By Marcus Wilson 21/09/2015


I read the ‘Rental Nightmare‘ article on stuff.co.nz last night. Some of the stories are horrific indeed, and I’m reasonably confident that the writer has deliberately sought out the worst situations rather than the most common situations. But one cannot deny that a great deal of housing in New Zealand is sub-standard. In housing-deficient Auckland, in particular, families are forced into cold, damp homes because there is nowhere else they can afford. 

I’ve been in NZ eleven years now, and I have still to wrap my head around why this is. It seems that up to the 1970’s, houses were designed with three underlying assumptions: 1. New Zealand has a warm climate, 2. New Zealand has a dry climate, 3. Everyone needs a large, detached house. While the first is arguably true for parts of the country for parts of the year, the second is true almost nowhere and unfortunately the consequences of the third are coming home to roost as Auckland has to struggle with the concept of high-density housing inside the city or yet more expansion on its already sprawling fringes.

So New Zealand is left with uninsulated, poorly ventilated, high-surface area housing, in which we put our most vulnerable families.

Back to the article. As I said, I expect sensationalization in an article like this. But I do take issue with the comment from Andrew King, whom the article says is representing the Property Investor’s Federation. The article reports him as saying (note that this is the words of the reporter, not Andrew King’s direct words):

He says tenants often do things that encourage mould, such as not heating homes and drying clothes on clothes racks.

Not heating your home isn’t clever. But there’s only so much money you can fork out on power bills, and when the house isn’t insulated the benefit you get per dollar spent on heating isn’t high. So it’s not at all surprising that some houses are left unheated. Money is better spent elsewhere.

But drying clothes on clothes racks? Where do you expect people to dry them? Outside on a washing line? Try doing that in Hamilton yesterday, or, I suspect having seen the forecast, for the next week. What about in a dryer? First, that assumes the tenants can afford to run a dryer, and that, secondly, the dryer is properly vented to the outside of the house. Putting a hole in the wall for a permanent vent is the landlord’s job. How many of them make that a reality? Venting the air into a room puts the same amount of moisture into the room that drying on a rack would do – but in a much shorter space of time.

So what’s the problem with drying clothes? Imagine a load of washing that leaves your machine after a wash and spin. How much water does it contain. A large load might contain around three kilograms of water. (That’s my estimate based on the weight of the laundry basket when laden with wet clothes compared with when the clothes are dry.) All that water needs to evaporate. How much air is needed to do that?

Let’s assume you are drying at 18 Celsius (in a student flat in Dunedin in winter, yeah, right). How much water can the air in a room hold? Consulting a psychrometric chart, you can see that at a relative humidity of 100% (the air holding as much water as it can) air can hold about 13 grams of water per kilogram of dry air. Roughly speaking a kilogram of dry air is about a metre cubed (1000 litres) in volume, so that’s about 13 grams of water in a thousand litres of air. So to soak up say three kilograms of water, you need about 250 thousand litres of dry air. The ambient air almost certainly isn’t dry – if it’s a relative humidity of 70% outside, the air is already containing 70% of all the water it can hold. So that boosts the requirement to around 800 thousand litres of air needed. Call it a round million litres of air to dry your load of washing.

Now, a small room (3 m x 3 m x 2 m) would be about 20 metres cubed, or contain about 20 thousand litres. You need therefore about 50 rooms-worth or air to provide enough capacity to suck the water out. If you dry your clothes in that room (and they will dry eventually) it helps considerably if the room is well ventilated. That allows the damp air inside to exchange with the slightly less damp (this is Hamilton) air outside.

So where does condensation come from? Let’s suppose you hang out your clothes on a rack on an 18-degree cloudy afternoon, with 70% relative humidity. Your house is uninsulated (and unheated), so it’s also 18 degrees inside your laundry-room. You won’t get condensation. The psychrometric chart will tell you that the dew point is about 15 C – that’s the temperature below which air of this humidity will start dropping its water content. Your walls are at 18C so it’s not a problem. But as you head through the night, the temperature outside drops. The clouds clear, and you go down to 5 Celsius, say. Your house is uninsulated, and your interior walls find themselves at 5 Celsius, with a room full of moisture laden air. The walls are well-below dew point, and now the water condenses onto them.

The problem then? The house is uninsulated and poorly ventilated. Fix the fundamental problem (a bad house), the walls stay above dew-point, and then there is no reason why you shouldn’t dry your clothes inside when it’s a damp day.

Features image credit: Efraimstochter – pixabay