Technical Study: Garage Conversions
The conversion of an underused garage into internal accommodation is a popular way to provide extra space within existing homes. Conversions can be a great way to provide additional living space such as a dining room, a guest bedroom, a cinema room or a utility room.
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The majority of garage conversions can be carried out under permitted development rights. This means that the development can be carried out without planning consent as long as the technical guidance for permitted developments is followed.
The key component of a garage conversion is that the space is to be changed from an unheated space to a heated space. An unheated garage is typically designed to be outside of the thermal envelope of the main house. This means that the envelope surrounding the garage is not thermally insulated, watertight or airtight. There are several upgrades required to the envelope before the space can become habitable.
Garages can be integrated, attached or detached from the house and the type determines the amount of building envelope that’s exposed to the outside. The entire external envelope of the garage will need to be upgraded and as such there will be a greater area of upgrades required to a detached garage than an integrated garage.
What are the Planning Requirements?
The majority of garage conversions can be carried out under permitted development rights. This means that the development can be carried out without planning consent as long as the technical guidance for permitted developments is followed. For example, to be permitted development any external materials used in the conversion should look similar to those of the existing house. If different materials are desired (such as a contrasting cladding or roofing) then planning permission would need to be gained.
It is essential to check whether permitted development rights apply to the property by contacting the Local Authority before carrying out any work. To be certain that the plans for the permitted works are compliant with planning rules, a Lawful Development Certificate (LDC) can be applied for. An LDC is not compulsory but it can give peace of mind by providing written confirmation that the works are lawful.
It is important to note that houses within conservation areas, listed buildings and some new developments often have their permitted development rights removed. In those instances a planning application would need to be submitted to gain consent for the works.
What are the Building Regulation Requirements?
The conversion of a garage normally requires building control approval through a full plans application or a building notice. This is because the work forms a material change of use and a change in energy status. All parts of the building regulations will apply to the works. Some key sections of the Approved Documents to consult when detailing the upgraded envelope are:
- Part L Conservation of fuel and power (Volume 1) – insulation to the walls, floor and roof should meet the minimum u-value requirements for existing elements shown in table 4.3.
- Part F Ventilation – new external windows and / or doors for purge ventilation and trickle vents for background ventilation will be required.
- Part C Resistance to moisture – the walls, floor and roof are to be detailed to protect from ground moisture, precipitation and condensation.
- Part A Structure – a structural engineer should ensure that the existing structure can take the weight of the upgraded envelope and design new foundations for any new walls.
- Part B Fire Safety (Volume 1) – means of escape should be provided off a hallway, otherwise it will need an escape window and smoke alarms.
- Part K Protection from falling, collision & impact – new low level glazing and glazing local to doors should protect against impact as outlined in section K4 & K5.2.
In addition, any heating systems, drainage and electrical work will need to comply with the building regulations. If the garage is bound by a party wall to an adjacent dwelling, acoustic requirements will need to be met and the requirements of the Party Wall Act will need to be followed.
The Need to Manage Moisture when Upgrading Thermal Elements
BS 5250:2021 is the key text to refer to with regards to managing moisture risk in construction elements. It is recommended reading for anyone designing thermal element upgrades as part of a garage conversion. The standard outlines the types of floor, wall and roof constructions where condensation risk calculations are required and the type of calculation methods that are most appropriate. Condensation calculations take the project location into account and so will have to be done for every situation and cannot be transferred across projects.
Upgrading the Walls
Garage walls are generally constructed of uninsulated single skin or double skin masonry. Before starting any works, the condition of the existing walls should be reviewed by a specialist to check that they are stable and defect free. Any issues with penetrating or rising damp need to be addressed prior to upgrading the wall.
New insulation will need to be added to the walls to improve their thermal performance. The new insulation can be installed internally or externally. Both instances require careful detailing to avoid exacerbating or introducing moisture problems such as surface condensation, interstitial condensation and mould.
Internal Wall Insulation
When insulation is added to the internal face of an external wall, the warm internal air temperature becomes isolated from the cold external wall. This causes the internal face of the masonry to be colder than it was before the insulation was added. This cold internal face of masonry becomes the critical junction and is at most risk of interstitial condensation forming where the warm moist air meets the cold surface. The colder the masonry, the less able it will be to dry out if it does get wet.
To insulate a single skin masonry wall, a well ventilated cavity (of at least 50mm) should be created between the existing wall and any new internal insulation. The cavity is essential to prevent moisture being carried to the new inner leaf. The internal insulation can be supported on an inner leaf of masonry, metal studwork or timber studwork. Where the new inner leaf is constructed from masonry, lightweight blocks should be used and a structural engineer should check that the slab can take the weight. As the inner leaf will be supported from the existing slab, it is unlikely that the cavity between the two leaves will extend 225mm below the DPC level. In this case, new cavity trays and weep holes should be added to the external leaf to prevent moisture passing into the inner leaf.
The wall upgrade can be detailed as a ‘moisture closed’ construction (that prevents moisture from moving in and out of it) or a ‘moisture open’ construction (that allows moisture to freely move in and out of it). In a moisture closed construction a vcl should be included on the warm side of the insulation and there should be no gaps between the insulation and the inner leaf. In areas exposed to very severe wind driven rain there may be a risk of moisture forming within the build up. Condensation calculations should be undertaken to highlight this or prove otherwise. If there is a risk of moisture, a moisture open construction which incorporates breathable insulation (such as wood fibre) could be utilised.
It is not recommended to add insulation directly to the inside face of a solid single skin masonry wall to upgrade a garage. Any insulation applied directly to a solid masonry wall should be part of a moisture open system to enable the wall to dry from both the inside and the outside. In moisture open systems there is a limit for how much insulation can be added to the wall before it becomes insufficiently permeable to water vapour and at risk of condensation. Research commissioned by Bristol City Council recommends that the target u-value for a solid wall with internal insulation should be 0.6 W/m2K or higher which would not meet the guidance within Part L (0.30 W/m2K for upgraded walls).
A double skin masonry wall is simpler to upgrade as the existing outer leaf forms a protective layer against the elements (rain, wind and solar radiation). Provided that the existing cavity is well ventilated, internal wall insulation can be applied directly to the inner leaf. There should be no gaps between the insulation and the masonry inner leaf to avoid water vapour getting in and condensing on the cold masonry. In areas exposed to very severe driven rain, the use of breathable insulation as part of a moisture open construction is likely to provide less risk of moisture than a moisture closed construction. Project specific condensation calculations should be carried out to assess the risk of moisture within the preferred construction.
External Wall Insulation
In the case of a garage conversion, it may not be desirable or possible (without planning consent) to insulate the existing walls externally. This is because it would change the appearance of the building envelope and bring the face of the external wall outwards. It would also require an extension of the eaves / roof covering to suit the new finished face. However, where it is possible to add insulation externally, this can often be the best way to upgrade the external walls as it does not carry the moisture risks associated with internal insulation. Where external wall insulation is applied, the outer face can be rendered or clad as desired.
Upgrading the Floor
In most instances the level of an existing garage floor is set flush with the external ground level (150-200mm below the internal level of the house). This is useful when it comes to upgrading a floor as part of a garage conversion. The difference in level provides a zone within which to insulate, whilst still being able to create a level threshold to the existing living areas.
The amount of insulation should be specified to meet the requirements of Part L. The new insulation can be fitted tight to the existing slab or it can be suspended above it. Where it is fitted tight to the existing floor slab, a damp proof membrane should be fitted below the insulation and a vapour control layer should be fitted above it. There should be no gaps below the insulation.
To raise the floor further, the insulation could be suspended between joists above the existing floor rather than fitted flush to it. In this instance, it is important to create a well ventilated void of at least 150mm between the new insulation and the existing slab. A vapour control layer should be fitted above the insulation and damp proof courses should be installed wherever the timber joists are supported from the masonry.
Upgrading the Roof
A garage roof can be flat or pitched. In both scenarios, insulation will need to be added above, below or within the existing roof structure to meet Part L requirements. As with the walls, insulating the roof externally (on the cold side) presents less risks of moisture forming within the build up.
For flat roofs, the insulation should be added above the existing roof deck to avoid any risk of surface condensation. A vapour control layer should be added immediately above the supporting structure to avoid the risk of interstitial condensation forming between the insulation and the waterproof covering. It should be wrapped around the edges and sealed to the new waterproof finish. Insulating the roof externally will raise the height of the roof, require an extension of the eaves and include a new roof finish. Because of this, it is essential to check whether the work can be carried out under permitted development rights or whether planning consent will be required with the Local Authority. Adding insulation below a flat roof is not recommended due to the difficulties in providing sufficient ventilation beneath the existing roof deck, especially in larger roofs.
For pitched roofs, the insulation can be added between and above or below the existing roof structure.
Where the existing pitched roof is in good condition it may be retained and upgraded from below. In this case it is assumed that there is a risk of interstitial condensation forming on the underside of the underlay as a worst case. As such, a well ventilated cavity of at least 50mm should be created between the existing roof underlay and the new insulation which will be laid between and below the existing roof joists. A vapour control layer should be installed on the internal face of the insulation.
Where insulation is added between and above the existing roof structure, the previous roof covering will need to be removed and replaced. An underlay with low vapour resistance should be added to the external face of the insulation. Where the roof covering is air permeable (such as discontinuously laid natural slates, clay and concrete tiles) there is no need to ventilate the batten space between the tiles and the underlay. If an air impermeable roof covering is to be used, then the batten space will need to be ventilated.
In both cases, a vapour control layer should be installed on the internal face of the insulation. It should be noted that adding insulation above the existing roof structure will raise the height of the roof, require an extension of the eaves and include a new roof finish. It is essential to check whether the work can be carried out under permitted development rights or whether planning consent will be required with the Local Authority.
Creating New Windows and Doors
It’s likely that new windows and doors will be added to the existing envelope of the garage to improve daylighting, ventilation and allow for escape in an emergency. It should be checked that the additional openings will not cause the total area of openings (windows, rooflights and doors) within the dwelling to exceed 25% of the floor area. If they exceed 25% of the floor area, Part L requires that compensating measures be taken to improve the energy efficiency of the dwelling. These measures would need to be agreed with building control.
Any new windows will need to meet the minimum u-values stated within Part L. The window should be positioned as close to the line of the external wall insulation as possible. Where the windows are not directly in line, it is necessary to insulate the reveals to prevent cold bridges from occurring. Any new windows should include trickle vents to provide background ventilation to the space.
If windows are required for escape, they should be sized to the requirements noted within Part B. A structural engineer will need to design new lintels to ensure that the wall is supported above.
Infilling the Garage Door
The existing garage door opening can create an opportunity to create a new window opening or it can be infilled to create a solid wall. Where a solid wall is to be created, a structural engineer will need to design the new foundations. The wall should be insulated in the same way as the upgraded walls to ensure continuity of the thermal envelope and avoid cold bridges. The external face of the wall may be clad in masonry to match the existing walls. In this case, the new masonry can be toothed into the existing brickwork to create a more seamless finish.
There are several factors to consider when detailing a garage conversion. Amongst other things, the upgraded envelope must consider thermal efficiency, moisture risk and ventilation. When detailed well, a garage conversion can create high quality living space. Poorly detailed conversions run the risk of introducing or exacerbating moisture issues. It is essential to design out any potential risks at the design stage through following the guidance within BS 5250 and undertaking condensation risk calculations where required.
Download the Details
Many of the details featured here can be downloaded from the Detail Library. For more all our garage conversion details follow the link below. They are available to all members, or can be purchased individually.
- BS 5250:2021 Management of moisture in buildings – Code of practice
- The Ministry of Housing, Communities and Local Government research study ‘Resistance to moisture in buildings’ which can be found on the www.gov.uk website
- LETI Climate Emergency Retrofit Guide
- A Bristolian’s Guide to Solid Wall insulation
- The Department for Business, Energy & Industrial Strategy guide ‘Retrofit Internal Wall Insulation: guide to best practice’ can be found on the www.gov.uk website
- ‘Permitted development rights for householders: technical guidance’ which can be found on the www.gov.uk website
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Written by Emma Thackstone. Emma is an architect. At the Detail Library, Emma helps the Detail Library with drawing new details and carrying out technical research.
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