What vapour control layer should you use in walls?

Vapour image

Vapour control layers are used to control the movement of warm, relatively moisture rich air from reaching a cold surface where it will condense.

There are four common options that can help control vapour reaching

300-500 gauge polythene fitted to the warm side (the inner side) of the insulation. Polythene is mostly used on timber or steel framed walls where foil-backed insulated plasterboard isn’t used (eg Kingspan Kooltherm K18 Insulated Plasterboard)

Foil-backed plasterboard
Is generally less effective, however it is quick and easy and is suitable for most buildings.

Foil-faced insulation
If you are using foil-faced insulation, then you can use foil tape on the joints where the insulation boards meet. Standard duct tape will not do the job!

Composite products
Composite products (such as Kingspan Kooltherm K17 & K18 Insulated Plasterboard) include integral moisture vapour control

As with any vapour control, workmanship is crucial. For example, taping and lapping joints for polythene, avoiding penetrations, and then sealing around any penetrations. You can get further advice for your project by calling technical services on 01544 387 382.

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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 “What vapour control layer should you use in walls?
  1. Hello,

    We are looking to use an insulated plasterboard on an existing wall and new roof, such as Kooltherm K18. I note there is a vapour barrier as part of the product, but can you tell me how you deal with vapour control at the joints?


    David Robins

  2. Hi, please could you recommend the best way to insulate internal bedroom wall.
    I have taken off all the plaster back to bare brick.
    Would it be better to use silver foiled 50mm kingspan, 1000gauge polythene, then 25 mm timber batons screwed through to brick but sealing screw hole.
    Then on top finishing off with plasterboard screw into batons.
    Or option 2 dot dabbing adhesive or timber batons using your Pir plasterboard 50 mm thick.
    Iam a carpenter to trade & used on my flat roof extension your 110mm TD deck kingspan which is amazing warmth.
    So will always be staying with kingspan products.
    Kind regards
    Martin Stuart

  3. Hi,

    I’m in the process of building a single storey Garden Room made out of timber frame with a flat roof.

    As I’m using Nilvent Breather membrane to wrap the walls in, can I not use this on the roof instead of a separate Vapour Control Layer? If not, what do I need for the roof?

    Also, using Kingspan Thermaroof TR27, does the EPDM roofing go directly on to this? If so, how do you dress the sides of the insulation that doesn’t have the glass tissue to stick to?


    • Hi James,

      Thanks for your questions.

      A breather membrane and a vapour control layer are two separate things, and therefore one cannot be replaced with the other. A breather membrane such as nilvent is applied to the outside of any insulation, acting as a weather barrier while also allowing vapour to escape from inside the building. A vapour control later on the other hand is applied to the inside of insulation in order to minimise the amount of warm moist air entering the building.

      Regarding the EPDM, yes this does go directly on top. An insulation upstand should be positioned at the sides of the insulation.

      Further details are available within the TR27 brochure, which can be downloaded on the product page of the website. This includes information on methods of installation, insulation upstands and vapour control layers.

      Good luck with your garden room!

  4. Hello,
    I would like to know if Kingspan K18 fixed to battens is suitable for use on the inside face of of a single skin brickwork garage conversion? If so do I need to install a breather membrane too?
    Thanks in advance

  5. Hi there,

    I’m in the middle of coverting my garage into an office and i have used insulation boards in between my rafters, my question is would i be best to tape the joints off and leave for my next stage of plasterboarding or should i cover the whole lot wth vapour barrier and seal it, going to be doing the same with the walls.
    Regards, Karl.

  6. Hi, Currently planning the insulation of new upstairs extention (completely new, not loft conversion). Construction is timber frame 90 studs, 50mm Cavity and 100mm block exterior with double render and dash (to blind the existing dash).

    Is there an alternative to the K18 insofar as can I get the insulation and VCL separate then take on the plasterboarding at a later date? Happy to go with K12 over studs but is there a suitable “stand alone” VCL?

    Many thanks

    • Hi Graeme,

      Thanks for your question. The build up you are suggesting – Kingspan K12 and a stand alone VCL – is fine, although please note we don’t provide VCLs. However we would still recommend use of Kingspan K18 due to the benefit of achieving all of the above in just one, simple fix – saving time and labour costs in the process.

      Hope this helps.

  7. I stay in a stone built farm cottage with the second floor being part of the roof void (believe its classed as a 1 1/2 storey property. The property is lathen plaster in construction. Upstairs I intend to remove the lathen plaster walls/roof and replace with a suitable insulation material and plasterboard. What products would you recommend for doing a good job on heat retention and ease of construction?

  8. Hi, I am about to insulate my external wall in my kitchen! Which is of brick construction. (Circa 300thick overall thickness)

    Can you tell me which would be the best insulation to use for this?…. I had thought about 50mm strapping to the walls with timber at 400/600ctrs and in-filling with 50mm insulation, then plasterboard screwed/nailed to the timber strapping?

    Would this suffice or can you point me in the right direction? (Ps keen DIY’er)

    Thanks in advance

  9. Hi. I’m about to install 100 mm. Kooltherm K12 between rafters in my pitched roof. I’m then thinking about adding another 25 mm. under rafters, to reduce thermal bridging, followed by a vapor barrier, then 45 mm. of mineral wool (to allow for electrical cables/installations/spots) between timber, finished with 2×15 mm. plasterboard.

    How can I fix the 25 mm. to the underside of the rafters?

    How can I then fit the vapor barrier onto the 25 mm. of Kooltherm K12? Usually I would staple the barrier onto the rafters, but that’s not possible with the 25 mm. inbetween. Can I staple the vapor barrier directly onto the 25 mm? Then fixing the timber to allow for the 45 mm. mineralwool?

    I’d appreciate your advice. Thanks in advance.

    • Thank you for your question Dennis.

      To comply with our BBA certification, it should be Kingspan Kooltherm K7 Pitched Roof Board installed between rafters, not K12. However, thermally, there will be no difference in performance. Please do note that K12 isn’t available in 25mm without a large order quantity, but K7 is. Alternatively, you could also use 37.5mm of Kingspan Kooltherm K118 Insulated Plasterboard.

      Any batten cavity you create, we suggest leaving this empty to run services through. If using K7 or K12, a vapour barrier could be fixed back with the same fixings that are holding the battens up, through to the rafters. If using K118, a separate vapour control layer wouldn’t be required as the product has an integral one from the unperforated foil behind the plasterboard.


  10. I’ve read the excellent questions and answers an my head is spinning …

    I’d like to build a shed to use as an office. The walls are (say) t&g/19mm. Does the vapour barrier go on first and then the foil covered slabs? Foil into the room or away?

    I apologise for the really dumb question …


  11. I’ve just stripped old plaster board off the ground floor walls of my living room. The walls are stone construction with no cavity. There is a stud frame set a couple inches away from the wall and there is currently a vapour barrier secured to the back of the frame. I want to insulate and replaster and have used your K18 board in other rooms upstairs, is this OK to use if I remove the old vapour barrier to allow rewiring? I also may need to insulate the floor once I dig down and investigation there is anything in place already, what product should I use?
    I also emailed this query to wingspan but didn’t get any response so hopefully this works.

  12. I need to upgrade the insulation in the external walls of my 1950s timber-clad timber frame house (basement has brick walls). Currently there’s only foil-backed plasterboard – and fresh air! Would welcome any advice on the most suitable form of Kingspan to use to avoid any damp or mould issues. Many thanks!

  13. Hello

    Thanks for all this useful info.

    I am planning to use kingspan k12 on the internal wall – wall will be framed and insulation board k12 will be installed between timber frame. on top of it will be k118 plasterboard.
    the advises below were to have all joints of the plasterboard sealed with sealing tape which is ok.

    but what about the joints of the K12 board? the joints between two pieces of k12 board – is that to be sealed with the tape as well?

    thanks in advance

    • Hi Mike,

      Sounds like you have it well planned! Kingspan Kooltherm K12 Framing Board does not need to be sealed between the joints. This is because the boards are between timber studs, so to create a seal you’d have to tape across the studs as well. Just ensure that you create a seal by taping and skimming the joints of Kingspan Kooltherm K118 Insulated Plasterboard.

      Hope this helps.

      • Thanks for reply. It might be well planned but execution is a different story.
        ok on your spec sheets for insulated plasterboards and framing boards, etc you mention that gyproc drywall sealer can be applied to provide VCL – if two coats are applied.

        Could you please advise if the drywall sealer can be applied on the ceiling?

        I have a dormer with low pitched roof (<20 degrees) – the roof joists are levelled and the rockwool insulation is installed between and over joists. and the ceiling is lined in normal plasterboard. the builder did not used plasterboard with VCL or did not installed any other polyethene sheet between plasterboard and insulation.

        So I didn't find conclusive information if I need VCL or not.

        However assuming I need VCL – and I don't want to remove installed ceiling plasterboard – can I use gyproc drywall sealer on the ceiling to provide VCL?

        would appreciate your help

        Thanks in advance


        • Hi Mike

          As this doesn’t concern our insulation we’re unable to advise. We suggest you speak to British Gypsum, who manufacture the sealant in question, to check whether their product will act as a vapour barrier in your construction.

  14. Hi
    Thanks for the information
    I have loft insulation 300mm rockwell between and over rafters
    Builder installed standard plasterboard on the ceiling
    Should there not be VCL between insulation and plasterboards?
    if required – can we put VCL paint on the plasterboard – would gyproc drywall sealer – 2 coats do the job?

    Thanks in advance


    • Hi Mike

      We only manufacture rigid foam insulation boards, so unfortunately are not the best people to ask about mineral wool. We can confirm that there should be a vapour barrier on the warm side of any insulation to avoid any condensation, but you should contact a mineral wool manufacturer for specific advice about your build-up.

      Sorry we can’t be of more help.

  15. Hi
    Trying to get a definitive answer.
    1930’s bedroom 1st floor Bay window.
    On Inside have Stripped back to timber studs with flush fitting
    Breeze block type construction.
    The external finish is render.

    Plan is insulate with two layers of 25mm Kingspan
    Then 12.5mm plasterboard
    Then skim over to finish.

    Do i apply 1:
    1000 gauge plastic to bare wall. Dpm
    Then batten ontop of studs with 25mm
    Roof batten
    3. Fix 2 layers of kingspan bent onto batten
    4. Fix plaster board through to. Stud
    Tape & joint or skim to finish

  16. Hi

    We are part way through a self build project, an oak framed house with SIP panels.

    The SIPS wall panels have a breather membrane to the outside, the spec for the internal side of these wall is timber battens and then VCL backed plasterboard or a vapour control membrane and then standard plasterboard, we are looking to use the second option – what product would you recommend for this vapour control membrane please?


    • Hello Jean

      We do not specify a brand or exact type of vapour control layer as there are many available on the market and we do not manufacture or supply one ourselves. However, when we run our U-value calculations we tend to use a 1000 gauge polythene vapour control layer, with a vapour resistance of 500 MNs/g, so a vapour barrier of this level of performance or similar would be acceptable.

      Hope this helps.

  17. The spec for my extension vaulted roof is concrete tiles on timber battens on breathable felt on 50 x 175 rafters. There is 100mm rigid insulation between the rafters finishing flush with bottom of rafters and 25mm of rigid insulation fixed to underside of rafters with 12mm plasterboard under that.
    I want to put in downlighters but suspect that I cannot cut into insulation.
    The low end of the vaulted ceiling is already very low to the top of a french door. Is there anything I can do to incorporate LED downlight fittings without creating a service void under the insulation?

  18. I have a 1920s brick house with cavity walls. The walls are plastered internally.
    I’d like to internally insulate the exterior walls.
    Can I use Kingspan mounted directly on the existing plaster or would I need batons?
    Would I need a vapour control layer?
    Which type of board would be best?
    I plan to skim over the boards.

    • Hi Patrick

      Kooltherm K118 Insulated Plasterboard can be used here.

      As the walls are cavity walls, timber battens would not be required and the insulated plasterboard can be mechanically fixed directly to the internal masonry. We suggest ensuring that the mechnaical fixings used are of a length that can penetrate the insulated plasterboard, existing plaster and then the internal masonry by at least 25 mm. A separate vapour barrier would not be required as this is integral to the insulated plasterboard.

      Hope this helps.

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