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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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    • CommentAuthorDur
    • CommentTimeAug 5th 2021
     
    Hi all

    I am sorry if this is a well trod subject but I am finding it difficult to figure it out from what I can find on existing posts.

    This a 1908 semi with (mostly) suspended wooden floors with about 350 mm from floor level to the oversite.
    We are looking to install underfloor heating with an ASHP down the line.

    The plan is to remove the existing floors and "fill in" with a slab / insulation/ ufh and screed.

    I would be very grateful for suggestions as to what the build up should be - thickness of insulation, slab etc.

    Should the insulation be under the slab to increase thermal mass (rest of house insulation will be internal)?

    Builders we have spoken to for a side extension that is planned seem to lean towards 150 PIR insulation on top of the slab with the UFH pipes pinned to the insulation and a screed on top.

    We would like a decent level of insulation in the floor to give the ASHP a chance

    Thanks
    • CommentAuthortony
    • CommentTimeAug 6th 2021
     
    The more insulation the better!

    With 350mm I would go 100 concrete on undisturbed base, skim/cut top 25mm off, no hardcore, 200 sheet insulation, 75mm liquid screed
    • CommentAuthorDur
    • CommentTimeAug 6th 2021
     
    Thanks Tony
    Sorry I don't follow the skim / cut part. Where is that removed from?
    I guess it has to be PIR to get the insulation levels with the available depth
  1.  
    200mm EPS would be fine
  2.  
    We put 150mm EPS in 2 x layers of 75mm each
    Which was cheaper than 100mm PIR
    for the same insulation value
  3.  
    Our slab is on top of the insulation but we have done the entire ground floor, probably slow response time due to the mass but hoping for a lower flow temperature. GSHP
  4.  
    What floor surface are you planning? We have used brick which has also added to the thermal mass.
    • CommentAuthorDur
    • CommentTimeAug 6th 2021
     
    Hi Dominic
    Floor surface most likely ceramic tiles over 75% of ground floor (ultimately!) with engineered wood laminate in the remainder (spare bedroom etc).
    We need to look for low flow temperature to give the ASHP a chance and we are aiming to put a thin layer of insulation onto the external walls (single storey, chalet/dormer bungalow, 9" solid walls mostly). There won't be a lot of thermal mass when done but I guess 75 mm screed will help a little?
    .
  5.  
    What Tony means by skim/cut top 25mm off, I would assume is to remove 25mm from oversite to get undisturbed base.
    I have put insulation under the slab, as per Dominic above, because I don't see the point of essentially 2 slabs (100 below insulation and 75 above).
    Also I would use EPS.
    350mm is quite a bit to fill so perhaps 225 EPS with slab over to level .
    I would also put 20mm EPS around the periphery between the slab and the walls.
    • CommentAuthorDur
    • CommentTimeAug 6th 2021
     
    Hi Peter
    That would give more mass which seems like a good idea.
    I guess there is a method of fixing the UFH tubes to the top of the slab and then a thin screed over to cover them. I'd better figure out depth required on the screed to allow for the tiles and adhesive. I guess 20 mm
    •  
      CommentAuthordjh
    • CommentTimeAug 6th 2021
     
    Posted By: DurI guess there is a method of fixing the UFH tubes to the top of the slab and then a thin screed over to cover them.

    I think it's more normal to wire the UFH pipe to the reinforcement in the slab and cast the slab around the whole lot. You don't need a screed on top depending on your intended surface finish.
  6.  
    Yes work out the thickness of your layers from the top down, then whatever is left fill with EPS

    Cable-tie the UFH pipes to reinforcing mesh e.g. A142 or D49 to suit the pipe spacing. Or you can get clip rails that screw/glue to the slab.
  7.  
    Oops cross-posted
  8.  
    Like so. It can be a bit OCD
      244BBD6E-F027-4F12-BFFB-64B289E1F76D.jpeg
  9.  
    We also put 75mm EPS around the edge of the slab, 100mm PIR on the walls
    • CommentAuthorDur
    • CommentTimeAug 6th 2021
     
    That's really useful thanks. So putting the UFH in the slab saves a bit of space so we can have a little more eps which is great. A good layer of insulation around the perimeter noted.
    • CommentAuthorjms452
    • CommentTimeAug 6th 2021
     
    Posted By: DurSo putting the UFH in the slab saves a bit of space so we can have a little more eps


    exactly - EPS is cheaper and lower embodied carbon than PIR - it should also give a constant insulation value even as it ages. Only down side is that you need it to be thicker for the same performance but with 350mm that's not a problem.

    2 staggered layers is considered better as less thermal bridging through imperfections (and it can bend a bit more).

    Insulation up-stands at the edges of about the same thickness as your internal wall insulation.

    Typically polyethene dpm between the insulation and the inner slab.

    Caveat: physicist not builder :devil:
    • CommentAuthorDur
    • CommentTimeAug 6th 2021
     
    Thanks JMS.
    Interesting - I was expecting the DPM would have to be under the EPS but I guess the EPS is not too worried by any damp. Is a vcl needed above the concrete ( once properly cured) or is the concrete a sufficient barrier in its own right?
    •  
      CommentAuthordjh
    • CommentTimeAug 6th 2021
     
    Posted By: DurIs a vcl needed above the concrete ( once properly cured) or is the concrete a sufficient barrier in its own right?

    The DPM acts as a VCL, so adding another would create a vapour-tight 'sandwich', which is generally a bad idea. Floors tend to have more problems dealing with liquids than vapours though - spills and floods etc, so that's usually the most important consideration. Concrete is moderately resistant to water and water vapour but can absorb them and let either of them through slowly. But the short answer is no, you don't need a VCL above the concrete.
    • CommentAuthorDur
    • CommentTimeAug 6th 2021
     
    Thanks DJH

    All makes sense. Good to have a plan - just need to get to that stage now!
    • CommentAuthorDur
    • CommentTimeAug 6th 2021
     
    Thanks JMS.
    Interesting - I was expecting the DPM would have to be under the EPS but I guess the EPS is not too worried by any damp. Is a vcl needed above the concrete ( once properly cured) or is the concrete a sufficient barrier in its own right?
    • CommentAuthorjms452
    • CommentTimeAug 7th 2021
     
    Posted By: DurThanks JMS.
    Interesting - I was expecting the DPM would have to be under the EPS but I guess the EPS is not too worried by any damp. Is a vcl needed above the concrete ( once properly cured) or is the concrete a sufficient barrier in its own right?


    As dgh says the DPM is above the EPS and acts as a VCL - I think this is a simplified version of your floor dgh?
    You don't want a second vcl above the top slab (it would make your tiling a pain!).

    Whether you need a dpm under the lower slab is down to your building control & builder.
  10.  
    The last floor I did I put the DPM under the EPS (and coming up the sides to the wall DPM) because I didn't want the builder stomping all over it with big boots and wheel barrows which is what would happen if it was between the EPS and the slab
    So the floor build up was soil (happens to be very sandy) DPM, ESP, slab then tiles as it was a kitchen but could equally have been any other floor finish.
    • CommentAuthorDur
    • CommentTimeAug 7th 2021
     
    OK - so the position of the DPM not critical.

    Having talked to a couple of builders I think the challenge will be to persuade them to do something different from what they normally do. Like many, I would be keen to do it myself if I didn't have to go to work!
    •  
      CommentAuthordjh
    • CommentTimeAug 7th 2021
     
    Posted By: jms452I think this is a simplified version of your floor dgh?

    Yes, we have four layers of 100mm graphite EPS with the DPM above the bottom layer. I think the logic is that in that position it is protected from damage by the hardcore etc underneath and from builders boots etc on top. Some people like it right at the top to prevent any concrete leaking down between the sheets of EPS.

    Our slab is quite complicated: it's nominally 150 mm but has some 100 mm downstand ribs to give it extra strength, plus of course a separate perimeter beam for the walls.
  11.  
    Posted By: DurHaving talked to a couple of builders I think the challenge will be to persuade them to do something different from what they normally do. Like many, I would be keen to do it myself if I didn't have to go to work!

    welcome to the club:bigsmile:
  12.  
    I expect one of the reasons that insulation-over-the-slab-then-pipes is more common is that it’s quicker to staple the pipes into the insulation or clip it into “egg-boxes” than to cable-tie it to mesh. Doesn’t mean that it’s the best for your situation.
  13.  
    Posted By: Peter_in_Hungarywheel barrows


    We were lucky with the orientation of the driveway we were able to pour the concrete straight in through the doors and windows off the chute, so only a bit of raking and tamping to get it level. No wheelbarrows required!
    • CommentAuthorDur
    • CommentTimeAug 9th 2021
     
    DJH - I just re-read the above and realised you said 4 x 100 EPS. I have now insulation envy, especially given the latest UN report!

    DC - I spoke to a builder the other day and he was suggesting insulation with slab on top and then eggboxes and 50mm screed. I can see that is going to reduce the labour bill significantly but then you lose a little insulation space. I don't know whether there would be much difference in the heat transfer into the slab with the pipes in a 50 mm screed - I presume it would be better in the slab. Builder default was 100 mm PIR so some work to do there/.
    • CommentAuthorphiledge
    • CommentTimeAug 10th 2021
     
    50mm screed sounds a bit thin with pipes reducing the thickness wherever they are. Our kitchen/diner has the pipes buried in a 5" slab over EPS. No screed as its finished in tiles so didnt need a super smooth finish. Its slow to respond but it stays warm overnight without the woodburner running so tiles are warm in the morning for barefeet😁

    I think 100mm PIR just meets building regs for refurb jobs so maybe thats where your builder has got that thickness from?? TBH if your builder is suggesting insulation to meet the regs, if thats what 100mm achieves, youve probably found someone who knows what they are doing
   
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