Installing and insulating a subfloor on garage concrete floor

Discussion in 'Builders' Talk' started by urf, Sep 21, 2016.

  1. urf

    urf New Member

    Hello

    First timer here. I am not a builder, just someone trying to do as much as I can diy-wise.

    I am in the middle of converting my detached, double-skin (no cavity) garage into an office/cave. It currently has a concrete floor (5.5m x 3.3m) and I'm not sure of the best way to build my subfloor. I've searched around a bit and cannot find any information on how best to tackle this. One friend, who is more experienced at diy than me, suggested the following layers:

    - DPM on concrete floor
    - Treated batons
    - Treated 3x2 joists at right angles to above, with Celotex/kingspan in between
    - Chipboard or OSB
    - Final finish, possible some kind of vinyl or linoleum, ie something that can take a bit of bashing but is easy to clean. Other budget-friendly suggestions welcome!

    Does the above seem ok? Would there be adequate ventilation, if any? Or is this not an issue?

    Are there any alternatives to the above that won't break the bank?

    Thanks for any help or advice.
    Urf
     
  2. BMC2000

    BMC2000 Screwfix Select

    I'd go DPM, foil faced insulation, 70mm screed. Floor finished with underlay & carpet.
     
  3. Urf, hardly any heat is lost through the floor, and you certainly won't have any draught issues...

    When I converted my own attached garage, I simply laid a DPM (taking it up the walls a good couple of feet) and then bedded some joists at 400 spacings in lines of mortar across the floor. My floor was sloping (typical old garage, to let any water run out) so my joists varied from 25mm to 150mm depths as best I recall. Bedding in a line of mortar allowed me to fine-tune the heights - tap them down using a long batten as a level guide - and it also didn't breach the DPM.

    (The lowest 'joist' height is only 25mm in my case (a single roofing batten!) as head-height was a bit of an issue as the garage roof and ceiling is sloping to the side, so I wanted to maximise this. I would normally have made the 'thinnest' joist a bit thicker than this, but in actual fact it has made no difference. The only issue I might have 'suffered' was warping or movement of such a thin batten since it was 'held' only by a bed or mortar, but I built a cupboard for the boiler over this and also a short 2-step staircase, so it's all weighed down nicely...

    I then cut sheets of 25mm Jablite - the cheapest possible insulation, expanded polystyrene - and filled in between the joists adding layers to fill the increasing heights.

    Once filled, I laid a softwood T&G floor on top because that's the finish I wanted.

    That's it - barely no effort, cost only some treated 'joists' (I may even have used 2x1 timber roofing battens, stacking and screwing them together to increase the height as necessary...), cheap Jablite, a sheet of DPM and a bag of cement.

    It is as warm as a warm thing.

    I could have put down the 'recommended' silly amount of Celotex, screeded, this and that - but it would not have any more perceptible or measurable warmth.

    Do go overkill on the insulation thickness, is what I'm saying. It won't make any difference... :)
     
  4. urf

    urf New Member

    Thanks bmc2000 and thanks Devil's Advocate for the comprehensive reply. I was given a load of celotex from a very generous friend who had lots left over from another project, so I'm planning to use that. Really like the line of mortar idea.

    Many thanks.
    Urf
     
  5. benben5555

    benben5555 Member

    Converting your garage needs to go through building regs and you'll also need to insulate the walls and loft. If you do tbe works without approval you'll not be able to sell your house in tbe future.

    Unless your friend has given you a celotex board that is 70mm it won't meet the right u value
     
  6. Joe95

    Joe95 Screwfix Select

    Remember when you install the damp proof membrane a good measure up the wall is to the skirting board, so take all your measurements and add them up, that will give you an idea of the height.
    Devils advocate has a great point about T&G flooring - its much easier to learn without having experience laying it!
     
  7. Mr Rusty

    Mr Rusty Screwfix Select

    Surely only if you want to formally make it a "habitable room" . If it's just a temporary subdivision of an outbuilding to be used as a hobby room/workshop you can do whatever you like, particularly if it is an internal timber structure. You can always take it out when you come to sell in the unlikely event it was an issue.

    I'd go with the DPM, battens, 2x3's with flooring over and insulation between. def better than a cold concrete floor.
     
    Last edited: Sep 29, 2016

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