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How to Insulate Roof Rafters with Celotex

How to Insulate Roof Rafters with Celotex


If your home has a pitched roof then the best way to insulate the space will be to position Celotex insulation board in-between or below the rafters. This works just as well in a new build as it does foran existing property, allowing insulation to be installed in a watertight roof without creating anydisturbance to a current restoration project. Below we explain how to insulation roof rafters with Celotex.


 Step one: Prepare the space for insulation
Clear the rafter space using a vacuum to remove any dust or debris that has settled. If you spot any dry rot, cracks or damage to the rafters or roof membrane, get these fixed by a professional first before proceeding. Ensure you also have good lighting and enough floor support as you will be working in the area for a while. Clear any large objects away from the rafters so they can be easily accessed.


Step two: Measuring up
Use a laser or tape measure to get the full dimensions of the rafters. Do this by measuring from the apex down to where the horizontal joists are met by the sloping roof. Multiply this by the roof width. And also measure the width of the rafters. Be sure to include the width of the Celotex insulation board you want to use. At least 100mm is recommended, with Celotex boards available in thickness ranging between 25mm to 200mm.


Step three: Create a space for air circulation
Before installing the first Celotex insulation board ensure a gap of around 25mm to 50mm is left between the material and the roof to allow air to pass through freely. This helps to avoid damp and condensation. You could also attach treated timber to the sides of the rafters to use them as a gauge to create the required gap.


Step four: Cut the insulation boards to size
A Celotex insulation board can be cut to size quickly and easily. And because they are lightweight you won’t have to worry about a heavy load being applied to the structure. If the board does not completely fill the full depth of the rafter, the low-emissivity foil facings should enjoy extra thermal benefit from the air space above them. When cutting the board make sure it is a snug fit so they canbe stapled to the rafters.


Can you insulate below the roof rafters?
You also have the option to install Celotex insulation board underneath, rather than in-between, therafters. This may be preferable if the rafters are too shallow or if you are not concerned about losing headroom. You also won’t have to worry about leaving a gap in the roof membrane or have to deal with any other issues like condensation.


Nail the boards to the underside so they can be fixed into place easily. Having someone on hand to help can also make the job much faster – and accurate – to complete. Once finished you may also want to add a thin layer of plasterboard that can be painted to create a more decorative finish.

There is a very useful U-Value calculator on the Celotex Website if you need to work out the best thickness before you start.

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