Warm Flat Roof Insulation Between Joists

The insulation is immediately below the waterproofing layer.

Warm flat roof insulation between joists.

The vapour control layer is above the decking but below the insulation. 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. Unless the joists are exceptionally deep there is also likely to be a requirement to install insulation to the underside of the joists as the upper 50mm of joist space is usually required to be left clear for ventilation purposes. Good question theoretically when building a warm roof the vaper control layer goes on the warm side of the insulation.

There are two main types of flat roof that require different approaches to installation. A warm roof is defined as being when the all or the majority of the insulation is above the joists or deck. In creating the cold flat roof air is allowed to circulate freely between the joists and there is no insulation. The roof itself is therefore kept warm.

A cold deck flat roof will typically require insulation to be installed between the roof joists to comply with building regulation requirements. 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. In the warm flat roof construction there is insulation packed into every possible space where air can flow.

A cold flat roof is defined by insulation being installed between and if necessary underneath the timber joists. A warm flat roof is defined by insulation being installed on top of the roof deck. The insulation boards should be laid with the plywood face up. 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.

Warm deck insulation 1. Insulation is placed in between the joists underneath the roof surface above. Here the roof deck is maintained at the same temperature as the internal conditions hence warm deck roof. So a solution i would like to propose is.

Between and under joists. 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.