Insulating a garage: How to insulate the walls, ceiling and floor

Insulating a garage is not a complicated process, often easier than insulating the house, but to get it right, you need to know how the garage is constructed and the reason for insulating it.    

If an integral garage is making the bedroom above it (or the lounge beside it) cold, what you actually want to do is insulate the house from the garage, as you're not particularly looking to keep the garage warm but rather to prevent it affecting the comfort in adjacent rooms. 

However, if it is an integral or detached garage conversion that is being insulated to provide more living space then it will want the same level of insulation as the house itself. 

Here's what you need to know about the process. 

How to Insulate Garage Walls

Garage walls are typically single-skin brick or block, which presents the problem of rainwater penetration as single skin masonry is not great at keeping rainwater out. Usually, in the uninsulated garage, any moisture penetrating the wall will mostly be evaporated away and not be an issue. 

Installing insulation will prevent air movement on the wall and moisture penetrating the wall does become a problem. If the internal surface of the wall is obviously damp then its cause – poor brickwork or pointing, cracks in the render, failed gutters and the like – needs dealing with before any insulation is installed. 

The options for insulating are to treat the internal or external surface. The advantages of external wall insulation are that it retains more of the floor space and, most importantly, provides weather-proofing to the wall. 

However, internal wall insulation is more likely as it is usually simpler and cheaper. There are three ways to do it. 

1. Fix the insulation directly to the wall

Kingspan and Celotex offer products specifically designed for this method, with insulation bonded to plasterboard and with a vapour barrier. If the surface of the wall is in good condition, this is a relatively inexpensive and quick method of insulating. The boards are glued directly to the wall (using a specific adhesive), together with mechanical fixings (screws). The boards themselves are quite expensive at £9/m2 for Tektherm polystyrene to £19/m2 for Celotex PL4040, but this is offset by the speed of installation. 

2. Fix battens to the wall which can carry the insulation

This provides a gap between the wall and the insulation ensuring that any moisture penetrating the wall is unable to reach the insulation. This is also a good method where the wall is particularly uneven. Cost will be broadly the same as with the first method as it will use the same materials, plus the cost of battens. 

3. Construct a new stud wall inside the existing wall

This is generally only done when the existing wall is particularly subject to rainwater penetration as it is the most expensive option and no more effective. The stud wall will generally be 100mm thick and need a 40mm ventilated cavity between it and the wall meaning that it will significantly intrude on floor space.

4. Apply insulating plaster directly to the internal wall

This is most useful on stone walls, where breathability is important and would involve natural insulation, either a hemp or cork-lime mix (the hemp or cork providing the insulation) or layers of lime plaster sandwiching a cork or wood fibre insulation.

This option will not get the low U-value – typically around 0.5W/m2 which is about the same as 50mm polystyrene will do – but it achieves all that the other options provide at broadly the same cost, provides a solid fixing for pictures, cupboards, etc, and being breathable, deals with moisture penetration.

Where the objective is to keep rooms above or adjacent to the garage warm then the only wall that need insulating is the party wall with that room. That wall will be a cavity wall and will require the cavity to be filled with insulation first. Obviously there is no potential for this to cause any moisture penetration problems as the external surface of the wall is not exposed to the elements. The option exists to add more insulation to the external surface but this is largely unnecessary as the wall is sheltered and the cavity-fill is sufficient.

Integral garages often have cavity walls and where it is being converted to habitable space then cavity-fill is an option. This wall can be insulated internally but external insulation would only be effective if the cavity is also filled.

How to Insulate the Roof/Ceiling

Insulating a roof or ceiling will follow the same rules as insulating a similar area in the house. In the case of the flat roof it may be necessary to drop an existing ceiling and install insulation between the joists. 

In the case of pitched roof installing insulation between rafters, ensuring that there is a ventilation gap between the insulation and the underside of the roof covering. In both cases a rigid foam insulation is likely to be easiest to install and be most effective.

Plasterboards can be reinstalled to provide a ceiling, if desired, or the joists or rafters imply battened across to hold the insulation in place. Cost and U-value will be broadly the same as for the walls.

How to Insulate a Garage Floor

It is safe to assume that the floor is uninsulated, and that it is solid concrete, laid on hardcore. Getting the garage to habitable standards means doing something with the floor, if only to comply with building regulations, but it is difficult to justify the cost of excavating the floor and relaying the concrete, merely to install insulation. A more practical option will be to place insulation on top of the floor and how thick that is will be affected by the ceiling height. 

Optimum insulation thickness would be 100mm of rigid foam insulation, giving a U-value of 0.22W/m2K. Minimum ceiling height in habitable rooms would be 2.1m. It may be that there is insufficient ceiling height for 100mm of insulation, or that it presents problems elsewhere — at door thresholds and the like. In which case install what is practically possible. Even insulating a floor with a 10mm insulation will make the floor feel warmer if not doing much to improve the U-value.  

How to Insulate a Garage Door 

If the garage is to be habitable space then it makes more sense to remove the door and infill the opening with an insulated timber frame or masonry wall. 

If it is to be retained as a vehicle access, and insulation is still needed, then there are specific materials available, like Therma Wrap, Superfoil, or Weather Stop. These are all glued to the inside of the door – either self-adhesive or with adhesive pads – and is a simple DIY job, costing around £60 for a single garage door. 

In addition it would be wise to install draught-proofing and Weather Stop (amongst others) also provide sealing strips which are again a DIY job and costing around £50. Even if the rest of the garage is not to be insulated then treating the garage door in this way will make the garage a warmer, more welcoming space at minimum cost.  

How Much Does it Cost to Insulate a Garage?

The options for insulating are to treat the internal or external surface. The advantages of external insulation are that it retains more of the floor space and, most importantly, provides weather-proofing to the wall. 

Starting price for insulation materials will be around £350 for sufficient polystyrene and fixings for a single garage, plus installation costs which will be significant. That will provide 50mm polystyrene which will then need to be clad or rendered to provide a waterproof finish. 

While there is potential for the skilled DIYer to take on the project it is not simple and probably best left to the professionals. In which case budget £100/m2 for the finished project.  

Do I Need to Insulate a Garage?

The method and amount of insulation will vary with the intended use for the garage. If it is to be used for a garage conversion idea such as a utility room or home gym, say, it will need less insulation than if it is to be used as living space or a bedroom. In the first case the 50mm insulation discussed above would be adequate. If it is to be used for accommodation then it would be wise to increase to at least 70mm Kingspan or Celotex or 100mm polystyrene. 

It is not uncommon for the garage to be repurposed, at least in part, as a plant room, to accommodate a boiler or heat pump and hot water cylinder, perhaps with solar panel control gear. In which case insulation is not really necessary although consideration might be given to pipe runs from and around the garage so as to ensure they are not in the way if there is a change of mind and the garage is to be used as accommodation. 

Is Insulating a Garage Worth it?

If the garage is being converted to accommodation space then there is no option. It will be necessary to comply with building regulation, but also to be sufficiently comfortable to actually live in. 

In financial terms the questions of 'payback' or 'return on investment' do not really occur. The project is extending the living space and will therefore add to the heating bill. Insulating the garage will minimise that and improve thermal comfort of the garage and any adjacent rooms, but it will not reduce the overall heating bill.