Following on from Garage Conversions Part 1: Compliance blog post, we are looking at the key considerations for your garage conversion, with a focus on improving the fabric performance of your floor, walls and roof.

*Note: Since the creation of this post, Celotex GS5000 has been replaced with Celotex GD5000.

To achieve a good quality garage conversion there are three main areas for design and construction professionals to focus on:

  1. The first is designing and building a good fabric performance to limit heat loss through the building envelope.  This means using the correct thickness of insulation to meet the target U-values for the walls, roof and floors, whilst continuing the line of insulation around junctions and openings to limit heat loss and maintain air tightness. The specification of good quality energy efficient windows and doors is also key as they account for the remaining parts of the new thermal envelope.
  2. The second factor to consider when considering the overall energy efficiency is the choice of fuel and heating systems and thirdly the use of lighting and electrical appliances.
  3. As Celotex are insulation specialists, the focus of this blog will be on how to achieve a good fabric performance through the garage floor, walls and roof with a few comments about thermal bridging and air tightness.


The floor of a garage is usually a concrete slab which is in direct contact with the ground.  It will form part of the new thermal envelope and therefore require insulating to meet 0.25W/m2K U-value.  The finished floor level of the concrete floor is usually lower than that of the main house which means there is room to lay Celotex over the slab.

Celotex FR5000 can be laid across the floor slab with the thickness required dependant on the ratio of exposed perimeter to floor area.  This is worked out by dividing the exposed internal perimeter in linear metres by the internal floor area in m² to give the perimeter/area ratio.

The floor will need to be finished with a layer of screed or timber boards to form a stable surface for foot traffic and furniture etc. The screed is a minimum of 65mm thick and typically a standard sand and cement mix.  The screed laid above the insulation forms a thermal bridge or path of heat loss where it contacts the external wall.  This can be eliminated by placing perimeter edge insulation vertically around the edge of the floor slab. The up stand should have a minimum R-value 0.75m2K/W which can be achieved using Celotex TB4020.

An alternative to laying 65mm screed is a 22mm tongue and grooved floating chipboard floor.  Laid to manufacturer’s instructions this also provides a stable surface but offers a thinner floor build up which is ideal if space is a premium. A perimeter up stand is not required as the chipboard floor doesn’t present the same path of heat loss. Instead Celotex is butted tightly up to the wall.   Any decorative finishes such as carpet or laminated timber boards can then be laid on top of the screed or chipboard floor.

What about membranes…

There are two types of membranes required when upgrading a garage floor.  The first is a damp proof membrane. It’s hard to know if one was used when the floor was first constructed, so in the absence of this knowledge it’s a good idea to lay one over the slab before installing the Celotex.  This will stop any moisture from below ground rising to cause dampness and damage.

The second membrane is a 1000 gauge sheet of polythene. We recommend laying this over the room side of the Celotex before finishing the floor with a layer of screed or timber boards. This not only acts as a vapour control layer to minimise the risk of interstitial condensation forming on the concrete slab but also stops liquid screed migrating between board joints to form thermal bridges.  It also prevents the chemicals in the wet screed reacting with the foil facer to form tiny air bubbles which may potentially weaken the screed.


Garage walls are usually solid masonry and because they were designed to enclose an unheated space they are commonly ‘single’ masonry and 100mm wide. To make them more stable brick piers are often built into the wall.  These are generally ‘double’ masonry and 200mm wide. This means they occasionally protrude into the floor area.

When upgrading the walls thermally, because the walls are only 100mm thick its key to design an internal lining system that stops the moisture coming in from outside. The easiest method of dry-lining the wall is to fix 25mm x 47mm timber battens which are lined with dpc strips directly to the wall. This method has the advantage of providing a small cavity between the masonry and internal linings as well as a substrate to fix the insulation board.

Celotex GS5050 is fixed to the battens to insulate the walls. This is a thermal laminate made of 50mm of the highest performing Celotex PIR insulation laminated to a 9.5mm plasterboard.  This high performing insulation board has the advantage of giving 0.3W/m2K while taking up a small amount of floor space.

The brick piers if left exposed will bridge through the thermal envelope and become a path for heat loss and present cold spots where surface condensation may form. The line of insulation should be taken around any brick piers rather than butting up either side of them. The idea here is to include them within the thermal envelope.  For the same reasons the insulation should be taken into the reveals and soffits of door and window openings.  Celotex PL4015 can be used for these smaller areas. This is 15mm of Celotex laminated to 9.5mm plasterboard.

The thermal performance of the junction between the wall and floor is maintained because the Celotex GS5000 is continuous with either the Celotex TB4020 used for the perimeter up stand or Celotex FR5000 under the chipboard floor.  As GS5000 is mechanically fixed as opposed to dot and dabbing to the masonry, the continuity of the air barrier around this junction is achieved by sealing the gaps between the skirting board and floor.


A vapour control layer is always required on the room side of the insulation. It stops warm moist air getting behind the insulation and condensing on the surface of the masonry wall to form interstitial condensation. Celotex GS5000 has an integral vapour control layer. It is formed by sealing together the tapered edges of the plasterboard with scrim tape and jointing compound to form an effective barrier with a high vapour resistance. It is equally important to seal around the edges and joints of any service openings such as electric sockets and switches. Sealing between the boards joints and around the edges and openings effectively creates an air tight barrier as well as maintain a vapour control layer and helps minimise air leakage and paths of heat loss.


Single storey garages more commonly have a roof which is flat in construction. The roof is waterproofed with a high moisture resistance membrane such as a bituminous felt. Insulation can be installed between the joists but because the roof covering has high moisture resistance Building Regulations require a 50mm gap for the path of ventilation immediately below the plywood deck. The 50mm path of ventilation is to reduce the risk of interstitial condensation forming on the underside of the plywood deck and settling on the timber rafters. This means the thickness of Celotex FR5000 required is the depth of the joists less 50mm required for the path of ventilation.

The joists are usually not deep enough to fit the thickness of insulation required to achieve the 0.18W/m2K U-value plus allow for the 50mm ventilated airspace above. Celotex GS5000 can be installed as a second layer of insulation to the underside of the joists.  It provides a continuous layer of Celotex to meet the target U-value and because it is has a 9.5mm plasterboard finish it also forms the internal ceiling of the new room.

This prevents having to increase the depth of the rafters by fixing timber battens to the bottom of them between which the extra thickness of Celotex can be installed. The use of GS5000 as a continuous line of insulation also limits the unfavourable impact of repeat thermal bridging on the U-value allowing for a thinner solution and so saves on valuable headroom.

The thermal performance of the junction between the flat roof and wall is depends on how they are constructed.  The main principle to follow is the insulation used to line the wall internally meets and/or overlaps with the insulation fixed to the underside of the joists.  All gaps are sealed along the length of the junction to maintain air tightness.

The importance of a vapour control layer…

Condensation is formed when warm moist air rises and condenses into a liquid on contact with the colder surfaces above the insulation. As warm moist air naturally rises the risk of interstitial condensation with any roof construction needs to be carefully managed. So for this reason a vapour control layer is doubly important when upgrading a garage roof.

As mentioned before, Celotex GS5000 has an integral vapour control layer and blocks warm moist air rising into the pitched roof structure.  However, warm moist air by its very nature will always find the passage of least resistance so the use of a 50mm ventilated air gap between the joists in conjunction with a vapour control will effectively manage the small amount of condensation that will inevitably form

…And finally

The process of a garage conversion passes through a number of stages from the initial idea, obtaining planning permission (if required), preparing a detail design and getting approval from the local building control. It’s at this stage the search for a good builder starts and then the final stage is the construction process.

There are two stages which present the best time to ensure a quality energy efficient conversion.  This is the detailed design stage where the correct materials and services are not only specified but also drawn to show they are used together without conflict or compromising a good fabric performance. The second stage is the construction process, that the builder is knowledgeable of the materials specified and is fully skilled in the correct installation. This allows for the as built standards to be equal to the design standards. Both these factors will affect the overall energy efficiency of the conversion and if done to correctly will bring about a new, sustainable and comfortable habitable space.

For advice on your project get in touch with the Technical Centre, or find out more about how Celotex GD5000, FR5000, TB4000 and PL4000 can improve the fabric performance of your garage.