Flat Roof Insulation Between And Over Joists

Bs 5250 2011 the british standard for the control of condensation in buildings does not recommend installing insulation between timber joists as well as above the deck.

Flat roof insulation between and over joists.

The rule of thumb for foil faced boards like thermapitch tp10 or kooltherm k7 in this scenario is that you can have no more insulation under the joists than you have between the joists. Lay mineral wool between the joists and then fit rigid insulation boards across the joists with floorboards on top. The insulation is immediately below the waterproofing layer. Learn more about warm roof cold roof construction.

To protect the roof from the potential of condensation occurring unseen within the construction it s important to remember that the layer of insulation installed over the rafters should as general rule be double the thickness of the insulation layer between the rafters. 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. Where insulation depth is restricted by the flat roof construction it may be necessary to install the roof insulation between and below the joists to create a cold deck structure. This method of insulation can also be used to upgrade the thermal performance of an existing roof without affecting the waterproofing system.

Flat roof insulation between and under joists a cold deck flat roof will typically require insulation to be installed between the roof joists to comply with building regulation requirements. Can insulation be placed between and over flat roof joists. If insulation is placed on top of the flat roof joists and as such the joists are not fastened to the boarding above the depth to breadth ratio of the joists should not exceed a ratio of 4 to 1. The majority of domestic cold flat roofs use insulation between and under the joists.

The vapour control layer is above the decking but below the insulation. Constructing a warm roof on a new extension with parapet walls i will have surplus amounts of insulation left over and am considering adding this insulation to the underside between the joists to double up the insulation would this install have any negatives.