El proyecto arquitectónico

Ventilation through the subfloor is calculated as described in Section 5.5.5. The relationship between subfloor humidity and ventilation is provided in Figure 6.32. There is a negative slope as expected and as seen in the literature but the correlation is very weak. Similar ventilation studies in the literature show a stronger correlation (Kurnitski 2000).

Figure 6.32: Subfloor centre relative humidity vs ventilation

The net moisture flow through the vents is calculated as described in Section 5.5.5 as a function of the subfloor and outdoor air properties and the ventilation. The net moisture exiting the vents is shown in Figure 6.33. The uncertainty is moisture flow is approximately 15%. This is based on the uncertainty in indoor and outdoor specific humidity, the assumed uncertainty in ventilation rate of 10%, and the uncertainty in air density resulting from the errors in the temperature and humidity measurements. The moisture flow is sometimes negative, indicating that 24% of the time the net effect of the vents is to introduce and deposit moisture into the subfloor. The moisture flow is higher in the warmer months than in the cooler months. This contradicts theories that vents can be problematic in summer (Kurnitski and Matilainen 2000).

Figure 6.33: Net moisture exiting subfloor vents

The vents tend to increase subfloor moisture when the relative humidity is high and decrease moisture when the relative humidity is low, as shown in Figure 6.34. When the outside humidity drops below about 70% the vents primarily decrease subfloor moisture.

Figure 6.34: Net moisture exiting vents versus outside relative humidity

Evaporation was calculated as a function of the net moisture flow through the vents and the subfloor cavity moisture storage rate as shown in Equation 5.4. The moisture storage rate is calculated to be negligible in all cases. This was not unexpected as the specific humidity was observed to vary very slowly with time, as shown in Figure 6.25(b). Hence the evaporation term is equal to the net moisture exiting the subfloor via the vents. As the wood moisture content was also observed to vary slowly with time, as shown in Figure 6.31, any moisture exchange between the subfloor air and the wood elements in the subfloor is negligible. Thus, the source of evaporation in the subfloor can be considered to be solely from the ground.

There are two methods for moisture to enter the subfloor air as shown in the control volume of Figure 5.20. Moisture can enter from the ground via evaporation or from the vents via direct

transport. These two sources of moisture are compared in Figure 6.35. In nearly all cases, 99.6% of the time, more moisture enters the subfloor from the vents than from the ground. On average the vents introduce 8 times the moisture than the ground does. This relationship varies with the ventilation. With a low ventilation rate of 5 ACH, the vents bring in approximately four times the moisture than does the ground. At a mid-range ventilation rate of 12 ACH this ratio grows to 10, and the ratio increases as ventilation rate increases.

Figure 6.35: Subfloor cavity moisture sources versus ventilation

This high vent-to-evaporation ratio is comparable to that found in the British study described in Section 2.5.2 (Hartless and Llewellyn 1999) where it was found that the vents’ contribution of moisture was an order of magnitude greater than the contribution from the ground evaporation. The relationship between ground moisture evaporation and ventilation is shown in greater detail in Figure 6.36. The evaporation rate, averaging 5 g/m2_{/hour over the test period, is at the lower end}

of the range observed in the published studies from New Zealand and Finland (Abbott 1983; Bassett 1988; Kurnitski 2000; Trethowen 1994, 1998) as described in Section 2.5.2.

The data in Figure 6.36 show no correlation with an R2 _{of 0.05 and a slope of near 0. The}

uncertainty of 15% in evaporation rate is not enough to account for this lack of correlation. This differs from the literature, which shows evaporation to have a weak correlation with ventilation but a clearly positive slope (Kurnitski 2000). However, as ventilation is only one contributor to the evaporation potential as described in Section 5.5.5 and there may be substantial confounding between ventilation and the other contributors to evaporation potential, the lack of a strongly positive correlation between evaporation and ventilation does not indicate an unexplainable phenomenon. Nevertheless, the relationship between evaporation and ventilation was investigated further and is documented in Appendix A.5.1.

The evaporation potential is the product of ground surface vapour pressure deficit and wind speed. The ground surface vapour pressure is a function of the ground surface temperature and amount of moisture at the surface. However, the thermocouples measuring ground surface temperatures are not reliable. In addition, the soil moisture content has been observed to be lower than the critical value at which it can be treated as free water, and this alters the relationship between ground surface vapour pressure and temperature. Thus, the evaporation potential cannot be determined with great accuracy. An estimated evaporation potential is calculated using ground temperature at 150 mm deep and assuming the vapour pressure is equal to that of fully saturated conditions. The observed evaporation versus evaporation potential is provided in Figure 6.37. The scale and units of the evaporation potential are arbitrary and thus the value of slope has no meaning. However, the slope is clearly positive as expected and the correlation R2 _{of 0.30 is better than}

observed in Figure 6.36. Thus, the calculated value of evaporation is within expectation.

Figure 6.37: Ground moisture evaporation vs. evaporation potential

The net enthalpy flow through the vents is calculated as described in Section 5.5.5 as a function of the subfloor and outdoor air properties and the ventilation. The net enthalpy exiting the subfloor via the vents is shown in Figure 6.38. The energy flow is often negative, indicating that 35% of the

time the net effect of the vents is to increase the energy content of the subfloor cavity air. The net amount of energy exiting the vents is higher in the warmer months than in the cooler months.

Figure 6.38: Net energy exiting subfloor vents

There is no strong correlation between the net energy through the vents and subfloor ventilation. The strongest correlation between the energy flow and any environmental parameter is with the outdoor specific humidity, as shown in Figure 6.39.

Figure 6.39: Net energy exiting subfloor vents versus outdoor specific humidity