A non-treadable inverted roof is a flat-roof construction in which the thermal insulation is installed above the waterproofing membrane, reversing the conventional sequence of a warm roof. In this configuration, the waterproofing layer remains beneath the insulation and is therefore protected from direct exposure and temperature variations.
Extruded polystyrene (XPS) is particularly suited to inverted roof applications because of its combination of high compressive strength, low water absorption and suitability for exposure to the conditions occurring above the waterproofing layer.
For non-treadable roofs, the XPS insulation is typically protected by a filter/separation layer and a ballast layer, commonly washed gravel. The ballast protects the insulation from direct UV exposure and contributes to keeping the roof build-up stable against wind-induced uplift.
X-FOAM® HBT boards, with their shiplap edges and compressive strength of ≥ 300 kPa, are suitable for this type of application. The insulation is installed above the waterproofing layer, creating a continuous thermal layer while the membrane performs the waterproofing function beneath it.
Because rainwater can penetrate between and beneath the insulation boards in an inverted roof, the effect of rainwater flow on thermal performance must be considered when determining the required insulation thickness.
