Case Study – Canopy Junction

Canopy junction

Temperature with thermal break

Relative humidity with thermal break

Temperature without thermal break

Relative humidity without thermal break

The challenge

A new apartment building will have a concrete roof with a concrete upstand around its perimeter and a concrete canopy attached to the upstand. Canopies and balconies draw heat out of the building envelope in winter and are a potential cause of surface condensation inside the building and interstitial condensation in the building fabric. The architect wishes to know whether a thermal break must be placed between the canopy and the upstand or whether the thermal break can be omitted and the roof, upstand and canopy can be cast in the same mould.

The solution

Atkinson Science carried out a risk assessment for surface condensation, interstitial condensation, mildew and mould growth with and without the thermal break in accordance with BS5250:2011. The assessment method recommended in the Standard assumes that the diffusion of heat and vapour through the building envelope is one-dimensional. In the canopy junction the diffusion of heat and vapour is significant in two coordinate directions and a two-dimensional method must be used. Atkinson Science used a computational fluid dynamics (CFD) program to extend the one-dimensional method recommended in the Standard to two dimensions. February is the coldest month and provides the conditions most likely to lead to surface and interstitial condensation. Using the 2-D method we computed the distribution of temperature and relative humidity in the canopy junction for the month of February with and without a thermal break.

When the thermal break was in place we found that the surface temperature everywhere inside the building was at least 8.0°C above the 80% RH temperature (at which mould growth can occur) and at least 11.2°C above the dew point, so we concluded that there is no risk of surface condensation or mould growth in the building. In addition, we found that the relative humidity everywhere in the structure was no higher than 82.9%, so we concluded that there is no risk of interstitial condensation.

When the thermal break was removed and the canopy, upstand and roof were cast in the same mould, we found that the surface temperature everywhere in the building was at least 7.3°C above the 80% RH temperature (only 0.7°C lower than before) and at least 10.5°C above the dew point (again, only 0.7°C lower than before), so we concluded once again that there is no risk of surface condensation or mould growth in the building. In addition, we found that the relative humidity everywhere in the structure was no higher than 82.9%C (the same as before), so we concluded once again that there is no risk of interstitial condensation.

We attribute our findings to the large mass of concrete in the junction. The concrete is easily able to absorb the cold temperatures from the canopy and prevent them from reaching the inside surface of the building. In addition, vapour from inside the building is easily absorbed in the mass of concrete and only reaches the cold outer surface of the building in low concentrations that are below the saturation vapour pressure.

The benefits

By extending the risk assessment method recommended in BS5250:2011 to two dimensions, Atkinson Science were able to demonstrate convincingly that a thermal break is not needed in the canopy junction and that the roof, upstand and canopy can be cast in the same mould.