Articles for the Home Consumer

Good Roof Ventilation: An Essential Safeguard For Your Family Home

Few of us think about the importance of our home’s roof ventilation system, or the hidden dangers of poor and bad ventilation in general.

The hidden dangers of poor roof ventilation include:

  • Poor air quality in the home, which can cause illness;
  • Condensation in the roof space, which leads to mould infestation even in a brand new home. Eventually, this causes structural damage.

Some of these hidden dangers are caused by modern home construction.

Roof ventilation and airtight modern home construction

In past eras, building engineers weren’t concerned about roof ventilation. Few homes were airtight and big gable roofs provided large roof spaces in which air could freely circulate and ventilate.

In an article in Issue 152 of Build magazine, Building Physicist Stephan Rupp suggests that the way homes are built today and the way we use them has changed:

“… (Today) buildings are significantly more airtight… Roof styles are changing as well. Skillion-type roofs are very popular, with a much reduced cavity volume.”

Not only do many new homes have decreased roof cavity volume, they’re also likely to be crammed with insulation. Without sufficient air circulation and a good roof ventilation system, insulation traps and retains moisture, increasing the dangers of mould and rotting timbers.

Poor roof ventilation can lead to moisture and mould infestation

In modern roof construction today, even brand new homes with inadequate ventilation can have moisture and mould.

A report on mould infestation, by Tim Dorrington of Minz, revealed high mould infestation in new, unoccupied homes. How could this happen?

He theorizes that mould growth develops when framing timbers are manufactured, then stored. Before they’re shipped to a building site, the timbers are stored in sheds open to wind and rain. Then, those same moisture-laden timbers are wrapped and stored on pallets. He suggests that: “trapped moisture will create condensation within the pallet and increase the moisture content” of the wood.

Finally, when the framing timbers are used with inadequate roof ventilation, there’s nowhere for the released moisture to go. So the airless roof cavity quickly grows mould in a new home, even before anyone moves in.

No matter how new or old your home may be, insulation is also a factor. Without a good roof ventilation system, insulation can lead to increased vapor condensation and moisture in your roof.

Vital: your roof’s insulation needs good air circulation

BRE’s Good Repair Guide reports:

“(In modern homes) … the risk of condensation in the roof has increased, due to a combination of factors. Houses are much better insulated… the air can hold larger quantities of water vapor and more finds its way into the roof space.”

This water vapor condenses into moisture and this builds up over time: wet insulation can’t do its job. Eventually, this causes damage to the home. By the time the damage is discovered, it’s usually well advanced.

How can we mitigate the unwanted effects of modern home construction methods?

Roof ventilation is the key. That starts with passive roof ventilation.

Passive roof ventilation gives you a healthier home and more secure home naturally

What’s “passive” roof ventilation? Basically this system uses various non-mechanical processes (via vents) which encourage good air circulation. When stale air and moisture are drawn out of your roof, they’re replaced by fresh air.

An article, by BRANZ Senior Physicist Manfred Plagmann, emphasizes the necessity of measuring air flow in a roof. However, measurement processes can vary, and some are unreliable:

“Some devices or methods work well with low measurement uncertainties – less than 10% – while others show systematic errors of 50% and more.”

It’s also vital to ensure that fresh air from outside a home (rather than exhaust air from the home) is circulated in, and vented from, the roof cavity. As Stephan Rupp reports:

“… avoid situations where exhaust air… exiting through a ridge vent, is sucking up moist air from the living quarters below. The replacement air needs to be fresh – outside air coming through vents around the eaves for instance.”

Be guided by your roofing contractor to provide the best roof ventilation system and products for your home.