Half build complete... Need some guidance!

Discussion in 'Builders' Talk' started by TonicUK, Jul 18, 2018.

  1. TonicUK

    TonicUK Member

    Hello all,

    Long time lurker. Not much in the way of posting but I have become stuck.

    I have recently instructed a builder to complete me a half build extension, so a shell only. I will project manage the rest.

    I understand that the next step would be to get my cables and pipes in then followed by insulating.

    I understand that for a extension the required u-value for ceilings is 0.11. However, looking at ecotherm 100mm - http://www.ecotherm.co.uk/Download....3M5DzDa5SLztRRC5nf+nl&Bestandsnaam=Eco-Versal(for inbetween rafters on a cold pitched roof) with a u-value of 0.23 and a second layer of insulated plasterboard (ecotherm ecoliner 50mm - http://www.ecotherm.co.uk/Download....QIM3TFJLMJJpzlgYnvM/WH&Bestandsnaam=Eco-Liner) at 0.34.

    How do I know if the two combined meet building regs? Including if I use any PU foam for any gaps..?

    I intend on using rockwool flexi to insulate internal walls and the builders will be insulating my external cavity.

    Also, is there anything else I need to be wary of?

    Thank you very much. I am a tradesman but not much experience with general building!
     
  2. rogerk101

    rogerk101 Screwfix Select

    First thing ... I would studiously avoid using insulated plasterboard. It's overpriced and much more difficult to work with than separate insulation and plasterboard.
    Second thing ... there's no such thing as too much insulation ... you might pay a bit more up front to maximise on insulation, but it'll pay itself back many times over with dramatically improved comfort and lower heating bills.
    By far the best way to insulate a roof (extension or otherwise) is as follows.
    Fill the gaps between the joists/rafters with a rigid polyurethane insulation board cut to a snug fit. I would use at least 150mm of insulation, ideally 200mm, so would double up the joists/rafters if necessary.
    Use 30mm insulation boards to cover the joists/rafters, so they're no longer a thermal bridge.
    Use insulated foil bubble wrap as a vapour barrier.
    Add the plasterboard, screwing into the joists/rafters with extra long (~75mm) plasterboard screws, with load spreading countersunk washers.
    Finally it's skimming and painting.
    I've done so many ceilings/roofs like this, and have the beautiful comfort and low heating bills to prove that it works.
     
    TonicUK likes this.
  3. TonicUK

    TonicUK Member

    Thanks for the response!

    I will consider if additional timbers will be within budget because it sounds like a no-brainer to me! Plus, I suppose if I am using thicker insulation board I will be hitting the target without a problem.

    I hadn’t considered that I need a vapour barrier between insulation and plasterboard. Is it necessary and could you point out which ones you’re talking about for me to do some research on them?

    Further to my additional question - how do you ensure you are meeting building regulations?
     
  4. Dr Bodgit

    Dr Bodgit Super Member

    If you need 0.11 using .23 and .34 then this is the calculation

    1/.23 + 1/.34 = 7.28

    1/7.28 = .137

    So its not quite enough.
     
    TonicUK likes this.
  5. TonicUK

    TonicUK Member

    Thank you. That is very helpful.

    I was wondering.. the u-value required for new roofs does that include the roof tiles, plaster, etc?
    In which case would it meet the requirements?
     
  6. Dr Bodgit

    Dr Bodgit Super Member

    I'm no expert but I don't think so, the u-value is for the insulation that's put in and just that

    Roof tiles are not going to have much, if any, insulation value plus they let in air so no insulation at all!
     
    TonicUK likes this.
  7. TonicUK

    TonicUK Member

    Thank you. Back to the drawing board!

    May I ask any general builders out there:

    What is the typical go-to with a cold pitched ceiling to meet building regs (in England)?

    Bearing in mind this project has a very tight budget!
     
  8. Jord86

    Jord86 Screwfix Select


    Based in Wales, but should be similar, 4" celotex/king span between rafters, tri Ido/ alutherm quilt stapled over rafter area, battens fixed over top to take plasterboard and provide gap for services.
     
  9. rogerk101

    rogerk101 Screwfix Select

    That would not get you past the building inspectors I've used in Oxford - neither private nor council.
    Even if it did, the difference in materials price between this and what I suggested above is negligible compared with the total materials + labour price of the whole job ... and it becomes even more negligible when you look at the total cost of ownership, i.e. upfront costs (capital expenses) and running costs (operating expenses).
    Do the right thing ... and do the thing right!
     
    TonicUK likes this.
  10. TonicUK

    TonicUK Member

    I do like the idea for battens to provide a gap for services!

    I have just had a second thought... If I intend on fitting downlights in the ceiling am I required to create a cavity in the ceiling to house it? Instead of cutting notches out of the insulation?

    And anyone with any further advice on how to achieve 0.11 u-value for a cold pitched roof?

    Thank you
     
  11. Jord86

    Jord86 Screwfix Select


    Do the right thing??? Who are you to comment I've advised the wrong thing? Not once have I suggested doing the wrong thing, what I have stated is how the majority of vaulted extension roofs are built at least in South Wales and a few dozen I've personally built myself over the last ten years right up to vaulted two lean to's I cut on earlier this year to architect specified drawings, 6x2 rafters, 4" celotex between, 2" air gap, foil quilt and battens over the top, forgetting figures and values common sense and experience alone should tell you how warm the room is likely to be. 8" celotex is ludicrous, price wise and the work involved in deepening the rafters to get the insulation flush, and you ideally require a 2" airflow void at the top of the rafter too, how is that to be achieved without using massive depth rafters or battening then counterbattening and losing even more of the ceiling height?

    It's also debatable how much of the celotex becomes a waste once it goes past a certain depth, if for arguments sake 4" is warm enough, then what's the point in packing more and more in if the effects are negligible anyways? One plus point of the tri-Iso quilt is that it's also a vapour barrier as well as an insulator, so that's two jobs in one done.

    Plus, the OP's words, "bearing in mind this project has a very tight budget," 200mm/8" celotex costs roughly £100 a sheet.
     
    TonicUK and Deleted member 11267 like this.
  12. Jord86

    Jord86 Screwfix Select

    Even if you fitted battens you would still need to notch out the insulation so it doesn't foul the back of the spotlight, unfortunately.
     
    TonicUK likes this.
  13. TonicUK

    TonicUK Member

    So, notching out the insulation is what you are supposed to do? Instead of dropping th ceiling to create a cavity under the insulation with enough space to house some spotlights?
     
  14. Jord86

    Jord86 Screwfix Select

    It can depend on the spotlight, how far the back protrudes, you could use 2x2 instead of 2x1 but that's more money and ceiling height lost, I've used a 100mm holesaw the last few times, taken accurate measurements of the centres of your spots, slit the foil in a cross, folded back and drilled through the celotex to create the void, once plasterboard is fitted drill through with the correct size holesaw for your spots and they should all fit neatly. You really really won't notice any supposed cold spots coming through the sides of your spots, or other insulation worries.

    This is based on you using LED spots, as halogens heat up and I'd imagine the electrician you'll use would have to cut a big chunk of the celotex out to allow the heat to escape. But halogens are so old fashioned and inefficient compared to LEDs, I'm sure you weren't going to use them.
     
  15. stevie22

    stevie22 Screwfix Select

    Where did 0.11 come from?

    Should be 0.16 for a pitched roof at ceiling level or 0.18 under rafters. 0.18 for flat roof. Optimum depends on rafter size and spacing. Celotex have a very useful calcultor on their wbe site.

    Can't agree with the old "can't have too much" line: it costs, takes up space and the energy savings are minimal
     
    Jord86 likes this.
  16. TonicUK

    TonicUK Member

    Thanks for the response. I found 0.11 after a little while on google. Perhaps I was wrong?

    Where does 0.16/0.18 come from? I will have a look on their website!
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice