BRANZ went into the field to solve the problem of a failed roof cavity in a new build in Tauranga. Because of the parapet design, a novel approach was taken that used a solar fan to ventilate the cavity.
A CURRENT Building Research Levy-funded project focuses on understanding exactly why some roof cavities are failing due to moisture accumulation and what can be done about it. This has resulted in several case studies that are often quite involved but are important for us in gaining a better understanding of the issue.
Depending on the individual measurements required, setting up an experiment that will run for some weeks or months is a complex undertaking. Here, we look at a situation where we tested an intervention in the form of a solar-driven fan. The measured data provided insights into the dynamic roof space environment, which suggested such a solution could work, and the data was then used to show its effectiveness.
Understanding roof space conditions
In BRANZ case studies, humidity and temperature are logged inside the roof cavity and the living space. Readings are taken every 5 minutes to get a good understanding of the dynamics of heat and moisture flows throughout the building. Because the inside climate is generally affected by the outside climate – for example, wind is a major driver of ventilation (see Build 174, How airtight do houses need to be?), a weather station is usually used as well.
The rate at which inside air is replaced with outside air under any given wind condition depends on the airtightness of the building. A blower door measurement can provide an estimate of air infiltration, but using tracer gas techniques gives a better understanding. By releasing a tracer gas at a constant rate inside the building and watching how the concentration changes, we get an idea of how the air infiltration rate varies over time.
If the concentration drops quickly, air inside the building is being replaced with air from outdoors at a rapid rate. In the case studies, we used this technique to understand air movement into the roof cavity. This helps us understand where the moisture sources and moisture sinks are and why, under certain circumstances, this moisture can build up to become a major problem in locations such as the roof cavity.
Tauranga case study
The methodology described was applied to get to the bottom of a failed roof cavity in a newly built residence in Tauranga.
During the final stages of completion, the builder noticed significant amounts of water on the underside of the roofing underlay (see Figure 1). The adjacent timber structure also appeared wet.
After requesting guidance through the BRANZ technical helpline, the case was picked up by the Building Performance Research Team. As it related to a current research project, the problem was looked at in more detail.
Causes of roof space moisture
Often, roof cavity moisture issues are a result of a combination of things. For example, a new relatively airtight house may have a high internal moisture load due to inadequate ventilation. Combine this with an air-leaky ceiling and a relatively airtight roof space, and moist air can easily find its way into the roof space and condense on the cold underlay.
In this case, without the building even being occupied, the high moisture load was caused by the wet trades, in combination with the concrete slab still curing. The ceiling still had open penetrations for downlights, and the winter temperatures inevitably led to condensation on the cold roofing iron with its underlay. To avoid the risk of mould, the moisture needed to be removed quickly.
New approach to actively ventilate
Usually, passive ventilation elements would have been an option, but the parapet design wasn’t suited to this, so a novel approach was tried with a solar fan.
In previous roof moisture projects, it had been noticed that large amounts of moisture were released from building materials during the day. Moisture had been stored in the materials – timber and roofing underlay – and the increased temperature from the sun caused the evaporation of this stored or condensed water.
What if we could actively ventilate during these times, while the moisture has been driven out of storage?
This process can be achieved with a ventilator that is operated by a photovoltaic solar panel – the fan does not need any connection to the domestic electricity supply and only operates during the daytime. Commercial systems are available, including flashing kits for different roof profiles. Figure 2 shows such a commercial system installed on the roof of the Tauranga property.
Acting as an exhaust fan to the roof space, the supply air is provided by ventilation openings at the eaves of the building. Both the exhaust fan and the inlet openings are placed appropriately to ensure good cross-ventilation in the roof cavity.
Avoid fans that operate continuously day and night as they draw in cold, moist outside air during the night. This might lead to more condensation on the underside of the cold roof cladding.
Moisture removed during daytime
Figure 3 shows the initial unvented case followed by the vented cases after installation of the ventilator at the end of January. The absolute moisture content is plotted, expressed as grams of water in a kilogram of dry air. The advantage of this quantity over relative humidity is that absolute moisture is independent of temperature and thus more appropriate when looking for sources or sinks of moisture.
The graph shows the daily cycles when moisture is being released into the air by evaporation driven by the higher daytime temperature in the cavity. The moisture content in the roof space is typically higher than outside due to the moisture sources.
After installing the solar ventilator, the peak moisture levels are significantly reduced and are much closer to the outside levels. Moisture is being removed from the roof cavity during the daytime. Figure 4 shows the same data in a different form. The number of readings where the roof cavity air has moisture values above 15 g/kg are much reduced for the vented case.
Passive vent openings often sufficient
While we have gained valuable data from our case study, solar-driven ventilators will not be necessary for most residential roof spaces.
Under most circumstances, passive vent openings will be adequate to increase resilience against accumulated roof moisture levels.
Solar-driven ventilators a possible alternative
However, as in this case with the parapet roof design, sometimes the vents are difficult to install or may not result in an efficient air exchange of the cavity due to their location.
Time was also of the essence in the Tauranga house to remove the moisture quickly to avoid mould growth.
Active daytime-only ventilation might also be an option for buildings with a high moisture load – schools for instance. The situation for schools is slightly more complex since classrooms often have suspended acoustic tiles forming the ceiling, and these systems are not very airtight.
A situation where the fan pulls up moist air into the cold roof cavity needs to be avoided. BRANZ is currently monitoring the effectiveness of a solar-driven ventilator at a school in the Wellington region. We will keep you posted on the outcome.
Articles are correct at the time of publication but may have since become outdated.