation of personal choice. The cost difference is going to be the additional thickness of the EPS as all other costs are the same. (over here EPS is priced /m3 so 200mm is twice the price of 100mm)
  7.  
    Okay that's good to know, anyway to remove the batts or I guess it's not worth the effort/cost?

    It's only really 2 smaller elevations anyway, so I guess if scaffold is up and we're looking to re-render it must have a benefit in EWI'ing?

    A quick look on EWI store suggests....

    Rockwool External 120mm - £39.83m2
    Rockwool External 90mm - £34.98m2

    XPS 100mm -£22.64m2
    XPS 60mm - £17.32m2

    EPS 240mm -£36.67m2
    EPS 120mm - £19.58m2
    EPS 100mm - £16.64m2
    EPS 60mm - £9.89m2

    So definitely cheaper to buy thicker for all materials in UK but there must be a maximum thickness where marginal benefit of extra depth comes into play?

    EPS looks the winner in terms of cost mind.... I'd imagine as with most people cost is the biggest factor
    •  
      CommentAuthordjh
    • CommentTimeSep 13th 2024
     
    Posted By: Victorianecothere must be a maximum thickness where marginal benefit of extra depth comes into play?
    Yes there is, but as PiH already said, its a personal choice. Once you've gone beyond whatever you're required to do to meet regs it all depends on your own goals and your assessment of how much its worth to meet those goals.
    • CommentAuthorwookey
    • CommentTimeSep 14th 2024
     
    I reckon a good rule of thumb is to make your walls at least U=0.15. So just work out how much EPS/graphite EPS/woodfibre that is given your existing/proposed cavity wall fill. ubakus says with 80mm of blown fibre you need 160mm of 0.04 EPS or 130mm of best 0.032 EPS. (not modelling fixings which is fine if you use the good ones)

    Better is to work out the whole plan with PHPP and you can easily model different thicknesses/costs and tradeoffs, with better windows/roof/floor allowing lower wall U-values, to decide where you want to end up, or just pick a standard like AECB level 2 or EnerPhit and design to that.

    I'll second everything Nick P said. Except that I favour using structural insulation (alma vert/compacfoam) for mounting windows in the EWI, rather than plywood boxes - but both methods work fine if you get the details right. The silicone render is great.

    I did 160mm/200mm woodfibre+silicone render (and XPS below ground) (on a similar with-cavity detached building but inland/East Anglia so a lot drier). XPS is annoyingly high-carbon but otherwise great. EPS is significantly cheaper and easier than WF but you will spread plastic bits everywhere. WF will give you loads of thermal mass/decrement delay and excellent breathability but you don't really need the latter if you have a cavity and working DPC. And you may not need the former if you don't have overheating issues.
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      CommentAuthorfostertom
    • CommentTimeSep 21st 2024 edited
     
    200 white EPS EWI plus cavity fill I reckon optimum, as good as any future eco-standard will need, diminishing returns to go further, only marginal extra cost (of the EPS itself) over 100 or 150. Also 200 EWI gives depth to easily install deeper eco-windows within insulation zone, not touching the inner masonry, and with generous (100ish) external reveal.

    EPS, as well as being cheapest, has great tech advantage of zero capillarity (no 'soaking up' or 'soaking in', is its own DPC, unlike wood fibre), as well as ready and complete re-drying when/if it does get wet, by interstitial condensation or hydrostatic pressure (standing flood water).

    Wookey, XPS (and all cellular plastics) is worse not better than EPS below ground - tests show that it does in fact gradually absorb water but then never re-dries. Of course, mustn't be in standing trench water/water table, hence Nick's recommendation of french drain, but that won't save you from flood water - nothing will!
  11.  
    What do you mean by 100 external reveal if using 200mm please Tom?
    •  
      CommentAuthorfostertom
    • CommentTimeSep 22nd 2024
     
    The distance by which the window frame is inset from external surface - typically 'half a brick' or 100mm. 'Reveal' is that 100 wide surface.
  13.  
    Okay I'm still trying to work out how one would hang a window or sliding doors 150mm out from the original outer skin?

    Can you really screw into structural foam and get a good fixing on say doors 3m wide?

    I'll get a picture up of my facade and highlight some queries

    Thanks
    •  
      CommentAuthorfostertom
    • CommentTimeSep 23rd 2024 edited
     

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