The thermal insulation of flat roofs is a particularly important issue both in winter chills and summer heats. The performance and longevity of flat roofs depend upon many factors including the position of the insulation within the structure. Due to its numerous advantages for flat roofing, the best solution is the inverted roof construction.
• protected water-proofing membrane,
• the roofing membrane is installed directly on top of the screed (concrete surface),
• simple build-up of layers without an additional vapour barrier,
• the installation of layers above the waterproof membrane is independent of weather,
• easy and fast to install,
• simple exploration and leak repair.
Inverted roofs comply with the building energy requirement that the thermal protecting hull should be unbroken and installed on the outside surface of the envelope structure. Thermal insulation efficiently reduces outbound heat flows in the heating period, while in the summer period, it prevents building structures from warming up, thereby significantly reducing (or, ideally, excluding) the need for air conditioning.
The closed-cell structure design of STYROFOAM™ XPS thermal insulation is perfect for inverted flat roofs:
• inverted roof with gravel ballast
• green roof
• terrace roof
• parking roof
• duo roof
The advantages of inverted roofs reveal themselves when compared to any other flat roof solution. Because of the danger of condensation, the complex structural design and the problematic breathability, double-shell cold roofs as a possible structural design solution for flat roofs, are not commonly used. Single-shell warm roofs with a straight order of layers, even though provide a perfect solution, have a shorter life expectancy due to the water insulation’s exposure to weather and its implementation without mechanical protection in general. Also, in the case of failure, it is harder to find and repair the damage.
ADVANTAGES OF INVERTED ROOFS
The inverted flat roof answers all physical, implementation, and operational/usage problems by placing thermal insulation above the waterpéroofing layer. The temperature of the thermally insulated waterproofing membrane is roughly the same during the whole year, and in addition to the appropriate mechanical protection, STYROFOAM™ XPS also provides protection against UV radiation. The reverse roof structure is the safest solution for waterproofing and for the operational spaces located under the roof.
The insulation protects the waterproofing layer from:
• wide temperature variations — daily 60 to 70°C and annually sometimes even exceeding 100°C (-20 to +80°C),
• the degradation caused by extreme weather conditions,
• mechanical damages during construction, use and maintenance.
The waterproof layer acts as a total vapour control layer, and, being on the warm side of the insulation, is maintained above dew point temperature so the risk of condensation is eliminated.
The inverted roof concept has other benefits as well. The insulation can be:
• installed in any weather
• added to or replaced without stripping the waterproof layer
• easily lifted and replaced/re-used if the building is altered, or even on another building.
LONG LIFE EXPECTANCY
The inverted roof insulation system has proven successful for more than four decades in Europe. The long-term behaviour of the system has been examined by independent institutes and building experts several times.
Their assessments can be summarized as follow:
The life expectancy of the membrane is higher, and the risk of failure is reduced due to the effective and permanent protection of the membrane.
With a correct and diffusion-open installation, no significant reduction of thermal performance can be expected.
INSTALLATION OF THERMAL INSULATION IN INVERTED ROOFS
In order to design an appropriate inverted roof and use it in the long-term, respect the following advice:
• The slope and design of the screed under the waterproofing membrane should comply with the professional guidelines. The use of waterproofing, e.g. bituminous plate, glued on the entire surface is recommended. In the case of utilised roofs, the use of PVC waterproofing membranes is to be avoided.
• The thermal insulation boards should be laid loosely, in an offset bond on the roof. The boards can be cut to size using a hot-wire cutter or thick-toothed saw on site.
• The filtering, separating and surface-stabilizing layer, that is, the geotextile or water-guide film should be a non-absorbent material to avoid the possible creation of a water film over the thermal insulation boards.
• The thickness of the ballast layer should be dimensioned for wind-suction, but it should be a 50-mm thick washed 16/32 gravel layer. Adjacent to attic walls and roof superstructures, concrete tiles may be used as additional load based on the calculation.