Air gaps, insulation and roofing membranes in pitched roofs.

Pitched roof, membranes and air gaps

We have a question about what air gap you should leave between the insulation and the roofing membrane in a pitched roof almost every day. Here’s some advice about what to do…

The gap you leave depends upon the type of membrane used. The membrane acts as a secondary waterproofing layer underneath tiles or slates on a pitched roof. It is designed to shed any water that gets through the tiles or slates.For refurbishments, where you may have a traditional bitumen-based roofing felt which has high vapour resistance, you should follow the guidance in the BS5250 Code of Practice for Control of Condensation in Buildings and leave a minimum 50 mm air gap underneath the membrane ventilated to external air. If the membrane is a breather membrane (eg Kingspan nilvent), then there are a number of choices. What is critical is to allow moisture to run from the top surface of the membrane. This can be achieved by either:

On a discontinuous surface

  • A minimum 10 mm gap between the membrane and tile / slate batten, although 25 mm is typical to maximize thermal performance.
  • If your building is built to NHBC standards, then a 50 mm gap is still required.

On a continuous surface (for example, laid flat on the insulation or sarking board)

  • An air gap is not required as long as there is a path for moisture to run down the roof.

More detailed standard specifications are shown in the Kingspan Kooltherm K7 Pitched Roof Board brochure.

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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 “Air gaps, insulation and roofing membranes in pitched roofs.
  1. I am aiming to insulate an attic bedroom. This is the present situation : The ceiling is pitched in line with the roof and is wall papered un- plastered plaster board., between the ceiling and the roof is rolled insulation , a gap and then roofing felt then slates. The ceiling is divided by 6-8 inch deep beams making 12 in all spaces roughly 8ft x 4ft each . insulated plaster board seems to heavy and difficult to use. What is the best way to add enough insulation board and finishing with plaster board? Thickness of insulation and does it need an air gap ?? I would be grateful for any advise , Thanks John

    • Hi John,

      This is a difficult one to answer. We would need to know what the u-value target is to advise you on the thickness of insulation required. Also we would need to know the thickness and performance of the existing insulation.

      If the roofing has an unbreathable felt, then a minimum of 50mm ventilation MUST be maintained beneath this membrane. There would not need to be an air gap between the existing structure and the proposed insulation internally.

      K18 insulated plasterboard would be the easiest product to install, as the added weight of the foam onto the back of the plasterboard is negligible when compared to the weight of the plasterboard itself. Alternatively, you could fit them in separate layers but it may be more difficult as you would need to fix the plasterboard through the insulation and into the rafters once the insulation has be fixed up. This requires you to hit the rafters and does increase the time of the project.


      Arron Chalmers,
      Technical Advisor

      Tel: 01544 387 382
      Fax: 01544 387 482

  2. I have a pitched roof on a bungalow that had new tiles, batons and ‘breathable’ membrane/felt installed within the last two years. This roof space also includes a dormer bedroom with plasterboard nailed to the diagonal roof timbers (4″ x2″ timbers) with no insulation at all between them. I wish to remove the plasterboard and insulate between the diagonal timbers with 100mm (4″) kingspan, then affix new plasterboard. Do I need to leave an air gap between the insulation and the breathable membrane, which would have to be less than 100mm kingspan or can I put the 100mm kingspan right up to the membrane?

    • Thanks for your comment Stuart.
      If there are tiles on battens, counter battens, and a taught breathable membrane on the outside of the dormer, then this can be filled with insulation without the need for an air gap.

  3. Hi I am currently installing 100mm kingspan between the rafters in my pitched/vaulted roof it states 50mm air gap but i only have 25mm with breathable felt which I presumed would be OK there is not much heat into the room at present… ever I took some down on one side the other day and the kingspan was soaked at one end (the furthest away from the house, possibly the coldest part)
    Naturally I took down some from the other side – and bone dry???

    Is it possible for frost to get in and thaw out thus causing condensation? We have had a lot of freezing fog the past week. I am wondering if that’s the case and one side of the pitched roof is colder than the other?

    It’s really breaking me and not sure if I should carry on. Any help would be really appreciated

    Thanks kind regards lee

    • Hi Lee

      Don’t worry – we will do our best to help you sort it out! If you have installed the pitched roof insulation correctly as per the product brochure (see either Kingspan Kooltherm K7 Pitched Roof Board, or Kingspan Thermapitch TP10) in theory there should no risk of condensation. You need to have installed a breathable membrane above the rafters, with a 25mm unventilated void below, before the installing the insulation between the rafters. If this is what you have done, everything should be OK from the condensation side of things. It’s best to have a qualified roofer take a look as there may be a leak.

      Hope this helps.

  4. Hi there I am putting 75mm kingspan inbetween rafters which will leave me a 25mm air gap I have got deep tiles and breathable felt but about a year ago a had to have tile vents installed because condensation will a 25mm air gap be ok?
    Any help would be great thanks

    • Hi Stuart,

      If there is a breathable membrane installed above the timber rafters with a 25mm air gap below so that the membrane can sag and provide a draining channel then cross-ventilation wouldn’t usually be required. 50mm cross-ventilation is only required below an unbreathable material, such as bituminous sarking felt or plywood.

      Hope this helps.

  5. Hi, I am re roofing a pitched roof I have used tile battens on counter battens and a breathable membrane on top of 25 mm insulation. To obtain the required insulation factor can I use 100 mm insulation between the 120mm rafters tight up against the 25 mm insulation and a plaster/vapour barrier board

    • Hi David

      If the 25mm of existing insulation is on top of the rafters to the external side of the roof, only a maximum thickness of 25mm of insulation can be used between the rafters in order to ensure no risk of interstitial condensation. The U-value that would be achieved by using this construction when using Kingspan Thermapitch TP10 would be 0.37 W/m²K. Kingspan Kooltherm K7 Pitched Roof Board would allow you achieve a U-value of 0.36 W/m²K. The new Kingspan Koolthemr K107 Pitched Roof Board would reach 0.32 W/m²K. Building regulations state that 0.18 W/m²K is a compliant U-value.

  6. Hi there is looking at insulating a 10 x 8 shed, the frame work is made from 3 by 2 and it’s fully tounge and groove. What sort of membrane and air gap should be used ?

  7. Hi I have a pitched slate roof. There are no counter battons. The roof timbers are 75mm, I have 75 mm kingsman to fit in between. What air gap do I need to leave between the breathable membrane and the insulation? (Assuming I’m going to need to batton the rafters)
    Many thanks

    • Hi Ben,

      If there are no counter battens on the roof then a 25mm gap should be left between the breathable membrane and the insulation to allow the breathable membrane to drape. If there were counter battens then no gap would need to be left as the counter battens would keep the breathable membrane taut to allow water run-off.

      Hope that helps.

  8. I am in the process of converting a garage into a summerhouse. The garage has a tiled slate pitched roof. I have used 50mm seletex insulation pushed up against the underside of the roof tiles, held in place by 2″ x 1″ batons. I then intent to fix plaster boards to the batons thus leaving a 50mm gap from the underside of the seletex and the plaster board. Is this the correct method of insulating the roof?

    • Hi John,

      Generally for pitched roofs, guidance would be subject to understanding the complete build-up. For example, if the construction contains a breathable membrane, it is acceptable to fully fill the timber rafters provided counter battens are installed either above or below the breathable membrane. If counter battens are not included a 25mm (minimum) un-ventilated cavity should be included between the insulation and the breathable membrane.

      If however a sarking felt is being used instead of a breathable membrane, a 50mm (minimum) ventilated cavity should be incorporated between the insulation and the non-breathable sarking felt. It is not necessary in any of the above constructions to include a cavity between the plasterboard and the insulation. We would recommend you consult the insulation manufacturer to confirm your specific requirements.

      Hope that helps,

  9. I have a chalet style house with bedrooms effectively being room in attic. 1920’s and no insulation. I am removing the existing plaster board from side of bedrooms where roof slopes. Roof style is based on Scottish traditional practice the roofing underlay is laid directly over a series of sarking boards fixed to the roof rafters with the roof slates nailed directly to the sarking boards.

    Plan is to insulate between roof rafters, I assume leaving a 50mm gap between insulation and sarking underside. So effectively 75mm of insulation for 125mm rafters. I can then overlay this with additional insulation under the rafters (with plasterboard where appropriate).

    1. Does this seem a sensible approach and would I use TP10 between rafters.
    2. Would I just use additional TP10 below rafters (with tape to ensure good membrane seal between boards) where I do not need plaster board as in lower eaves.
    3, Is there a recommended insulation product that includes attached board where plasterboard is needed (i.e up area of sloping roof). This cannot be too thick as it would encroach on space.

    Any advice would help ensure I am on right lines.

    • Hi Steve,

      Please be advised the following construction build-up would follow Kingspans recommended approach: a 50mm ventilated void between the insulation and the top of the rafters, assuming the ‘roofing underlay’ is a non-breathable material such as a Sarking felt. Kingspan offers two insulation products for pitched roof applications, Thermapitch TP10 and Kooltherm K107, either of which would be suitable for between and under the timber rafters. Alternatively you could consider Kooltherm K118 Insulated Plasterboard to install below the timber rafters. Regarding appropriate thickness’ of insulation to achieve your target U Value, Kingspan offers an online U-value Calculator to assist with this.

      Hope this helps,

      • Thanks, gone with K107 70mm between rafters (quite long wait to get hold of it unfortunately) + K118 under rafters for plasterboard in bedroom. I will extend the K107 between rafters into the attic area but will not be needing plasterboard there. What would be the recommendation to complement this under rafters in attic area without spending too much unnecessarily (still want decent insulation). Do I need a separate vapour control layer if I do not use K118 or is there another insulation option I could use under rafter?

  10. I’ve an uninsulated flat roof, about 40 sq mts. Recently done. Felt onto ply and osb. Rafters are about 21″ apart, and about 3.5″ exposed. I like the exposed beams ‘look’, but if I insulate, what do you recommend?

    • Hi David

      If you were to insulate between the joists, creating a cold flat roof, a 50 mm fully ventilated void would need to be created between the underside of the ply and the insulation that will be installed. This ventilated void is required in order to avoid a condensation build-up on the underside of the plywood deck. Once the ventilation has been introduced, the highest performing product that can be used between the joists is Kooltherm K107 Pitched Roof Board. See the product literature for guidance on how to install.

      Hope this helps.

  11. Hello, thanks for that. I’m confused about why the 50mm airspace above the PIR has to be ventilated.
    If I put a vcl below the PIR, so it is ‘waterproofed’ above it, how does condensation occur?

    Apologies for being thick!


    • Morning David

      Condensation occurs when warm air hits a cold surface, therefore an uninsulated roof deck or non-breathable membrane being above the insulation would result in the warm air condensing against it. This is why any non-breathable substrate or roof covering requires a 50mm ventilated space beneath it. The air flow within the ventilated cavity carries out any water vapour or residual moisture.

      Hope this explains it a bit more for you.

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