Remedies: Moisture

Villas have generally cope well with moisture because their structure allows air to circulate. But as they are upgraded and become more airtight, moisture can become an issue.

On this page:

Weathertightness

Villas will let in water at some time, whether it be a cladding failure such as a rotted or split weatherboard or a storm that is more severe than usual. Villas have been able to deal adequately with the occasional water incursion thanks to the inherent durability of the framing and the fact that there is significant air movement through the structure. Once insulation is added and spaces are hard lined, the villa becomes more airtight and therefore the design must aim to keep out the water.

One of the areas that should be addressed is how can the principles of E2/AS1 be incorporated into villa reconstruction.

The main issues are around:

the detailing of windows where the existing window has been removed or a new window is to be inserted into an existing wall
minimum roof pitches on lean-to construction – many are below the current minimum slope requirements
the lack of a wall and roof underlay ( see roofing and cladding for solutions)
the need for a cavity when matching existing - see regulation and compliance which details a path that may allow the cladding to be installed without a cavity provided it matches that existing.

Window flashings

For double-hung windows, one approach that may be used is to incorporate flexible flashing tape into a double-hung window detail as well as the sill tray flashing.

A number of designers also use details where the flexible flashing tap is extended to include the space containing the window weights. To facilitate this, a boxed enclosure is created that also allows an air seal to be formed around the opening.

Across the sill, it is a feasible option to dress across the sill trimmer and turn the tap down the face of the wall cladding. In doing this, a back-up to the sill tray is provided, and any water that gets onto the tape can be drained down the face of the cladding rather than behind it, as is the case in E2/AS1 direct-fixed weatherboard details.

Traditional window and other head flashings have simply been inserted into the lap or joint of the weatherboard immediately above the opening . As long as the flashing remains in good condition, the detail appears to have generally worked well at protecting the top of the window.

Modern details that require the flashing to be lapped under the wall underlay cannot be applied to an existing villa wall unless the cladding is being removed – BRANZ believes that, where a window matching existing is being inserted into an existing wall, using the same detail would appear to be a logical solution. The difficulty is neatly dressing a flashing to the profile of a rusticated.

Internal moisture

Draughty villas are less prone to internal moisture problems than newer, more airtight houses because of the air that moves through gaps around windows and doors, through chimneys and even floorboards.

But some previous renovations may have impacted on this. As a house becomes more airtight generally as a result of hard lining with plasterboard, internal moisture can become a problem from cooking steam, bathroom steam and unflued gas heaters or clothes dryers.

It is also not uncommon for there to be a musty smell, which often can be traced to:

a damp subfloor and the migration of the moisture through a draughty floor
a leak from an internal gutter or through the roof or wall cladding.

One solution to a damp subfloor is to lay 0.25 mm polythene sheeting to completely cover the ground under the house. Ensure that the ground is shaped so no water accumulates on top of the polythene. Polythene sheets should be lapped a minimum of 150 mm, preferably taped, and tightly butted up to foundation walls and piles. Weigh down the sheets with bricks or concrete to avoid them being displaced by air movement from subfloor ventilation.

Renovations generally make the villa more airtight. Design solutions to remove moisture should include extract fans in kitchens and bathrooms ducted to the outside, and more insulation, which keeps temperatures higher. Windows that can be opened to allow ventilation without providing a security risk are another obvious solution. Security stays, trickle ventilators or special catches that allow a window to be securely held open can be installed in new or existing windows.