It's not generally necessary to insulate a detached garage in the UK if you don't plan to heat it. In many cases, however, your garage will be attached to your house, sometimes with an adjoining door. Attached garages can be a weak spot in an otherwise well-insulated home, potentially allowing heat to escape through walls or floors. Strategic insulation can turn your garage from a liability into an asset.
The time and expense of insulating your entire garage is not generally worth it but insulating the walls or ceiling where your garage adjoins the house may be. This is particularly important in older houses, as the floors above garages were often left uninsulated. Placing insulation around doors can also cut down heat loss, especially if there is a door from the garage to the house.
If you have a garage beneath one or more of the rooms in your house, you have two possible options for insulation. You can insulate the floor of the room above or the ceiling of the garage. Since insulating the floor above might require ripping up existing flooring, it's often easier to insulate the garage. Fix rock wool or fibreglass roll insulation between the ceiling trusses and cover with foil-backed plasterboard panels.
Walls where a garage adjoins your house can be a major cold spot. Again, the problem is likely to be worse in older houses, where there may only be a thin concrete wall without even a cavity to prevent heat loss. Insulate garage walls where the garage adjoins your house using rigid foam boards, rock wool or fibreglass roll insulation under foil-backed plasterboard. Foam board or tiles on the interior side of the wall will also help prevent heat loss.
Main garage doors
The main door to the garage sheds a lot of heat. Most doors are thin, made of a single sheet of metal or (in the case of older garages) thin wood planks. If you're losing a lot of heat from your house via your garage, consider replacing the door with something more solid. You should keep the door closed as much as possible during the colder part of the year and ensure that there are no gaps around the door when it's closed.
Internal garage doors
If there is a door between your garage and your house, you should insulate it almost as thoroughly as an exterior door. Thin doors, especially with glass panels, will allow heat from the house to escape into the garage even when the door is closed. It may be advisable to replace the whole door. Put draught excluder all around the door and install a brush strip along the bottom to cut down on heat loss.