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General: Plinth detail - cavity wall

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  1.  
    • CommentAuthorgravelld
    • CommentTimeJan 15th 2018
     
    Next one: plinth insulation.

    As a reminder, the target of this retrofit is a 1950s cavity walled property:
    - Existing partial fill mineral wool insulation
    - No air bricks
    - All solid floors

    I don't actually know how deep the foundations are, or the nature of the footings.

    Here's a first attempt at the foundations then.
    - EWI on the wall, then continued in EPS below the foundation (see below)
    - Air barried continued from existing render with a S+C coat on the plinth as low as possible.

    I have opted for EPS on the plinth, even though XPS is often suggested. My understanding is that EPS is more "fault tolerant" in that it may get wet, but it can dry.

    Questions (other than points others raise):
    - I have separated the plinth and wall insulation, even though it's all EPS. Is this required or can I just continue it down?
    - Do I need to mark the existing DPC, and how to treat it?

    Refs: https://retrofit.support/detail/20/
      plinth.JPG
    • CommentAuthorgravelld
    • CommentTimeJan 23rd 2018
     
    Just wondered if anyone had any comments about this... Not enough detail to comment on?
  3.  
    The EWI installed by others on a job of mine was separated from the plinth layer (of high density EPS) by an alu base-rail. I don't really like this as it introduces a thermal bridge, and if cut boards are used, the possibility of voids.

    2 installations I worked on had EPS (in one case HD and in one case graphite) immediately below the main EPS. One had no drip to the plinth, and on the other I used an angle-bead as a drip (see pic), with the EPS slightly relieved so that the joint was otherwise all tight. I sat the EPS on the HDEPS with low expansion foam as 'belt and braces'.

    I am not sure what you mean about marking the dpc, and how to treat it.

    I am intrigued, though this is not part of your Q, how the cavity is partly filled. Given that it would not have been filled in 1955, how is there a vertically-defined gap to fill? I can understand there being the odd void due to slumping or 'picking up', but I don't understand how retro-fitted EWI could leave a fillable vertical gap.

    Have I misunderstood that bit?
      DSCF5830 - Copy.JPG
    • CommentAuthortony
    • CommentTimeJan 23rd 2018
     
    Nice job, I worry about air in the cavity bypassing the EWI if wind can get in there cooling the block adjacent to the edge of the slab.

    How many houses need the same treatment?
    • CommentAuthorgravelld
    • CommentTimeJan 24th 2018 edited
     
    Thanks Nick. Nice pic.

    Posted By: Nick Parsons
    2 installations I worked on had EPS (in one case HD and in one case graphite) immediately below the main EPS. One had no drip to the plinth, and on the other I used an angle-bead as a drip (see pic), with the EPS slightly relieved so that the joint was otherwise all tight. I sat the EPS on the HDEPS with low expansion foam as 'belt and braces'.

    I've not seen mentioned before using different _types_ of EPS for above/below DPC. In fact I haven't seen much mention of types of EPS at all other than some have lower conductivity. So why would different types be used, something to do with how it deals with water?

    Posted By: Nick Parsons
    I am not sure what you mean about marking the dpc, and how to treat it.

    The DPC is always the first subject builders bring up (well, three of them...) when I talk about doing this part of the project. I'm wondering whether it has to be continued somehow from the blue course of bricks across the EPS and how this should be drawn.

    Posted By: Nick Parsons
    I am intrigued, though this is not part of your Q, how the cavity is partly filled. Given that it would not have been filled in 1955, how is there a vertically-defined gap to fill? I can understand there being the odd void due to slumping or 'picking up', but I don't understand how retro-fitted EWI could leave a fillable vertical gap.

    The house is extended many times. There's actually as much extension wall as original. The original wall is retrofit fully filled EPS bead. This wall pictured is actually more like 1980s build. We will need to add the white layer, ideally something like walltite but pouring EPS if not.
    • CommentAuthorgravelld
    • CommentTimeApr 25th 2018
     
    I wanted to update with some findings about the old XPS v EPS debate.

    The study often cited to suggest EPS is better long term is http://www.epsindustry.org/sites/default/files/EPS%20Below%20Grade103.pdf . However that has the obvious caveat that it's by... "epsindustry.org".

    An impartial (I think) study was conducted at http://thepathtosustainableliving.blogspot.co.uk/2013/10/xps-vs-eps-which-holds-up-better-in.html which led to XPS being declared the winner:

    "Well I dug up the samples today and delivered to BCIT for testing. I first weighed them and it is very clear that EPS absorbs a LOT more water than XPS. The samples were continually wetted for the last 8 months. The XPS samples all were 130 grams when buried and came out at 158/173/180 grams. The EPS went in at 139/133/133 grams and came out at 511/447/494 grams. So proof is in, XPS is more appropriate for below grade."

    Also, see what they did at Golcar Passivhaus: https://www.greenbuildingstore.co.uk/golcar-passivhaus-ground-floor-foundations/

    "We have also changed the type of polystyrene we’re using in the cavity, below the cavity tray down to the foundation strip. We have discovered that extruded polystyrene is more water repellent than the expanded polystyrene we used at Denby Dale."

    No info about how they decided this... I have tweeted Bill Butcher!

    I think I'm being won over to XPS now...
  7.  
    I couldn't see the results of the test, what grade of EPS was used and any difference expected between different densities?
    Any idea about how long the water took to come out of the EPS and XPS?
    What was the insulative degradation after the water take-up?

    The EPS study (above) conducted by the epsindustry.org quotes 'In August 2008, independent testing evaluated.........' and whilst I would not expect this organisation to publish an unfavourable result I would not expect them to produce a fraudulent result. The epsindustry.org has other studies, obviously favourable to EPS but again I doubt fraudulent. Any one know of tests done by the XPS industry?

    I knew a racing sailor once who, being worried that the EPS buoyancy in his boat would take up water and so add weight (and slow him down) put some EPS in a dustbin of water with a brick to hold it down and after 6 months there was no added weight.
    • CommentAuthorgravelld
    • CommentTimeApr 25th 2018
     
    Fair points, I added a comment asking what grade/type.
    • CommentAuthorTimSmall
    • CommentTimeApr 26th 2018
     
    I spent hours and hours researching this when I did my groundworks, and came to the conclusion that both types were susceptible to water uptake when there was sufficient vapour drive, so that trying to avoid that was the most important thing (e.g. by designing the DPM placement appropriately so that the wasn't vapour drive towards the cold side of the insulation). e.g. In a thread a few months ago I posted a link to a paper which had some wufi vs. sample measurements on some inverted green roofs, which is worth a read I think...
  10.  
    Posted By: TimSmallI spent hours and hours researching this when I did my groundworks, and came to the conclusion that both types were susceptible to water uptake when there was sufficient vapour drive

    If that is the case then, what becomes important is the insulation value when some water has been driven in and the speed the water can come out once the vapour pressure has reduced.
    • CommentAuthorgravelld
    • CommentTimeApr 26th 2018 edited
     
    Posted By: TimSmall(e.g. by designing the DPM placement appropriately so that the wasn't vapour drive towards the cold side of the insulation)
    The DPM isn't placed in this situation is it - this is outside of the DPM at the plinth. Do you mean this thread? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15568

    I don't really understand the vapour science. Is vapour drive the 'speed' and direction in which vapour moves given temperature deltas, vapour permeability of materials etc?
    • CommentAuthorgoodevans
    • CommentTimeApr 27th 2018 edited
     
    Posted By: gravelldI don't really understand the vapour science

    Think of it as vapour / gas diffusion - the vapour will travel to an area of lower concentration.

    Concentration is the problematic word here - in air the concentration should be measured in absolute humidity (or its equivalents: partial vapour pressure, specific humidity) but not Relative Humidity.

    However where there is a solid involved (wood, concrete, eps) the solid eventually reach a equilibrium moisture content (emc) and is most closely related to the relative humidity of the air at is boundary (often there are two boundarys to consider - outside and inside). For wood if the relative humidity is too high it rots - for concrete, block and brick it's not so critical.

    When liquid water is involved (as tested above) it is different again - it will make a massive difference depending on if the blocks remain submerged or are periodically submerged with free draining gravel around it (as in your case).

    How liquid water moves throughout a solid differers - and it is the case that a small pinprick in a waterproof coating will let in plenty of water but will take ages to evaporate out as vapour through the same hole (think of a bituminous flat roof with a puddle and a small hole in it).
    • CommentAuthorgravelld
    • CommentTimeApr 27th 2018
     
    Thanks. Why is it RH for the solid and AH for the gas? If water evaporates from the surface of the solid shouldn't it be following the same rules as for the gas?
    • CommentAuthorgoodevans
    • CommentTimeApr 29th 2018
     
    It seems not (and I don't know why) - the moisture content of wood relative to the moisture content of the air has been well tested and reported - and the dominant variable is the RH but temp/AH does have an effect. The same is true for concrete as well (but each material has a different relationship of RH to MC).

    Another almost related example is the way a saturated salt will keep enclosed air at a particular RH (almost independently of temperature) each salt gives a different RH value - see - https://www.engineeringtoolbox.com/salt-humidity-d_1887.html.

    Vapour transmission in building is interesting, difficult to visualise, not well tested and very difficult to model with other additional complications due to the long lag times of diffusion - but much faster transport of vapour due to infiltration, exfiltration and convection currents.

    For my EWI below dpc I have the following plan... EPS doesn't rot, If it is not submerged 99% of the time (French drain) then it won't take up alot of water and it will still be a good insulator.
    • CommentAuthorgravelld
    • CommentTimeApr 30th 2018
     
    Thanks - thought you'd already done your EWI or am I mixing things up?
    •  
      CommentAuthordjh
    • CommentTimeApr 30th 2018
     
    The point is that the water isn't a gas inside the solid, so there's no reason to expect it to follow gas laws. RH is a way of describing part of the relationship between the gas and liquid phases of water, and it gets even more complicated when hygroscopic solids are around. There's likely a monolayer of water on the surface of pretty much every surface around you along with things like virions and bacteria and just plain dust.

    Where the water does pass through a solid as a gas, then it does obey the gas laws, modified for the extra collisions with the molecules of the solid.
    • CommentAuthorgoodevans
    • CommentTimeApr 30th 2018 edited
     
    Sorry gravelld - it's just theory from me at the moment - site clearance started last week.
    • CommentAuthorgravelld
    • CommentTimeMay 1st 2018 edited
     
    Posted By: djhThe point is that the water isn't a gas inside the solid, so there's no reason to expect it to follow gas laws
    I didn't realise this! That is to say: I thought it could be either.

    Posted By: djhand it gets even more complicated when hygroscopic solids are around.
    And hydrophobia affects speed of evaporation?

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