How to insulate under a suspended timber floor – a practical guide

Kingspan Insulation K3 Floorboard in Suspended Timber Floor

In this blog post we aim to provide a simple, practical guide to the best practice methods for fitting insulation boards, such as Kingspan Kooltherm K3 Floorboard, into a suspended timber floor.

This is a floor where the timber floorboards are attached to floor joists and the insulation suspended on timber battens between the joists. This type of floor is suitable for both new build and refurbishment projects.

U-Value/Thickness of insulation

We are assuming that you have already worked out the right thickness of insulation required to give you the U-value that you want (and that matches that required by building regulations/standards).

If you are unsure of the thickness of insulation board required then have a look at our blog post on calculating the amount of ground floor insulation required and then the Kingspan Kooltherm K3 Floorboard brochure gives the U-values for a range of thicknesses.

There are two main methods, depending on if you have access above the floor joists, or below the floor joists.

Installation from above the floor joists

  • The insulation boards should be installed before you start boarding the floor.
  • Cut the insulation boards so that they fit snugly between the floor joists. Measure the distance between the joists prior to cutting the boards as the spacings can vary.
  • To make sure the insulation boards are flush with the top surface of the joists, they should be supported on minimum 25 mm x 25 mm treated softwood timber battens, proprietary galvanised steel saddle clips, or galvanised nails partially driven into the side of the joists.
  • If you are using battens or nails, they should be placed at an appropriate height to suit the thickness of board being fitted, and nails should remain 40 mm proud of the joist.
  • The insulation boards should then be fitted between the joists so that they are supported by the battens / saddle clips or nails.

Installation from below the floor joists

  • The floor boards should be fixed over joists before you fit the insulation boards from below.
  • The insulation boards should be cut to fit snugly between the floor joists. Measure the distance between the joists prior to cutting the boards as the spacings can vary.
  • Push the cut insulation boards between the joists so they are flush with the underside of the floor boards.
  • Side–nail 25 mm x 25 mm timber battens to the joists or partially drive galvanised nails into the side of the joists in the appropriate position to hold the boards in place.

All methods of installation

  • If you are fitting two layers of insulation make sure that they are horizontally offset, relative to each other, so that (as far as possible) the board joints in the two layers are not in the same place.
  • All board joints should be tightly butted.
  • Make sure that insulation boards are fitted tightly between joists and any gaps are filled with expanding urethane sealant.
  • Any narrow gaps between a joist and perimeter wall should be insulated by specially cut pieces of board which in turn should be supported on blocks nailed to the underside of the joists. Gaps less than 25 mm wide should be filled with expanding urethane sealant. There should be no gaps greater than 50mm.

If you have any questions about this more details can be found in the Kingspan Kooltherm K3 Floorboard brochure or have a look at our structural quick guide.

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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 under a suspended timber floor – a practical guide
  1. I specialise in damp having problems with concrete & ashfelt floors having to dig out thinking of using King Span for under supporting wood floor can you forward any information on the product you would recommend. Regards Chris

  2. I live in a house built from stone (no cavities and no insulation). Over the last couple of years I have added a combination of rigid boards between rafters and wool between ceiling joists (depending on the area) as I carry out repairs and finances permit.

    The two downstairs living areas have suspended wooden floors (hall and kitchen have solid floors) and I now intend to lift the floorboards and place rigid boards between the joists. During a previous plumbing repair, I noted that there is a concrete base upon which brick walls have been built to support the floor joists.

    As the exterior walls have been built with stone (there are air vents in place) do I also need to place some form of insulation against the inside of the exterior wall; and what type of insulation board would you recommend. Regards Dermot

  3. I will be accessing my floor from the underside and only have a small gap in places.

    The joists are only 100mm deep with various services between em and on top of that the builders must have been drunk as very few are parallel and even fewer are equally spaced.

    Can I just use a 100mm mineral wool infil and screw a 50mm board to the underside? Or do the joists have to breathe?

    If I can, what board would you recommend, and can I just use screws and penny washers?

    • Hi Antony

      Are you able to let us know a little more information? We need to know whether this is a ground or semi-exposed floor, and if the joists are internal or exposed to the elements. Whatever the case, the insulation to the cold side of the floor will need to be the thickest layer to avoid any risk of condensation. Perhaps you could call our technical department on 01544 387 382 to further discuss?

  4. I order to increase the U-Value where there is limited depth of joist would it be possible to put 100mm Kooltherm K103 between the joists then overlay this with 20mm Thermafloor TF70 and sheet everything in 18mm ply?

    • Hi Shaun

      Depending on the P/A ratio of the floor in question, you may or not need an additional layer of insulation over the top of the joists. This also depends on the joist centres and the U-value target. It may be worth giving our technical department a call on 01544 387 382 to discuss getting a U-value calculation done for this specific floor. With regards to overlaying the joists, the TF70 would need to be applied to a solid substrate such as a chipboard or plywood, rather than just being laid directly onto the joists, this will avoid point loading.

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