How to insulate a garage conversion: Floor insulation

13 January 2016 Kingspan Insulation UK

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 0.22 W/m²·K
  • Wales 0.18 W/m²·K
  • Scotland 0.15/0.18* 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. 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 t 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 5 mm over a 3 m straight line.

  • Concrete Floor
    • This is where the insulation, such as Kingspan Kooltherm K103, 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 K103 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. 

Other articles in this series