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Insulation Between and Over Rafters

High performance PIR insulation boards for use between and over rafters.

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The benefits of insulating a building around the outside, protecting the structure and effectively creating a ‘tea cosy’ effect, are well publicised.

When it comes to pitched roofs, however, adding a thick layer of insulation to the rafters can result in a depth of roof construction that may not suit every site, meaning over-rafter insulation (sarking) only is not a viable option. Internal restrictions, like maintaining sufficient head height, can mean that fixing the insulation between and under the rafters is not an option either.

Where the benefits of over-rafter insulation want to be enjoyed while still creating a useable room in the roof space, it’s possible to insulate over and between the rafters. Due to the thermal bridging effect of the timbers, the overall thickness of insulation used to achieve a low U-value is not as efficient as over-rafter insulation only, but it remains a useful compromise solution.

To protect the roof from the potential of condensation occurring unseen within the construction, it’s important to remember that the layer of insulation installed over the rafters should be as thick, or thicker, than the insulation layer between the rafters. A common approach to avoid falling foul of this rule of thumb is simply to use two layers of the same thickness - making things easier when ordering materials too.

An over and between rafter insulation solution can be a feature of a new-build project. It is also suitable for retrofit with a substantial renovation of the roof. The loft space can be used as a habitable room, or offer a storage area with a more consistent temperature year-round. Any water pipes and tanks do not need to be insulated.

Celotex polyisocyanurate (PIR) foam insulation boards are easy to cut and handle. Once up at roof level, the between-rafter boards can be cut accurately to fit the rafter spacings. Being lightweight, they do not impose significant additional loads on existing structures. Where the insulation does not fill the total rafter depth, the low-emissivity foil facings can gain additional thermal benefit from the air spaces they face into. 

 

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