How to calculate the amount of insulation needed on a ground floor

K3 Floor Insulation

Unlike roofs, walls and intermediate floors, U–value calculations for ground floors cannot be calculated with reference to the construction detail alone. Heat loss from ground floors depends upon a number of factors including the ratio of the exposed floor perimeter to the total floor area.

It also depends on the size of the floor area that you are covering. For buildings with relatively small ground floor areas (primarily domestic properties), if the ground floor is left uninsulated, the thermal performance will be poor. Complete insulation of the ground floor will improve the thermal performance.  However for buildings with large ground floor areas (primarily non–domestic buildings), complete insulation of the ground floor may be unnecessary. Instead either insulating the perimeter strip or insulating the floor in vertical or horizontal strips may provide adequate thermal performance.  If this is the case then we suggest you contact our technical department for more information about the required thickness of insulation.

To work out the amount of insulation required for a floor, first you need to calculate the Perimeter Area (PA) ratio. This is calculated by dividing the exposed perimeter given by the floor area.

Floor dimensions should be measured between the finished internal surfaces of the external walls. This should include non-usable heated spaces, such as ducts and stairwells, but not unheated areas outside of the insulated fabric, such as garages or porches. The exposed perimeter is the length of the walls that connect to unheated spaces, such as garages or the outside world. If you are calculating this for an extension to an existing building, the floor dimensions should be taken as those of the extension.  The smaller the P/A figure the less insulation is required; for example, a large area with a small exposed perimeter will have less heat loss and, therefore, will require less insulation. For a PA ratio less than 0.1 we would suggest that the floor does not require insulating fully and our technical department will be able to provide more guidance with this.

Type of floor
The next consideration is the type of floor construction, which is likely to be one of several options.

  • Suspended Timber
  • Beam and Block
  • Solid Concrete (insulation below floor slab)
  • Solid Concrete (insulation below floor screed)
  • Suspended Concrete Floor

Each of these different types of flooring has a different construction build up and gives a different U-value for a specified thickness of insulation. The building standards/regulations page on our website gives guidance on what this u-value should be.

The easiest way to work out the required thickness of insulation required to achieve the specifed U-value is by looking at the product information in the Kingspan Kooltherm K3 Floorboard brochure. This has tables of U-values for different thicknesses of board as well as a brief guide to fitting the insulation. When you have settled on the thickness of board required (and if you are still not sure you can contact our technical department for more guidance), then you can get the boards from a stockist.

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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 calculate the amount of insulation needed on a ground floor
  1. Question on calculating the perimeter to area ratio for floor insulation :-

    I notice that a top ratio of 1.0 is used then going down in 0.1 increments.

    I made up a quick table to get an idea of the relevance to insulation thickness.
    To start I used four exposed walls so a 1×1 area will give a ratio of 4!
    Not until you get to a 4×4 area do you get a ration of 1.0!

    Now being a ratio there is no system of measurement, i.e feet or meters so EVERY square floor could be 1.0!!!

    Can you help on this as I cannot find anything relevant in all the Building Regulation of Approved Documents?

    • Thanks for your question Steve.

      We deliberately finished at a P/A ratio of 1 because the vast majority of calculations fall within the 0.1 to 1 range. This is because most building footprints aren’t square and many have one side that is unexposed (eg a semi-detached house). We did look at this issue when we created our free online uvalue calculator to find out where we should stop. So, we looked back at the last few years and the number of calculations we had performed for floors and nearly all were within the 0.1 to 1 bracket.

      Not all is lost though. We do carry out calculations for P/A ratios over 1 for free – simply call our technical line on 01544 387 382.

      Thanks for a great question.


  2. How much kingspan insalation would I need for a 140 square metre block and beam floor ?

    How much would this cost roughly ?

    • Hi Paul,

      Thanks for your question. You also will need to calculate the exposed perimeter of your 140 square metre floor in order to determine how much insulation is needed. The exposed perimeter is the length of the walls that connect to unheated spaces, such as garages or the outside world.

      Once you have this information, you will find our online U-Value Calculator to be of use.

      We advise that you contact your nearest stockist for information on the cost.

  3. Are all concrete floors in new build domestic houses fitted with Kingspan insulation or similar by law..Building Regs as my floor is giving off high moisture readings and is very cold. Comment appreciated. Thanks.

  4. I have several ground floor self contained studio flats, which need updating to comply with the April 2018 minimum EPC ratings. There are several alternative measures but one being to insulate over the existing wooden floors. Your guidance will be welcome. Thanks.

    • Hi Peter

      Given the number of different options available here, your best option would be to call our technical services department on 01544 387 382. A member of the team will be happy to discuss your requirements.

    • Hi Mumin

      Your best bet is to call our technical services department on 01544 387 382. They will be able to provide a condensation risk analysis if needed, and give you any advice on the topic.

      Hope this helps.

    • Hi Charles,

      If your P/A ratio is not a round number then we would advise that you contact our technical department on 01544 387 382 so that they can provide you with a calculation to your exact P/A ratio. However, if you are using the U-value Calculator, we would advise that you round up as this way you will overcompensate rather than under-compensate.

      Hope this helps.

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