Insulation Between Flat Roof Joists

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.

Insulation between flat roof joists. Thread the twine between the joists attaching it to the sides of the joists with drywall screws or roofing nails. Depth to width ratio should not be greater than 6. 1 2 timber strength class and joist size. Warm deck insulation 1.

The insulation boards should be laid with the plywood face up. Insulation is placed in between the joists underneath the roof surface above. 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. How to insulate a flat roof.

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. Staple a 4 to 8 mill. 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. This method of insulation can also be used to upgrade the thermal performance of an existing roof without affecting the waterproofing system.

It is preferred to have the bulk of the insulation between the joists to reduce any risk of condensation. This is used for refurbishment e g. The majority of domestic cold flat roofs use insulation between and under the 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 width ratio should not be greater than 4.

Measure and cut 2 by 4 inch lumber to fit the gaps between each set of joists. Measure and mark the joists at each end of the roof about 4 feet in from the edge. 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.

Between and under 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. Lay your chalk line across your two pencil marks and snap a line across all of the joists in between. The insulation is below the deck but above the plasterboard.

The insulation should not bulge past the joist faces. A cold deck flat roof will typically require insulation to be installed between the roof joists to comply with building regulation requirements.