Closed cell polystyrene foam thermal insulation for utilised flat roofs
It is often a puzzle to create a straight or inverted flat roof – there are typical cases when the latter is clearly preferable. However, in order to reach the benefits, some fundamental aspects have to be taken into account in both material selection and structural solutions.
How does an inverted terrace roof come together? On the reinforced concrete slab first the slope formation, then the insulation against rainwater is installed. On top of this is the thermal insulation material – expediently closed-cell polystyrene foam (XPS). As the next layer, surface-stabilizer separator layer is required, which may be geotextile or water deflector, on which a bedding of at least 3-4 centimetres of stone grit should be placed, which acts as a vapour diffusion and drainage layer. Then comes the cladding, which in the classic case can be made of concrete tiles, but also small-element paving, or thick ceramic tiles or wood can be layed.
Why do we recommend the use of closed-cell polystyrene foam for this purpose? This is mainly because if the thermal insulation is placed above the waterproofing, the latter is placed in a wet environment, and the XPS, thanks to its crystalline, closed cell material structure during the extrusion process, takes up very little water. (1-3 % by volume, depending on thickness)
One of the advantages of the XPS is its good thermal insulation (λ = 0.033-0.035 W/mK). The high load capacity of the material, i.e. its compressive strength, is sufficient to ensure that even a large public traffic surface can be constructed. If we want to briefly summarise the advantages of an inverted roof, we must first and foremost mention the protection of the waterproofing, both from mechanical effects and against UV radiation. Such a structure of the layer order is also favourable from the point of view of building-physics: there is no need to install a separate vapor barrier membrane, or since the waterproofing is on the warm side of the thermal insulation, there is no need to fear of condensation here. It is also favourable that, since waterproofing is replaced first, the installation of additional layers can be carried out regardless of weather conditions. Finally, it’s worth thinking that in case of changing functions – or even storey addition – it is easy to break down and recycle the material. This can also be beneficial during renovation.
In the construction process, it is important that the waterproofing is provided a mechanical protection after the installation of thermal insulation: even scaffolding can be placed on the XPS material, alongside with other contruction materials, equipment, or tools for installing air-condition systems and other machines on the roof. In the case of utilised roofs such as terrace roofs and green roofs, it is also important for the future function that the layer order is load-bearing and the waterproofing is protected.
In these cases, it is therefore worth creating an inverted roof:
• if a slab with a high thermal mass (typically reinforced concrete) is built,
• if a large or heavy mechanical equipment is placed on the roof (and therefore there is considerable construction and maintenance traffic),
• and if a utilised roof – green roof, parking roof, terrace roof with a higher public traffic – is built.
During installation of the Ravatherm XPS, it is worth paying attention to:
• if there is a parapet wall
• complete internal drainage
• appropriate material selection (type, thickness; it is recommended to fix waterproofing with full-surface flame melting to the slab)
• appropriate slope formation,
• installation of water deflector membrane or geotextile between the ballast and the thermal insulation.
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