Moisture control & ventilation

Controlling moisture and ventilation is essential to creating the right atmosphere for health and comfort. We look at the usual methods of control with an emphasis on energy efficiency.

Controlling moisture build-up

Moisture accumulates in the house from people bathing, showering, cooking, breathing, and watering plants. It also comes into a home from outside. Some appliances, such as unvented gas heating, clothes dryers, dishwashers and washing machines also produce excess moisture into the air.

The moisture builds up quickly and is worse in modern homes which are built to be almost airtight. On cold days you can see the moisture when it collects on the windows as condensation. It is also absorbed into fabrics and building materials. The problem with this moisture build-up is that it can cause mould and mildew on walls and fabrics, which is not only unsightly, but can trigger allergies. For example, dust mites - the source of one of the most powerful biological allergens - thrive in damp conditions.

If you are building a new house, there are a number of chemicals and resins present in many building materials which continue to leak into the indoor atmosphere for many months after you move in. This can be managed with good ventilation.

The options for managing moisture are:

Good ventilation.
Window joinery with built-in drains - that allow condensation to drain to the outside.
Keeping the house warm and dry through heating.
Extraction fans in bathrooms and kitchens.
Good insulation to keep the home warm and reduce condensation and mould growth.

Options for ventilation

Ideally the air in a home should be ‘renewed’ every two hours, even when you are not at home. Some simple options are to fit aluminium windows with passive air vents or to fit security stays that allow the windows to be left ajar. When windows and doors cannot be left open, consider mechanical ventilation systems and air conditioning systems:

Heat recovery ventilation systems (HRV units) take fresh outdoor air which is then carbon filtered and pumped through a heat exchanger using the heat from the indoor air. As the stale air is removed from the house it passes through a heat exchanger preheating the outdoor clean air. Some heat loss is experienced which needs to be made up.
Forced air ventilation blows dry air into your house from the roof space above the ceiling. It works best where there is a decent amount of space in the roof. It must also be dry and preferably warm on sunny winter days.
Extractor fans are used in places like the kitchen and bathrooms to remove steam.

For more information about ventilation systems go to consumer.org.nz. Note: you need to be a member to access this information.

Keeping cool in summer

Keeping a house cool in summer is also an important consideration. Measures to control overheating include:

Controlling heat from the sun by reducing window areas or using tinted or reflective glazing, and shading.
Increasing ventilation – cross-flow ventilation, extractor fans and ceiling fans and passive vents.
Increasing insulation, especially in roofs.

Mick added a 50 square metre living space/sunroom in a recent renovation to his 115 year-old home. The room is orientated to the north for maximum sun, is well insulated, and has double glazing. He installed a ducting system that transfers the warm air from the sunroom around the house. It can be reversed on hot days so the cool air in the south rooms can be used to cool the hotter north rooms.

The house is heated by a single fire which also is connected, via a hot water cylinder, to hot water pipes in the concrete slab, which heat the floor when the fire is going. The concrete floor slab in the sunroom is insulated by 200mm thick polystyrene.

Glazing to control temperature

Window glazing options to control temperature in the home include:

Double glazing (argon-filled, with low emissivity and UV-filter coatings).
Heat absorbing (tinted) glass.
Heat reflective (coated) glass.
Low emissivity glass (transmits the short-wave radiation of visible light but reduces the rate of long-wave radiation transmission).
Window design to allow airflow through the house.

Double glazing can be fitted throughout the house or just in the living areas where people spend the most time.

The New Zealand Standard NZS 4223:1&2: 1985 provides requirements and recommendations for the selection and installation of glass, and human impact safety. See also NZS 4218:2004 which requires double-glazed windows in new houses in the South Island and North island Central Plateau. These Standards can be purchased from Standards New Zealand, contact 0800 782 632 or go to www.standards.co.nz.