Flat Roof Insulation Between Joists

How to install insulation in a flat roof with timber deck insulated between and below joist.

Flat roof insulation between joists. Install quinn term ql kraft insulation boards below the joists using suitable mechanical fixings. Warm deck insulation 1. 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. 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.

How to install insulation in a flat roof with timber deck insulated above the joists. Good question theoretically when building a warm roof the vaper control layer goes on the warm side of the insulation. The majority of domestic cold flat roofs use insulation between and under the joists. The insulation is below the deck but above the plasterboard.

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. To form a vapour control layer seal the joists between the boards. Colde deck insulation 1. 1 2 timber strength class and joist size.

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. However in many situations the insulation may need to be primarily or entirely below the flat roof membrane and between the roof joists if so what are the best options. A cold flat roof is one where the insulation is between or between and below the roof joists. Insulation is placed in between the joists underneath the roof surface above.

It is preferred to have the bulk of the insulation between the joists to reduce any risk of condensation. 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. Both these variants require different approaches to insulation which will be covered in this guide. Depth to width ratio should not be greater than 6.

This is used for refurbishment e g. 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 width ratio should not be greater than 4. How to insulate a flat roof. A cold deck flat roof will typically require insulation to be installed between the roof joists to comply with building regulation requirements.

At pitched roofs with the insulation between joists we know a back vented roof is a great way to keep the roof functioning well as stated in a prior 475 blogpost.