Garage conversion wall insulation

Discussion in 'Builders' Talk' started by John Williamson, Jan 9, 2022.

  1. Could someone please sense check this insulation for a solid wall for a garage conversion. I have asked building control but they said they weren't interested.


    Things I'm not sure about.

    • I have a very small air gap but I think this is countered by the breather membrane, does this make sense?
    • Using uncoated PIR (thinking this General-Purpose-Polystyrene to save quite a few pounds) between the battens because the outer layer of PIR will be coated and act as the vapour barrier.

    Apologies have asked this question before but slight change here in the makeup so want to make sure I get it right.

    Thanks in advance
  2. Jimbo

    Jimbo Screwfix Select

    Polystyrene is half as effective as PIR so cannot be substituted without affecting the u-value.
    John Williamson likes this.
  3. So glad I asked. Thanks a lot. Will not skimp on the ££s in that case
  4. ElecCEng

    ElecCEng Screwfix Select

    Why bother with an air gap if it’s so small and unventilated? You have a design with 6 separate materials arranged in a complicated way. I would reduce the complexity by:

    - Omitting the breather membrane, it’s doing nothing butted up against the blockwork.

    - Full depth PIR between the studs, seal with alu tape, this will be your vapour barrier.

    - Insulated plasterboard on top of the PIR. Tapered edge is easy to tape and fill and you can save on a plasterer.

    The above will cost less as it will take far less time to install and requires fewer trades. You’ll end up with the same or better U value.
    John Williamson likes this.
  5. Thanks for the response. So first thing is all the trades will be me so it's kind of same same in that sense. There's a reason for not using insulated plasterboard. I need to move a boiler to this garage way before I will have time to convert the room so need to just build up a smaller section of finished wall months before the rest of the wall will be done. By using regular foil backed PIR I can simple butt the next board up to this one and seal the gap with foil tape. If I use insulated PIR the joint will not have as solid a joint. Also it's more expensive than plasterboard on it's own with PIR.

    In terms of the breathable membrane, it's there because Kingspan say it should be there (or OSB board)

    The thin air gap will give me half the depth for services I need and then I only need to do minimal chasing.

    Not saying anything you've said is wrong, just explaining my logic
  6. ElecCEng

    ElecCEng Screwfix Select

    It's just advice, no criticism on your part :)
    I suspected DIY hence offering a simpler solution.

    The Kingspan recommendation has the membrane on the inner skin of timber framework before an air gap. The idea is that moisture condenses on either the membrane or inside of the blockwork then gets vented out. With no gap between the membrane and blocks it is unlikely to be in the correct place unless you do further calcs to work out the dew point.

    For the boiler you’re presumably installing 1 board width (1200mm) or two stud gaps (600mm ctrs). In that case it doesn’t matter what you put behind the boards, as even if you change your mind you’re starting with new section after the finished stud.

    Insulated plasterboard tends to come in thicknesses equivalent to multiples of boards , so you could double up behind the boiler. It’s fairly common practice to have a double layer on external walls and will increase the fire resistance.

    It may come down to a cost balance. Compare the cost and u values of:

    - Your option
    - Block->PIR->insulated PB
    - Block-mineral wool->insulated PB
    - Block->mineral wool->foiled PB

    You should be able to make an informed choice based on that.

    Hollowing out PIR or compressing mineral wool slightly is not arduous and wont affect your u values a huge amount, so I wouldn’t cling on to your air gap unnecessarily.

    Cables will need to be derated if in insulation but in the right position it shouldn’t affect them too much. One thing that shouldn’t be done is have PVC cables in contact with polystyrene. It leeches out some of the plasticiser over time and makes them brittle, which can cause faults and fires. Various ways around it.

    What you often see in videos, NHBC guidance etc is standard designs which in general meet the building regs in most of the UK, meaning building control and developers don’t need to do complex designs or calcs. Although it’s often stated that you must have this or that, the regulations probably don’t actually define the requirement.
    John Williamson likes this.
  7. Honestly can't thank you enough for taking the time to write all of this. Always shocks me the generosity of people with their knowledge on here. This was all a massive help. Particularly the bit where "you’re starting with new section after the finished stud". So as this is a small section it will give me time to try it out and enable me to price up the full garage. I was kind of worried that I make that big decision now without being informed (or knowing what a pain the first bit was

    Definitely leaning towards Block->PIR->insulated PB as cost not nearly as much of a factor as a warm room.
    ElecCEng likes this.
  8. ElecCEng

    ElecCEng Screwfix Select

    No problem. Do post pics of your progress if you have time. So much experience amongst these forums, worth asking if you have even the smallest query!


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