When thermal insulation is installed above the waterproofing membrane of a roof deck and secured by a ballast, it is referred to as a protected membrane or inverted roof.
The benefits of this system lie primarily in protecting the waterproofing membrane from the effects of weather and ultraviolet radiation, which maintains the membrane’s integrity, as well as thermally insulating the mass of the roof slab, which then remains at a relatively constant temperature, moderating the internal temperature of the building below.
Additional benefits include the use of the roof surface for traffic and parking, as well as for green planted roofs. Trafficable inverted roofs can be designed to be suitable for pedestrians, light or heavy vehicles, and even helicopter landing pads.
Most inverted roof thermal insulation solutions in Southern Africa are for concrete-decked residential and commercial applications, including homes, hospitals, blocks of flats and offices. These are primarily designed for use as suitable for foot traffic and planting for roof gardens, or access to rooftop building services such as HVAC, elevators and solar systems.
The standard density extruded polystyrene foam board (XPS) manufactured in South Africa (36kg/m3) has proven itself suitable for use for inverted roof with light-vehicle traffic. Isoboard imports appropriate higher board densities, which have higher compressive strength, from its parent plant when required for heavy-vehicle traffic. As one is effectively building a road on top of a building to accommodate the movement of vehicles, this is the design province of an engineer.
Other ballast types include interlocking bricks, paving flagstones, gravel or even cement screeds for warm deck systems. There are proprietary systems, where supporting rings are laid over the insulation, which enable flagstones to be placed onto the rings, braced on four rings, one in each corner. The small gaps created between each flagstone facilitate drainage from the flagstones and the XPS. The benefit of using XPS is it has the lowest water absorption – moisture ingress being the downfall of thermally insulated buildings – and will have the longest durability.
An additional benefit of many of these interventions is that XPS thermal insulation can be retrofitted to an existing roof, which may have been built before energy efficiency was a priority. This intervention saves the future integrity of the protective waterproofing membrane, and will add thermal efficiency to the roof system. This is very helpful when creating a roof garden or trying to make best use of available space. The board and ballast are comparatively lightweight, which is often important when retrofitting to an existing building.
Isoboard has South African Board of Agreement assessment for the inverted roof or protected membrane application, Certificate 2000/277, certifying Isoboard as ‘Fit for Purpose’, valid as of the year 2000. Isoboard has been privileged to supply Isoboard XPS to thermally insulate the concrete decks of many prestigious building projects, including hospitals, embassies, residential flats, office blocks and campuses. After 28 years and many thousands of square meters, it is safe to say Isoboard is proven effective in this application.