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Insulation for Between and Under Joists

Celotex high performance thermal PIR insulation for use between and under joists.

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Traditional flat roofs were constructed with little or no regard for accommodating insulation - it either wasn’t a factor, or energy efficiency requirements were not as developed as they are now.

Installing insulation after the fact can prove challenging, to say the least. Timber boards and a bituminous roof covering are impermeable to the moisture vapour generated inside a building. Positioning insulation between timber roof joists causes a condensation risk on the underside of the roof covering - unless a flow of air removes the moisture first.

A suitable level of cross-ventilation is widely recognised as difficult to achieve. Many roofs can’t easily accommodate vents at opposite ends, but a ventilation opening at only one end is not enough. Even where it is possible, if the roof is too long then the rate of air flow suffers.

Problems can occur unseen within an ineffective cold roof. They occur gradually, with moisture potentially building up over many years. A warm roof design should always be the preferred option (in Scotland, the wording of regulations actually means cold roof constructions are not allowed), although they can face their own constraints.

Where a cold roof is the only option, however, it makes sense to aim for the most efficient solution possible. The depth of insulation is restricted by the height of the joists - for example, 150mm (6-inch) flat roof joists can only take 100mm (4 inches) of insulation between them, to leave the required ventilation space above. That is likely to mean fixing an extra layer of insulation across the underside of the joists, which puts pressure on head heights in all but the most generously proportioned rooms.

Lightweight, rigid polyisocyanurate (PIR) foam boards, such as those manufactured by Celotex, are among the most thermally efficient, commonly available insulation products. They are lightweight and easy to cut, helping with installation from inside the room while keeping thicknesses to a minimum. An insulated plasterboard can be used for the below-joist layer, providing thermal insulation and the ceiling finish in one operation.


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Celotex products to consider

1200 x 2400mm
0.022 W/mK

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