Flat Roof Insulation: Insulating a flat roof with timber deck insulated between and below joists: cold deck insulation

Plasterboard

AVCL

Mannok Flat Roof PIR insulation boards

Mannok Flat Roof PIR insulation boards

Roof joists

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Vented airspace between quinn therm and timber deck

Timber deck

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Waterproofing layer

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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.

One of the most essential considerations in a cold deck flat roof construction is the requirement for adequate ventilation in the form of a ventilated cavity of at least 50mm between the underside of the timber deck and the roof insulation below. This ventilation is necessary to prevent condensation from forming below the roof deck, which can cause rotting to the timber deck and compromise its load bearing capacity.

Thermal bridging also needs to be carefully addressed when installing insulation in a cold deck flat roof in order to achieve the required U-Values and improve the thermal efficiency of the structure.

Choosing the right insulation boards

In the Mannok insulation range, there are a number of suitable insulation products for use in this particular type of roof construction. We recommend a combination of Mannok Therm Roof / MR boards and Mannok Therm Laminate-Kraft / MLK insulation boards.

Mannok Flat Roof insulation is foil faced on both sides and should be used to insulate the flat roof between the joists to achieve optimal thermal performance.

Using Mannok Therm Laminate-Kraft / MLK insulation to insulate below the roof joists will also help achieve optimum thermal performance and it also eliminates the need to install an additional vapour control layer and a separate plasterboard layer. Both the vapour control layer and plasterboard are in-built in the Mannok Therm Laminate-Kraft / MLK board, making it much easier and more efficient to install the insulated structure.

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

Insulating between the joists

1. Measure the space between the joists where the insulation will be placed accurately and cut the Mannok Therm Roof / MR insulation boards marginally wider than the space to ensure a friction fit upon installation.

2. Fit the cut Mannok Therm Roof / MR insulation boards tightly between the joists. A gap of 50mm must be left between the upper surface of the insulation layer and the timber deck above to form the ventilation cavity. Ventilation openings must be left at each end of every air space. The openings must be equivalent in area to a continuous opening of 25mm along each side.

Insulation below the joists

1. Install Mannok Therm Laminate-Kraft / MLK insulation boards below the joists using suitable mechanical fixings.

2. To form a vapour control layer, seal the joists between the boards.

3. Set the boards onto a wide (30mm) bead of vapour resistant sealant applied to the under surface of all the joints and cross noggins or alternatively a separate AVCL can be installed.

 

Depth of insulation and U-Values

The depth of insulation is dependent on the U-values required and the depth of the roof joists, always allowing for a minimum 50mm air gap. The below U-Value table shows the thickness of insulation to use to achieve a range of U-Values using Mannok Therm Roof / MR and Mannok Therm Laminate-Kraft / MLK PIR insulation boards. To calculate U-Values specific to your roof construction, use our free U-Value calculator.

Required insulation thickness of Mannok Therm Roof / MR:
Mannok Therm Roof / MR insulation between and below joists: Ventilated cold roof

Mannok Therm Laminate-Kraft / MLK below joists Mannok Therm Roof / MR Insulation between joists (mm)
75 100 125 150
Thickness (mm) U-value (W/m2K)
37.5 – – 0.19 0.16
42.5 – – 0.18 0.16
52.5 – 0.19 0.17 0.15
62.5 0.20 0.17 0.15 0.14
72.5 0.18 0.16 0.14 0.13

100mm x50mm studs @ 400mm centres. 100mm Therm between Studs

U-value results above based upon pitched roof construction shown:

12.5mm plasterboard; AVCL; Mannok Therm Roof / MR; roof joists; Mannok Therm Roof / MR between joists @ 400mm centres; timber deck; waterproofing layer (built-up felt shown). Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.

Required insulation thickness of Mannok Therm Roof / MR:
Mannok Therm Roof / MR insulation between and below joists: Ventilated cold roof

Mannok Therm Laminate-Kraft / MLK Insulation below joists Mannok Therm Roof / MR Insulation between joists (mm)
75 100 125 150
Thickness (mm) U-value (W/m2K)
37.5 – – 0.19 0.16
42.5 – – 0.18 0.16
52.5 – 0.19 0.17 0.15
62.5 0.20 0.17 0.15 0.14
72.5 0.18 0.16 0.14 0.13

100mm x50mm studs @ 400mm centres. 100mm Therm between Studs

U-value results above based upon pitched roof construction shown:

12.5mm plasterboard; AVCL; Mannok Therm Roof / MR; roof joists; Mannok Therm Roof / MR between joists @ 400mm centres; timber deck; waterproofing layer (built-up felt shown). 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 Mannok team.

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  • Flat Roof Insulation
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