How to insulate a garage conversion: Part 2 – Wall Insulation

hammer and wooden floor

In the first post in this series we looked at the building regulations that have to be complied with when carrying out a garage conversion including the U-values required and today we will be looking at wall insulation.

This is important as when you are converting a garage to be a more habitable room you need to make sure that the walls are properly insulated so you create a room which stays warm when it is needed. The U- values you need to achieve for walls are shown below:

England Wales Scotland
Wall 0.28 0.21 0.19/0.22

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.

The existing construction of the garage wall will have an impact on the type and thickness of insulation chosen. Garages are usually built with a brick or block single skin solid wall, without any sort of insulation. Assuming that you are not going to tear down the walls and start from scratch, there are two methods of adding insulation to the garage wall; either to the inside (Internal Wall Insulation – IWI) or the outside of the wall (External Wall Insulation – EWI) and we will look at each of these in turn.

External Wall Insulation (EWI) on a garage conversion

Adding insulation, like Kingspan Kooltherm K5, to the exterior of the wall (under a suitable render system) is probably only suitable on a detached garage as adding the extra thickness of insulation and render to the outside of a garage that is attached to the side of the house may look odd, unless the entire house is being insulated and rendered in the same fashion. One advantage of adding the insulation to the exterior is that it maximises the amount of space within the room.

If space is limited and external wall insulation is the preferred solution then Optim-R vacuum insulated panels, a thinner solution, could be used to insulate the room to the standards required.

For more information on EWI solutions visit our website.

Internal Wall Insulation (IWI) on a garage conversion

The other, and probably more common, solution is to add the insulation to the inside of the garage. The simplest method of doing this is by using insulated plasterboard (like Kingspan K18 Insulated Plasterboard). These plasterboards are mechanically fixed to timber battens, at 600mm vertical centres, with either drywall screws at 300mm centres or large headed galvanised clout nails at 150mm centres. These fixings should allow for at least 22.5mm penetration of the timber. The timber battens should be protected by a strip of damp proof course, placed between the batten and the wall. Timber noggins should be positioned horizontally at floor and ceiling level and at a maximum of 1200mm vertical centres. This method provides insulation, plasterboard and a vapour control layer in one product.

If there is an impervious finish on the exterior of the wall, such as timber cladding or a waterproof render, which prevents moisture penetration then an insulated plasterboard (like Kingspan K17 Insulated Plasterboard) could be fixed to the wall using dot and dab method. We would recommend that an exposure risk analysis is carried out by our technical services department before using this method as this does not apply in some areas of the country.

The other option is to build a timber frame system with a breathable membrane and insulation (using insulation like Kingspan Kooltherm K12 framing board) between the studs and plasterboard. The plasterboard could be insulated plasterboard (K18) if extra thermal insulation is required. This method of insulating the garage wall is probably unnecessary unless the walls are too uneven to fix the plasterboard and need straightening out, although plastic shims can be used behind timber battens if only minor adjustments are needed. The full details of fixing this can be found in our quick guide.

Wall between house and garage

When you have a party wall between the garage and a heated space (such as the house) both sides of this wall will be heated spaces and no heat loss is assumed through this construction. There is no requirement for thermal insulation between two heated spaces.

Doors and Windows

When insulating around doors and windows you will need to put timber battens around the opening to support the board edges and fix the insulation to these battens. For the window reveals, we suggest screwing 25mm of Kooltherm K18 around the reveal to prevent cold bridging from happening within this area.

Next time we will be looking at how to insulate the roof, so check the construction of your garage roof ready for this. 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 2 – Wall Insulation
  1. We moved to a barn conversion last year where the garage had been converted to a kitchen in 2011 we cannot see any insulation between the outer and inner walls and it,is very draughty
    Should the building inspector have checked this?
    It is very cold in winter and we are thinking we will have to put in insulation and remove the plaster board but are worried about the cost
    We rang the council but they have not been helpful and say we cannot claim from the builder as we did not order the work.
    We live in Lancashire in Colne
    Do you have recommended builders who would do the work?
    The survey did not highlight the problem either
    We can see the outer wall through the boiler which is housed behind a cupboard

    • Hi Ann,
      First of all, don’t worry. Secondly please be aware that Kingspan Insulation does not manufacture, sell or install the cavity wall insulation I think you may be referring to (beads of polystyrene or fibre blown into the cavity through holes drilled in the wall). We only manufacture rigid boards which are either installed when the building is erected or during refurbishments. Having said that, you do have plenty of options (hence the don’t worry bit!).

      1. The National Insulation Association ( with the blown in sort of cavity wall insulation. They may also help you get some redress if the installer was one of their members. Their website also provides a list of installers too if your walls are suitable.
      2. Building inspectors are often not-on site for refurbishments and renovations – it really depends upon where you are or whether the work was undertaken by a ‘competent installer’ who can effectively sign-off work on behalf of building control. However, you should have or would have a certificate if this was the case.
      3. You have a number of other options for insulating your walls:
      a. Internally insulate using pre-insulated plasterboard to achieve a U-value of at least 0.30. A product such as Kingspan Kooltherm K17 would be used for this. You’d need around 62.5mm of insulated plasterboard for this, but do check with us for your particular construction).
      b. Insulate externally using a product as as Kingspan Kooltherm K5. You’d need around 50mm thickness for this (but contact us to check your exact construction).
      Both of these options need careful detailing around hot flues from your boiler.

      I hope this helps. If you need any more help please call our technical services team on 01544 387 382.

  2. I’m planning a garage conversion, there is a cavity between the external brick and internal block, but there is no insulation in the cavity. Can you please advise on solutions other than blowing insulation into the cavity.
    Thank you Jon

  3. Hi,
    I have bought an old unusual house and it is very cold, we have a large double height (very cold and draughty) garage with glass roof and old barn doors attached to our hallway. The wall between this hallway (part of an open plan downstairs) appears to be just a stud wall with plasterboard on the house side and timber cladding on the garage side. Hence the house is very cold, what would be the best way to insulate this wall and keep the house warm. bring down the plasterboard and insulate between with kingspan? there are no electrics on this wall at all so no wires to deal with.

    • Hi Joe,

      Thanks for your message. This is a very common wall build-up and you’ll be pleased to know there are a number of options. You’ll need to know the depth of your studs and the distance between each stud centre (normally 400 or 600 mm). There are two basic options: One is to insulate only between the studs. The other is to insulate between the studs and to add a layer of insulated plasterboard on the house (warm) side as well. As we don’t know you stud depth or centres, I’ve included links to two calculations on the U-value calculator which will illustrate both, but you will need to adjust them to suit your stud depth and centres. Please also note the use of breather membranes and ventilation to avoid condensation risk.

      Insulating only between the studs.
      Insulating between the studs and the use of insulated plasterboard.



  4. Hi we have just added a single brick 102.5mm porch to existing house. Plan is to dot and dab 37.5mm k17 boards to the single brick then skim.the existing house has insulated cavity walls. Do we just dot and dab moisture board onto that aspect?

  5. Good Morning.

    A very quick question – I have a garage converted from an outbuilding, it has single skin brick walls.

    I would like to insulate it, but keep it as a garage, will fixing K18 board to battens (as per your information in this topic) comply with building regulation?

    Would I have to inform building control that I am insulating the garage if I am not changing its use, all I want to do is make it warmer and have flat walls to fix cabinets and tool boards to, yet ensure it is as dry as possible.

    • This is a very common question. The issue is whether your outbuilding is a habitable space – from the sounds of it, you are keeping it as a garage, so it is therefore not a habitable space and you are not changing its use. We cannot answer for Building Control, and sometimes the advice differs, so our best advice would be to ask them (they normally have a free helpline).
      72.5mm of Kingspan Kooltherm K18 Insulated Plasterboard on battens will achieve a U-value of 0.28 on a single skin brick wall.

  6. I have a 15ft square garage ( which used to have a toilet and a coal store which were removed by previous owners) It has an extremely uneven floor and a pitched roof full hip- pyramid style) with double roman marley tiles and a cromar vent 3 breathable membrane. It has double brick walls. The garage door has been replaced with a small back door and the remaining gap bricked up, I would like to use it as a workshop however condensation is a huge problem , I will be installing two small windows and hope to have a wood burning stove eventually, Can you recommend suitable products, unfortunately I cannot find any evidence of a damp proof course in the walls or floor?

    • Hello and thank you for your comment.
      Sounds like an interesting project. For a start you will need to get to the root cause of your condensation – the source of the moisture. This needs to be resolved first before you do anything else.

      For the floor, we’d suggest some sort of leveling compound or you may wish to dig out the floor and add a damp proof course. Depending upon how heavy the items are in your workshop will depend upon the type of insulation solution you choose. For heavy duty work (eg driving cars in) then Styrozone would be the best option. For lighter duty, then either Kingspan Kooktherm K3 between battens or under screed or even Kingspan Therafloor TF70 if you went for a floating floor option.
      For the walls, I’m presuming that this is a solid wall (ie no cavity between the two layers of brick). In this instance you will need to add Kingspan Kooltherm K18 Insulated Plasterboard on battens or metal framing.
      For the roof, again I’m presuming that you will not want to re-roof the building, so ‘between and under’ is the option here. As you have a breather membrane, this can be an unventilated choice. Use Kingspan Kooltherm K7 Pitched Roof board between the rafters with Kingspan Kooltherm K18 Insulated Plasterboard lining the building beneath.

      All these options can be found on our free U-value calculator.

  7. I would like to internally insulate a room in a Victorian house using the dot-dab method.

    The K17 applications, though, suggest that a single skin wall needs to be rendered on the outside, whereas my house is not.

    The wall does not suffer any damp problems. Could I use K17?

  8. I am about to convert an integral garage to a utility room . the external wall is single skin brick wall
    i want to use the thinnest option to get the u value . i am going to cut a door in as well . outside is a protected east facing side entrance .
    can you let me know thinnest option
    rich jones

    • Thanks for your question Richard.
      We will need to know more about your construction, geographical location, local exposure and condition of the wall to advise you properly on this.
      Contact us on 01544 387 382 and one of our technical advisors will ask you a few questions and recommend the best options for you based on your answers.

  9. Hi,
    My garage is attached to the house at one end and I am converting it into living space. It looks like I will have to install load-bearing timber framing. The designs for timber framing imply that the timber framing is built before the external brickwork. As I’m converting the garage, the external brickwork is already there. I can’t see how to make it fully. I can pre-fabricate sections of timber framing, but how do you attach wall ties to the membrane and OSB sheathing?

    Many thanks,


    • Hi David,
      Thanks for your question. This is really one that we would recommend you talking to a wall-tie manufacturer (eg Ancon) for advice on how they would recommend doing this. It is also worth getting the advice of your structural engineer as you say the timber frame will be load-bearing.

  10. Hi,

    I have read so many diffrent things regarding this.

    I have a garage that is attached to the house and has a bedroom above it..

    I have just insulated the garage ceiling with 100mm between joists and 25mm under joists , tapped an fired boarded. Bedroom above garage is warmer.

    The next problem is as follows…

    The garage is attached to the house by 2 walls.. 1 connecting to the main hallway and 2nd wall connecting to downstairs toilet. These areas are freezing even when the heating is on!

    Inside the garage are untouched breeze block walls… so would it be worth insulating the garage walls that connect to the house ?

  11. Solid wall insulation
    Hi we are renovating a 250 year property. It is lower than street height, and has solid walls. It has been Damp Proofing Injected and tanked. We have been told different ways to insulate the solid wall as condensation apparently is a huge issue and the walls need to breath. Please can you advise the most efficient way of this bearing in mind it still gets a bit damp in wet weather.

    • Hi Sally,

      Thanks for your question. You have a couple of options here. We would recommend the following:
      – Using our Styrozone insulation, which is often recommended when renovating basements due to its resistance to ground moisture penetration
      – Fixing our Kooltherm K18 insulated plasterboard to timber battens

  12. Thanks i’ll have a look at these products. Should I use metal battens as wood could/ will absorb moisture, or is that overkill?

    • Hi Sally,

      We understand your concern about moisture, and a steel channel system can be used if you are worried. However we usually advise that you line your timber battens with a damp proof course before fixing them to the masonry wall, which will protect them against moisture penetration. You can then fix the Kooltherm K18 Insulated Plasterboard to the battens. Our Structural Quick Guide (see pages 38 – 39 for advice on how to install the K18 Insulated Plasterboard) will help. Hope this helps.

  13. Hi

    I have a utility room with a single skin brick wall, which is freezing!

    I am looking at using the K18 product on batons. The room is already narrow and so wish to use as thin a product as possible.

    I was thinking of using 32.5mm K18. Will this make a significant difference.

    What thickness would likely be required to achieve a similar rating to a double skin cavity wall?

    Thank you in anticipation


    • Hi Paul,

      Thanks for your question. First things first, ensure that your wall is absolutely watertight and you sort out any problems (eg poor mortar joints; any damp penetration etc).
      Yes, 32.5mm of Kooltherm K18 Insulated Plasterboard will make a difference, but won’t meet Building Regulations or Standards in England, Wales or Scotland. For these countries you will need 72.5mm, 82.5mm, and 92.5mm respectively. To achieve the equivalent of a new build wall, you’ll need at least 122.5mm. Have a play for free on our U-value calculator.

      regards, Peter

  14. I am also considering K18 to insulate an existing single skin 215mm face brick garage. I have used the online calculator which recommends a minimum of 102mm insulation.

    The online stores I have looked at to source your product suggest that K18 comes in a maximum thickness of 60mm. Is this true, and if so how do I achieve the 102mm?

    Kind regards

    • Hi Simon,

      Firstly, it’s great that you’ve been using the U-Value Calculator and have found the information you need from it.

      Kooltherm K18 Insulated Plasterboard does run in thicknesses over 60mm. However the 102.5mm board you are looking for (which is 90mm of insulation topped with 12.5mm of plasterboard) is subject to a minimum order, so it is unlikely that you’ll find it on any shelves. It’s certainly worth checking with your nearest stockists though.

      If you don’t manage to get hold of it, we would advise using 90mm of our Kooltherm K12 Framing Board instead. You could then line this with 12.5mm of plasterboard, which will allow you to achieve that 102.5mm thickness you need.

      Hope this helps.

  15. Hi

    I am planning to convert a 2.5 meter by 5 meter single brick walled garage (attached to the house on one wall) into a study room.

    My main concern is Damp proofing. Should I use a damp proof that runs over the floor and up the walls? Also do I have to leave a gap between the damp proof and the insulation?

    I will be making a raised timber floor. Will it be a problem having the wooden battons touching the walls or floor?


    • Thanks for your question – you’re right to be thinking about damp proofing correctly.

      In regards to the floor, a damp proof membrane should be laid across the ground floor concrete slab. The timber battens should then be laid on top, with Thermafloor TF70 installed between them.

      Now to the walls. We wouldn’t recommend that you cover the entire wall with a damp proof membrane. Any insulation placed internally to the this could then potentially cause a risk of condensation. Instead, a vapour barrier should be installed on the internal, warm side of any insulation.

      Hope this helps.

  16. Hi I am looking to insulate my garage its single skin on 3 sides and attached to the house on the fourth.

    The problem being its built out of stone and the inside walls are really uneven I am talking inches not MM.

    What products would I need and how would you advise attaching the insulation/plasterboard?

    • Hi Tom

      Thanks for your question.

      We suggest perhaps fixing Kooltherm K18 Insulated Plasterboard onto battens that vary in size to level out the surface of the wall. The product literature gives fixing advice, but to give an overview – the battens should first be lined with damp proof course, then fixed to the wall. The insulated plasterboard should then be fixed to the battens with long dry wall screws. Note that additional fixing advice should be sought from a fixing manufacturer.

      Hope this helps.

  17. Hi
    I have a garage made of single skin breeze block ,and corrugated roof, the walls get damp, I want to convert part of the garage to a hobby room.
    Could you advise me on the best product to use to achieve a resolution to the dame and to form so insulation
    Thanks for your re ply

    John mulford

    • Hi David

      Thanks for your question.

      If you want to use 9mm plasterboard that is fine. However we usually recommend 12.5mm because it is more robust. You’ll have to bear in mind the impact choosing 9mm over 12.5mm will have if you are considering applying any fixings to the wall.

      Hope this helps.

  18. Hi,

    We currently have a garage that has been poorly converted to a kitchen 10 years ago before we bought the property.

    We currently have standard plasterboard attached to the walls by the dot and dab method. Can we attach insulated plasterboard over the current plasterboard or should we be stripping back to the brickwork and starting from scratch?

  19. Hi, I’m planning my garage conversion however wanting to keep the garage doors with 1.5-2m of garage space. It’s unclear how this wall separating the new garage space and new living space should be constructed. I’m planning a timber frame rather than masonry (as it’s well protected in terms of weather) with insulation (unsure of spec) between the studs and plasterboard both sides. Have you any advice on this scenario?

  20. I want to insulate my wooden garage doors, and also seal the edges. Which Kingspan would you recommend. I need to be able to open and close the doors. I’d like the best insulation I can get.

    Many thanks

  21. Just had a concrete section garage type shed built in Nov but need to seal and insulate the wall and roof area. This is to keep it dry not insulation to keep warm etc, temp heater will be used when in there. Just looked at K118 would this do the job and could it be mounted using the correct sealant to stick it up. the wall are 2 foot sections.
    Any advise would help.

    • Hi Steve

      Kingspan Kooltherm K118 Insulated Plasterboard cannot be fully adhered to a wall. Depending on what adhesive is used, either two or six additional fixings will be required, as well as bonding agents etc. Take a look at the product brochure, which advises on how to install depending on your chosen adhesive.

      If the garage is solid concrete, it might actually be more beneficial to fix DPC lined battens to the wall first, and then mechanically fix the insulated plasterboard to these. This will create a cavity to prevent any water ingress coming into contact with the insulation on the external walls.

      If the concrete garage ceiling requires internal insulation, and the space above is external or unheated, then 50mm cross-ventilation will be required first to prevent a condensation risk. Insulation above the concrete roof would be recommended if ventilation is not possible – take a look at Thermaroof TR26 LPC/FM and Thermaroof TR27 LPC/FM for further information.

  22. Hello I’m currently converting my detached single brick thickness flat roofed garage into an office. Which is the correct thickness of thermal plasterboard to use for the walls & what is best suited for between the rafters which have a 120mm recess.

  23. Hi

    Our conservatory is about 15-20 years old. Both sides are single skin brick walls, currently with rock wool between battens and plasterboard on top. We are having the glass all replaced with more energy efficient stuff but I wonder is it worthwhile improving our wall insulation too. If so what should I use? Going to insulate the floor too. We are trying to make a room that we can use in winter too!


    • Hi Donal, and thanks for your question.

      The floor insulation can be Kingspan Kooltherm K103 Floorboard if the build up is beneath screed or concrete, or between timber joists. If you are using a timber floating floor it would have to be Kingspan Thermafloor TF70.

      Unfortunately we can’t advise any further as to which floor construction you should choose, as you will need to opt for something that suits the existing structure. Take a look at the respective product literature however, which should be able to help you decide which is more suitable.

      When it comes to the walls, adding rigid board insulation to the existing construction may be difficult. The most efficient way would probably be for you to remove the existing plasterboard and fix Kingspan Kooltherm K118 Insulated Plasterboard to the wooden battens. Ensure that the insulated plasterboard is much thinner than the mineral wool that is currently between the battens in order to avoid a n interstitial condensation risk.

  24. Hello
    We are looking to start converting our single skin garage that is attached to the side of our house in April but are looking to add 2mtrs to the front of the garage to bring it level with the front of the house. Can I use a single skin on the new wall and Kingspan K118 the lot or will the new walls need to be cavity? If they need to be cavity, how do you change from the Brick Cavity to Kingspan?

    Thanks in advance


  25. Hi. Converting a detached garage to a workshop. Non-habitable but I’d like to insulate to a good but economic standard. My question is about build-up of the internal insulation for the outer walls, which are single skin brick 100mm in good condition approx 15 year old.

    Rather than plasterboard internal finish, I would like to use water resistant ply or OSB for convenience of fixing tools etc to it. I was going to use 75mm PIR insulation boards.

    Can I put the PIR boards direct against the inner brickwork, then a 25 x 50 batten framework with fixings through the battens and insulation into the brickwork. Then a vapour layer and the final ply/OSB internal finish screwed onto the battens?

    So, build up is :
    Vapour control layer
    25mm battens
    75mm PIR
    100mm Masonry

    This would seem to keep the battens away from any moisture through the masonry. Does this setup have any advantages/disadvantages over the method you suggest (battens against the masonry, then the PIR and internal finish fixed through to the battens).

    • Thanks for your question.

      The insulation should be fixed to the batten rather than directly to the masonry. This is because with a single skin of masonry there is a risk of moisture ingress from driving rain. This moisture can track through the masonry and come into contact with the insulation. The 25mm batten provides a cavity to lift the face of the insulation away from the wall, thus protecting it from moisture. The battens should have strips of damp proof course behind them, in order to protect them from the moisture.

      Hope this helps.

  26. Have a question. I have an internal garage in a house built in 1929. I don’t want to convert the garage but want to insulate the walls and ceiling because they transmit too much heat out of the house. Is there a way to do this easily and cheaply and without losing much space?

    • Hi Reed,

      Without knowing the exact build-ups for the wall and ceiling it’s very difficult to specify the most appropriate insulation. Initially we would recommend looking at our online U-value calculator where you can input the makeup of both constructions, whilst this tools main function is to provide U-value calculations, it will also identify suitable insulation products for your constructions. Alternatively, please contact the Kingspan Insulation technical department on +44 (0) 1544 387 382 or email them at, to receive further guidance.

      Hope that helps,

  27. Hi
    I am wanting to convert my garage into a living space. It is detached from the house and has a pitched roof and the walls are double bricked(225mm thick) with no cavity. We live in Bournemouth and would like to know what insulation would you advice.

    Thank you

  28. I live in a mid terrace house. My house has two garages under the sitting room. One is mine, accessible from inside the house, the other belongs to a neighbour. I intend converting mine to a home gym. The party walls are made of breeze block with a layer of brick either side. I intend using the batten method to install insulation. What thicknesses would you recommend?

  29. Hi.
    I live in a new build home (2015) which has a detached single skin red brick garage. Currently anything stored in the garage goes mouldy within a month or so.
    The garage I want to convert to a usable workspace as self employed and want to ensure that there is no more mould etc. How do I do this from start to finish. Thanks.

  30. Hi I am building an office. Two walls will be single skin brick with doors and windows in- for a pretty exterior. The other 2 walls will be block. I want to insulate so understand I need to batten at 600 intervals and at floor ceiling and 1200 but what boards and or insulation is best. Block walls will not be rendered as they will be under cover

  31. Hi, I have a triple detached garage and I am wanting to build a stud wall to separate 1 garage and make the rest a non-habitable space. This is subject to building regulations. Can you please tell me the best way to build the stud wall? I want to use timber, no water enters the garage so minimal risk of damp. Also want to leave on the garage doors and internally brick up and insulate, can you please tell me how this would be acheived

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