Discussion in 'Builders' Talk' started by Jack001, Jun 3, 2021.

  1. Jack001

    Jack001 New Member

    Hi all,

    I’m currently in the process of getting a rear single story extension done. My builder has said it can be done in all block, both outer and inner leaves, is this correct?

    He’s asked me to order 100mm blocks which will make the two skins internal and external with 50mm Kingspan insulation in the cavity.

    Is this acceptable and will it pass building control regulations when they come to inspect the build?

    Thanks in advance.
  2. spannerw

    spannerw Active Member

    What has your architect specified and what do your submitted plans show?
  3. Hans_25

    Hans_25 Screwfix Select

    Why isn't the builder ordering the blocks? If you order them and get the wrong ones, or not enough, or too many, its your problem. As above - what do the plans show? Builders typically have accounts with the major suppliers so can get them cheaper.

    100mm blocks come in two strengths, 3.6N and 7.2N, 3.6N is the norm especially for a single story extension, but the stronger ones may be needed to support RSJ over an opening for example.

    What is the finish on the outside, is it being rendered?

    What have you contracted the builder to do exactly? This should include getting building reg approval, no idea if 50mm PIR is sufficient, sounds a bit thin.

    I can see this scenario going south very quickly.
    spannerw likes this.
  4. Jack001

    Jack001 New Member

    The plans state 100mm Celcon HI 7 blockwork, but when speaking with the architect he stated nobody actually uses them and hasn’t had anything fail on build regulations for 10 year.

    The builder is a friend of a friend and charging me labour only, told me to get the materials myself at cost as most builders will add money onto the price, used a friends account so getting them cheaper anyway!

    He gave me a list of materials and just told me to order them.

    The block’s i have got are 7.2N, the back of the house is coming out with a steel beam going across.

    The finish on the external walls is going to be k rend across the whole house once extension is completed.

    The builder is building up from the footings the shell to water tight and i am sorting all internal trades sparks, plumbing, plastering etc.

    The architect drawings state 50mm cavity wall insulation so guess thats all it needs?
  5. jonathanc

    jonathanc Screwfix Select

    What u value are you aiming for you on the walls? Does the builders proposed construction achieve the same or better?
  6. Jack001

    Jack001 New Member

    it’s doesn’t say on the drawings the exact value, just value for walls, windows, doors etc, is the higher the better or lower the better?

    As my bifold doors require 1.6W/m2k is requires
  7. jonathanc

    jonathanc Screwfix Select

    You need to achieve whatever is specified for any thermal element on the plans so if you order different stuff then make sure it meets or exceeds the u value: lower the better
  8. Jack001

    Jack001 New Member

    Okay i see, will they do a thermal testing once the extension is completed?
  9. Hans_25

    Hans_25 Screwfix Select

    No, you'll need to demonstrate that the design and actual materials used conforms to the requirement.
  10. Jack001

    Jack001 New Member

    How would i do that though?
  11. Hans_25

    Hans_25 Screwfix Select

    You're better off asking your Builder that, he should know and have done this 100s of times.

    If you've employed your builder on a labour only basis, then you're going to be responsible for all the Building Reg issues, this is just the tip of the iceberg.
  12. stevie22

    stevie22 Screwfix Select

    Yes you can use all block. Celcon blocks can cause trouble with sand cement render so many builders will opt for dense concrete to the outer skin. If, however, you have a wall on the boundary then it is usual to use face brick to avoid maintenance issues. 3.6N Celcon are fine because you size the padstones on any beams to suit.

    But look, the architect is he guy you've paid to design the thing so it's him you ask. He is demonstrably stupid of course if he says to you, a customer, that he designs things but accepts that people ignore that design!!

    Have drawings been submitted to BC and approved? If so then build to that and all good. All insulation details should be on the drawings and the norm for doors is 1.8W/sqmK and lower is better. Your 1.6 should be fine BUT if the design has more than 25% floor plus covered area as new glazing the basic U values don't apply.

    %0 cavity will not accept 50 Celotex: full fill is usually glass fibre

    It sounds to me that your builder is OK but the architect is the problem.
    Jord86 likes this.
  13. Jack001

    Jack001 New Member

    Hi Stevie,

    Thanks for the reply,

    I’m going for dense concrete outer and inner, outer will be K Rend and inner will be plaster boarded and skimmed, with 50mm cavity insulation. I’ve seen that Celcon thermal blocks are not very good for screwing things into and solid block would be better as we want TV on the wall and kitchen fitting onto it.

    Drawings have been submitted by the architect but not too sure if they have been approved yet. Is there anyway of checking?

    What do you mean by 25% floor plus covered area?

    We won’t have 0% cavity, the cavity will be big enough to have 50mm Celotex and i’ve ordered insulation clips to hold them together.

    To be honest the architect is decent, always replies to emails and text to help me out with stuff, he said people don’t usually use the Celcon thermal as they’re weaker as stated about and not very good for internal fixings and stuff and said with 2 skins and cavity wall insulation there isn’t much thermal difference anyway.
  14. stevie22

    stevie22 Screwfix Select

    Personally I'd go for Celcon to the outer if you're using K-rend because the insulation value is better than concrete. No problem fixing to Celcons with proper fixings

    Check with BC if they have approved the drawings.

    The 25% comes from the basic approach to extension heat loss. The 4 build elements (wall, floor, roof and doors/windows) are ascribed a maximum "U" value and if these are met and the area of doors/windows is less than 25% of the new floor area plus the area of any door/windows covered by the works then no further calculations are required.
  15. jonathanc

    jonathanc Screwfix Select

    two skins of dense block is not going to get the thermal efficiency of dense outside and thermal block inside. Get it wrong and you risk tearing it down. As to building control application: what have you paid for? If you have not paid the council or a private firm to take on the Bc activity then nothing had been done. If you have paid and appointed building control then chase the plans
  16. Jack001

    Jack001 New Member

    My extension is going to be opened by taking out a middle wall and the full back of the house, so even with the extension up to scratch, the existing house may not be and cause the U Value to come down anyway?
  17. Jack001

    Jack001 New Member

    The architect submitted the plans and i’ve currently paid the council £180, i pay the rest upon the first inspection which will next week when the excavation starts.
  18. stevie22

    stevie22 Screwfix Select

    The existing house doesn't come into heat loss (unless you need to compensate for excess loss from the extension when you can improve existing).
  19. Jack001

    Jack001 New Member

    I don’t understand because they did a heat loss calculation on the extension it would also use the existing house as it’s all one big room?
    Confused o_O
  20. jonathanc

    jonathanc Screwfix Select

    So that’s the approved plan route. Ask the architect if they have approved the plans or not. Forget the heat loss calcs. You have plans that specify u value of the walls: you need to meet or exceed this else it will not pass. So do you know whether your proposed materials will have sufficient thermal efficiency?

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