ColdFlat Roof - Technical Detail

Flat Roofs

Cold Flat Roof

Insulation between joists

OmniFit® Slab 35 / Rocksilk® RS45 

 

  • Insulation slabs knit together at joints, reducing the potential for cold spots and loss of thermal performance

  • Friction Fit

  • Quick and easy to install

 

Our OmniFit® Slab 35 and Rocksilk® RS45 Slabs are Non-combustible with a Euroclass A1 reaction to fire rating.

Knauf Insulation Flat Roof GRP TimberDeck-Solution-UK

Products

OmniFit® Slab 35 is a Glass Mineral Wool slab, designed for use in multiple applications in both timber and steel frame construction, that offers excellent thermal and acoustic performance.

Rocksilk® RS45 is a Rock Mineral Wool slab, designed for use in multiple applications in both timber and steel frame construction, that offers excellent thermal and acoustic performance.

Typical construction

Cold flat roofs consist of insulating between joists, or between and below joists to meet building regulations. A cavity should be left between the insulation and the waterproofing layer for ventilation purposes.

Often chosen for refurbishment applications, our insulation solutions for flat roof applications provide both excellent fire and acoustic performance, in addition to high levels of thermal Performance

Installation

OmniFit® Slab 35 or Rocksilk® RS45 are gently pushed between the timber joists ensuring the slabs are close butted with no gaps. The insulation will friction-fit and can be easily cut with a sharp knife or saw to suit joists at any centres.

Insulated plasterboard can be used where greater thermal performance is required to underdraw the timber joists. These are mechanically fixed and installed in accordance with the manufacturers instructions.

Condensation control

To reduce the risk of interstitial condensation, a AVCL should be provided on the warm side of the insulation, to the underside of the joists. A cross-ventilated void, not less than 50mm deep should also be provided, between the slab or deck and the insulation following the guidance in BS 5250:2021

Ventilation opening should be provided to every roof void along two opposite sides of the roof and should be equivalent in area to a continuous opening of not less than 25mm at each side.

Waterproofing

Waterproofing layers should be fixed in accordance with the manufacturer’s recommendations.

Typical verge detail

Fire performance

Both OmniFit® Slab 35 and Rocksilk® RS45 are non-combustible insulation with the best possible Euroclass A1 reaction to fire classification, and are manufactured using Knauf Insulation's unique bio-based binder, ECOSE® Technology. This means that they are non-combustible and will not contribute to the development of a fire or flame spread.

Typical U-values of cut to falls flat roofs

    U-value (W/m2K)
    Knauf Insulated Plasterboard Thickness (mm)
Joist depth (mm)

OmniFit® Slab 35 / Rocksilk® RS45 thickness (mm)

35

50 65 75
250 200 (2x100) 0.16 0.15 0.13 0.13
225 175 (100+75) 0.18 0.16 0.15 0.14
200 150 0.20 0.18 0.16 0.15
175 125 (75+50) 0.23 0.20 0.18 0.16
150 100 0.27 0.23 0.20 0.18

Contact our Technical Support Team