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Green Building Bible, Fourth Edition
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Products: Insulation under a contrete slab

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  1.  
    • CommentAuthorTriassic
    • CommentTimeDec 17th 2014
     
    The architect has specified Celotex GA4000 70mm thick or equivalent for the insulation below a floor slab. As the architect is now on holiday I was wondering what you would consider to be 'equivalent' to Celotex GA4000? I was thinking EPS 70 at 100mm thick, but is 100mm thick enough?
  2.  
    0.07m/.022W/mK = R = 3.182

    0.1/0.038 (IIRC)= R = 2.63. You'd need just under 125mm EPS

    Note, however, that if you Google, you'll get Lambda (Th Conduct'y) values from 0.030 - 0.038.
    • CommentAuthorTriassic
    • CommentTimeDec 17th 2014 edited
     
    So looking at my local builds merchant web site ..

    Celotex GA4000 2400 x 1200 x 70mm = £25.75 each sheet

    EPS 70 2400 x 1200 x 125mm = £17.47 each sheet

    So quite a saving?

    As an aside, I think me must be sponsored by Celotex as that's all he specifies in his designs
    • CommentAuthorringi
    • CommentTimeDec 17th 2014
     
    How much is it going to cost to dig down 55mm more?
    • CommentAuthorTriassic
    • CommentTimeDec 17th 2014
     
    Posted By: ringiHow much is it going to cost to dig down 55mm more?
    Nothing other than my sweat and toil :smile:
  6.  
    EPS should be safer under slab as water won't bother it. I think there may be a bit of room left in the price quoted for negotiation downwards depending on how many sheets you are after. Its not an awful lot of insulation going under the floor, I would be trying to get at least 2x100mm sheets for only a few quid more. The 100mm sheets most likely to be the volume sellers so better chance of discounts.
    • CommentAuthorSaint
    • CommentTimeDec 17th 2014 edited
     
    Posted By: TriassicThe architect has specified Celotex GA4000 70mm thick or equivalent for the insulation below a floor slab. As the architect is now on holiday I was wondering what you would consider to be 'equivalent' to Celotex GA4000? I was thinking EPS 70 at 100mm thick, but is 100mm thick enough?


    If its white EPS you'll need 120mm, grey EPS you'll need 100m. 70kPa is half the compressive strength of GA4000 at 140kPa so its not an equivalent. Why not just use what the architect specifies?
    • CommentAuthorTriassic
    • CommentTimeDec 18th 2014
     
    Posted By: SaintWhy not just use what the architect specifies?
    The Architect tends to specify the most costly products for each element in the design and then covers his ass by stating "use 'costly product' or equivalent". It's been suggested by the QS that using the equivalent product could save us between 10 and 20% of the total build cost.
    • CommentAuthorTriassic
    • CommentTimeDec 18th 2014
     
    Posted By: SaintIf its white EPS you'll need 120mm, grey EPS you'll need 100m. 70kPa is half the compressive strength of GA4000 at 140kPa so its not an equivalent. Why not just use what the architect specifies?
    Just checked the Structural Engineers drawings, the slab is not load bearing, it's simply the ground floor slab, located inside the load bearing walls. The specification calls for a 150mm concrete slab over 70mm Celotex GA4000 or equivalent.
    • CommentAuthorCWatters
    • CommentTimeDec 18th 2014
     
    UFH?
    • CommentAuthorTriassic
    • CommentTimeDec 18th 2014 edited
     
    Posted By: CWattersUFH?
    No, I'm building a small 7.5 x 4.5m one bedroom cabin and I thought UFH would be overkill.
  12.  
    So is/will be an architect=overkill
    for a one bedroom cabin
    surely?:shocked:
    • CommentAuthorSigaldry
    • CommentTimeDec 18th 2014
     
    0.22 W/m²K U-value is hardly overkill - underkill if anything...

    (assuming it's detached with P/A ratio of 0.71 from advised dim's.
    • CommentAuthorTriassic
    • CommentTimeDec 18th 2014
     
    Posted By: orangemannotSo is/will be an architect=overkill
    The cabin is part of a larger self build project. I had the choice of a second hand caravan or self build a cabin in the garden to live during the build. Having talked to a couple of local self builders, one of whom has been living in a 'temporary' caravan for 4 years, I was convinced to go for the cabin.

    Also access onto site is tight and delivery of a residential caravan would have entailed a road closure, a crane costing £800, the purchase cost of the van and delivery charges. It looks like I'll be saving quite a chunk of cash, which can then be spent on the main house.
  15.  
    For a bit of digging, though, why not go for 0.1-something for floor U value? Warm feet tend to lead to lower demand temps further up.
    • CommentAuthorborpin
    • CommentTimeDec 18th 2014
     
    For insulation never touch merchants - go to one of the insulation specialists - I used Encon. They will deliver but be sure to ask for a Mechanical Off Load (MOL) delivery (unless you have a forklift of course). They also cannot be beaten for plasterboard if you are buying in quantity. Make sure you get a name to speak to so always deal with the same person. Make it clear you will be buying lots more in bulk and you will get good prices.

    Never occurred to me not to use PIR. Wish I had - had loads of depth to do it :(
    •  
      CommentAuthordjh
    • CommentTimeDec 21st 2014
     
    Posted By: borpinFor insulation never touch merchants - go to one of the insulation specialists - I used Encon. They will deliver but be sure to ask for a Mechanical Off Load (MOL) delivery (unless you have a forklift of course).

    For plasterboard etc sure, but for EPS? I unloaded an artic full of EPS with the driver one gorgeous summer's day last year. It was quite enjoyable. The packs of boards are easily manhandled.
    • CommentAuthorborpin
    • CommentTimeDec 22nd 2014
     
    Posted By: djh
    Posted By: borpinFor insulation never touch merchants - go to one of the insulation specialists - I used Encon. They will deliver but be sure to ask for a Mechanical Off Load (MOL) delivery (unless you have a forklift of course).

    For plasterboard etc sure, but for EPS? I unloaded an artic full of EPS with the driver one gorgeous summer's day last year. It was quite enjoyable. The packs of boards are easily manhandled.
    Certainly for PIR, much quicker to unload with MOL. Thick sheets surprisingly heavy (doubled up).
    • CommentAuthorcjard
    • CommentTimeDec 25th 2014
     
    So, by using EPS you save £7 per 2.88m2
    And your shed is about 32m2
    So over the 12 sheets you'll need you save about 85 quid?

    Penny wise, indeed
  20.  
    EPS is well proven in ground slab applications. PIR, in particular the sheets specified will have extra costs associated with them - in particular you can look forward to spending twice as much on DPM as you need a layer under the PIR (you cannot let it get wet under the slab) and a layer above it (you need to keep concrete off its foil face). So extra material, extra labour.
    • CommentAuthorTriassic
    • CommentTimeDec 25th 2014 edited
     
    Posted By: cjardPenny wise, indeed
    Also Pound wise.

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