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I got a phone call today from a lovely lady, she is an investor, one of those good landlords.

She said this house has a mould problem. I had visions of long flowing locks of spiney mould. The fear began to grow – I put an extra organic dustmask in the car.

Imagine my surprise, when I was showed the mould issue, a little mildew, barely visible, unless the light caught it just right. She had treated it with the off the shelf cleaners several times, and was a little mystified that it kept coming back.

Now this problem was far more low level than I was anticipating. The owner was wise to try and find an intervention early.

Sometimes with condensation issues, I place data loggers throughout the house, record temperature and humidity for a few days straight, because good data is key to getting a good intervention.

Previous tenants had not had this issue – because it was a single person. Current tenants are a family – two adults breathing at night generate a fair bit of moisture. Current tenants were also forgetting to wipe down the wet windows and open the curtains in the day time!

The house is a 1940s brick state home. Positioning for sunlight is particularly woeful – it is built straight facing onto the street.

One bedroom, the one in question, will almost never see the sun. It is on the southern tip of the house. In addition, it has a partial skillion roof – where the gib is directly fixed to the bottom of the rafters. Though insulation has been retrofitted, r4.0 in the roof in general, this area is very difficult to insulate effectively. It has none at present.

The temperature differential between the insulated ceiling, and the uninsulated portion is very significant as a percentage (yeah, we could argue that zero kelvin should be the reference point, but earth never experiences that temperature – so) using 273.15 degrees kelvin as zero, which coincides nicely with the freezing point of water. 9 degrees Celsius to 6 degrees Celsius is a 33% differential. Infrared is great in these circumstances – instant diagnosis on one third of the condensation equilibrium.

If you are familiar with the World Health Organisation recommendations, you should already see a problem – recommended minimum temperature is 18 degrees inside.

So why doesn’t that landlord just get a heater, sheesh?!

Funnily enough, she has actually installed a fireplace, installed a big heat pump, and purchased a dehumidifier for the tenants to use. But they only work if they are used.

Remember, the house is well insulated, at least in the ceiling and subfloor.

To understand the issue with condensation completely requires a PhD in voodoo wizardry – Start here. But suffice to say, like cold milk left on the bench, cold surfaces cause condensation to form. Temperature gradients are the issue. To prevent condensation, either heat the air, which allows it to hold more moisture before condensation occurs or dry the air, which cracks everyones lips etc.

Heating is expensive, we want to keep it in the house once heat is generated.

So what else can she do?

The law of diminishing returns kicks in if we aren’t careful!

My suggestions for her in this house.

Install a positive air pressure system. There are pros and cons – Some systems cost a lot more than they are worth. Some off the shelf systems are fundamentally useless. What is the middle ground?

I can vouch for DVS systems – they have a few different systems, heat exchangers can be beneficial if the house is otherwise relatively well sealed. For the house we are talking about, it has a concrete tile roof, with no underlay; the ceiling is quite draughty. A simple ventilation system will help a bunch.

Also consider moisture master, smartvent – systems I can speak about from experience.

If a new Passivhaus certified home gets an arbitrary score of 100 on heating efficiency,  then a rough (ad hoc, and specific to this house, not scientific) guide to the benefits of various interventions are below.

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Now the funny thing, there are overlaps – the gold standard would of course be to reline the room, with building paper and as close to an airtight barrier as could be installed, then good insulation (consider packing the walls to add extra!) then a sealed internal lining – plywood or similiar, with taped joins and penetrations, Replace the windows with evacuated, thermally broken new double (or triple) glazed units, and uprate the ceiling insulation, installing building paper under the tile roof, and then installing a ventilation system with heat recovery, and probably ducted heat.

Downside is, that would be well more than $30,000 – a years rent – if you are freehold, that might be acceptable, but if you are paying a mortgage, that is going to hurt.

So what are the reasonable interventions that won’t cost the earth?

First, in this situation, you can poke some insulation into the skillion portion – it will be difficult, but worth the effort. The insulation cannot touch the roofing material, and needs t be moisture impervious in this eave location. My favourite insulation is technobond. It is nice to work with, not itchy or dusty, doesn’t sag, and its made in Dunedin!

Second, I would look for second hand double glazing units, but if I couldn’t find any, then I would install polycarbonate.

Thirdly, I would install a good quality ventilation system. What I am looking for here, is a sensor operated system, ideally that can route airflow around the house as needed. It should have lagging/insulated piping (airlines lose a lot of heat in a short distance).

A forth suggestion to my friendly Landlady – She is giving the tenants a significant weekly discount to allow for heating.

It really annoys me to read some of the common media and comments that all landlords (with extra acid thrown at Asian landlords – guess from what extraction the subject of this post is?) don’t care about their tenants.

I suggested that she consider renting the house with “free firewood”. If the house is kept warm, it is much less likely that condensation can form – that of course much reduces the likelihood of mould ever forming.

Tenants come in all shapes, sizes and variations. If they are trying to save, maybe for their own house deposit, then it is entirely possible they will skimp on heating costs – Maybe, they simply cannot afford to heat a house. As landlords, oftentimes we are blessed with the ability to help people in these situations. You can’t turn on the heat for them, but you can remove every reason they have to not warm the place up.

So, insulation works, I’ll go into more blah blah blah on that in another post.

Making a shiney diamond out of an old state home is an expensive business.

These are a few interventions which can work in an economic fashion.

Hopefully you feel a bit more informed about those now – if you have specific questions, give me a bell, or an email or facebook or whatever, and I will try and talk you through options in your situation.