How to insulate a garage conversion: Part 3 – Roof Insulation
In the last few blog posts we have looked at the Building Regulations and Standards required for insulating a garage conversion as well as the different methods of insulating the walls.
The next step is to look at the insulation requirements for the roof of the building. This is a key component of creating a warm habitable space from what is often a cold and drafty garage.
If the existing roof of the garage is pitched or if you are intending to build a space above the garage, with a pitched roof, then we suggest you look at our quick guide on pitched roof insulation which covers this area in a bit more detail. The space created above the garage would also have to be fully insulated in a similar fashion to the garage.
When a previously insulated room above the garage exists before the conversion, then roof insulation will not be an issue, and you will not have to add extra insulation to the garage ceiling.
The U-values needed to achieve this on a flat roof are shown below. We are only looking at how to insulate a flat garage roof in detail as this is the most common construction.
U-Values for Extension of Existing Buildings in W/m2.K
*The first number applies where the existing building has U-values worse than 0.70 W/m.K in the walls and worse than 0.25 W/m.K in the ceiling.If the U-values are better than those figures the second number applies.
Most garages have flat roofs, which means that there is less than 10 degrees of pitch in the roof. However a flat roof does not mean that it is perfectly level, in fact a flat roof should always have a certain degree of pitch or fall in it to allow water to drain off it. If water lies in pools on the roof this could damage the fabric of the roof, causing problems in the room below. If water is pooling on the existing roof then this should be rectified at this stage, and the fabric of the roof checked to make sure that it has not been damaged. If any part of the existing roof needs replacing this may affect how you decide to insulate the roof.
Again, the roof, like the walls, can have the insulation added to the inside or the outside of the construction. These two types of construction are known as warm roofs, where the insulation is above and between the roof joists, whereas cold roofs are where the insulation is between and below the roof joists.
For a cold roof, where the insulation, such as Kooltherm K7, is fixed between the roof joists, the roof needs to be ventilated by leaving a 50mm gap above the insulation to create an air gap which is vented to the outside. This will prevent condensation forming in the roof. Then, if necessary, further insulation can be added below the roof joists to reach the required U-Value. This is commonly in the form of insulated plasterboard.
For warm flat roofs, where insulation is added above the roof joists, the first area to check is the condition of the existing waterproofing on the roof. If this is in a good condition, then it can just be swept clean and the insulation either mechanically fixed or glued on top of the existing roof. Then a waterproofing layer can be added on top of this.
If the waterproofing is leaking then this should be removed and any further damage below rectified. Depending on the extent of this it may be necessary to replace the entire roof. In this case the insulation can go between and above the joists. The full details of this method can be found in our quick guide.
The next (and final) post in this series will be about insulating the floor of the garage. If you’ve missed the other posts on this topic they can be found below:
- How to insulate a garage conversion: Part 1 – Overview and Building Regulations/Standards
- How to insulate a garage conversion: Part 2 – Wall Insulation.
- How to insulate a garage conversion: Part 4 – Floor Insulation.
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