We have a fibro cladding house covered with polystyrene & then cement rendered so house is insulated but we are having a lot of problems with major condensation on every window in winter only which is also leading to mould… We have looked into 2 options of a dehumidifier for $800 or installing a ventilation system which would be similar to an air conditioning duct in every room, the system is by Ventisand we were quoted approx 4k. We like the idea of the Ventis system as to this is a problem occurring on every window so to move a dehumidifier to each room daily would be a pain & emptying water tank daily but we are concerned whether on rainy periods the Ventis system will work as well (as it extracts heat from your roof, filters & pushes through vent ducts). Have you heard about this system & what are your thoughts ideas on how to resolve our issue permanently?
Our expert’s answer
Thanks for your question.
Window condensation is brought about by the temperature difference through your windows – the cold air from outside your home meeting the warm air inside your home on your window is what makes condensation form.
At a cost of $4,000 a ventilation system might fix your problem but it may not be the most cost effective way of doing so. You might find secondary glazing – a kind of after-market double glazing – will work better for you. It might not cost that much more – it may even be cheaper – and it won’t have the energy costs associated with running a dehumidifier or ventilation system. You should also save money on heating and cooling in the future as double and secondary glazing help insulate your windows, which are where a lot of heat transfer happens. If you are set on the ventilation system, consider getting quotes from other similar systems such as HRV (heat recovery ventilation) – you might even find you can play them off against one another to get a better price.
These days, the most efficient form of heating is using a heat pump which you may know as a split system reverse cycle air conditioner. Sized correctly for your rooms, a number of these, used in the rooms you live in only, will keep you warm while minimising your energy bills – they also dehumidify the air which would address your mould issue in the rooms in which they are used. If you go this route, make sure you keep the temperature setting at a maximum of 20 degrees C in winter (18 is better) as every degree adds 10% onto your heating bills – in summer, 26 degrees is good unless you have ceiling fans, in which case 29 or 30 would be adequate. Make sure when buying a heat pump that the CoP/EER is 4.5 or greater (this is the measure of performance enhancement the heat pump achieves – higher numbers are better) – these figures are stated in the specifications. Ceiling fans can also be very effective in winter as, in reverse mode, they mix the hot ceiling air with the rest of the air in the room.
A special note from building designer Dr. Chris Reardon
It sounds like the house was badly built in terms of positioning of breathable v non-permeable membranes. The polystyrene cladding should have been packed 10mm clear of the frame to allow condensation to exit. Prior to this, the frame should have been wrapped with a breathable membrane. I would assume the frame was wrapped with reflective foil (not breathable) and the polystyrene fixed hard against it. This leaves water vapour nowhere to exit. The result is that the dewpoint forms on the windows and, I would bet – also in the walls.