Roof ventilation works very well to regulate both temperature and humidity levels in your roof cavity. It provides several benefits including electricity costs reduction, increased air comfort and longevity of your insulation/roofing timbers.
As hot air rises, an unventilated roof cavity can become exceedingly hot reaching temperatures during summer months between 50C and 80C. Moisture generated through cooking, showering and washing can also rise to the roof cavity. Internal heating during winter months can also load moisture into the roof space via condensation and all of these moisture-generating activities create an ideal breeding ground for mould and mildew if the roof space is not ventilated effectively. A moist, warm, dark and potentially mouldy roof cavity with no ventilation will impact upon the effectiveness of your roof insulation, the services run within the roof cavity (e.g., A/C ducting, plumbing, electrical etc.) as well as the structural integrity of a timber framed roof over time. The variations in the temperature cause expansion and contraction of roof joints, which may shorten the expected lifespan of your roof and cause cracking at the ceiling below.
How Does Roof Ventilation Work?
Roof ventilation stabilises the temperature and humidity levels in your roof during summer and winter. An extractor fan is used to remove the stale air from inside the house, while roof vents allow fresh air to enter the roof cavity via under eave vents or gable vents. In summer, the roof ventilation system extracts hot air and replaces it with cool dry air while in winter, cold moisture laden air is extracted and replaced with clean outside air. The air movement provided by the roof vents prevents moisture from building up in the roof cavity and results in a decrease in humidity.
Powered and Solar Roof Ventilators
Powered (mechanical) ventilation can be achieved in a number of ways. Firstly, in homes that are not air-conditioned, a whole of home natural ventilation system can be used to control the temperature, but these can be quite expensive. Generally, mechanically powered, or solar powered extraction fans installed along the ridge of the home in conjunction with the installation of intake vents is a very effective way of ventilating a roof space. A sufficient number of intakes for the roof vents need to be installed for this method to work effectively.
Most Powered or Solar roof ventilators are thermostatically controlled for temperature as well having humidity control to activate the fan once a pre-set temperature or humidity setting has been reached. The effect is to reduce the amount of time air spends in the roof cavity thereby reducing the amount of heat and moisture build up in the cavity. Powered vents are tied to the property power grid and use electricity. Current solar vents operate quietly on their own power reducing potential operating costs; however, some also have a back up to power so that they can still operate when there is not sufficient sun to operate from solar energy. The running costs of these units is usually very little as they will run from solar energy most of the time unless there is a lot of cloud in the sky or temperatures/humidity settings are breached during the evening causing the fan to activate and run from mains power.
Roof Ventilation Benefits
If you’re wondering if roof ventilation is necessary, all you need to do is take a look at the list of benefits to see how roof ventilation could help enhance your home. These benefits include:
Reduce moisture build up in the roof cavity and prevent mould and mildew from forming
• Maintain a more constant temperature in the roof cavity and prevent the expansion/contraction of roof joints
• Improving the lifespan of your roofing materials (including timber and insulation)
• Improving the effectiveness and lifespan of the services contained in the roof cavity by having them operate in a more controlled environment in the roof cavity. (A/C ducting, plumbing, electrical etc.)
• Reducing electricity bills by allowing your air conditioner to work more efficiently
Types Of Roof Ventilation Systems
While all roof ventilation systems work on the same principle, there are different types of roof ventilation and components. Each of them has its benefits and installation considerations. Each system must have an exhaust vent to release air and an intake vent to allow fresh air in.
Exhaust vents allow hot air to exit your roof cavity. They come in various forms and can be passive or active and vertical or horizontal. Passive vertical exhaust vents allow air to exit via convection. While passive horizontal exhaust vents harness the wind and allow cool fresh air to blow in and warm stale air to blow out. Active systems use wind or electricity-powered exhaust vents to extract air out of the roof cavity. Exhaust vents can be installed on the roof’s ridges (peak where two areas meet), off ridges (below the roof crest) or in the form of a box vent or wind turbine.
The intake vent is how fresh outdoor air enters the roof cavity. There are 3 main types of intake vents:
• Under Eave (soffit) vents: Installed at the bottom (under side) of the covered eaves.
• Static vents: Installed at the low end of the roof.
• Gable vents: Installed on house gable exterior walls (only for gable roofs)
All of the above intake vents can be used to create cross-ventilation through the roof cavity.
An intake vent (under eave vent, gable vent or static roof vent) is an essential component of an effective roof ventilation system which allows fresh air to be drawn into the roof cavity to flush out the hot or moisture laden air trapped in the cavity via the roof vents.
Static vents are vents with no moving parts which can be installed in multiple locations along the ridge of the roof and can be used as an intake vent if installed low down on the roof when under eave vents or gable vents cannot be installed. Static vents rely upon temperature and the natural flow of air to provide a passive vertical exhaust allowing air to escape through heat stack as the hot air rises. They are generally relatively small in size, which means they release less air than other types of exhaust vents.
Wind-driven turbine ventilators remove heat and moisture from your roof space by harnessing the power of the wind. The wind causes the head of the vent to spin creating high and low pressure either side of the head which draws air up and extracts it out of the roof cavity. Wind turbines can be very effective if provided with “free air” from intake vents. The added bonus is that they have zero running costs. They do need to be installed in an area of the roof that provides good airflow to function. (Generally installed along the ridge of the roof and the number required will be dependent upon the size of the area being serviced and the availability of free air)
Ridge vents are a very popular type of exhaust vent (especially in commercial warehouse type buildings) because they can be easily installed along the roof ridge in a continuous length. They are also able to release larger quantities of air than other passive static exhaust vents because they can run the entire length of the ridge. Ridge vents use natural convection to extract air from the roof cavity, making them simple and efficient.
Factors That Affect Roof Ventilation’s Performance
While all ventilation systems will improve the airflow in your roof cavity, some roofs will allow better ventilation than others. Some factors that can affect your roof ventilation system’s performance include:
• Roofing material (Metal V’s Tile)
• Roof sarking
• Roof structure (timber V’s metal)
• Roof Pitch and air volume within the roof cavity
Different roofing materials have different thermal properties, and some allow for better airflow than others. Metal roofs tend to retain more heat than tiled roofs. They are also much more sensitive to temperature fluctuations and may result in expansion and contraction when the temperature changes. The size of the roof combined with roof pitch will dictate the air volume contained within the roof cavity and the larger the volume the longer it will take to heat load. Metal roofs are generally larger in volume than tile roofs so this can be an advantage, however, a metal roof with metal roof structure will heat up much quicker and respond to temperature fluctuations much more.
Insulation can slow down the transfer of heat and effectively control the temperature of your roof space. Insulating your roof will effectively reduce hot air build-up and make your ventilation system much more effective. For metal roofs it is suggested that you insulate beneath the roof sheeting as well as on top of the ceiling.
Sarking is a membrane that sits under your roof tiles. It can help improve the performance of your insulation and the thermal performance of your roof while providing an extra layer of protection from the environment. While sarking has some benefits, it can negatively impact the ventilation of your roof as heat can potentially become trapped between the sarking and your insulation and result in moisture build-up.
A well-installed roof ventilation system can offer multiple benefits including mould reduction and improving the longevity of your roof. There are plenty of roof ventilation system options that would be suitable for your house. Contact Calidad Industries today to discuss installing a roof ventilation system in your home.