crying windows

At this time of year many homes in New Zealand suffer from crying windows and mildew on the curtains. Community chat sites are full of queries about the best sort of heat pump, the best way to bleach curtains, whose electricity bill is the highest….

The majority of the housing stock in NZ have  low levels of insulation and heating – this causes a vast amount of condensation on cold single glazed windows, also unhealthy moulds on ceilings in bathrooms and wardrobes. Many studies have linked the high rate of  poor respiratory health to the level of indoor dampness. Other effects of various moulds and mildew can include asthma, headaches, eczema and skin irritations. Dust mites also thrive in a high humidity above 50% in order to survive.

A recent BRANZ seminar for our Architects ‘continuing education’ provided a good summary of why this occurs and how to reduce it in your home. There are a variety of solutions available to suit both renters and home owners.

  • 1. Elimination of unnecessary moisture sources. Now while we can’t stop breathing ( which releases 3 litres of moisture a day)- using unflued gas heaters not only release toxins, but also 1.6litre of water per 1kg of gas burnt. Drying clothes on a clothes rack also produces 5 Litres of water per load – keep the washing outside where possible! Repair any leaks from external sources eg. through roof, walls, or plumbing fixtures.

  • 2. The removal of the moisture at its source using extract fans in the kitchen, bathroom and a vented dryer (vented to the outside) is essential. Look around for fans that have ‘humidity’ sensors that kick in when it senses a high moisture level, or that have timers to operate for a short time after you leave the room. Covering the underneath of your existing house with polythene prevents subfloor dampness that creates high indoor humidity. Check around the house – is there any vegetation covering the ventilation spaces or grilles at the bottom of the cladding? Is there ponding water near the house that needs to be drained away?
  • 3. Ventilation – This can be either as simple as opening windows, or mechanical fans/forced air systems. In the old days opening windows to ‘air out’ the house with fresh clean air was an important part of the housekeeping ritual. Today – with many of us working and more security conscious – houses are shut up for longer periods of time, leaving the moisture trapped inside.With Passive ventilation – Opening windows on both sides of a room provides good cross flow ventilation – in our house we utilise the stack effect where the downstairs windows open and the warm air flows out the upstairs windows to remove  it from the house. In new houses trickle vents are an option around the window frames which allow additional ventilation to occur without having the window open. While this takes a little bit of thought and operation by the home owner – passive ventilation is FREE!  Supply ventilation are mechanical systems where fresh outdoor air is forced into the indoor space via a diffuser, this creates a positive pressure in the house forcing moist air through gaps around doors/window /joinery. Active ventilation & heat recovery ventilation systems supply outside air and extract stale/moist air to the outside and can heat, clean and dry ventilated air. There are some Positive pressure ventilation systems available that draw air from the roof space – however these do not comply with the Building code G4 ventilation standards as it is not considered ‘fresh outdoor’ air. Also if you have moisture in your roof space via a leak, or it has moved there from a damp subfloor space through open paths in the wall cavities, then the system will pump moist air into the house – making the situation worse. While these systems can be controlled and target specific areas to also warm the house they can still be expensive to install and have ongoing running costs.
  • 4. Heating – If the water vapour content remains constant, warmer air temperature will reduce relative humidity. Also if the surfaces of a room are warm the water vapour is less likely to reach dew point and condense on them. The easiest way of doing this is with SUNSHINE, the site, location and orientation of windows. I am slightly evangelistic about this as we live in a house that is purely heated by the sun  falling on the insulated concrete slab (thermal mass)  that radiates the warmth. We put in an efficient wood burner, with the idea of having a sustainable way of heating the house in winter, but it has turned out to be an optional extra rather than an essential as the sun does all the work! For a mechanical heating option there are the heat pump systems – these can still be expensive to purchase and run (especially if you haven’t insulated first!), but they basically warm the air in a more ‘efficient’ way than standard electric or ‘gas heaters and maintain the indoor temperature at a healthy and comfortable level.
  • 5. Insulation –  keeps you and your house warm in winter and cool in summer. The timber-framed buildings in New Zealand generally use glass wool, wool, polystyrene or polyester insulation within the frames, but it must be installed properly. If you leave just a small gap between the insulation and the framing, or squash the product in – then you are likely to have halved the insulating performance of the product. Double glazing has only recently become compulsory in New Zealand, (jaw drop moment for those northern hemisphere dwellers…) but for new houses take it one step further and use thermally broken aluminium windows where the internal surface of the window remains closer to the ambient indoor temperature, preventing the air near the window from cooling enough to form condensation.

I hope that helps cheer up your crying windows a little  – Stay warm and dry this weekend!


Branz – Internal Moisture Seminar 2012.

Image –



Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s