INVERTED ROOF INSULATION

Inverted roof also referred to as a 'secured membrane' is a type of roof construction sequence where the waterproofing layer lies beneath the insulation instead of above it. Insulation boards are loose-laid on top of the membrane and then weighted down with paving slabs, tiles, stones or gravel ballast. Due to the open-jointed roof finish, the insulation can be easily lifted to allow inspection of the waterproofing system. Inverted roof is also called upside down roof as rainwater percolates down through and between the insulation boards to the membrane where it is drained away through rainwater outlets. An inverted roof system doesn’t require an additional air and vapour control layer (AVCL). The main benefits of the inverted roof method is that the membrane is protected by the insulation from the expansion and contraction caused by weather fluctuations such as frost or UV exposure. Inverted roofs ensure that the complete roof construction, including roof covering, is kept at warm temperatures during winter months and at moderate temperatures during the summer months. An inverted roof removes the need for mechanical fixings or adhesives to secure the roof build-up. On other hand the main disadvantage of inverted roof is the fact that the insulation is likely to be wet a lot of the time, with some moisture trapped beneath the boards. Designers also need to be sure that sufficient weight is placed on the boards to prevent movement. Logically, this can only be achieved using insulation specifically designed for this purpose. This material must be highly pressure-resistant and be unaffected by freeze and thaw cycles. The closed cell structure design of XPS thermal insulation is perfect for inverted flat roofs such as green roof or terrace roof. On the other hand rigid extruded polystyrene insulation such as Kingspan GreenGuard, has minimal water absorption due to its closed cell structure. This makes it the only material suitable and approved for inverted roof applications.

INVERTED ROOF INSULATION PRODUCTS

• Kingspan Optim-R
• Kingspan Greenguard GG300/500/700
• Kingspan Styrozone H 350 R
• Kingspan N300/N500
• IKO enertherm XPS 
• XPS Soprema SL
• Foamglas® Invatherm™
• ProTherm G XPS X 300 SL
• ProTherm Quantum^® Plus⁺
• BauderGlas
• Polyfoam Roofboard Extra
• Polyfoam Roofboard Super
• Syrofoam
• Finnfoam

INVERTED ROOF DETAIL

Rain that falls on the roof percolates through the ballast and between the insulation boards until it reaches the membrane. It then drains away through rainwater outlets. Condensation is on the warm side of the insulation. The build-up of the roof means that some water is almost always trapped beneath the insulation boards by capillary action. The inverted roof build-up design comprised the following details:

• Ballast
• Insulation
• Liquid-applied polyurethane waterproof membrane

INVERTED ROOF VS WARM ROOF

In a warm roof, the insulation material is protected on both sides. First, the roof covering protects the insulation from rain and wind. Second, a vapour barrier protects the insulation from moisture from the inside of the building. On the other hand, an inverted roof is a type of warm roof that is installed upside down. Insulation is simply placed over the roof covering converting roof to green terraces. However, an inverted roof will lose more heat and you will therefore have to use more insulation to obtain the same performance compared to a warm roof. To avoid moisture issues, the inverted roof must also have a greater pitch than a warm roof.

INVERTED ROOF DRAINAGE

Specifications for inverted roofs currently focus on several issues - in particular correct drainage. It is essential that roof falls and drainage paths are correctly designed to avoid ponding and subsequent risk of silt build up, causing stresses in freezing conditions and to reduce water entry in the event of a waterproof layer failure. Drainage points need to be located at the lowest point of the roof, to facilitate effective removal of rainwater. Care is needed to identify the locations of these. For example, since roofing decks will deflect between spans, mid-span may be the lowest point of the roof, rather than the edges or column supports. Where a water flow reducing layer is used, drainage must be provided at the two levels: at the water flow reducing layer level and at the roof deck waterproofing level.

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Picture credit:https://fibran.si/