Insulated plasterboard behind kitchen units

Discussion in 'Builders' Talk' started by Felics, Jan 15, 2020.

  1. Felics

    Felics New Member

    Hi all,

    So I am having a full renovation done on my house (walls being taken out, new plaster etc etc.).

    I've looked into some options and realised there are many different ways to insulate the walls internally. The builder has taken the walls back to brick and has mentioned a few options. Ideally I dont want to eat into the space of the room too much (let say 5-7cm at most). I've settled on the following method.

    1) New thin coat of sand/cement to the brick wall.
    2) Attach timber battens to the wall and back with DPM
    3) In between the battens, tightly fit 50mm insulation board (Kingspan foil backed etc.). Seal around the edges with foam and tape with electric tape.
    4) Over this, place a vapour control barrier
    5) 12.5mm plasterboard over this.

    I will also treat the outside of the brick with ronseal etc. The builder has said there is no evidence of damp.

    Does the above sound reasonable?

    Also, I am doing the same in the kitchen. My Q here was how will I hang the units and sink? Do I need to take anything into account when hanging rads and anything heavy?

    Thanks in advance.

    Read more:
  2. rogerk101

    rogerk101 Screwfix Select

    Don't bother with step one. Why render when it's all going to be covered up anyway?
    I assume the DPM will be continuous right the way across the wall, and the timber battens will be fixed through it into the brick wall. This is fine, but I would also squirt a bit of builders silicone into the hole of each fixing.
    No need for foil backed insulation if you've already got the DPM in there. Unclear to me what the electrical tape is for.
    What sort of vapour control barrier were you thinking of? You've already got the DPM to prevent moisture getting into the insulation from behind. You'll have the kitchen units and splashbacks preventing water vapour getting into the insulation from the front, so another vapour barrier is probably overkill, but it a kitchen which can get quite steamy and it's not a big expense so no harm in adding it.

    Given the choice between internal and external insulation, external should always be the preferred option. It's much better thermal performance (no thermal bridging at internal walls), and it allows the entire masonry fabric of the house to become a thermal mass (longer to heat up but once heated, with good insulation on the outside, MUCH longer to cool down). Internal insulation can create thermal stresses on a building, with some parts of the masonry at 20 degrees and others at outside temps, which can be as low as -10 degrees. This can open up cracks that were never seen on the building before ... not serious, but why go there if there is a better alternative?
  3. Felics

    Felics New Member

    RE step 1 - it will be a parge layer.

    DPM was planned to be put only behind the battens and yes this would be drilled straight into the wall.

    ELectrical tape would be to ensure all the insulation is taped up together after placing into the battens.

    I was just thinking of a plastic sheet , nothing fancy.

    Yes - the only reason I dont want to render the outside as I like the brick facade. But will probably repoint and reseal.
  4. carl24bpool

    carl24bpool Active Member

    use aluminium tape or duct tape, not insulation tape.
  5. Mr Rusty

    Mr Rusty Screwfix Select

    Is it a solid wall or cavity wall? If it is solid, I would treat internal insulation with caution due to the possibility of interstitial condensation - read heritage house

    do NOT seal your bricks externally - they need to breathe.

    If you have a ventilated cavity then IMHO internal insulation is fine - the construction becomes similar to a modern timber framed house with an external brick leaf and an internal insulated structure separated by a ventilated cavity. I would use insulation backed PB fixed directly to the wall. This material includes an inbuilt VCL and is easy to fix. IMHO battens are a PITA because if the wall isn't totally flat, the battens won't be and the wall won't be - insulated PB can be adjusted on the depth of the dabs.

    DON'T put a dpm on the cold side of any insulation - any water vapour that gets through will condense on this barrier - you want any condensation inside the wall structure to get taken away by the cavity ventilation. The idea of the VCL on the warm side is it helps to stop water vapour getting into the wall structure to begin with.
    koolpc likes this.
  6. Mr Rusty

    Mr Rusty Screwfix Select

    Rads can be hung on long fixings through the insulated PB to the wall. If you do know exactly where they are going you could install some timber behind. Same with kitchen cabinets. Install a timber rail where you want to fix cabinets, and if using insulated PB, just chop out the insulation to match. Use expanded foam to seal any gaps.

    You don't need to take base units/sinks into account - they don't hang on the wall.
    koolpc, Felics and masterdiy like this.
  7. Felics

    Felics New Member

    Ahh good to know Mr Rusty - i will let you know how it all goes
  8. Felics

    Felics New Member

    It is a solid brick wall with NO cavity.

    How else could i then insulate then? From what I've read the above method would work on a solid wall? Do you think its better to avoid insulation altogether?
  9. Mr Rusty

    Mr Rusty Screwfix Select

    I have never tried to insulate a solid wall internally. I have successfully internally insulated an edwardian house that does have a well ventilated cavity with insulated PB with no problems. My opinion FWIW is that the guidance on heritage house is good - not tainted by commercial requirements to sell damp systems etc. If I had a solid wall house, I think I would be looking at breathable lime plaster and traditional finishes. I agree that all the problems with solid walls are that they get damp internally and can't dry out because the internal/external finishes are waterproof.

    If I decided I had to insulate a solid wall internally, I think I would build an insulated stud wall leaving a cavity between that and the solid wall, and then insert airbricks to that cavity, again so the structure would be more like a modern timber frame with a brick ext cladding, internal timber frame and ventilated cavity. BUT this would take 100mm min out the room - I've only done this to build an insulated office inside a single skin garage (which was very successful) but never on a house.
  10. Jord86

    Jord86 Screwfix Select

    Look at EWI, External wall insulation, though forget it if you have period details on the outside as they’ll be covered over.
  11. Felics

    Felics New Member

    Yeh - dont want to hide my period features really.

    I think I'm going to have to insulate onto the walls as mentioned and hope its all fine.

Share This Page