Garage ceiling insulation

30 Jan 2011
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United Kingdom
I'm going to get the ceiling of my part-integral garage renewed by getting rid of the old plasterboard and putting new plasterboard up. However, i'd like to take the opportunity to insulate the ceiling void which meets my bathroom floor above.

A builder has suggested using Celotex in the void. Would this be better than using mineral wool? What i'm after is ensuring that my bathroom above the garage keeps as much heat as possible during the Winter months. Currently there is no insulation in the garage ceiling below the bathroom.
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A typical insulation detail in this location would ask for either 270mm+ of fibreglass wool type insulation or 130mm+ of celotex or a mixture of the two.

As most floor joists are less than 270mm then either a mixture of the two or insulated wholly with celotex, is most practical. Fitting a foil sandwich underdraw of celotex (complete with all joints foil taped) will act as both a vapour barrier and deal with cold bridge etc.

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