Preventing condensation and mould in roof cavities

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Eave and ridge seals.

By William Lin

As New Zealand’s leading manufacturer of permeable roofing underlay, Thermakraft is often asked to explain why water droplets sometimes form on the underside of underlay in ceiling cavities at colder times of the year.

The air in a ceiling/roof cavity will always contain a certain amount of water vapour, which travels up from the living areas below.

Breathing, cooking, un-flued gas heaters, laundering and showering all generate water vapour that escapes into the roof cavity in various ways.

In older homes, water vapour escapes up through down lights, ceiling access holes, and unsealed lining joints. In newer homes and totally renovated homes (which are more airtight), water vapour diffuses into the roof cavity through solid permeable materials, such as wallboard and plywood.

If the roof cavity is poorly ventilated, the air up there becomes over-burdened with moisture. And when the temperature drops outside, this moisture can condense into water droplets (dew) on the underside of roof cladding and underlay — just like warm moist air condenses on a cold glass of water.

Underlay plays an essential role in helping water vapour to escape

Roofing underlay, whether it is building paper or a high-tech synthetic underlay, is in close contact with roof cladding.

And when the temperature drops outside, the cold energy is transferred to this underlay directly beneath the roofing iron or tiles.

High-quality, permeable underlays are designed to hold excess moisture overnight (absorbent) then release it back into the atmosphere in the form of water vapour when the roof warms up the next day.

While the membrane is able to effectively repel liquid water from the outside and prevent the roof from leaking, its permeability allows water vapour to escape from the inside to the outside.

However, there’s a limit to how much moisture an underlay can absorb and release. If the condensation forming on the underlay is more than the underlay can effectively hold and release in a 24-hour period, water droplets may form.

If this continues over a period of time, conditions are ripe for the development of mould. In this case, improving roof cavity ventilation is the only answer.

Black or Kraft paper products exhibit the same condensation problem as synthetic underlay products. However, due to their high absorbency and dark colour they tend to disguise the moisture.

When touched by hand in an under-ventilated roof cavity, the paper will feel wet. Black paper products allow water vapour to escape to the outside. However, the process takes longer than it does with advanced synthetic underlay products.

Roof condensation is a side effect of modern building practices

Until relatively recently, New Zealand houses ventilated themselves.

They either had purpose-built air vents, or they naturally leaked air due to old-fashioned building methods.

Think about the classic New Zealand villas and bungalows, which are basically wooden tents, and almost as breezy.

These old homes usually have passive roof vents, plus their non-airtight construction allows water vapour to escape easily from gaps around the edges of roof cavities.

The other extreme arrived in the 1990s, when monolithic cladding came along to create air-tight boxes that retained heat, but also moisture.

The result was a generation of homes that had internal dampness and mould, which extended to their roof cavities.

Today, the New Zealand Metal Roof and Wall Cladding Code of Practice (NZ MRM COP) V3.0 Section 10, details the need to manage internal moisture in the building design, to ensure homes stay healthy. Ventilating roof cavities is part of this design process.

Ventilation needs to be active and passive

Effective ventilation is where air is removed from the inside of the building and expelled to the external environment. This is necessary for the liveable space and the roof cavity.

If your roof space is too airtight, the drying of condensed surfaces takes too long or, in a worse case scenario, never happens.

However, only a small proportion of new builds incorporate ridge vents and eave vents to ensure water vapour is able to escape easily from the roof cavity.

For homes that are airtight, there are various ways to retrofit roof cavity ventilation that will benefit the entire home.

Best practice for home ventilation is “balanced pressure ventilation” or “balanced heat recovery ventilation”. This involves actively drawing air from the outside of the house and delivering it to the inside.

Exhaust outlets are also installed inside the building to expel moist air to the outside. Active ventilation adds resilience to the building’s internal moisture management design. However, passive/natural ventilation within a roof cavity is still critical.

Timely design prevents the problem

Managing moisture in the roof cavity to prevent unhealthy moisture levels and mould growth is a two-part process that involves effective ventilation and high-quality permeable underlay.

In both cases, good decisions need to be made in the design phase of a new build or renovation project.

• Story courtesy of RANZ’s Rooflink magazine.

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