Roofs can account for a lot of heat loss, so if you’re converting your garage, it’s important to ensure it is well insulated. In most cases, garage roofs use a flat roof construction. In this article, we’ll look at how to properly insulate these roofs.
How to assess the condition of your garage roof?
In most cases, garages feature flat roofs. Despite their name, virtually all flat roofs will have a very low pitch (less than 10º) to ensure that rain and snow run off and drain away. If the roof has been fitted with an insufficient pitch or has bowed over time, this can lead to water ponding, potentially damaging the roof structure. If water ponding is occurring, there will usually either be standing water on the roof or clear staining patches where water has gathered and evaporated.
In addition to issues with water ponding, you should also check the roof surface and guttering are clear of debris and that the existing waterproofing system is in good condition. Keep an eye out for any damage or tears in the waterproofing or signs of bubbling under the surface, which can be a sign that water has entered the construction and evaporated. You should also check the fascia boards and gutters to ensure there are no leaks or signs of damage or rotting.
If issues are present, you will need to strip the roof and address these before fitting any insulation. Whilst this may be time consuming, it will also ensure the roof performs well for years to come.
Should I fit insulation above or below the garage roof deck?
When insulating a garage roof, we would always recommend fitting the insulation above the roof deck. This is known as a warm roof construction because the structure of the roof is insulated. The advantage of this approach is it reduces the risk of moisture condensing on the cold building structure and leading to damp issues. If the existing roof deck is in good condition the insulation can also usually be fitted directly above the existing waterproofing system.
In contrast, if you fit insulation below the roof deck (known as a cold roof construction) you will have to ensure there is sufficient ventilation within the roof space to remove any moisture. This means there must be at least a 50 mm gap between the insulation and the roof deck and ventilation channels fitted at the fascia boards. This can be quite tricky and time-consuming to get right so it is almost always advisable to stick with a warm roof approach.
How much insulation do I need for the flat roof of my garage conversion?
Most garage conversions are classed as an extension of an existing building in the Building Regulations/Standards. This means the roof will need to achieve a minimum level of thermal performance (known as a U-value) The requirements vary depending on which country/area you live in:
| England | Wales | Scotland* |
Flat roof U-value (W/m2K) | 0.15 | 0.13 | 0.12/0.18 |
*The first number applies where the existing building has U-values worse than 0.70 W/m²K in the walls and worse than 0.25 W/m²K in the ceiling. If the U-values are better than those figures, the second number applies.
You can use our U-value calculator to estimate what thickness of insulation you will need to reach these U-values. Keep in mind that the calculations are specific to the insulation product used and required thicknesses may vary depending on the performance of the product.
How to insulate a garage roof
Once you’ve ensured the deck is clean and in good condition, start by fitting a vapour control layer over the deck. You should ensure the layers overlap each other by at least 150 mm and are turned up at the edge of the roof to the height required for your waterproofing system (this should be included within the manufacturer's guidance).
A rigid insulation board such as Thermaroof TR26 can then be fitted above the membrane with suitable fixings. The boards should be lightly butted and break-bonded. A 25 mm thickness of the insulation needs to be fitted around the perimeter of the roof to prevent thermal bridges. Alternatively, if the roof has no parapet, a timber edging batten of the same height as the insulation should be used to fix the fascia board to the guttering system. Any roof lights or ventilator kerbs should also be insulated to the same level. The roof membrane can then be fitted following manufacturer guidance for the system.
What should I do if my garage roof isn’t draining properly?
As mentioned earlier, if water is ponding on the existing roof deck, it will usually be necessary to strip it and replace it. One way to ensure good drainage on the replacement roof is by fitting a tapered roof insulation system. These systems feature angled boards that not only insulate the roof but also channel water away. We offer a dedicated tapered design service for these systems.
Do I need to insulate the ceiling of my garage conversion if it’s below the existing house?
Many interior garages lie beneath the existing house, so for a colder isolated space it’s common practice to isolate the ceiling. However, in a garage conversion, the ceiling becomes part of the existing occupied house. Since you wouldn’t insulate the ceiling between the living room and an upstairs bedroom, there is similarly no need to insulate the ceiling of an interior garage conversion that has insulated walling and/or flooring.
