How to insulate a garage conversion: Part 1 – Building Regulations/Standards

hammer and wooden floor

Converting a garage is an easy (and often affordable) way of creating more space in a house without carrying out major building work.

A key part of the work will be adding insulation to the garage in order to make it a warm, comfortable living space.

This is a common project which we often get asked questions about, whether it is to convert the space into an office, bedroom, dining room or play room, so we thought that we should go into a bit more detail. There are a number of factors to consider when insulating a garage conversion and in this series of blog posts we will go through it all stage by stage.

Areas to consider

There are three main areas to consider insulating when converting a garage into a habitable living space; the floor, roof and walls. They all have specific challenges and solutions and we will look in more detail at the insulation requirements for each of these over the next few weeks. This week we will be looking at the Building Regulations or Standards that will have to be met when carrying out this work.

Building Regulations

Converting a garage into a living space will usually be considered as an extension to an existing property under the building regulations/standards and there will be a requirement for the work carried out to meet the specific thermal standards for each application specified in these for the location of the property, as different standards apply in England, Scotland and Wales. You may also wish to meet higher standards than these, for example if the rest of the property is built to Passivhaus Standards then it is likely that the new room from the garage conversion will be insulated to the same standards.


Basically each element of the garage conversion; walls, floor and roof, will have to be constructed and insulated so that its U-Value (rate of heat loss through that element) meets or is less than the value stated by Building Regulations/Standards. These U-values, applicable to an extension of an existing building, are shown in the table below.

England Wales Scotland*
Wall 0.28 0.21 0.19/0.22
Floor 0.22 0.18 0.15/0.18
Flat Roof 0.18 0.15 0.15/0.18

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.

Please note that different values apply for refurbishments and new builds – we always recommend you check with local building control before starting any work.

In our product literature we give the U-Values for a range of different construction build ups and insulation thicknesses. More information about the building regulations/standards can be found on the Knowledge Base section of our website.

Existing Construction

Before starting planning the conversion it is important to check what the existing construction of the garage is, as the type and thickness of insulation you use will depend on this; the first thing to check is the construction of the wall as next time we will look at the best methods of insulating them.

The other posts in this series will be coming soon and when they go live we will add the links 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 1 – Building Regulations/Standards
  1. how do we check that kinspan has been put in our garage conversion to a kitchen as indicayed on the architects plans it was conveted in 2013
    and we bought the house in 2014
    it was passed by the building control in 2013 and they will not let us know the detail
    the kitchen is very cold and draughty in winter!!

    • Hi Ann,

      I hope you received my reply I posted on 30th June. Probably the only way of checking is by drilling a hole and seeing if the cavity has Kingspan fitted (it will say ‘Kingspan on the board). If the wall was insulated internally using insulated drylining then you can often tell by looking behind electrical sockets or any other penetrations into the wall.
      Hope that helps give you a steer.

  2. we are considering converting the rear of an attached garage into a bathroom , the garage walls are 100mm dense blocks we can construct the separating wall as a cavity wall we could add 75mm to the outside with insulation and cladding the rest would have to be internally. The existing roof joist are 100mm at 400 centres under a felt covering the PA is 1.The floor is solid concrete without a dpc your suggestions would be very helpfull the property is in the south west Thanks

    • Hi Tony,

      Thanks for providing us with so much information. Our Leading Technical Advisor, Naomi, has taken a look at your project – her response is below:

      There are a few options to meet domestic extension building regulations in England.

      For the ground floor, lay a damp proof membrane over the existing concrete, lay 75mm of Kooltherm K3 on top of that, and lay a polythene separation layer and 65mm of screed on top of the insulation to achieve approximately 0.21 W/m²K. This calculation can be downloaded from the online U-Value Calculator and the build up is shown in the K3 product literature (available to download here).

      To insulate between the roof joists, 50mm ventilation will be required between the felt covering and the insulation to be installed between the joists. If the joists can be extended by 25mm, then 75mm of Kooltherm K7 between joists and 62.5mm of Kooltherm K18 fixed below will achieve approximately 0.18 W/m²K.

      For the existing blockwork wall, fixing 72.5mm of Kooltherm K18 on DPC lined battens would achieve approximately 0.28W/m²K. There are a number of blog posts on using insulated plasterboard which I’m sure you’ll find of use. You can also take a look at the build-up on the U-Value Calculator and in the K18 product literature (available to download here).

      If the existing wall is to be insulated externally, then 70mm of Kooltherm K5, covered with Nilvent breathable membrane, with a batten and cladding finish, would achieve approx. 0.26 W/m²K.

      If the new separating wall is constructed with 2 x 100mm dense blockwork, then 50mm of Kooltherm K108 with a 50mm clear cavity will achieve approx. 0.26W/m²K.

      Hope this helps you Tony,


  3. Hi I want to make my garage into a room I heard that you got to leave a gap between the felt and inserlation what size is the gap so am I right in saying I leave a gap for the air flow baton it then put loft inserlation up then the plaster board up if so do I have to put a air brick in or any think or the other way I was going to do it was just leave a gap and put inserlated plaster board up 20-30mm so ther would be a gap then the the plaster board

  4. help please.

    Plan to convert a attached garage into living space with a vaulted ceiling

    I have 150mm rafters. plan to install 100mm between rafters (600mm centres) and then 62.5mm insulted plaster board. what would the u value be?

    walls. garage is single brickwork so i was planning to put up 150mm stud and a 9mm stirling board outer with breathable membrane . again insulate between studs with 100mm. is this enough insulation or will i need to use insulated plaster board as well?

    floor. is a solid concrete raft. can i get away with 50mm and screed finish? height is a possible issue

    thanks in advance


  5. Hopefully this won’t be too long winded!

    I have a detached (from the house) single skin brick garage, with a corrugated concrete sheet roof and a concrete slab floor. It ajoins my neighbours similar garage with a shared breeze block wall. It has the usual aluminium up and over door, a side door and small window at the rear, but has no detectable damp proof course above ground level.

    I want to thermally insulate and damp proof it in order to have a dry and useable space in which to store tools and machinery and to also use as an exercise space (bicycle turbo trainer and a rowing machine).

    I was planning to use Kooltherm K5 on the exterior walls with a suitable render system in order to maximise the space inside, and probably either “dot and dab” plasterboard on the interior or fit ply panels.

    My question (eventually!) is though, what would you recommend for insulating the roof and the floor? Do I need to think about a membrane/suspended chipboard floor for example? I’m concerned about how best to manage the junction between the external ground level and the K5/render inside and/or out.

    • Hi Bernie, and thanks for your question.

      If you want to have a floating chipboard floor, your best bet is to use Thermafloor TF70. There is guidance on how to install in the product literature.

      Kingspan Kooltherm K5 External Wall Board on the walls is fine, and a good idea to maximise inside space.

      With the roof, it depends whether you want to insulate internally or externally. If internally, 50mm ventilation will be required first to avoid condensation risk, and then Kingspan Kooltherm K7 Pitched Roof Board underneath. If externally, you can use either Thermaroof TR26 or Thermaroof TR27 (depending on how you are planning to waterproof) – here it’s best to seek guidance from the product literature and perhaps a contractor too.

      Finally, we suggest speaking to local building control to determine where they are happy for insulation to be installed (for example, if you are in a listed building there may be some objections to external wall insulation) and how thick it needs to be so as to meet building regulations in your area.

      Hope this helps

  6. Hello,

    We are planning to convert our garage into an habitable room. The house is two years old and the garage sits within the property. The right hand side wall is a party wall and the other two internal walls are to our corridor, the front wall (external wall) is to the front elevation. We are planning to replace the door with matching upvc window to match existing. Our question is what insulation do we need for the walls considering the walls are all internal apart from the front elevation and for a concrete floor?

    I look forward to hearing from you soon.

  7. Hi, I have a detached double garage with a single skin brick wall and an ‘up and over’ metal garage door. The floor is of cement finish with the ceiling having exposed rafters and no insulation. I wish to use this space more for storage and always observe my garage freezer lid is frozen over and cardboard boxes are somewhat damp in winter. In summer, I can’t stand the spiders!

    Please what can I do (in terms of insulation) to make the garage more user-friendly?



  8. Hello I wonder if you can offer some guidance please ?

    We are self converting an integral garage to a room, as the garage is integral the walls are brick with a cavity the floor is concrete and because this is an integral garage there is a damp proof course on the whole house already, above it there are two more floors, the living room and bedrooms above that – Its a town house.

    Id like to know your recommendation for insulation of the floor in order to meet the building regulation U Value ??

    I believe as walls are already cavity walls I am not required to insulate them further ? And naturally there are two more stories above so no insulation is required on the ceiling either ?

    Many thanks

  9. Hello

    I have a garage attached to the house, the remaining 3 sides of the garage are:

    – rear = single skin brick with window
    – side = single skin brick
    – front = up and over garage door

    The roof construction is flat roof with felt and exposed joists from the inside. The floor is concrete which is about 5 inches lower than the house.

    I would like the make the garage into a warmer storage space. Can you help with the products required?

    What would I need to insulate the floor, walls, ceiling and if possible tips on insulating around the door.

    If I wanted to insulate the floor would I need some typeof damp proof membrane/damp proof paint before the insulation?

    Thank you for your advice.


    • Hello Jeeten

      If possible, it would be best to create a warm flat roof by insulating above the felt finish. Thermaroof TR27 could be used for this, with the existing felt being used as a vapour barrier if it is still in good working order. You would need a layer of additional felt above the insulation, or a single ply membrane.

      If a warm flat roof is not possible, a 50mm cross-ventilation would be needed below the roof finish to then be able to install Kooltherm K7 Pitched Roof Board between the joists without causing a condensation risk.

      For the single skin brick walls, Kooltherm K118 Insulated Plasterboard on damp proof course lined battens would be the best option.

      Finally, the floor. If it doesn’t already have a damp proof membrane then one would need to be installed. Assuming your garage is not going to be used to park vehicles, you could then lay:

    • Kooltherm K103 Floorboard on top, and then screed on top of that
    • Thermafloor TF70, and then chipboard on top that
    • If you are parking vehicles then you will need to use Styrozone, which has a higher compressive strength.

      Please refer to the relevant product literature for installation advice and guidance. I have linked to these below:

    • Thermaroof TR27
    • Kooltherm K7 Pitched Roof Board
    • Kooltherm K118 Insulated Plasterboard
    • Kooltherm K103 Floorboard
    • Thermafloor TF70
    • Styrozone
  10. Hi.

    I am converting my integral garage into a room. I need to check if the outside wall has insulation. It is brick cavity then breeze block.

    Do I just drill a hole into the block to check? If so how big would it need to be to see?


  11. I have plans to convert my double garage into living area which requires a insulated stud wall to be built on 3 walls, the plan shows 11mm osb and a membrane on the back side of the wall does this mean the panels have to be pre constructed then lifted and secured in place ????

    Thanks in advance

    • Hi Ken

      We need some more information on your build-up to advise, as it’s not clear what construction you have. Please could you call into our technical department to discuss? 01544 387 382


  12. Hi

    I have integral garage with two bedrooms above with no insulation in the floor these two floors are are very much cooler on the floor the than the other bedrooms to house i
    was planing on removing the ceiling in the garage to gain access.
    Can you recommend the best kingspan product to use, also does there need be a gap for ventilation?,the joists run from back to front the house is around 25 years old.

    • Hi Neil,

      As this is a floor, no ventilation will be required. We have two types of insulation that can be used between the timber joists our standard PIR insulation Thermapitch TP10 or our higher performing phenolic foam product Kooltherm K107. Both products can be cut and friction fit between the timber joists.

      Hope that helps.


  13. I am converting my integral garage into a kitchen. It is a semi- detached built in 1995. One of the walls which is a party wall is constructed of 150mm thick thermalite blocks with a 90mm unfilled cavity. This wall is the seperating wall from next doors integral garage which isnt heated What is the thinnest insulated plasterboard that i can dot & dab to meet current building regs?

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