Warm Deck Flat Roof Insulation Between Joists

How to install insulation in a flat roof with timber deck insulated above the joists.

Warm deck flat roof insulation between joists.

The insulation is immediately below the waterproofing layer. Saint gobain insulation uk manufacture and supply glass mineral wool slabs and rolls along with rigid polyisocyanurate pir boards that can be considered suitable for a wide range of applications across flat roofs whether you re looking to insulate at joist level or above the roof deck in your flat roof construction. 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. The roof itself is therefore kept warm.

A warm flat roof is defined by insulation being installed on top of the roof deck. Warm deck insulation 1. Here the roof deck is maintained at the same temperature as the internal conditions hence warm deck roof. 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.

The vapour control layer is above the decking but below the insulation. Good question theoretically when building a warm roof the vaper control layer goes on the warm side of the insulation. 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. The insulation can be adhered mechanically fixed or loose laid.

No ventilation is required for these types of roofs. A cold flat roof is defined by insulation being installed between and if necessary underneath the timber joists. 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. 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.

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. 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. 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. A flat roof where the insulation sits below the waterproof membrane and above the structural deck is a warm roof deck.

Warm flat roofs do not need to be ventilated as the insulation is.