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Flat Roof Insulation: Insulating a flat roof with timber deck insulated above the joists: warm deck insulation

Timber deck

Vapour resistant sealant

Quinn Therm QRFR-PLY PIR insulation boards

Waterproofing layer

Quinn Therm QRFR PIR insulation boards

Capping

  • Timber deck

  • Vapour resistant sealant

  • Quinn Therm QRFR-PLY PIR insulation boards

  • Waterproofing layer

  • Quinn Therm QRFR PIR insulation boards

  • Capping

The best method of insulating a flat roof is by using a warm deck flat roof construction method. The main advantage of a warm deck construction versus a cold deck construction is that there is no need to include a ventilated cavity in the warm deck construction. It also provides better thermal performance and makes it easier to eliminate thermal bridging.

Using PIR rigid insulation boards faced with plywood removes the need to fit a separate timber deck and is suitable for use in domestic buildings where a timber deck is preferred.

Quinn Therm QRFR-PLY PIR Insulation is a high-performance rigid board, with a rigid thermoset insulation core and faced with a low emissivity composite foil facing on the inner side and 6mm water and boil proof (WBP) plywood on the outer. It is designed for use in flat roof construction, with the deck, insulation and vapour control layer all-in-one. This means faster and easier installation and can help achieve excellent U-Values.

How to install insulation in a flat roof with timber deck insulated above the joists: warm deck insulation

1. 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. The insulation boards should be laid with the plywood face up.

2. The Quinn Therm insulation has a very high vapour resistance and can be formed into a vapour control layer by sealing the joints between boards.

3. Fix the Quinn Therm insulation boards with mechanical fixings. Use low profile headed screws, long enough to give minimum 35mm embedment into the timbers. Fixings should be at 200mm centres around the board edges (300mm centres on any intermediate timbers) and set at least 10mm from board edges and 50mm from corners. Fixings for adjacent boards should be staggered to the same joist or noggin and screw heads must finish flush with the surface of the plywood.

4. Lay the waterproofing layer on top of the Quinn Therm insulation layer as soon as possible to prevent water damage to the roof insulation. In poor weather conditions, the insulation boards should be protected by polyethylene sheeting until the waterproofing system is installed.

5. Pack the joist space at the eaves and the verges with insulation

6. Install vertical edge insulation at parapets to the inner face of the wall, ensuring the wall insulation continues at least 150mm above the surface of the Quinn Therm QRFR-PLY boards.

Note, access to flat roofs constructed with Quinn Therm QRFR-PLY laminate should be for maintenance and repair traffic only.

What thickness of insulation do I need to use?

The required thickness of Quinn Therm flat roof insulation will depend on the desired U-Values. The below U-Value table shows the required insulation thickness based on the typical warm flat roof construction outlined. To calculate U-Values and insulation thickness specific to your project, use our free online U-Value calculator.

Required insulation thickness of Quinn Therm QR and QRFR-PLY:

Quinn Therm QR between joists Quinn Therm QRFR-PLY above joist (mm) U-value (W / m2K)
- 76 0.25
- 96 0.21
- 106 0.19
- 116 0.17
- 131 0.15
25 131 0.14
30 131 0.13
50 131 0.12

* Thickness of Quinn Therm QRFR-PLY includes 6mm WPB plywood Joists at 400mm centres.

U-value results above upon flat roof construction shown:

Plasterboard; timber deck; Quinn Therm QRFR-PLY plywood laminate on vapour resistant sealant strips(over joists at 400mm centres); waterproofing layer. Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.

Required insulation thickness of Quinn Therm QR and QRFR-PLY:

Quinn Therm QR between joists Quinn Therm QRFR-PLY above joist (mm) U-value (W / m2K)
- 76 0.25
- 96 0.21
- 106 0.19
- 116 0.17
- 131 0.15
25 131 0.14
30 131 0.13
50 131 0.12

*Thickness of Quinn Therm QRFR-PLY includes 6mm WPB plywood Joists at 400mm centres.

U-value results above upon flat roof construction shown:

Plasterboard; timber deck; Quinn Therm QRFR-PLY plywood laminate on vapour resistant sealant strips(over joists at 400mm centres); waterproofing layer. Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.

Need technical support?

For help and advice on any of our products and how they might fit your needs please contact our Technical by Quinn team.

  • Floor Insulation
  • Pitched Roof Insulation
  • Flat Roof Insulation
  • Wall Insulation