How to insulate a garage conversion: Part 3 – Roof Insulation

hammer and wooden floor

In the last few blog posts we have looked at the Building Regulations and Standards required for insulating a garage conversion as well as the different methods of insulating the walls.

The next step is to look at the insulation requirements for the roof of the building. This is a key component of creating a warm habitable space from what is often a cold and drafty garage.

Pitched Roofs

If the existing roof of the garage is pitched or if you are intending to build a space above the garage, with a pitched roof, then we suggest you look at our quick guide on pitched roof insulation which covers this area in a bit more detail. The space created above the garage would also have to be fully insulated in a similar fashion to the garage.

When a previously insulated room above the garage exists before the conversion, then roof insulation will not be an issue, and you will not have to add extra insulation to the garage ceiling.

Flat Roofs

The U-values needed to achieve this on a flat roof are shown below. We are only looking at how to insulate a flat garage roof in detail as this is the most common construction.

England Wales Scotland
Flat Roof 0.18 0.15 0.15

U-Values for Extension of Existing Buildings in W/m2.K
*The first number applies where the existing building has U-values worse than 0.70 W/m.K in the walls and worse than 0.25 W/m.K in the ceiling.If the U-values are better than those figures the second number applies.

Most garages have flat roofs, which means that there is less than 10 degrees of pitch in the roof. However a flat roof does not mean that it is perfectly level, in fact a flat roof should always have a certain degree of pitch or fall in it to allow water to drain off it. If water lies in pools on the roof this could damage the fabric of the roof, causing problems in the room below. If water is pooling on the existing roof then this should be rectified at this stage, and the fabric of the roof checked to make sure that it has not been damaged. If any part of the existing roof needs replacing this may affect how you decide to insulate the roof.

Again, the roof, like the walls, can have the insulation added to the inside or the outside of the construction. These two types of construction are known as warm roofs, where the insulation is above and between the roof joists, whereas cold roofs are where the insulation is between and below the roof joists.

For a cold roof, where the insulation, such as Kooltherm K7, is fixed between the roof joists, the roof needs to be ventilated by leaving a 50mm gap above the insulation to create an air gap which is vented to the outside. This will prevent condensation forming in the roof. Then, if necessary, further insulation can be added below the roof joists to reach the required U-Value. This is commonly in the form of insulated plasterboard.

For warm flat roofs, where insulation is added above the roof joists, the first area to check is the condition of the existing waterproofing on the roof. If this is in a good condition, then it can just be swept clean and the insulation either mechanically fixed or glued on top of the existing roof. Then a waterproofing layer can be added on top of this.

If the waterproofing is leaking then this should be removed and any further damage below rectified. Depending on the extent of this it may be necessary to replace the entire roof. In this case the insulation can go between and above the joists. The full details of this method can be found in our quick guide.

The next (and final) post in this series will be about insulating the floor of the garage. If you’ve missed the other posts on this topic they can be found below:

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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 garage conversion: Part 3 – Roof Insulation
  1. Hi I am going to insulate my garage roof that has been recovered with E.P.D.M.
    This has meant that my garage is colder, the build up is E.D.P.M over new plywood sheet on top of the rafters that are 150mm deep.
    Can I put sheet up in between the rafter and make it a warm roof, as I will have difficulty in trying to vent the roof as I can only get to one side to put vents in.
    Can you advise as what I can do .



    • Hi Michael,

      Thanks for your message. What you are describing is a cold roof (where insulation is between the rafter). A warm roof is where the complete deck of the roof is warm – for this insulation would be over the rafters. As you have newly waterproofed the deck, your only option is to insulate between the rafters – a cold roof. You must have ventilation both sides through the cold roof otherwise you will cause condensation. Try for ventilation solutions. Otherwise your only option is to go for the warm roof.

  2. Hi,
    I wish to convert my garage into a second living room. My garage is part of the house, we have a bedroom above it. Will I have to insulate the ceiling of the garage?

  3. Hi,

    I’m looking to convert my standalone garage and as part of that I want to completely replace the flat roof (the old felt and boards are damaged). I was planning to create a new warm deck roof and the Thermaroof TR31 would appear to be the best solution. I have a couple of questions that I hope you can help me with:

    1. Is the Thermaroof TR31 suitable for finishing with EPDM rubber membrane.
    2. The application guide states that the VCL should be maintained by applying a bead of water vapour resistant, non–setting, gun–grade mastic sealant. I’m having trouble finding a suitable product. Can you recommend some examples please?



    • Thanks Jeff,

      Regarding compatibility of Thermaroof TR31: Most often the product is used with three layers of partially bonded felt. You would need to discuss with the membrane manufacturer whether their product is compatible. Thermaroof TR27 would be our recommended product for EPDM.

      You also asked about a suitable product for the sealant. Here’s a link to a supplier.

  4. Hi

    Our garage is integrated beneath a bedroom. The bedroom above is much colder than the rest of the house so we’d like to insulate the garage below. How is it best to so this? Do we add kingspan board beneath the ceiling? If so what type? Is fireproofing an issue (we don’t use the garage for a car). How do we attach the insulation board to the plastereo ceiling? Many thanks

    • Hi Victoria,
      We’ll need to know a little more about your constructions, but if we assume that the floor/ceiling is timber joists, then the ideal would be to insulate between the joists with Kingspan Kooltherm K7. As for fireproofing, typically this would mean two layers of 12.5mm plasterboard, but you should check with the plasterboard manufacturer themselves what they recommend (it can be different between manufacturers).
      So, I hope this has helped, but give us a call on 01544 387 382 and we’ll get you the specific advice you need for your project.

  5. Good morning. We have a double detached garage with a pitched roof. We have extended it to the intention of using it as an entertainment area, housing a snooker table, seating area and one or two items of gym equipment. The new part also has a pitched roof. I want to insulate the roof, walks and floor. The walls are brick and block, the floor concrete and the roof; a normal tiled roof. Please can you guide me through which types of Kingsspan insulation I will need as I want to get it right the first time! I have read your step by step guide, but I want to make sure everything is spot on!

  6. Hi,
    I have recently bought a new build (constructed in 2015) In Scotland, where the bedroom above the integrated garage is considerably colder than other rooms.

    I have had a number of visits from the site manager who has confirmed that the fireboard and mineral wool insulation in the garage ceiling is compliant with Building Regs.

    However, he mentioned that I could further improve the insulation capabilities of the garage and therefore improve the temperature within the bedroom above the garage by adding an additional layer of insulated board on top of the existing fireboard.
    Would the Kooltherm K17 be the correct product for this type of job?


    • Hi John,

      Thanks for your question. We would advise mechanically fixing Kooltherm K18 Insulated Plasterboard beneath the ceiling of the semi-exposed floor (heated bedroom above, unheated garage below) that already contains mineral wool. However do note that this will need need to exceed the performance of the existing insulation so as to reduce the risk of interstitial condensation. Feel free to get in touch with our Technical Department who will be able to offer further guidance here.

      • Thanks for that, however a couple of follow up questions.
        Re: the K18 insultated plaster board, are the widths detailed in the specs inclusive of the plaster board i.e. When the spec says 50mm is this 50mm of insulation plus the 12.5mm of plaster board?

        As I am unconcerned about the finish of the ceiling is there any reason why I would be unable to use the Kooltherm K7 on the flat internal ceiling? I am happy to sacrifice the minor insulating properties of the plaster board in order to have a lighter product.

        Thanks for your help

        • Hi John,

          To answer your questions:
          1) The Kooltherm K18 Insulated Plasterboard product literature provides for product thickness as a whole. To use your example of 50mm, you will see in the literature that we provide for a 62.5mm product thickness – 50mm of insulation plus 12.5mm of plasterboard.
          2) If you didn’t want to use Kooltherm K18, we would usually advise one of our soffit boards for structural ceilings: Kooltherm K10 and Kooltherm K10 PLUS (the latter is more aesthetically pleasing). However you could indeed use Kooltherm K7 instead, which is available thinner thicknesses ‘off the shelf’ than K10, but you will have to bear in mind that the thinner the board is, the higher the risk of condensation – especially as you have existing mineral wool between the joists.

  7. Hello,
    I have garage at the bottom of my garden which is part of a block of garages, it currently has the old asbestos roof and I am replacing this, probably with a tin roof however I would like to insulate the roof as I intend to use it as a dry storage and workshop.
    Any advice on the best way to insulate the roof would be appreciated.

  8. Hi
    just moved into our new purchase that has a bedroom over unheated garage, the floor is concrete slab type construction and is not insulated leaving the floor very cold, is a floating floor insulation the only choice but am I am able to insulate underfloor within the garage?
    thank you

  9. Hi, i have a stand alone garage im converting into a granny annex. The roof will be a new fiber glass roof.
    1. What thickness celotex insulation is needed for the roof?
    2. Also does all the plaster board have to be fire baord?
    3. And on the floor which is already concrete. Can i put a floating celotex floor with chipboard on top, or does have to be screed?

    Many thanks Dave

    • Hi David,

      Thanks for your question. We are able to answer with regards to our Kingspan insulation products.

      1. We’d need a little more information from you to give a specific thickness of insulation. However, if your roof construction is a typical 12.5mm plasterboard, 100mm timber joists, 18mm plywood, polythene vapour barrier, 120mm Thermaroof TR26 mechanically fixed with metal fasteners, 18mm plywood, and 1mm polyester GRP, it would achieve a U-value of 0.18.
      2. It would be recommended to speak to a local building control officer as to whether they need to achieve a fire rating or not, but we believe some plasterboards achieve a 30 minute fire rating. British Gypsum, or any other plasterboard manufacturer, might be worth contacting for advice on that.
      3. Thermafoor TF70 can be laid on top of the existing concrete floor, as long as there is an existing damp proof membrane below the concrete, or a new one should be laid down first. Then, over a polythene separation layer, either 65mm sand cement screed or 18mm minimum T&G chipboard can be laid over the top, which are both acceptable floor finishes for domestic loads. Please take a look at the Thermafloor TF70 product literature for more info and advice.

      Hope this helps.

  10. Hi I have built a single story rear extension with flat roof (EPDM and 100mm GA boards) but when it rains its really load, can I put acoustic insulation in the voids or will this cause condensation?

    I have already planned to use BG acoustic plasterboard on the ceiling.


  11. Hi we wish to have an out building converted to a habitual room it’s about 2.5×2.5m with solid walls and concrete roof. the ceiling height is 2.5m enough room for internal insulation I think.A room does have a widow.

  12. Hi,
    We have a double bedroom extension above a garage. We’d like to insulate the roof of the garage to make the bedroom warmer. The bedroom floor/garage roof is concrete. What would be the best way to insulate?

  13. Hi, I have a concrete panel garage which I am converting into a man cave, I have removed the concrete panel roof and replaced it with OSB3 with a liner on top, sealed up the joints in the panels and now I am suffering from condensation on the roof! I have some kingspan insulation boards to use. The OSB3 is laid on 3×2 treated timbers attached to the metal trusses of the garage. What is my best route to use the kingspan boards and help stop the condensation please? Thanks

  14. Hi,

    I own a first floor apartment, the ground floor beneath is my open air car port (all 4 sides)

    I have noticed that I get a lot of cold coming up from underneath (even with thick underlay and carpet)

    I have unscrewed one of the carport ceiling boards to see what was behind and find out if there is any option for insulating.

    Behind the 5cm board is a 8-10cm void and then concrete. (With 2 lots on wood baton that the boards attach to)

    Would I benefit adding some type of foam or changing the boards?

    When looking at the building from the outside I can see that there are small air vents placed between the concrete level and my flat so would not be effected if filling the void underneath.

    Thanks in advance for any advice

  15. Hi I have recently re roofed my garage roof which has pantiles. I covered with OSB 3 and covered with a breathable membrane and new tiles. I wish to insulate with 50mm Kingspan would I need to leave a ventilation gap of 50mm or would I be able to glue to the OSB.

  16. Hi,

    Hope you can advise. we have an attached garage that is being converted. it has a cold flat roof with joists running across (from the house to the side of the garage).

    I am looking at using K7 between the joists (14″ gap, 7″ deep), and then maybe K18 to finish beneath this. would these choices sound right to meet current building regs?

    As I cannot get a vent at both ends of the joist runs, is it ok to have a vent at just one end of each joist run?



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