Warm Deck Flat Roof Insulation Between Joists

Good question theoretically when building a warm roof the vaper control layer goes on the warm side of the insulation.

Warm deck flat roof insulation between joists.

Here the roof deck is maintained at the same temperature as the internal conditions hence warm deck roof. Warm flat roofs do not need to be ventilated as the insulation is. If you put insulation under the decking and between the joists you lower the temperature of the decking and lower the due point thus bringing it closer to the danger point of being in the building. The roof itself is therefore kept warm.

A flat roof where the insulation sits below the waterproof membrane and above the structural deck is a warm roof deck. The vapour control layer is above the decking but below the insulation. Cold deck roof a cold deck roof is where the insulation is placed between the joists rafters or in between the ceiling joists in the case of a pitch roof. Set the quinn therm qrfr ply insulation boards onto a wide 30mm bead of vapour resistant sealant applied to the upper surface of all the joists and cross noggins.

Cold flat roofs are typically avoided as you d need to install ventilation which can prove problematic for many reasons relating to space labour costs etc. A warm flat roof or a built up roof bur is one where all the insulation is above the joists deck making them part of the warm fabric of the building. A cold flat roof is defined by insulation being installed between and if necessary underneath the timber joists. The underside of the rafters in this case will need to be finished with a suitable vapour control layer vcl and plasterboard to provide a suitable internal finish to the roof.

The insulation can be adhered mechanically fixed or loose laid. No ventilation is required for these types of roofs. A warm flat roof is defined by insulation being installed on top of the roof deck. The insulation is immediately below the waterproofing layer.

A warm deck roof is where the insulation is placed on top of the rafters joists and the roof covering is then placed over the insulation. How to install insulation in a flat roof with timber deck insulated above the joists. Alternatively battens can be secured to the underside of the joists and gmw fitted between these in much the same way as between the main roof joists. A warm roof is defined as being when the all or the majority of the insulation is above the joists or deck.

By majority the rule of thumb is that in a domestic house not flats approximately a third of the total thermal performance of insulation can be placed below the deck with two thirds above.