Case Study – Balcony Junction

Balcony junction

Temperature without extra insulation

Relative humidity without extra insulation

Temperature with extra insulation

The challenge

The apartments in a new residential block will have a concrete floor slab and a concrete balcony slab. The two slabs will be separated by a thermal break but there will be mild steel plates above and below the thermal break to support the glass doors leading onto the balcony and to support the insulation below the balcony. The mild steel plates are a potential cold bridge that might cause surface condensation in the apartments and interstitial condensation in the building fabric.

The solution

Atkinson Science performed a risk assessment for surface and interstitial condensation in accordance with BS5250:2011. The assessment method specified in BS5250:2011 assumes that the transfer of heat and moisture through the building envelope is one-dimensional, but in the balcony junction heat and moisture may diffuse both parallel to and normal to the slabs. To solve this problem we used a computational fluid dynamics (CFD) program to extend the method in BS5250:2011 to two dimensions. We used this 2-D approach to calculate the temperature and relative humidity in the balcony junction for the month of February when the environmental conditions are most likely to lead to surface and interstitial condensation.

Surface condensation occurs when the temperature on internal surfaces is at or below the dew point and consequently the relative humidity is 100%. However, mildew and mould growth can occur at temperatures above the dew point. Guidance on the conditions that allow mould to germinate and grow is given in CIBSE Guide A, 2006. We can assume that if the surface relative humidity is over 80% for less than three hours a day then mould growth is unlikely, but if it is over 80% for more than six hours a day then mould growth is very likely.

We found that the lowest surface temperature in the apartments was 16.2°C and occurred in the ceiling cavity. This temperature was 3.7°C above the dew point of 12.5°C, so we concluded there was no risk of surface condensation. However, the temperature was only 0.3°C above the temperature 15.9°C at which the relative humidity is 80%. We found that the relative humidity everywhere in the building fabric was no higher than 88.8%, so we concluded that there was no risk of interstitial condensation.

To be certain there was no risk of mildew or mould growth we repeated the computations with an extra layer of insulation inside the ceiling cavity. We found that the lowest surface temperature was still in the ceiling cavity and rose to 18.9°C, i.e. 6.4°C above the dew point and 3.0°C above the 80% RH temperature. The highest relative humidity value rose to 91.6%, still well below 100%. We concluded that with the extra insulation there was no risk of surface or interstitial condensation and in addition there was no risk of mildew or mould growth.

The benefits

Atkinson Science discovered a potential risk of mildew and mould growth in the ceiling cavity of the apartments. We showed that adding an extra layer of insulation inside the ceiling cavity eliminated the risk and the possibility of expensive repairs in the future.