Warm Flat Roof Insulation Between Joists

There are two main types of flat roof that require different approaches to installation.

Warm flat roof insulation between joists.

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. Between and under joists. 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.

Warm deck insulation 1. 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 deck sometimes this is called a hybrid warm roof. Insulation is placed in between the joists underneath the roof surface above. A warm flat roof is defined by insulation being installed on top of the roof deck.

Making a warm flat roof construction but instead of putting the insulation above the structure joists as seems to be normal i propose to include the joists effectively as insulation i believe wood is quite good for that and install a proper vapour control layer underneath the joists. 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. Insulation material is placed on top of the roof rather than in between the ceiling and the decking. 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 cold deck flat roof will typically require insulation to be installed between the roof joists to comply with building regulation requirements. In the warm flat roof construction there is insulation packed into every possible space where air can flow. The insulation boards should be laid with the plywood face up. The vapour control layer is above the decking but below the insulation.

The insulation is immediately below the waterproofing layer. A warm roof is defined as being when the all or the majority of the insulation is above the joists or deck. Good question theoretically when building a warm roof the vaper control layer goes on the warm side of the insulation. Here the roof deck is maintained at the same temperature as the internal conditions hence warm deck roof.

A cold flat roof is defined by insulation being installed between and if necessary underneath the timber joists. The roof itself is therefore kept warm.