How to insulate a garage conversion: Part 4 – Floor Insulation

hammer and wooden floor

In the final post in this series on insulating a garage conversion we are looking at the best methods of insulating the floor, to ensure that the cold concrete floor of a garage becomes the basis of a warm welcoming room.


Like the roof and walls that we have previously looked at, there are U-values set by Building Standards/Regulations which need to be achieved when refurbishing a garage. These are shown below.

England Wales Scotland
Floor 0.22 0.18 0.15/0.18

U-Values for Extension of Existing Buildings in W/m2.K
*The first number applies where the existing building has U-values worse than 0.70 W/m.K in the walls and worse than 0.25 W/m.K in the ceiling.If the U-values are better than those figures the second number applies.

P/A Ratio

The first consideration when working out the insulation required for a floor is to calculate the Perimeter/Area (P/A) Ratio. This is the perimeter of the exposed walls divided by the floor area. (If you want to read a bit more about the P/A ratio and how to calculate it in detail click here). You need to know this in order to work out what thickness of insulation you will need in order to achieve the right U-Value to meet Building Regulations/Standards. The actual thickness of the insulation will depend on the type of floor (concrete screed / suspended timber etc.) and the type of insulation selected.

Floor Level

The other consideration for floor insulation is the level of the floor in relation to the rest of the house. If there is a step up from the garage into the main part of the house (assuming that the garage isn’t detached) then the insulation and new floor build up can be added to make the new rooms at the same height to the rest of the house. However if the current floor of the garage is level with the rest of the house then you may need to dig out the floor in the garage in order to add the required damp proof course, insulation and floor build up.

Space Saving Vacuum Insulation

If space is an issue in a conversion then Vacuum Insulated Panels (VIPs) such as Kingspan’s OPTIM-R range, can be used to provide a high level of insulation in a much thinner space. This is much better in tight spaces as it provides optimum performance insulation without the thickness.

Other solutions

If space for insulation is not so much of an issue then there are several solutions, depending on the construction of the floor and the difference between the floor height of the garage and required floor height of the new room. It is likely that a damp proof layer will need to be added to the floor before insulating.

The main floor constructions are detailed below. These all assume that the existing floor of the garage is a solid concrete slab and any irregularities in the floor do not exceed 5mm over a 3m straight line.

  • Concrete Floor
    • This is where the insulation, such as Kingspan Kooltherm K3, is laid on top of the existing concrete slab and final layer of screed added above this. The insulation should be loose-laid with joints lightly butted. A layer of polythene should be over laid on the insulation boards to stop the wet screed from penetrating between the joints of the insulation.
    • When insulating the floor a strip of insulation board should be laid vertically around the perimeter of the floor to prevent cold bridging. (See our blog post on insulation upstands for more information about why this is necessary.)
  • Insulation between timber joists
    • This solution is where insulation, such as Kingspan Kooltherm K3 is added between timber joists. This solution works well when the floor needs to be built up to match the existing house level. The insulation is cut to fit between the joists, and supported on softwood battens so that it is flush with the top of the joists. Floorboards are then fixed to the joists above the insulation.
  • Floating floor
    • This is where insulation, such as Kingspan Thermafloor TF70 is loose laid on top of a concrete slab and timber floor boards laid on top. The surface of the slab should be smooth, flat and free from projections. A thin layer of sand mortar or levelling compound can be used to achieve a level surface and to prevent the boards from slipping. The insulation should be overlaid with a polythene sheet to act as a slip layer and vapour control barrier.

Whichever way you insulate the floor it is important to make sure that the floor level will be the same as the rest of the house unless you want a step up or down into the new room created by the garage.

This is the end of our series of blog posts about how to insulate a garage conversion. We hope you have found it useful, but if you still have questions then our quick guides and product literature go into more detail. If you’ve missed the other posts on this topic they can be found below:

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

0 comments on “How to insulate a garage conversion: Part 4 – Floor Insulation
  1. Would I have to allow any difference in the insulation product used between a house that is typically very warm versus customers that tend to prefer a colder atmosphere? Just conscious because my father in law lives like an eskimo and the temperature difference is at least 10 degrees. I’m thinking this would make a difference to the moisture in the air?

    • Hi Andy,

      Good question. In short, it does not matter which insulation product is used. Instead the key factors relate to a couple of things: First is to remove excess moisture ( a ‘ventilation strategy’) from your building (extractor hoods over cookers, extractor vents in bathrooms etc), as excess moisture hitting a cold surface will likely mean condensation. Secondly, it’s about your heating preferences as any moisture would be quicker to condense in a colder building as the surfaces it comes into contact with will by definition be colder.

      The best method would be to keep relative humidity at around 40 to 50 per cent and rooms to be around 21 Centigrade.

      Hope this helps.


  2. I was going to lay a garage floor as follows 225mm type2,100 mm concrete,100 mm celotex,100 mm concrete screed but am concerned that it may not be strong enough, I would be grateful for your comments.

  3. Hi, I’m looking to convert part of my integral garage into a utility room for the wife, there is only 90mm difference between the kitchen floor and the concrete floor of the garage. Which type of flooring would you recommend for that height? I was thinking probably the floating floor. Any ideas would be appreciated, cheers

    • Hi Andy,
      The thickness of insulation would depend on your P/A ratio (search our blog for a definition of this). A timber floating floor would be ideal. The U-value for a ‘retained element’, in other words your old concrete garage floor, is 0.25. I’ve attached a link on our UValue Calculator to the sort of thickness you will need – approximately 70mm. This would allow for 20mm chipboard flooring and would be within the U-value required.
      Hope that helps.

  4. Hi, I am going to use your 70mm foil/foil insulation boards under a floating chipboard floor to upgrade the concrete floor in a garage conversion but I have seen conflicting information about whether I need to leave an expansion gap around the edges. Is it necessary to leave a gap in the insulation or just the floorboards, or neither?
    Also do I need a separate vapour control layer or does taping the joints with aluminium tape suffice?

    Many thanks

    • Hi Richard,
      You do not need to leave a gap between the insulation and the wall, nor do you you need to tape the boards. However the boards should be tightly butted. You DO need to leave an expansion gap between the edge of the chipboard flooring and the perimeter walls – 2mm for every metre of run or a minimum of 10mm overall – whichever is greater. You will need to add a vapour control layer over the insulation (under the chipboard flooring). Also make sure your concrete slab has a Damp Proof Membrane – it should do, but if there isn’t lay one on top of the concrete and below the insulation.
      More installation instructions can be seen on our website. Just click on the product brochure:

      Let us know how you get on.

  5. hi
    I am converting my garage to a kitchen. I need to raise the floor by about 100mm. what thickness insulation would be best? was thinking of putting a screed over it to bring it up to 100mm??

    • Brian,

      Thanks for the question. We’ll need to know quite a bit more information:

      1. The floor area and the Exposed Perimeter to work out the P/A ratio.
      2. Type of screed being used as some can be thinner than others meaning more space for insulation.
      3. Location of the project to understand the U-value to be achieved

      Once you have that information, then you can either contact us again or use the online U-Value Calculator.


  6. Hi,

    I have a garage that is fully breeze block on one side (left), half breeze block (1m up from floor) and then wood cladded on the other two sides (right and back). The front of the garage has a normal up and over door. What is the best insulation I could use on the 3 different walls? I am loathe to breeze block the other sides unless i really need to.



  7. My client is converting an integral double garage to open plan kitchen/dining/family room.

    The floor level is 240mm below house floor level.

    The ceiling height is 2240mm…how can we achieve the u-values for the floor in the garage conversion with minimum impact on floor to ceiling height?

    The exposed internal perimeter linear measurement is 15.60m
    Area is 42.00 sq.m

    Look forward to your comments/advice.

    Many thanks


  8. My garage is two storey.
    By builder has built internal walls, a new staircase and new doors and frames.
    Now we’ve realised we need to insulate the floor.
    Does he need to take everything apart and start again of is there a way round it?
    Will the reulations inspector ‘fail’ the project if we do not put in insulation.
    Please help.

    • Hi Giyan

      Unfortunately we can’t tell you whether the inspector will or will not fail the project if insulation is not put in the floor. We can however tell you how you might be able to insulate it.

      If your floor is made of concrete with a damp proof membrane below, you could install Kingspan Kooltherm K103 Floorboard above (if it now a living space) or Styrozone H350R (if it is still used as a garage) with a polythene separation layer and circa 75mm sand cement screed above.

      Hope this helps.

  9. I am converting a internal double garage into bedrooms. The floor area is 36m2 and the external wall length is 18m. I want to lay insulation then timber flooring on top. I have limited ceiling hight so what thickness insulation would I need to achieve the Building Regs UV please?

  10. I have an 8ft x 18.5ft integral garage that we are converting to a dining g room. The current garage floor is approximately 130mm lower than the main house. Can you recommend which of you products would be best under a screed?

  11. Hi,

    I would like to install a floating floor above insulation on a concrete floor, but not sure how I secure the floor boards together. Are they screwed to each other? I would like to put underfloor heating then laminate floor on top so need the floor boards to be secure so any movement would not damage the heating wires. any advice would be welcome.


    • Hi Basil,

      Please be advised that Thermafloor TF70, a Kingspan floor insulation suitable for Floating Timber Floor applications, should be loose-laid break-bonded, with joints lightly butted. Please refer to page 10 in the TF70 product literature for further guidance.

      Hope that helps.


  12. Hello, we are converting our garage and are constrained a low room height, so need a slim floor insulation solution. We will add electrical underfloorheating and then tiles. Currently we have uninsulated concrete floor that needs to be evened out in some areas. The building instructor said to use 60-80mm insulation and then about 75mm of screed, but that’s impossible height-wise. We would like to put the insulation on top of the screed: 50-60mm tile backer board, then the cable mats, and then adhesive and tiles. Is that a good solution? Is there any literature available to back that in discussions with the building inspector? Thansk

    • Hi Gina,

      We recommend that our insulation products are either used beneath screed or beneath the concrete slab, this ensures that there is a solid structural substrate above the insulation to spread foot traffic loadings as well as furniture loadings etc. Our thinnest insulation product would be OPTIM-R. The thickness would depend on the U-value that you need to achieve and the exposed perimeter and area of the floor, it would be beneficial for you to contact our technical services department to obtain a U-Value calculation, please call 01544387382 or e-mail (e-mails must include exposed perimeter and area of the floor, full build-up of the floor and full project address, as well as the target U-value, if you do not already know the target U-value building control should be able to assist).

      Hope that helps,

  13. We would like to insulate a garage floor (used as a workshop) for warmth and for standing comfort. It is currently made up of a concrete slab with ceramic tiles over. Ideally we are looking to install a rubber flororing over any insulation. What would you recommend please?

    • Hi Andrea

      The rubber flooring could not be installed directly to the insulation. Instead the insulation will need to overlaid with a substrate such as a screed or chipboard finish. The flooring could then be applied to the substrate.

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