How to insulate a pitched roof at rafter level

Loft conversion using Kingspan Kooltherm K118 Insulated Plasterboard

When insulating a pitched roof at rafter level there are a number of key design considerations which will affect the build-up of the roof and the choice of insulation. In this blog post we explore some of the options available.

Firstly, just to make it clear, when we are talking about rafter level we mean the timbers that support the pitched roof. We aren’t talking about joists, which are the flat horizontal beams at ceiling level.

The main decision to make when insulating a pitched roof is if you are going to have a warm roof or a cold roof. This will depend on the construction on your roof, and if it is a refurbishment, whether the roof tiles are going to be removed.

The main difference between the two is the location of the insulation.

  • A warm roof is where it is above and between the rafters (so the rafters are kept warm).
  • A cold roof is where the insulation is between and below the rafters (and the rafters remain cold).

For a new build, or if a detached dwelling is being re-roofed, then there is scope to choose the build-up of roof that suits your purposes best, however if the roof insulation is being added to an existing roof, for example as part of a loft conversion, then a cold roof is the most likely solution as you can’t insulate above the rafters.

Warm Pitched Roof

To create a warm pitched roof, insulation such as Kingspan Kooltherm K107 Pitched Roof Board should be fitted between and over the rafters. As this requires fitting insulation above the rafters the roof covering will have to be removed in order to fit the insulation. This roof type will also require a breathable membrane, like Kingspan nilvent, to be fitted above the insulation.

An example warm pitched roof build up can be seen on our online U-value Calculator.

Cold Pitched Roof

In this case we would suggest fitting Kooltherm K107 Pitched Roof Board between the rafters and Kooltherm K118 Insulated Plasterboard to the inside face of the rafters. Usually this construction is a ventilated roof which will have sarking felt. Ventilated roofs require a 50mm air gap above the insulation, timber stop battens can be nailed into the side of the rafters to give the correct air gap when installing the insulation. The insulated plasterboard should then be placed with the long edges running across the rafters.

View an example cold pitched roof build up on our online U-value Calculator.

Building Regulations

When refurbishing a pitched roof in England or Wales, there is a requirement to reach a U-value of at least 0.18 W/m2.K. For a complete re-roof in Wales the U-Value required is 0.15 W/m2.K. Scotland is similar, but you can be required to have a U-value of as low as just 0.13 W/m².K. Detailed guidance on Building Regulations is available here.

When choosing the insulation for you pitched roof, it is important to ensure that the thickness of insulation used achieves the required U-value for your area. This can be done using our U-value Calculator or by contacting our Technical Services department, who can provide advice about thicknesses and also provide a Condensation Risk Analysis which ensures that, with the chosen insulation and construction build up, there is no risk of condensation occurring within the roof build up.

Further information on how to install the pitched roof insulation products mentioned above can be found in the respective product literature:

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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 a pitched roof at rafter level
  1. Is there any technical/thermal difference between the plain and the printed side? If so which side should face into the loft when placing between rafters?

    There is no mention of close-boarded lofts i.e. where the rafters are covered by 9inx2in/22cmx5cm planks with separation of about 1/8th inch/2-3cm inch then sarking, then clay tiles. What, if any, should the gap be between the insulation and the planks.

    • Hello J Russell,
      Thanks for your question. Kingspan Insulation Kooltherm K7 Pitched Roof Board is printed on both sides, so there is no difference in thermal performance.
      The other part of your question refers to sarking boards (a popular way of constructing roofs in Scotland). Insulation can be fitted directly underneath the sarking boards if you have a breathable membrane (eg Kingspan nilvent) on the top of the sarking boards. If you are using felt, then you would need a 50mm ventilated gap between the underneath of the sarking board and the top of the insulation.


  2. I have a semi detached property with a hip roof, it has slate tiles & no sarking, the rafters are 100mm deep. Can I insulate as a cold pitched roof without any sarking?

    • Hi Paul,

      You can as long as there is no rain getting through, but you would need a 50mm ventilated space between the insulation and the slate/tiles then another layer of insulation on the underside. You may need to batten out your rafters in order to meet building regs. However, we would always recommend a membrane to prevent any chance of rain getting through.


  3. My Architectural technician who has prepared my drawings for the BCO has suggested: Tyvek breathable roof covering, with 50mm Air gap.
    150mm * 50mm C16 rafters @40mm C/C, With 100mm K7 between the rafters.
    53mm K18 Pitched board over face of rafters and 13mm Plasterboard with 3mm skim finish.

    Can I get rid of the air gap and use 150mm of K7 and the rest as listed OR do I need to increase the rafters to 200mm to keep the air gap with 150mm of K7. Thanks

    • Hello Garry,
      It’s not a simple answer I’m afraid as it could depend upon a number of things eg whether your warranty provider stipulates that you need to leave a gap or if the architectural technician has reasons to believe that a gap is necessary because of the building type, location, room use.

      This aside, yes, it is possible to fully fill the rafters. You would need an additional set of counter battens above the breathable membrane before the tiling battens and tiles installed. This is to provide a drainage layer. More details can be found on

      It really would be worth talking this one through with one of our advisers on 01544 387 382 to go through the specifics of your project.

  4. Hi Ive checked an dmy roof has been fitted with 50mm kingspan on the outside and on top of the srking. The apartment is period property. Is this amount of insulation adequate for this roof/? I’ve tsaken the ceilings down and would insulate between the rafters, if necessary?


    • Hi Martin,
      Your insulation will not be adequate for building regulations purposes in whichever nation of the UK you are in. However, it’s not just as simple as simply adding 50mm between the rafters underneath. This is because we would be concerned about the risk of condensation. It is worth talking this one through with one of our technical advisors on 01544 387 382. they will want to know what part of the country you are in, the type of breather membrane or felt used and the type and condition of the sarking board. Hope this helps.

  5. Hi could you please tell me what the gap between the insulation and the eaves in a cold roof should be as I have had my roof insulated by a company and they have laid it right up tight to the eaves which I don’t think is right.

    • Hello Robert.
      The gap is set out in the rules – “BR262: Thermal bridging: Avoiding risks.” Published by the BRE (Building Research Establishment). You are right to be concerned. BR262 states that there should typically be a 25mm gap between the insulation (at ceiling level) and the eaves. This is to allow for ventilation and air flow.

  6. Hi ,
    Looking for some advice,i have just bought a Bungalow ,and i plan to do a loft conversion and make 2 rooms with a few dormas included ,the loft currently has mineral wool on the floor between the joists ,i was going to replace the wool with a 100mm kingpan between the joists and floor it ,is it practical to do that before the room conversion of the loft as it makes sense for the heat to rise up from the ground floor , and what way should the silver side face if i’m doing the room conversion

  7. Hi, we want to convert a pitched attic in the Netherlands into an additional bedroom and study. Currently it has ceiling insulation (at the floor of the attic, which we will leave) but there is no attic ceiling insulation. However we also want to keep the old wooden rafters exposed (large wooden beams) for a decorative effect. Currently between the rafters are wooden boards (that are cold to touch) and we would place any insulation directly on top of this. Ideally we want to use a hard plasterboard K18 and it seems that we would need to install K7 and then K18 on top of this…but we are not sure if this is correct if we are placing both between the rafters? thanks

    • Hi Ross,

      There are a few things to consider here including the type of roof covering and the requirement for ventilation of your roof. You will likely need to remove the insulation at ceiling level as there could be condensation issues.

      You will need to talk this through with us. If you are based in the Netherlands, please contact our Tiel office. If you are in the UK, contact us here.


  8. Hi, I have a converted bungalow with 3 loft rooms added in roof space maybe 25yrs ago… the roof construction is small tiles nailed/clipped straight on to horizontal feather board type of slats.. no felt anywhere, I cannot afford to update the roof tiles for modern regs… but could I insulate from inside the loft using kingspan between the rafters ? what type would you suggest please? and leaving a 50mm ventilation gap between roof tiles and similar gap down to ceiling in rooms below? any advice appreciated

  9. Hi, i have a detached garage approx 5m x 5m single skin brick wall with tiled pitched roof. We are looking to insulate the walls and roof. Will the Kooltherm K7 be ok to insulate the walls and roof. The roof has rafters that we want to insulate in between the rafters then board.

    • Hi Lee,

      Thanks for your question. Kooltherm K7 Pitched Roof Board is fine for the pitched roof insulation between the rafters. Guidance on installation methods can be found in the product literature, which can be downloaded here.

      However, Kooltherm K18 Insulated Plasterboard is your best option for the walls, which is mechanically fixed to timber battens. You can also install this product beneath the Kooltherm K7 you have installed between the rafters of your pitched roof in order to increase thermal efficiency and reach a better (lower) U-value. Take a look at the Kooltherm K18 product brochure, which can be downloaded here – it gives guidance on how to install to walls and beneath rafters.

      You might also want to have a play on our free U-value calculator, which will give you and idea of how much insulation you need to reach a specific U-value.

  10. I have a boarded loft with insulation between the 4″ joists and is used for storage. The loft gets hot in summer and cold in winter. I’m considering between rafter insulation (space between rafters varies between 400 and 420mm, one at 390mm and one at 450mm!) to avoid the wide temperature difference, and improve the overall insulation of the house. What should I use and would this be of sufficient insulation to make the existing inter-joist insulation redundant? We have had cavity wall insulation done the best part of 30 years ago.
    Many thanks in anticipation.

    • Hi Tim

      We recommend Kingspan Kooltherm K7 Pitched Roof Board. This would need to outperform the existing insulation between the ceiling joists to avoid risk of interstitial condensation. Ideally, we need to know what material is above the rafters (for example, is there a breathable membrane, sarking felt or another material?) and whether the roof is ventilated before advising how much insulation install.

      Once you have this information, please call our technical team on 01544 387 382 for a condensation risk analysis and advice on how much insulation.


  11. Hi,

    I’m insulating the rafters of my 3 bed semi with 40mm leaving an air gap of about 50mm above the rafters. (this may seem a little thin but i already have 50mm in the ceiling joists plus rockwool). However, I’m wondering what the best way is to attach them? I understand the long edge should run along the length of the roof line and i have bought some silver roof tape to connect the boards together after they’ve been put up. Is a simple nail through the boards and into the rafters the best way (perhaps with a small wooden batton to act as a washer) or should i use some L or S tags at the bottom of each one as you run down the rafters which then means no nails / holes through the boards?

    Please let me know what options you advise.

    Thanks in advance


    • Hi Rod

      You will need to be mindful of how much insulation you plan to use, and where it is placed within your roof. If you already have an abundance of mineral wool and Kingspan insulation at horizontal ceiling level, and then the insulation is placed between or underneath the pitched roof rafters, this will have to outperform the insulation at horizontal ceiling level to avoid an interstitial condensation risk.

      The highest performing insulation should be on the unheated side of the construction. If the insulation between and under the pitched roof rafters exceeds the performance of the insulation at horizontal ceiling level, then in theory there shouldn’t be a risk of condensation as long as the 50mm air gap is cross-ventilated.

      Kooltherm K7 Pitched Roof Board can be friction fit between the timbers, and then either the same board or Kooltherm K118 Insulated Plasterboard can be fixed beneath the timbers with long dry wall screws.

      We suggest you call our technical department on 01544 387 382 to perform a condensation risk analysis.

  12. I have had a 4m x 3m extension built with a sloping roof (vaulted Ceiling) which has been insulated between the rafters using 125mm insulation with a 50mm gap above and plasterboarded and skimmed.
    4 led downlighters have been fitted and recessed into the insulation, having researched this I now know this is not recommended. After the latest cold spell I have noticed damp spots and damp tracking lines on the ceiling and around one of the light fittings. Again after researching I have discovered I have no VCL in place and therefore warm air is rising and condensating on the breathable membrane and dripping down.
    My questions are:
    Is the warm air just rising through the downlighter voids
    If I remove the lights and fill the voids wirh insulation will this cure it
    Does the warm air radiate through the non foil back plasterboard and rise through the gaps in the rafter insulation and condensate
    Rather than take down the ceiling and fit a VCL is it possible to over board with a product which will create a VCL
    Best Regards

    • Hi Graham

      In order to advise correctly we need a little more information from you. Is the 50mm air cap cross-ventilated? What breathable membrane has been installed (and is it actually breathable)?

      If there is no cross-ventilation underneath an unbreathable material, such as sarking felt, then condensation will form and have no way of leaving the construction. This will then cause damp and mould problems.

      Adding more insulation where the downlights are is unlikely to solve a condensation issue. Adequate ventilation and the installation of vapour barriers should help however.

      We advise contacting a competent builder or a local building control officer, who could visit your property and suggest what is best to do next.

    • The roof has been built using:
      Tyvek breathable felt
      25mm slate batons
      15mm over fascia vent
      Not sure what ‘cross ventilation’ is but there are no vents down the side of the building and the only vents are the ones above the fascia
      Will fitting some vented roof tiles help the airflow

      Best Regards

      • Hi Graham

        Unfortunately it’s going to be really difficult for us to advise in this instance. It would be best if you contacted a local builder or your building control officer, who will be able to visit your property and advise a fix.

        Sorry we can’t be of more help right now.

  13. I want to insulate an existing cathedral ceiling from the inside that is boarded and plastered. I want to know if I can insulate over the existing ceiling and what would I need to do to comply with building regs. I don’t want to remove ceiling?

    • Hi Anthony

      Whether or not you can add insulation below the existing ceiling will depend on the existing structure. How much insulation is there already? Where is this located? Is the roof ventilated? Are the materials above the roof rafter breathable?

      Once you have the full construction build-up of the roof, we advise contacting our technical team on 01544 387 382 to run a condensation risk analysis, which will determine whether additional insulation could be installed and whether this would increase a condensation risk.

  14. Hi. I am a sculptural blacksmith renting a unit on a farm with an asbestos roof. It has been there for 80 odd years (I’ve been there for 6yrs), there are leaky patches when there is heavy rain and, at this time of year, heavy condensation in most places, hence droplets that moisten the surface of all of my stock!
    It is looking less likely that my landlord will clad the roof to stop the leaking so I am looking to do what I can to protect my equipment and am wondering if using your panels on my ceiling would do the trick.
    I’d appreciate your feedback as I don’t know what else to do? Thanks in advance.

    • Hi Julie, and thanks for your question.

      Simply adding insulation to the underside of an existing asbestos roof is not going to fix the condensation issue, and may in fact make it worse! The roof will require adequate ventilation first (seek guidance from a specialist such as Glidevale). Once a 50mm cross-ventilation has been achieved, you could then fit Kingspan Kooltherm K7 Pitched Roof Board between timber joists or alternatively in a continuous layer.

      Hope this helps.

  15. Hello,

    I have a pitched roof with 170mm rafters. If I insulate between the rafters with 100mm or 120mm kingspan, I understand that I will need further insulation internally, such as the insulated plasterboard.

    The roof requires bracing though and I wondered whether it would be acceptable to brace on the underside of the rafters between the 100/120mm insulation and the insulated plasterboard?

    Would this create a thermal/condensation problem?



  16. We are having a new roof over our kitchen. It will be a pitched loose rafter roof. We are looking at warm construction with insulation over and between rafters. However we want to leave the lower portion of the rafters on display as they are deep rafters 225mm. All details show plasterboarded and vapour control under the rafters. We only want to board between rafters. would we have to run VCL over plasterboard and then up and over each individual rafter?

      • Hi, and sorry about the delay in our response.

        In this situation we would recommend that you use a foil faced insulation between the rafters, such as Kingspan Kooltherm K17 Pitched Roof Board. The edges of each board should be foil taped to improve the vapour resistance of the insulation.

        It is not possible to achieve a continuous vapour barrier if using a polythene VCL, so taping the edges is certainly the best option.

        Are you able to call our technical department on 01544 387 382 so that we can run a condensation risk analysis?

  17. Hi I’ve just bought a property and the loft has been renovated . The joist are 100mm and been filled with 100mm celotex tight upto bitchumen felt then a bat ton on the bottom of the joist and plaster boarded is this OK if not what do u recommended I do to fix it cheers

  18. Hi,

    We have moved into a new home. It’s a terrace back to back just over a hundred years old. There is a room in the attic but no insulation at all. We are getting the roof refreshed, at which point the roofers are going to install a breathable membrane. Once this is completed I want to add insulation to make this a useable bedroom. The rafters are only 75mm deep with 330mm between. How thick can I go with insulation between the rafters considering the new membrane that will be going in and should I add insulated plasterboard below this, i.e. in the room. If so how should i fix the plasterboard to the rafters to avoid cold bridging.

    • Hi Graeme

      Sorry about the delay in our response, we’ve had to run by our technical team.

      If the rafters are 75mm deep, there are two options for how much insulation can be installed between the rafters. If there are counter battens and tiling battens with a breathable membrane, the rafters can be fully filled. If there are no counter battens then a 25mm gap will need to be left at the top of the rafters to allow the membrane to drape, and therefore only 50mm of insulation can be installed.

      Insulated plasterboard will need to be added depending on the U-value you need to achieve. Generally for the upgrade of a roof like this, you would aim to reach 0.18 W/m²K. Our online U-value Calculator can be used to calculate the depth of insulated plasterboard required to achieve this U-value, depending on the construction you have opted for from the two options above.

      Hope this helps.

  19. I am about to have a 1920’s Bungalow re-roofed. There are two rooms in the loft with ceilings of plasterboard mounted directly on to the rafters.the rafters are 3″(75mm) deep. It will not be possible to remove ceilings to fit a vapour control barrier. The roofer has suggested fitting 75mm kingspan between the rafters with counter battens and breathable lining below the tiles. Will this be okay, or do yo have any other suggestions?

    • Hi Thomas,

      The proposed installation of the insulation is in line with our recommendations. The thickness of insulation will be fine if you are just upgrading an existing roof, and do not need to achieve U-Values in line with building regulations. But if building control are involved in the project, it would be sensible to liaise with them to ensure that they would be happy with the proposed insulation thickness, as 75mm of any of our pitched roofing boards alone would not achieve building regulation U-Values.

      Hope this helps.


  20. Hi
    I’ve constructed a warm flat roof with 100 mm kingspan and was wondering would an insulated slab on the ceiling below improve the u value

    • Hi Brian

      Adding insulation beneath the roof deck would improve the U-value, however it will also put the roof at risk of interstitial condensation. We recommend that you contact our technical services department on 01544 387 382. They can run a condensation risk analysis so that you avoid this condensation risk.

  21. Living in England, I am having the roof tiles relaid and a breathable membrane installed in a cold loft space. The rafters are only 70mm deep, and there is 100mm rockwool underneath the loft floor.
    What thickness insulation can be inserted between the rafters and what type, ditto any insulation under the rafters to bring the the ‘cold ‘ loft up to modern standards?
    Are there any other considerations?
    Many thanks.

    • Hi Graeme

      Kooltherm K107 Pitched Roof Board is our highest performing insulation product to use between the rafters. Depending on whether the roof is counter battened or not, you may need to leave a 25 mm cavity in the top of the rafter to allow the breathable membrane to drape, which will reduce the amount of insulation that can be installed between the rafters. In this case you would need to under draw the rafters using Kooltherm K118 Insulated Plasterboard to increase the U-Value.

      Take a look at our product literature for guidance on this build-up.

      Hope this helps.

  22. Hi,
    I’m currently having my garage converted into a room with a vaulted ceiling, the roof has been built using 7×2. What size insulation is best to use?
    Also we would like to use spot lights in the ceiling when it is completed, how are they to be fitted? Thank you

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