Flat Roof Insulation Between Joists

1 2 timber strength class and joist size.

Flat roof insulation between joists.

The insulation boards should be laid with the plywood face up. 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. The insulation is below the deck but above the plasterboard. Insulation is placed in between the joists underneath the roof surface above.

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. Depth to width ratio should not be greater than 6. It is preferred to have the bulk of the insulation between the joists to reduce any risk of condensation. Warm deck insulation 1.

To form a vapour control layer seal the joists between the boards. This is used for refurbishment e g. 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.

Colde deck insulation 1. 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. 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. Refurbishing an existing timber joisted flat roof which has no insulation.

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. A cold deck flat roof will typically require insulation to be installed between the roof joists to comply with building regulation requirements. Install quinn term ql kraft insulation boards below the joists using suitable mechanical fixings.

How to install insulation in a flat roof with timber deck insulated between and below joist. Good question theoretically when building a warm roof the vaper control layer goes on the warm side of the insulation. A cold flat roof is one where the insulation is between or between and below the roof joists. Both these variants require different approaches to insulation which will be covered in this guide.

The majority of domestic cold flat roofs use insulation between and under the joists.