Our building code was amended recently to include roof ventilation. This is a good thing and very well intended but like many things, can lead to disaster if not done properly.
In doing my energy reports I am noticing that roof vents are now common on drawings. This is a sign of good intentions. However, the problem I see is that high level roof vents,(usulally whirly birds) are shown but without low level ventilation. This is the disaster.
99.9% of our houses are not air sealed at the ceiling plane. If you install high level roof ventilation without low level roof ventilation what happens? YOUR ROOF SUCKS.
You have now created a situation where the roof vents are sucking air out of the conditioned space of the house through ceiling penetrations, (recessed downlights, exhaust fans, service penetrations etc.).
Remember Building Performance Lesson 4? Air out equals air in. If you suck air out of the roof, the replacement air has to come from somewhere. If there are no low level roof vents connected to outside air, the replacement air will come via the path of least resistance, which is in most cases through your downlights. You have now not only decreased the energy efficiency of the house but you have also created a potential moisture problem by increasing the moisture load in the roof cavity…… This is exactly the thing roof ventilation is supposed to prevent!
Last winter I was brought in to advise on a house that had a water damaged ceiling and water dripping from the downlights. In an attempt to fix the water leak the builder had already replaced the section of roof twice. Unsuccessful and still unable to find the leak they decided to install a whirly bird but found that the problem got worse.
The problem was that the downlights were channeling moisture laden air from inside the house into the roof cavity. The roof cavity was small, had foil sarking and metal roof battens. The warm, moist air condensed on these cold roof structure surfaces then dripped onto the ceiling insulation and through the downlights. The installation of a whirly bird without low level ventilation just resulted in drawing even more moisture laden air from inside the house consequently making the problem worse.
Once we addressed the vapour permeability, the thermal breaks, sealed the downlights, improved the ceiling insulation cover, and installed low level roof ventilation the problem went away.
The purpose of roof ventilation is to mitigate moisture accumulation and condensation in the structure. Low level ventilation such as eave ventilation must be of a greater surface area than the high-level ventilation, especially when whirly birds or forced air ventilation applies. The attic space will then be slightly pressurised and will prevent air from being sucked out of the house. This is also why an air sealing strategy is important at the ceiling plane. If moisture doesn’t get into the roof space there will be no problem.
Roof ventilation is a good idea but only if you do it properly. Understand why you are doing it and how to do it as part of a whole-house performance strategy. Don’t be the victim on an unintended consequence.