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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthormatt1
    • CommentTimeJul 15th 2018
    I have just finished installing my floor insulation ready for the ufh pipework and screed. I have put in 25mm kingspan upstands around all of the internal and external walls. Will these be sufficient to allow for screed expansion or do should I also install some of the self adhesive 10mm foam edge strip as well?
    • CommentAuthortony
    • CommentTimeJul 15th 2018
    I would use low density eps instead of Kingspan on internal walls it will probably be OK though
    • CommentAuthormatt1
    • CommentTimeJul 15th 2018
    Thanks Tony. What is the reason for that?
    • CommentAuthortony
    • CommentTimeJul 15th 2018
    Expanded polystyrene is a lot more squashable than pir and good insulation is needed more on outside walls.
    • CommentAuthorFred56
    • CommentTimeJul 16th 2018
    Put the foam is as well. Kinspan is too hard and it does not compress It's compressive strength is typicallly 130 kPa, EPS usually 70 kPa.

    Screed shrinks as it dries but thermal expansion is quite large. You need something that can flex with the movement. The kingspan and foam may be obscured once the wall is plastered and the skirtings on.
    Posted By: Fred56Put the foam is as well. Kinspan is too hard and it does not compress It's compressive strength is typicallly 130 kPa, EPS usually 70 kPa.

    Why put foam in to mitigate the hardness of kingspan? just put EPS in to start with.
    • CommentAuthorFred56
    • CommentTimeJul 17th 2018
    Basic building industry eps is too hard. Consider the compessible foam you put in movement joints in long concrete walls. It's the same principle. It needs to be sufficiently compressible and elastic. I can see why Matt wants the perimeter insulation although I don't know what is wall construction and insulation is.
    I suppose there may be really soft EPS, Tony did say low density. I don't know how hard packaging EPS is. Tony has more experience than most and have sources.
    the usual EPS for EWI is EPS 70, EPS 250 or 300 is used in passive slabs, for expansion joints I would use EPS 30.
    • CommentTimeJul 17th 2018
    Posted By: Peter_in_Hungarythe usual EPS for EWI is EPS 70, EPS 250 or 300 is used in passive slabs

    EPS 250 or 300 is used for high load areas under walls etc, but EPS 70 is used for the bulk of the floor area. It's certainly not what I would describe as compressible or flexible. I've no idea what the actual requirement is for the matt's use case.
    • CommentAuthorJeff B
    • CommentTimeJul 18th 2018
    This may be a naive question but if screed expands to such an extent that you are worrying about what grade of PE to use around the walls, how come the walls my house (which have no such upstand insulation) aren't bulging outwards under the expansion forces?
    • CommentAuthorJeff B
    • CommentTimeJul 21st 2018
    Any thoughts anyone or perhaps my question was too embarrassingly naive to be answered!
    From the internet (so it must be true!!)
    An average value for the coefficient of thermal expansion of concrete is about 10 millionths per degree Celsius (10x10-6/C), although values ranging from 7 to 12 millionths per degree Celsius have been observed. This amounts to a length change of 1.7 centimeters for every 30.5 meters of concrete subjected to a rise or fall of 38 degrees Celsius.
    so to put that in a domestic situation where the temp range is say 10 deg then the expansion of the said concrete would be 17mm*10/38 = 4.474mm but you are unlikely to have a 30.5m room, lets say the room is 6m so the expansion would be 1/5 of 4.474 = 0.894mm
    So in normal circumstances from the above I would not expect the expansion of concrete in a domestic situation to be more than 1mm.
    Assuming I can count !!!
    I would not expect this to cause a problem.....but then what would I know!
    • CommentAuthorgyrogear
    • CommentTimeJul 21st 2018

    I have a 12-meter room, and the slab has not busted the gables off yet..

    • CommentTimeJul 21st 2018
    There are expansion joints in concrete, and brick, structures though (don't know the implications, just saying ...)

    Also not all screeds are concrete, maybe it is screeds made from other materials that are an issue?

    Also not all houses have concrete (or even masonry) walls, especially internally. Maybe it is the differential between timber and screed that causes a problem?

    i.e. maybe there is a reason for expansion at the perimeter in some circumstances, and either through confusion or for simplification the 'necessity' has been extended to all screed situations?
    • CommentAuthorgoodevans
    • CommentTimeJul 22nd 2018
    I have often wondered about expansion and contraction joints etc.

    On the one hand the foundations on most builds have no expansion joint - but the walls that sit on them often do. One reason for this I think is the modular nature of bricks and blocks and cyclic heating and cooling each day a morter joint may expand but doesn't contract back into it's space and gradually ratcheting outwards each heat cool cycle.

    My SE has signed off on my blockwork not requiring an expansion joint on 16m of wall (because the blockwork is sheltered from the effects of sun and cold by ewi and morter bed reinforcement is used throughout

    And, in addition, no expansion joint in the continuous concrete screed layer (with 1 layer of a252 mesh) with a total size of 16m x 9m floating on 225mm of EPS. With a 40 degC temp range possibility the slab will need to expand up to 6.4mm (3.2mm each end) - I will be making sure to have an up-stand in place that can compress.
    Just had a word with my SE son-in-law (Hungarian) who said that it is usual to put 20mm of EPS between the slab and the wall to allow for the expansion. When I asked which grade he said it doesn't matter much. (To be honest I suspect that what ever was specified on the plans the builders would put in what was to hand because builders are like that)
    • CommentAuthorJeff B
    • CommentTimeJul 24th 2018
    Thank you all for your replies. I wasn't trying to be the "devil's advocate", just genuinely intrigued! If I was building my place from scratch I would certainly have perimeter insulation from the heat-loss point of view anyway, if not to take up expansion of the screed.

    Maybe Fred56 has hit the nail on the head when he says that the screed shrinks as it dries - if it shrinks a few mm during the drying process, then this would leave a sufficient gap to accommodate any thermal expansion later on and why the gable ends of my house have not cracked?
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