Realising the true long-term performance of an inverted flat roof

It can be immediately obvious when the physical design characteristics of a newly constructed building fail to reflect an original architect's vision. However, other properties, such as a shortfall in thermal performance, may not be so apparent, yet arguably are even more important.

This is why we must pay close attention to U-value calculations and the build-up of our inverted roofing systems; the decisions made in the design stage will dictate the long-term thermal performance of a building. Everyone involved in a construction project needs to trust the robustness of the U-value calculations and understand the nuances of their chosen building materials if they are to be confident that a structure will perform as envisaged.

In the case of inverted flat roofs using extruded polystyrene (XPS) insulation boards, there are several design factors that need to be considered. If left unchecked, you can easily end up with a building that falls short of expectations when it comes to thermal performance.

Recognising the differences between design and declared lambda values

All insulation products suffer from a degree of variability in their thermal conductivity. This is where we can see the real value of the European Products Standards thermal test results; the statistical analysis offers a consistent and reliable approach making it much easier to compare different materials.

The declared lambda, defined in EN 13164 and sometimes referred to as the 90/90 value, not only considers the performance of a product as it rolls off the production line but any degradation in performance over the expected twenty-five-year lifespan of the insulation board. The name is derived from the fact that there is a ninety per cent confidence level that at least ninety per cent of a product will achieve the stated conductivity value.

There is a temptation to use the declared lambda as the design value, however, in the case of an inverted flat roof, this is highly problematic as it will omit several important correcting factors. This oversight can cause you to significantly underestimate the thickness of insulation necessary to achieve a target U-value.

Considering the impact of moisture absorption

EN 13164 also refers to how any moisture trapped within an insulation system will impact its thermal efficiency as water is a much more effective conductor than dry air. The diffusion and freeze/thaw rates should therefore be used to determine the potential for moisture absorption over time, providing us with a moisture conversion factor for use in our U-value calculation.

Location, location, location

It's not just about whether a location is hot or cold, expected rainfall levels can have a significant impact on the performance of an inverted flat roof due to the rainwater cooling effect. While the placement of insulation above the waterproofing layer affords several advantages compared to a warm roof, this also means that rainwater is an additional heat loss mechanism; while relatively small in volume, the rainwater that percolates through the ballast, making its way beneath the insulation boards, will remove heat from the building fabric as it flows towards a drainage outlet.

Rainfall levels vary significantly across Ireland, with it not being uncommon for western coastal areas to experience twice as much rain as locations in the south and east of the country; this is due to the westerly winds and proximity to the Atlantic Ocean. Of course, there are plenty of other things that impact rainfall levels in specific areas, including the presence of natural geographical features such as hills or mountains, and bodies of water.

As rainfall increases, so does the heat loss from the building. A correction factor (Delta U) is applied to the U-value calculation, as specified in I.S. EN ISO 6946:2017 to account for this. It is important to realise that Delta U is increasingly important as the U-value becomes smaller – particularly relevant considering the exceptional levels of performance offered by the latest generation of XPS insulation boards and the continued emphasis on fabric-first design.

The use of a water flow-reducing layer (WFRL), positioned between the insulation boards and ballast, can reduce water flow to less than five per cent. This is a significant improvement over a build-up without one; for calculation purposes you are required to assume seventy-five per cent of rainwater would normally flow under underneath edge-profiled interlocking. When you consider how much it reduces the Delta U penalty, it would be foolhardy to not include a WFRL.

The lingering effect of condensation

As the waterproofing layer is on the warm side of the insulation boards, there is a significantly reduced risk of interstitial condensation, due to the boards effectively functioning as an efficient vapour control layer. The potential for surface condensation is also largely mitigated as the boards keep the temperature of the waterproofing at a level similar to the building's internal temperature.

Of course, the potential for condensation is not completely eliminated, with sudden drops in temperature able to cause water vapour to condense within the roof build-up. This phenomenon can happen in both the summer and winter months, and the previously mentioned rainwater cooling effect can even be the cause of this condensation.

Concrete decks have a reduced risk due to their large thermal inertia because of the duration and extent of deck cooling. Unfortunately, lightweight decks face significant risks unless equipped with adequate mitigation.

The first instinct of an architect might be to place some of the insulation below the deck within the ceiling void – creating a hybrid of the warm and inverted roof concepts – especially when grappling with design constraints. However, this can further enhance the risk of condensation by increasing the thermal resistance below the waterproofing layer!

Ultimately, it is incredibly difficult to identify the tipping point where condensation effects might emerge, and there is virtually nothing that can be done to offset this risk; ventilation of the insulation layer might on the face of it appear to be a solution, however, this short-circuiting reduces the effectiveness of the whole build-up.

This is why all the U-value calculations prepared by the team at Ravago are accompanied by a condensation risk analysis, drawing on our years of inverted roofing experience. By speaking with people who have intricate knowledge of a particular product you can identify the build-up that will provide optimal performance with minimal condensation risk.

Don't shy away from asking for help

As you can see, there is a lot more than meets the eye when calculating the U-values for inverted flat roof systems. Fortunately, an array of tools is available to assist with U-value calculations, and building product manufacturers like Ravago have technical teams who can provide expert insight and support.

It is always worth leveraging the expertise of building product manufacturers, taking advantage of their innate familiarity with their products. They're also able to provide guidance on any unique factors that might affect your specific application.