HOW DO POINT THERMAL TRANSMITTANCES MEASURE HEAT LOSS FROM FLAT ROOFS?
A point thermal transmittance measures heat loss through an isolated thermal bridge, and is expressed as a chi value. A point thermal bridge can occur anywhere in the building fabric, but for this blog post we’ll refer to common examples found in flat roof constructions.
A chi value is a counterpart to a psi value, which measures linear thermal bridging heat losses. Like psi values, chi values can only be calculated using numerical modelling (or 3D modelling) techniques. It is not possible to calculate a chi value using software designed to calculate U-values by the combined method.
What are examples of point thermal bridges in flat roofing?
A common point thermal bridge we’re asked about is when a concrete plinth forms part of the structural roof deck. The plinth, or a series of plinths, is usually required to support rooftop plant whose weight is too much for the roof’s thermal insulation layer to bear.
The plinth(s) must obviously extend high enough above the roof’s finished surface to provide a sufficient upstand, and they therefore cause a break in the continuity of the insulation layer.
When a situation like this occurs in a roof design, our technical helpdesk is often asked if one of the following two solutions can be adopted.
Carry out a U-value calculation for the flat roof as normal, but include a bridging percentage in the insulation that equates to the area of the roof covered by the plinth(s).
Calculate the U-value for the insulated roof as normal, and a separate U-value for the roof build-up through the plinth. The idea is to then do an area-weighted U-value calculation to adjust the performance of the roof as a whole.
What are the issues with addressing point thermal bridges like this?
The first option effectively treats the concrete as a repeating thermal bridge, even though it doesn’t repeat regularly. (Typical repeating thermal bridges in building fabric include mortar joints in masonry, or timber joists in a roof.)
Not only is it an incorrect application of the combined method, the calculation method is also likely to fail because of a discrepancy between the upper and lower limit values used to work out the U-value.
The second option is problematic because the combined method is designed to calculate heat loss through whole construction elements, and not small areas of unique construction. It might provide a representative heat loss for a small proportion of the (already relatively small) plinth detail, but it doesn’t account for the interaction between the edge of the plinth and the insulated roof build-up.
This interaction needs to be thermally modelled, which is why an appropriately qualified person should be engaged to produce chi value calculations.
What are the advantages of calculating point thermal bridging chi values?
The primary benefit of obtaining numerically modelled chi values is that it provides accurate point thermal transmittance values, in contrast to the two options discussed above.
A follow-on benefit is that, like with linear thermal bridging, numerical modelling also produces surface temperature factors for the detail in question. That means it’s possible to assess any condensation risk at the points of increased heat loss.
The U-value of a flat roof shouldn’t exceed 0.35 W/m2K at any point, in order to avoid the risk of surface condensation inside the building. Sometimes, despite the best design intentions, it’s impossible to avoid a scenario where that threshold isn’t met across the entire roof – for example, if structural requirements, like the plinths we’ve been talking about, need to take precedence.
In such unavoidable situations, the point thermal bridge, and the number of them, should be minimised as far as possible.
Because point thermal bridges are undesirable, it’s even more important to get accurate thermal modelling carried out in order to help assess the heat loss, and any potential consequences of it, at the detail in question.
The U-value of the roof as a whole, meanwhile, can be adjusted by multiplying the chi value by the number of point thermal bridges per square metre, and adding the result to the U-value.
For U-values calculated in accordance with the combined method, and advice on dealing with tricky detailing issues, contact us to discuss your current project.