The BREDEM- 8 Loudon model has been used to examine the impact of seasonal,
environmental and geographical variations in the prediction of relative humidity. A typical dwelling has been modelled and the variations in the predicted relative humidity over the heating season have been examined. The environmental variations in relative humidity within a dwelling have also been analysed by examining the difference between the internal airspace relative humidity and the internal surface relative humidity in a typical modern and a typical traditional dwelling. This has been carried out to test the traditional assumptions regarding the specification of a critical airspace relative humidity as an indication of mould risk. The impact of geographical region on predicted relative humidity has also been tested by modelling a typical dwelling and varying its location in the UK.
7.3.1 Variation anaiysis dweliing specifications
The impact of seasonal, environmental and geographical variations in relative humidity has been tested using two dwelling type specifications, a modem well-
insulated semi-detached dwelling and a traditional poorly insulated semi-detached dwelling. The basic specifications of these dwelling types are shown in Table 7.2.
Input parameter Insulated dwelling Uninsulated dwelling
Degree Day region Thames Thames Regional wind speed 4 m/s 4 m/s Exposure of dwelling Average Average Floor area 40 m^ 40 m^ Wall area 90 m^ 90 m^ Glazing area 8 m^ 8m^ Ground floor U value 0.45 W/m^K 0.8 W/m"K Wall U value 0.45 W/m^K 2.12W/m^K Roof U value 0.25 W/m"K 1.9 W/m"K Glazing U value 2.8 W/m^K 4.7W/m^K
MVHR None None
Number of extract fans 2 2
Heating system Gas boiler with balanced flue Gas boiler with balanced flue Controls Room thermostat and programmer Room thermostat and programmer Hot Water Cylinder 110 litre spray foam insulated 110 litre spray foam insulated Cooking Hob/oven combination: gas/electric Hob/oven combination: gas/electric Low energy lighting None None
Occupants 4 4
Z1 heating weekdays 9 hours 9 hours Z1 heating weekends 16 hours 16 hours Z2 heating weekdays 7 hours 7 hours Z2 heating weekends 11 hours 11 hours Z1 demand temperature 21°C 21°C Equipment usage level Average Average Drying clothes indoors Never Never Indoor plants 4 4
Moisture production rate 7.0 kg/day 7.0 kg/day
Table 7.2 - Details of the dwelling specifications used in the variation analysis
7.3.2 Seasonal variations in relative humidity predictions
The BREDEM- 8 Loudon model is designed to predict the monthly average
relative humidity in the airspace and on all surfaces in both zones of a dwelling. It is therefore able to determine the variations in the predicted relative humidity throughout the heating season. The modem well-insulated semi-detached dwelling shown in Table 7.2 has been modelled using the BREDEM- 8 Loudon model and
the results o f the monthly predictions o f relative humidity are shown in Figure 7.6.
Surface RH limit 80% 80 - è 70 = 6 5 - Airspace RH limit 70% 55 50 1 2 3 4 5 6 7 8 g 10 11 12 Month — 6 — Zone 1 air - O - Zone 2 air — 6 — Zone 1 floor surface — Q— Zone 1 wall surface — A— Zone 1 window surface — e — Zone 2 floor surface
- O Zone 2 wall surface - * - Zone 2 ceiling surface
A Zone 2 window surface — X— Zone 1 Cold Bridge
(TDR=0.3) X Zone 2 Cold Bridge
(TDR=03)
Figure 7.6 - Monthly relative humidity predictions for a typical modem semi detached dwelling
The results shown in Figure 7.6 show that there is a significant variation in the predictions of relative humidity for each month of the heating season. The most critical period for high relative humidity occurs very early in the heating season during the month of October. Outside the heating season the predicted conditions may be less accurate due to varying internal temperatures and ventilation rates. The predictions between October and May, the heating season normally assumed, indicates that the relative humidity predictions are highest at the start of the season and gradually reduce as the season progresses. This trend is also reflected in the monitored relative humidity data collected in the 36 dwellings and is counter to the British Standard for condensation in buildings (BSI 1989), which suggests undertaking the calculation during the coldest period of the year, December and January, ignoring the period of highest risk.
7.3.3
Environmental variations In relative humidity
predictions
The risk limits that are generally assumed for mould growth, an airspace relative humidity of 70% or a surface relative humidity of 80%, have been indicated in
Figure 7.6. The predictions indicate that the highest relative humidity in both zones occurs on the window and cold bridge surfaces, which is expected since these are the coldest surfaces in the dwelling. Ignoring these predictions, since relative humidity on window surfaces should not be a problem until condensation occurs and cold bridges should not occur in well designed and constructed dwellings, the difference between the surface relative humidity and airspace relative humidity predictions in all months is relatively low. Therefore, in a modern well-insulated dwelling the difference between the airspace and the surface relative humidity can be significantly less than the 1 0% difference
assumed by the British Standard for condensation in buildings (BSI 1989).
The traditional poorly insulated semi-detached dwelling shown in Table 7.2 has also been analysed and the results indicate that the difference between the airspace and the surface relative humidity in a poorly insulated version of the same dwelling is significantly greater than in the well-insulated dwellings.
The average difference between the airspace relative humidity and the wall and floor surface relative humidity in the well-insulated dwelling over the heating season is 2%. For the poorly insulated dwelling this difference is 10% and at severe cold bridges there can be a difference o f 2 0% in relative humidity between
the cold bridge and the airspace surrounding it. It follows that the risk limit o f 70% specified by the current British Standard for condensation in buildings is only relevant in traditional dwellings that are poorly insulated. Modem well- insulated dwellings can have an airspace relative humidity greater than 70% and still not be at risk from mould growth at the surfaces where the relative humidity can remain below the critical 80% relative humidity.
These results show the importance o f assessing the relative humidity where mould grows and dust mites live. In assessing the risk o f mould growth this requires an examination of the surface conditions o f relative humidity. In assessing the risk from dust mites it requires an examination o f the relative humidity in the micro environment within which they exist. At the moment there is little information
environments where dust mites live. Risks associated with the dust mite are currently based upon empirical findings regarding the population o f dust mites and the ambient conditions o f relative humidity. Further work is clearly required in the link between the ambient conditions and the micro-environmental conditions of relative humidity.
7.3.4 Regional variations in relative humidity predictions
The external climate is known to have an impact on the internal relative humidity in dwellings. Regional variations in the climatic parameters of temperature, vapour pressure and solar radiation can be accounted for in the BREDEM- 8
Loudon model. The BREDEM- 8 Loudon model can therefore be used to examine
the impact of geographical location on the prediction o f internal relative humidity.
The traditional poorly insulated semi-detached dwelling shown in Table 7.2 has been used to assess the impact of geographical location on the risk of mould growth. Ignoring windows and cold bridge surfaces, the critical relative humidity for mould growth is assumed to be either a surface relative humidity greater or equal to 80% or an airspace relative humidity greater or equal to 70%. Figure 7.7 shows the risk periods for mould growth in the poorly insulated semi-detached dwelling for 8 different locations throughout the UK. A similar analysis carried
out on the well-insulated semi-detached dwelling identified no critical relative humidity periods for any o f the geographical locations modelled.
The results show that the critical risk period occurs early in the heating season or before. In addition to this, generally, the further North a dwelling is located the earlier in the year that critical risk period occurs, and the longer its duration. However, these results must be interpreted with caution since the ability o f the model to predict relative humidity outside the heating season has not been tested as part o f this study. Nevertheless, for each geographical location examined, only the external climatic variables have been altered and therefore although the predictions outside of the heating season must be interpreted carefully, the general trends of risk have been identified.