Thickness of battens compared to Celotex boards

Discussion in 'Eco Talk' started by diy_nixy, Jan 30, 2019.

  1. diy_nixy

    diy_nixy Active Member

    I want to insulate internally. Battens on walls vertically + cellotex boards in between. Silver tape the gaps to keep airtight. Then plasterboard over top.

    I was going to use CLS battens as its cheap. 63mm depth. But insulation boards are either 50, 60, 70mm etc.

    1) Which thickness should i use for boards? 60mm?

    2) Or should I try and get 50mm rough sawn timbers and use 50mm celotex boards.

    3) Any recommendation on insulations boards would useful too, there seems to be so many - Celotex, Kingspan, Ecotherm etc.

    4) Is this a reasonable place to buy online? I did notice some places on ebay too which sell 'seconds'.

    (House is 1950 detached with cavity walls. 1930's style of house. But both inner and out leaf are red brick which makes it generally cold).
  2. b4xtr

    b4xtr Active Member

    why don't you look at insulated plaster board, can be stuck with foam
  3. sospan

    sospan Well-Known Member

    Insulated plasterboard, easier to fit and includes a vapour barrier. If you go for the better quality (non polystyrene) boards it will still work out cheaper than insulation, battens, plasterboard.
  4. andyscotland

    andyscotland Member

    As you have a cavity wall I'll third insulated plasterboard, dot and dab, much easier and works out cheaper by the time all in. And gives you a continuous layer of insulation - insulation between battens means you lose a lot of heat through the wood.

    If for some reason you don't want to do that, next best would be insulation boards resting against the wall, joints all foil taped as a vapour barrier. Hold them in place with 25x50 vertical battens + noggins screwed through the insulation to the brick. Then plasterboard on the battens. Still gives a continuous insulation layer, the air gap provides an extra bit and gives you a space to run cables, socket backboxes etc which can be a bit more of a faff with insulated plasterboard.

    Thickness wise - the more the better, depending how much space you can afford to lose! I believe in England you are supposed to achieve 0.30W/m2K if upgrading an existing wall, in Scotland 0.22W/m2K. EcoTherm's calculator says you'd need 92.5mm insulated plasterboard (80mm insulation) to get 0.22, 72.5mm (60mm insulation) to get below 0.30. If you have a service cavity you may be able to use slightly thinner boards. Ecotherm's technical desk are very helpful for doing calculations and advice.

    Broadly speaking PIR boards (EcoTherm / Ballytherm / Quinntherm) used to all be the same at 0.022W/mK, Kingspan are more expensive but better performance (e.g. their insulated plasterboard is 0.018) so you can use a thinner layer. However Celotex got caught out over-claiming on the performance of some of their boards so some are now officially 0.023 which isn't a massive difference but may be enough to tip you just over into needing the next board size if anyone's checking. Generally fine to go for whatever's most economical at the time.

    InsulationExpress aren't bad for online pricing (I used their site when I was estimating early in my project as they came up OK). However it's probably also worth asking local merchants - particularly if there's any that specialise in insulation - to price your job once you have a list of materials. I got great service and much better pricing than I'd found online from a local company even on a one-off not-enormous order.
  5. diy_nixy

    diy_nixy Active Member

    Thanks all for the advice. I was thinking of insulated plasterboard as well. However I was worried about getting it up the stairs and around corners in one continuous 8 x 4 sheet. If I end up cutting it down then it may get messy trying to patch them back together. At least with battens I can squirt some expanding foam and patch up any holes and gaps. Foil tape over the joints and then plasterboard sheet to finish. The plasterboard join lines would be different to the joins in PIR boards so perhaps easier to conceal.

    Maybe I should consider insulated plasterboard again. Still thinking what to do?

    (Also will try and see if local merchants can offer a good price once I have a complete material list).
  6. andyscotland

    andyscotland Member

    Ah ok. It depends how awkward the space / corners / etc are but yes I think if you're not confident you could fully butt and seal the insulated boards then I think it would make sense to do the insulation as a separate layer. You definitely want to avoid cold spots on the wall surface and/or any gaps in the vapour seal.

    In which case I'd probably:
    • put the insulation direct on the wall with a bit of adhesive if necessary just to keep it in place while you work
    • seal up any gaps and foil tape joints
    • Then fix battens over - drill through the insulation into the brick, pop in a hammer-in-fixing with a bit of sealant before you hammer it closed to fill the gap around the screw shaft / head.
    • Then screw plasterboard onto the battens the usual way.
    That gives a useful service cavity that will also provide a bit more insulation, virtually eliminates any thermal bridging on the plasterboard screws, minimises the number of things penetrating the insulation and should allow you to get a nice neat finish.

    I've tried the other way before (batten then insulation then plasterboard) and it was very hard to stop the screws churning a much bigger hole into the PIR as they passed through before they hit the batten and started to pull straight again. Plus the long screws were much more expensive than normal 38mm for going straight into a batten.

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