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Insulating integral garage ceiling

Discussion in 'DIY Disasters' started by Paulie67, 4 May 2021.

  1. Paulie67

    Paulie67

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    Hi All

    I wondered if you could assist me please as I am in the middle of insulating the ceiling of an integral garage/utility room and making a bit of a hash of it!

    There is a bedroom above the garage which gets cold.

    I have put 100mm of PIR between the joists right up against the floor boards and then started to put a second layer of 100mm PIR to fill the joist cavities completely. On this second layer I have cut out channels for where electrical cables run through the joists. Is this okay?

    I hadn't realised at the time of installing the second layer there was a variance in the depth in the joists - between 180mm and 200mm.

    Rather irrationally over the weekend I have trimmed off some of the second layer PIR (whilst in situ) to make flush with the joists taking off the foil on the underside/garage side. Unsurprinsigly this created a lot of dust and loose fibres so I covered the exposed face with aluminium foil tape to stop the dust.

    I was planning to then buy some additional 25mm PIR to fix to the underside of the joists and tape up before plasterboarding. I know this is effectively insulating the cold garage which will remain as cold garage for the time being. However in the future I want to convert the garage to a habitable room so was trying to plan ahead. I appreciate this will need building control sign off if I were to do this but is the above likely to be okay or cause me any problems such as vapour control? Or should I avoid putting this 25mm PIR up?

    I appreciate in hindsight I shouldn't have trimmed the PIR like that (lesson learned) but as I had already bought and fitted the 100mm it would have been wasted otherwise. Is trimming the PIR like that going to cause any issues given that I plan to cover it with the 25mm PIR? I am hoping to avoid removing the trimmed second layer if possible as will be a waste.

    Many thanks in advance.
     
  2. ^woody^

    ^woody^

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    Cutting the insulation is not a problem, nor is lining the joists underside with more insulation. This does not require insulation as part of any garage conversion, so wont be inspected.

    Have you sealed all gaps around joists where the enter any walls? That is the biggest source of heat loss.

    Have you considered that sealing in any electrical cables can in some circumstances lead to over heating? Get advice in that.
     
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  4. SFK

    SFK

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    Ready for woody to shoot me down on this - but as yo have a bedroom abaove a grage you have regs that you need to follow now.
    The plasterboard on your garage ceiling needs to have a fire rating - cannot remember if 60mins or 90mins.

    Woody - am I right (or wrong) that a garage below bedroom needs one of the following: one sheet of red fireboard board skimmed to seal gaps, or two sheets of normal plasterboard, overlapped at joints and skimmed to fill gaps. Both of whcih provide required fire rating.

    SFK
     
  5. Paulie67

    Paulie67

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    Thanks so much chaps really appreciate your help.

    The first 100mm layer sits above any cables and it is only the second 100mm layer I’ve had to channel out for cables. I will get our electrician to check out everything before going further however his initial response was it should be fine as i channelled out enough space around the cables.

    I’ve only sealed the gaps where the joists meet the external wall with foam and foil tape. Presumably I should have used mastic on the two sides and the underside of the joists. In which case I’ll need to remove a bit of the pir and seal it properly and reinstall! I planned to use the pink plasterboard and use intumescent sealant to seal the perimeter to the walls.

    Thanks again guys!
     
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