Garage Conversions

garage conversion before
garage conversion after
Garage Conversion Before
Garage Conversion After

For many of the house owners in UK, the garage conversion is one of the most economical ways to add a new habitable room to your property and it also increases the house value. Convert a garage and enjoy the extra space on the ground floor without reducing the natural daylight into other rooms. Building an extension is proving much more popular than moving home. The cost of building an extension is much more than garage conversions. Modern living with more people working from home, children with computers and gaming machines makes a home extension very attractive. Moving the dining room into the garage conversion allows you to have a kitchen extension in both existing kitchen and dining room.

Your options are endless but here are some ideas to get you started:

  • A family room
  • A bedroom with or without en-suite facilities
  • A playroom
  • A home office/study
  • A kitchen/utility room
  • A dining room
  • A home cinema

Garages are relatively easily converted to usable rooms. They are usually of sound construction. The biggest consideration is replacing the garage door with a window and increasing the insulation to current standards. This guide will give advice on increasing the insulation requirements and the different option's available.


The building regulations impose minimum insulation standards for garage conversions, which are considered to be ‘material alterations'. The recommended Best Practice insulation standards (maximum U-values) set out in Table 1 provide for a better overall standard of insulation, thus reducing fuel use, fuel costs and carbon dioxide emissions. The cost of additional insulation may be offset against the savings from needing a smaller heating system (e.g. a smaller boiler and fewer, smaller radiators) resulting from the reduced heat loss.

U-Values Table

Wall 0.30
Pitched roof (horizontal insulation at ceiling level) 0.16
Pitched roof (inclined insulation at rafter level) 0.20
Flat roof or roof with integral inclined insulation 0.20
Floors 0.22

Types of insulation

Three types of insulation are commonly used in garage conversions:
• rigid insulation;
• flexible insulation;
• thermal lining boards.

Rigid insulation is usually a form of plastic foam board, e.g. polyisocyanurate board. Examples of the flexible type are glass fibre and mineral fibre quilts. Thermal lining board usually consists of mineral wool or plastic foam insulation bonded to plasterboard, and containing an integral vapour check. For a given thickness the rigid insulants usually have better insulating properties (i.e. lower thermal conductivity) than flexible types.

Insulating garage floors

When a garage is converted, the ground floor should be insulated to achieve the Best Practice U-value shown in Table 1 (i.e. to achieve a maximum U-value of 0.20W/m2K). The ground floors of garages are usually concrete, typically a ‘ground bearing’ slab, but in some instances suspended, pre-cast concrete ‘beam and block’ units may have been used. The finished floor level is nearly always lower than that of the main house. Where the floor level is required to be raised, to bring it level with the house floor level, this presents an ideal opportunity to add insulation. This can be done in a number of ways but the more common methods are to:

• lay rigid insulation on top of the existing concrete floor, and finish it with a concrete screed or timber boards;
• form a new suspended floor above the existing floor, using timber joists with insulation between them.

concrete floor insulation
Floor Insulation

A new suspended floor usually consists of timber joists with some form of boarded finish (e.g. chipboard, plywood, or tongue-and-grooved softwood boards). The joists can span between the walls of the garage, or bear on to the original concrete floor. Insulation, which can be either a rigid board or quilt type, is placed between the joists. If the joists are spanning between the walls the quilt type of insulation is usually suspended between them on proprietary plastic netting, which is stapled to the tops of the joists. Alternatively rigid board type insulation, cut to fit snugly between the joists, can rest on small timber battens which have been fixed to each side of the joists. These methods allow the thickness of the insulation to match the depth of the joists, resulting in good thermal performance.

timber floor insulation
Timber Floor Insulation

When undertaking work to ground floors it is important not to block any ventilation openings, e.g. air bricks, to ventilated floor voids. It is also a good idea to include a polythene vapour barrier above the insulation, beneath the boarding, in order to reduce the risk of condensation occurring in the interstices of the new construction.

Insulating garage walls

When a garage is converted, exposed walls should be insulated to achieve the Best Practice U-value shown in Table 1 (i.e. to achieve U-values between 0.35W/m2K and 0.45W/m2K). Existing walls can be insulated in one of three ways: externally, usually with a render finish; internally, behind plasterboard linings; or, if the walls are of cavity construction, by filling the cavity with insulation. External wall insulation can be costly, but is often appropriate if the wall requires remedial action, for example, to combat dampness, or where an existing render finish needs to be replaced. Proprietary external insulation systems typically consist of rigid insulation board or semi-rigid mineral fibre insulation mechanically fixed to the wall, with a high performance render coating over the insulation.

If the existing wall is of cavity construction and the cavity is not already insulated, it may be insulated by blowing mineral fibre or plastic bead insulation into the cavity. This is a specialist operation that involves drilling holes in the brickwork at approximately one metre spacing (horizontally and vertically), then blowing-in insulation material to fill the void. This work may be carried out from the inside of the garage, before any plaster or linings are fixed, so the external appearance of the garage need not be affected. The most common form of external wall construction for existing garages is solid masonry (brickwork and/or block work). Typically, walls consist of 113 mm thick ‘single' brickwork, with occasional piers to provide stability. This form of construction lends itself to placing the insulation internally, and this is the easiest and most cost effective method of the three, often referred to as ‘dry-lining’. There are however some disadvantages to internal insulation: there will be a small loss of internal space, and fixing heavy items such as kitchen cupboards to the walls can be difficult. There are two methods of insulating a wall internally: fix a proprietary insulating lining board; or fix a metal or timber framing system to the wall, place insulation between the framing members and then finish the wall internally with plasterboard.

drylining plaster dabs
Drylining Daps

Perhaps the easiest method of dry-lining a wall is to fix a proprietary insulating board to the inside of the wall. The board consists of a layer of plasterboard bonded to a layer of rigid insulation. It can be fixed to the wall mechanically or (more usually) by plaster dabs.

drylining studs
Drylining Stud

The alternative, composite method is to fix galvanised steel laths or timber battens to the wall, place insulation between them, and then line them with plasterboard. This approach can achieve a better standard of insulation than proprietary lining boards, but at the expense of space taken from the room. Insulation is usually 50 mm thick, but up to 100 mm of insulation can be included. Note that the insulation is ‘bridged' by the framing. This problem can be dealt with by combining the two methods of dry-lining. This method will deliver better thermal performance, consistent with Best Practice standards. Whichever dry-lining method is used, it is very important to seal the edges and joints in the lining board (and any service penetrations such as electrical switches and sockets) in order to prevent warm, moist air penetrating behind the lining, resulting in hidden condensation on the cold masonry surface. Sealing should be carried out with tape and/or a skim of wet plaster, before the linings are painted.

composite drylining
Drylining Composite

There are several other techniques for reducing thermal bridging of the insulation, and the consequent risk of surface condensation. Where the walls have attached piers, the insulated lining should be taken around them rather than simply butting up to the pier at each side. The insulation should also be taken into the reveals and soffits of door and window openings, and beneath window cill boards.

avoiding cold bridging
Avoiding Thermal Bridging

Insulating garage roofs

An existing garage may have a pitched or a flat roof but both types will usually be uninsulated. Roofs should be insulated to achieve the Best Practice U-value shown in Table 1 (i.e. to achieve maximum U-values of 0.13W/m2K). If the existing roof is of pitched construction (consisting of a timber structure supporting roofing felt and tiles or slates), and a new ceiling is to be installed, then the easiest way to insulate is to place mineral fibre quilt between the timber ties (or ceiling joists).The insulation can be supported by a new plasterboard ceiling. It is important to maintain a flow of air in the roof void, above the insulation, to prevent condensation forming. If the roof space is to be used for storage, bearer boards should be placed across the existing joists, to prevent the insulation from being compressed.

Insulation Ceiling

If the intention is to expose the shape of the original roof internally (i.e. to create higher rooms, without roof spaces above them) then rigid insulation can be placed between the existing rafters. Polythene vapour barriers and plasterboard linings can then be fixed beneath the rafters. However, it is essential to maintain a flow of air over the insulation, beneath the roofing felt and tiles, in order to reduce the risk of interstitial condensation. The building regulations specify that a 50 mm wide ventilation gap must be maintained, so the thickness of the insulation cannot exceed 50mm less than the depth of the rafters, and this may make it difficult to achieve the recommended Best Practice U-value.1 If the existing rafters are not deep enough to contain sufficient insulation to achieve the recommended Best Practice U-value (plus the 50 mm ventilation gap) there are two other options:

• supplement the insulation between the rafters by using a thermal board, instead of ordinary plasterboard, for the internal ceiling lining; or
• adopt a form of construction called a ‘vapour balanced' or ‘breathing' roof.

These options may be combined.

Vapour balanced ‘breathing' construction

A vapour balanced roof construction is one through which moisture is allowed to permeate, removing the need for ventilation of the roof construction. The impervious roofing felt is replaced by ‘breather felt’, and the 50 mm ventilation gap, the soffit and ridge ventilators and the polythene vapour barrier are all omitted. This simplifies the construction and leaves more space for the insulation. However because the existing roofing felt must be replaced by the special vapour-permeable type, the existing slates or tiles, and the tiling battens, must all be removed and replaced as part of the conversion.

If an existing roof is flat then there are several options and opportunities for adding insulation. The waterproof covering of a flat roof has a limited life, usually between fifteen and twenty years. If an existing garage roof is in need of repair or replacement then converting the garage provides a good opportunity to look at the options.

insulation rafters
Rafter Insulation

One option is simply to re-roof the area, leaving the existing timber structure in place and adding insulation between the roof timbers. This type of construction is called a ‘cold’ roof, and must be ventilated above the insulation (and beneath the external roof deck and finish) in order to reduce the risk of interstitial condensation. Very few roofs are constructed in this way because cold roofs are very difficult to ventilate adequately. Cold roof construction is not recommended. A ‘warm roof’ construction is one that has the insulation above the timber structure, with the waterproof external finish layer bonded directly to it. This is the most common type of insulated flat roof. Another option is to change the flat roof to a pitched roof. Some householders see this as a more aesthetically pleasing solution, which also significantly reduces the need for regular maintenance. The change can be made relatively easily, but it depends on the supporting structure (the existing walls, and their foundations) being able to carry the extra weight. Advice should be sought from a consultant structural engineer before this approach is adopted.

cold roof insulation
Cold Roof

warm roof insulation
Warm Roof

Glazed openings

Glazed openings (windows, roof windows and glazed doors) fulfil multiple functions. They provide views out, let daylight in and assist with ventilation. However, the heat loss through one square metre of a modern, high-performance double-glazed window is much greater than the heat loss through one square metre of insulated external wall or roof. Excessive glazing is therefore a cause of unnecessary heat losses. However, inadequate glazing (i.e. windows that are too small) can lead to rooms being gloomy. If occupants feel the need to switch on lights during the day then fuel use, fuel costs and carbon dioxide emissions will all be increased.

For garage conversions, the building regulations in each part of the UK specify maximum areas of glazed openings allowed (windows, doors and roof windows). In England and Wales the requirement is that the total area of windows, doors and roof lights must not exceed 25 per cent of the floor area of the new accommodation. Or, the area of openings in the enlarged dwelling (i.e. the original house plus the converted garage) must not exceed the area of openings in the original dwelling. Or, the total area of the glazed openings in the enlarged dwelling must not exceed 25 per cent of the floor area of the enlarged dwelling. The requirements differ in Scotland and Northern Ireland. This is an important consideration if the original garage door is to be replaced.The area of a single garage door is approximately 4m2, and the floor area of a single garage is usually between 12.5m2 and 18m2, so replacing the whole of the garage door with glazing will often exceed the maximum permitted glazed opening area. It is preferable to fill the door opening with new external wall (insulated to the Best Practice standard in Table 1), and to incorporate a conventionally sized window to provide daylight, ventilation and views out. Timber-framed walls, incorporating insulation between the framing members, are often used to replace garage doors. They are finished internally with plasterboard or thermal board, and externally with timber weatherboarding or render. The building regulations also specify maximum thermal transmittances (U-values) for new openings (windows, doors and roof lights. Adopting the better Best Practice standards for openings shown in Table 1 (i.e. maximum U-values of 1.8W/m2K) will reduce fuel use, fuel costs and carbon dioxide emissions associated with heating the converted garage. There are many possible combinations of frame and glazing types for new openings. Examples of window types that meet the Best Practice standards are as follows.

• Timber-framed windows with double glazing incorporating at least a 16 mm glazing gap, argon gas fill and one ‘soft' low emissivity coating.
• Timber-framed windows with triple glazing, 12 mm glazing gaps, and one ‘hard' low emissivity coating.
• Metal-framed windows (incorporating thermal breaks) with triple glazing incorporating at least 16 mm glazing gaps, argon gas fill and one ‘soft’ low emissivity coating.

Low emissivity coatings

Low emissivity (‘low-e’) coatings for glazing are of two main types, known as ‘hard’ and ‘soft’. The soft coatings provide better performance at little additional cost.

Gas filling

The most common form of gas filling for double and triple glazing is argon. Better performance can be obtained (at higher cost) by filing with krypton or xenon.

All windows and external doors must be weather-stripped, and should be equipped with good-quality locking mechanisms that ensure that the seals are compressed when they are closed.

Window energy ratings

The British Fenestration Rating Council (BFRC) Window Energy Label provides an objective standard against which the relative merits of different window types for a home extension can be judged. Selecting A or B rated windows also ensures that the windows achieve the manufacturer’s claimed performance, and that air leakage and draughts are kept to a minimum. Since February 2005 Band C and above windows are Energy Efficiency Recommended by Energy Saving Trust (EST).


In the UK, domestic buildings have traditionally relied on air infiltration through the building fabric to provide background ventilation. This is supplemented by extract ventilation fans or by opening windows when additional ventilation is required. Modern construction methods and regulations deliver a higher standard of airtightness, and it is no longer acceptable to rely on infiltration to provide background ventilation. The maxim is ‘build tight, ventilate right'. In garage conversions, the provision of appropriate, controlled ventilation is therefore essential, in order to ensure good air quality and avoid the risk of surface condensation. However, excessive ventilation results in unnecessary heat loss, and consequently increased fuel use, fuel costs and carbon dioxide emissions.

Ventilation falls into three main types.
• Background ventilation - provided by air bricks, trickle ventilators in window heads, or facilities to secure windows slightly open in a ‘slot ventilation' position.
• Rapid or ‘purge' ventilation - provided by opening windows, when there is a need to expel pollutants or admit fresh air.
• Extract ventilation - provided to expel moist stale air from ‘wet areas' (i.e. kitchens, bathrooms and utility rooms) in order to reduce the risk of surface condensation.

Minimum requirements for each type of ventilation are set out in the building regulations for each part of the UK. Energy efficient ventilation is achieved by providing ventilation only when and where it is needed. Wet areas must be provided with extract ventilation, in the form of electric fans or ‘passive stack ventilation'. Extract ventilation fans should be controlled by humidistat's, or wired to operate with light switches (with timed ‘run-on’).