Why use foil-faced insulation?

aluminium foil

Many of our products have foil facings on them and we were recently asked why and what the benefits were.

Methods of Heat Transfer

In order to explain, firstly we need to look at how heat is transferred. There are three main methods:

  • Convection
  • Conduction
  • Radiation

Heat loss from convection occurs through the movement of air and is limited by airtightness, suitable design and effective workmanship. Insulation works to prevent heat loss through conduction and materials chosen to make insulation have a low λ value or thermal conductivity. This means that they are good at preventing heat loss from conduction.

It is the last method of heat transfer – radiation – that is addressed by adding foil facings to the insulation.


Radiation is the process by which heat is emitted from a body and transmitted across space as energy. Radiation does not require any intermediate medium such as air for its transfer; it can readily take place across a vacuum. The rate of radiation emission is governed by:

  • the temperature difference between radiating and receiving surfaces;
  • the distance between the surfaces; and
  • the emissivity of the surfaces.

Emissivity is, in layman’s terms, how ‘shiny’ a surface is. In more technical terms it is the relative ability of a surface to reflect radiation, defined in relation to a theoretically perfect black body.

A theoretical black body would have an emissivity of 1 and would not reflect any radiation, and a perfect reflector would have an emissivity of 0 and would reflect all radiant energy. The reflectivity of a material is directly inverse to the emissivity. The lower the emissivity, the better the material is at limiting radiative heat transfer as it reflects it. For example for a material with a emissivity of 0.06, 94% of radiated heat would be reflected by it.

Foil is a low emissivity material.

The effect that different foil breather membranes can have be seen in the Kingspan Kooltherm K12 Framing Board brochure.

For a clear unventilated cavity, adding a low emissivity foil facing to at least one side of the cavity will improve, maybe even triple the thermal resistance. This means that adding a foil facing to insulation can significantly improve the U-value of the construction. This enables a thinner insulation to be used to achieve the same U-value.


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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 “Why use foil-faced insulation?
  1. Hi what are the pros and cons of this build up non foiled (2nds) insulation on top a dpm, then lenghs of 2×2 with a service void this 2×2 will then have a engineered floor secret nailed to it, this would be a floating floor

    Also what would be the best way of insulating a outside wall, I am having a air gap and service void built then again using rigid insulation boards in the infil or other alternatives could be considered

    Also what would be the best way to insulate a ceiling I have been given a large quantity of none foil rigid boards



    • Thanks for your questions Dominic.

      In a floor we would not recommend lengths of 2×2 as a service void as this would mean there could be significant point loading which may compress the insulation.
      For your second question, we’re not sure if this is a timber frame wall or a cavity wall you are referring to. If it is a timber frame wall, then there is advice in the Kingspan Kooltherm K12 Framing Board brochure or for a cavity board, then look at the Kingspan Kooltherm K8 Cavity Board brochure.

      Finally, for a ceiling we would recommend foil boards. There are different ways of achieving this. Again , the specific product brochure for Kingspan Kooltherm K7 Pitched Roof Board has a whole section on ceiling level insulation installation and advice.

      Hope this helps.

  2. URGENT PLEASE – Builder friend has offered me surplus Kingspan (without foil) for my extension under construction. It is for bedroom above garage – bedroom formed in the roof trusses. I have used foil backed Kingspan for joists in garage ceiling/bedroom floor and wondering if the non-foil 120mm would be suitable for the cum walls of the bedroom – there will be a gap of approx 2feet behind the kingspan. Apologies if I am not describing this well. Thanks I Trousdale 07920004734

  3. Hi, my partner is insulating his mobile home on the cheap and was going to wood panel the interior ceiling but has been thinking of just erecting some kingspan foil insulation and then maybe just covering this with a decorative polystyrene tile, I am wondering could he paint the foil backed foam instead of covering it and what would it be like to paint or maybe wallpaper??

    p.s we really are having to think outside the box due to financial constraints.

    Thanks in advance

    • Hi Sheila,

      Thanks for your question. First for the bad news: We wouldn’t recommend painting the foil insulation mainly because we feel you wouldn’t get a good finish – imagine painting a sheet of tin foil. Also, we wouldn’t recommend polystyrene tiles as they are fire risk, so perhaps best avoid.
      Now for the good news: We would recommend your partner to timber frame the ceiling. Adding insulation between the frame and then boarding over the top. This will not only give you a firm and secure substrate, but also be a surface that you can paper onto or simply paint if you wish.

      Hope this helps.

  4. hello there, after reading the comments would it be possible to achieve the same u value simply using a roll of foil over the non foiled foam. i have some non foil backed boards and wondered if i could put them in the extention floor with foil rolled over then concrete on top
    thanks in advance

    • Jason, thanks for your question.
      Foil only really has a benefit when there is an air gap; for example in a cavity wall or in a pitched roof. When you are adding concrete or screed on top of insulation (with a separation layer of course – see our K3 and TF70 brochures), then there is no gap and therefore no benefit. So, no need to add foil over the top.

  5. Hello,
    From the comments below, would it be fair to say that in insulating an attic at either floor and/or rafter levels, the following works:

    . Floor insulation with Loft Boards
    Insulation rolls in between the joist, Non foil board, Loft Board

    . Under Rafter
    Foil board in between the Rafters

    Non-foil is possible when there’s no air gap on either side of the board
    Foil when there’s a gap

    Thank you

    • Morning Steven

      Our leading technical advisor, Naomi, has taken a look at this for you. Her reply is detailed below:

      A foil faced product facing into an unventilated cavity does provide a lower emissivity value, which can add to the overall thermal performance of a construction. However, combining copious amounts of mineral wool at the horizontal ceiling level and insulating with rigid boards at the pitch can create problems with interstitial condensation. This can be made worse by having an unbreathable material above the rafters with no ventilation. As there are multiple factors to consider, it would be best to have a condensation risk analysis done by us to provide the best insulation option – contact our Technical Services Department for more information. You might also want to take a look at the Kooltherm K7 Pitched Roof Board product literature for more information – this can be downloaded from our website.

      Hope this helps.

  6. I am considering the installation of an under-floor water heating system. generally the systems provided by the under- floor water heating engineers seems reasonable, but the cost of the insulation required seems very expensive.
    I am therefore wondering if you can provide and have had experience in providing such insulating goods for an under-floor water heating system. The suspended floor area to be covered is 12m x 6m


  7. Hello I am insulating a pitched roof with 4 tunnels and have aquired kingspan non foil faced 140mm boards. Will these be ok to use or should I not. Please help.

  8. I’m replacing attic insulation destroyed by raccoons. Ceiling is wood nailed to 2 x 6 joists at 27″ o.c., on top of which are transverse 1 x 6s. Insulation runs between the joists underneath the transverse boards (forget what these board are called). 3/8″ plywood is on top of all, providing a floor for the attic.

    The insulation I am replacing is foil-faced, foil facing down. Will I be well-served by replacing with similar foil-faced insulation?

  9. I am currently renovating my property. My roof is a pitched slate tile roof. The roof has no felt or insulation. I cannot afford to have the tiles taken off and a new membrane put in. I am considering using your k107 pitched roof board in between the rafters and then a foil backed plasterboard to finish. Do you think this would work ? Would you put a breathable membrane between the plasterboard and the insulation ? Would you do something completely different ?

    • Hello Amanda

      If there is currently no membrane on the roof to provide weather protection, we do recommend leaving a 50mm ventilated cavity between the insulation and the top of the rafter. This allows for a ‘drying cavity’ for any external moisture that does come in through the tiles. Rather than using a breather membrane between the insulation and the plasterboard, we would recommend using a vapour control barrier instead. This helps to control the passage of water vapour from the warm side of the construction.

      Hope this helps you.

  10. Hi there. I have a quick question. I am building a large garden room. The construction method used is SIP panels. They will have an outer breather membrane, with battens and then cladding.

    The cladding itself needs to match the local “vernacular”, which in this case means square cut boards overlapping each other vertically, as opposed to more straightforward horizontal type cladding. Because the membrane needs air to circulate underneath it, I’ve been advised I will need two layers of battens: the first layer horizontally, the second layer vertically so that I can then nail the cladding on. None of this is a big problem.

    What I was going to ask is: although the SIP panels already offer a high level of insulation, is there any point in adding to it by fixing a further layer of 25mm foil-backed Kingspan Insulation panels between the first and second set of battens? Hope I’m making sense. Many thanks, Nic

    • Hi Nic

      If additional insulation is required in a thin layer (this will be determined by the U-Value that you need to achieve, and should be discussed with your local building authority) we would recommend that it is added internally and not externally in order to avoid a risk of interstitial condensation. However do note that additional insulation will only be required if the SIP panel itself does not achieve your target U-value alone.

      Hope this helps.

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