How to insulate a garage conversion and improve energy efficiency

Garage conversions are a popular way to expand the living space in a home, transforming a previously under-utilised area into valuable living space, such as a lounge, a kitchen extension or even a bedroom.

A successful garage conversion project requires careful planning, and one element thvat should not be overlooked is insulation. In many cases, an existing garage may not have any thermal insulation, meaning that the roof, walls and floor will need to be upgraded to make the space comfortable and warm, while ensuring all relevant regulations and standards are met.

Do garages need to be insulated?

Whether a garage needs to be insulated depends first and foremost on its intended use and whether it forms, or will form, part of the building’s thermal envelope. The thermal envelope is the layer of a building that separates heated or cooled indoor spaces from the outside. It includes external walls, floors, roofs, windows, doors and any internal walls, floors or ceilings that divide a heated space from an unheated one.

If a garage is detached and not designed to be used as a habitable room, insulation is generally not required under Building Regulations. In these cases, any decision to insulate is usually about comfort and maintenance rather than compliance.

As David Milner, Technical Team Lead at Celotex, explains:“Insulation can help to improve thermal comfort by reducing temperature fluctuations and raising internal surface temperatures, which may also lessen the likelihood of surface condensation. It should, however, be considered as part of a wider moisture strategy that also takes account of ventilation and airtightness.”

Where a garage is attached to the main dwelling but is not habitable, insulation will normally be required to the walls, floors or ceilings that separate it from conditioned areas of the home. In some older properties, these separating elements may not meet current standards, so additional insulation may be needed as part of refurbishment work. Building control will usually expect this to be addressed in line with Approved Document L.

In the context of a garage conversion, the situation is more straightforward. If the garage is being turned into a habitable space, then insulation is required. Approved Document L sets minimum standards for thermal performance to ensure the new living area is energy-efficient and comfortable.

What are the benefits of insulating a garage conversion?

The main reason for insulating a garage conversion, or any part of a home, is to limit heat loss through the building fabric. This improves the thermal performance of the construction, supports better temperature control, and contributes to overall energy efficiency.

Thermal insulation can raise internal surface temperatures, which may reduce the likelihood of condensation. As part of an overall moisture strategy, this can help limit the conditions where damp and mould might develop.

Insulation can also offer a degree of acoustic benefit by reducing sound transmission, although where noise reduction is a priority, specialist acoustic products should be considered.

David also notes that the benefits are not limited to the converted space itself. “An adjoining garage conversion that becomes part of the new thermal envelope can improve the comfort of the space itself and may also contribute to the overall energy efficiency of the home, assuming adjoining elements were not already insulated to a comparable standard.”

What product can I use to insulate my garage walls?


The most suitable insulation product for your garage conversion depends on how the garage was originally built. So the first step is to understand how the existing building was constructed.

For example, does the garage have solid walls or cavity walls?

If the garage was built before the 1930s, it may have been constructed with solid brick walls. These are most often around 215 mm thick and built without a cavity, although some garages may have been built with half-brick (102.5 mm) walls, typically supported by piers. Both types are generally less thermally efficient than modern cavity wall constructions and will usually require insulation to be added either internally or externally to meet current standards.

Rigid insulation boards, such as Celotex PIR (polyisocyanurate), can be used for these applications. With a low thermal conductivity (lambda value of 0.022 W/mK), PIR can achieve required thermal performance with thinner wall build-ups than many alternative insulation materials.

For solid masonry walls, adding insulation to the external face is generally the preferred approach for thermal upgrades. Fixing insulation to the outside of the property keeps the wall structure warmer, which can reduce condensation risk and simplify moisture management. It also helps improve performance at thermal junctions compared with internal solutions. The insulation is then finished with a breather membrane, vented cavity, and protective layer, such as render, cladding, or weatherboarding. EWI may also be considered where external walls show signs of wear, since it provides a new outer layer while improving thermal performance. Planning approval may be required, as EWI can change the appearance of the property.

Where EWI is not possible, internal wall insulation (IWI) may be used as an alternative. A holistic approach should also consider airtightness, ventilation, and moisture management to reduce the risk of condensation. It is strongly recommended to seek guidance from a certified retrofit assessor, or from organisations such as the STBA or SPAB, before specifying internal wall insulation. Internal insulation should only be applied to walls that are not directly exposed to driving rain or external weathering, such as walls that are protected by cladding or sheltered by other parts of the building. (See: Retrofit internal wall insulation: best practice for more detailed information.)

In line with this guidance, Celotex requires a ventilated cavity to the exterior when internal wall insulation is specified on exposed walls, such as those not protected by cladding or sheltered from wind-driven rain.

A typical ventilated IWI build-up might include:

• A metal liner system or treated timber battens, spaced to create a cavity against the wall.
• Ventilation openings to the exterior in line with BEIS Retrofit IWI guidance.
• A breather membrane fixed to the battens to help manage airtightness.
• Celotex GA4000 insulation boards cut to size, with joints and penetrations carefully sealed.
• Secondary battens to support lining boards and provide service runs.
• An internal lining board fixed to the battens as the finished surface.

Other ventilated IWI options are described in the Celotex Handy Guide (pages 26–27) and our BBA certificate which should be consulted for detailed installation guidance.

From the 1930s onwards, cavity walls became more common. These are formed from two layers of masonry with a gap between them. In some cases, this cavity may already contain insulation, either original or added later by injection. Where further improvement is required, continuous internal insulation with PIR boards can be considered as a way to enhance thermal performance and help reduce thermal bridging.

Before starting any refurbishment, check whether the structure requires other repair work first. If there are any issues with damp or mould, these should be resolved before undertaking the refurbishment to minimise the risk of future problems.

How do I insulate the ceiling of a garage conversion?

Of course, it’s not just the walls that need to be insulated in a garage conversion. The roof and floors also need insulation to meet thermal performance standards, and the type and placement depend on the existing structure and project requirements.

For existing pitched roofs, the most common approach is a ventilated roof with insulation placed between the rafters and a continuous layer added below them to reduce thermal bridging. For flat roofs, a warm roof build-up is generally preferred, as it keeps the structure warmer and simplifies moisture management. However, where the existing roof finish is being retained, a cold vented flat roof may be the more practical option. Depending on the garage size, the loft area above a pitched garage roof may also be converted into a livable area.

If the roof has a cold construction, insulation can be installed above the ceiling, leaving the loft or roof void unheated.

How do I insulate the floor of a garage conversion?

Garage floors are typically constructed as solid concrete slabs. For a conversion into a habitable room, rigid PIR insulation boards are commonly used, as they provide low thermal conductivity and good compressive strength for domestic floor loads. The insulation is usually laid over the concrete slab and finished with a screed or a suitable flooring system.

Where the space will continue to accommodate heavier loads, such as machinery or vehicles, a higher-strength material such as extruded polystyrene (XPS) may be more appropriate.

The importance of ventilation when insulating a garage refurbishment

While insulation improves thermal performance and helps retain heat, greater airtightness can also reduce the natural air movement through a building. Without appropriate ventilation, this may increase the risk of condensation and damp. Effective ventilation is therefore essential for maintaining healthy indoor air quality and reducing the likelihood of mould growth. Ventilation options include:

• Trickle vents in windows
• Mechanical ventilation or extraction fans
• Roof ventilation, such as eaves and ridge vents
• Breathable membranes beneath roofing materials British

Standards such as BS 5250 (Code of practice for control of condensation) and building regulations Part F (Ventilation) provide guidance on adequate ventilation to balance thermal efficiency and moisture control.

Why choose Celotex to insulate your garage conversion?

The Celotex range includes PIR and XPS rigid boards, which combine low thermal conductivity with options for thinner build-ups compared with some alternative materials. With a choice of thicknesses and suitability for different build-ups, these products can be beneficial in garage conversions where maintaining interior space is often a priority.

To find out more about our insulation products, please get in touch with a member of our team. We’ll be happy to discuss your project requirements, discuss typical applications or help with thermal calculations.