The prevalence of mould in New Zealand houses is a disgrace but it’s also not unique, as this American paper in the latest ASHRAE journal makes clear. The author claims about a third of older and newer single-family homes in the US north-west have visible mould, as these images graphically show. The article outlines causes but also proposes solutions, with a level of specificity that is very useful.
Photo: George Tsongas
As illustrated, mould can be caused by high indoor relative humidity (RH), localised cold surfaces (that is, low fRSI) and abnormally wet surfaces because of leaks, curing concrete or flooding). As I’ve said before … cold spots equal mould spots.
This aligns quite well with the preliminary data from the BRANZ HEEP2 results, discussed here.
The author provides some generic advice about managing moisture at the source, utilising effective ventilation and ensuring adequate insulation and heating to maintain comfortable indoor conditions. Then he gets down into some details that are real gems: how to choose and run a dehumidifier for best results and how to test whether a bathroom fan is actually working.
Advice on dehumidifiers
This is the most practical direction I’ve found about how to size a dehumidifier. Tsongas recommends a unit capable of extracting 17-24L of moisture per day, which is big enough for all but the largest houses.
Buy a self-draining model (ie one with a drain hose) or the ability to retrofit with one and use it in the laundry; here the hose can be securely routed to the washing machine drain. This saves having to remove and empty a heavy tank several times a day, which will get tedious really quickly. I’ve read accounts of units that beep loudly when the tank is full, no-one wants that happening at 3am.
I recommend you buy a model that has a humidistat so it will run automatically until the desired level of RH is achieved.
Advice for bathroom extraction
Here’s a simple way you can test the adequacy of the typical extractor fan in a bathroom. Turn it on and place a piece of toilet paper over the fan. Is enough air being extracted that the paper sticks? If not, it’s time to replace the fan (and possibly the ductwork as well). You can source extraction fans with humidistats too. Otherwise, have it installed with a timer that runs on for 15-30 minutes.
Reference
“Understanding Mold And Moisture Problems Inside Housing” by George Tsongas, ASHRAE Journal, May 2025.