Is the house on piles or on a slab? If on piles, is there a DPM that has been laid across the earth under the house? If on a slab, I am wondering if the DPM has been laid under the entire slab, including up the edges of the slab. You can probably check this with a moisture meter on the slab. Do you have any water accumulating around the outside of the slab? Do you have the windows open most of the time to each out the moist air, or do you use a mechanical ventilation system to do this?
I recall seeing a story a few years ago about a house on piles, where they had a ventilation system installed, and apparently the motor in the roof space was sucking up damp air up through the wall cavities, from under the floor space, which was damp, and it was pumping it into the house, causing moisture issues. I think it was compounded by there being a leak under the house.
@networkn Have you put a bucket or some other container under the condensate pipe from the heatpump indoor unit to see if it is actually discharging any condensation during winter? Since relative humidity is related to air temperature. In otherwords, if you warm the room relative humidity drops even though you haven't removed any moisture from the air. So my bet is that the heatpump is warming the room, then measuring the relative humidity level. It sees that it is lower than it's setpoint. So it thinks job done, So it doesn't go into the cycling mode like what Richms was describing. Therefore no moisture removed.
I say you will need to start using the dehumidifiers again during winter. Also for every litre of water the dehumidifiers remove, you get 0.6kW/hr of free heat. Google latent heat of water if you want to know why that is.
When living in Dunedin, I would leave a couple of windows open in the winter when everyone was out at work. Much cheaper and easier to heat a dry home than a damp one. Same practice to avoid condensation on windows and other damp issues. Like others have said, a house needs to breath.
I would get an expert in to review the home. My last place had a non ventilated bathroom. Washing machine was in it. Shower dome which are great. Little condensation on the walls. No mould. Current house has an ensuite, double non shower dome shower. Extractor fan. No mould.
Given that OP's home is not old, and his comments, it seems highly likely to me that mould is not coming from lack of ventilation, poor moisture practices, a home that is sealed too well. Particularly if a dehumidfier gets used at times. It must be inherent "in" the house. So a review by the probe guy might at least lead you to a room where the moisture is seeping into the house, and thereby become airborne and affecting everything.
I am not sure if it's just that once we got mould, we didn't get ALL of it, so it's spreading again?
Bleach does not kill mold. It, well, bleaches it.
Molds have roots (hyphae) and bleach’s ion structure stops chlorine from penetrating porous materials, therefore when you spray all the chemicals are going to stay on top of your surface while the water part of the solution just soaks through.
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There are experts, and there are experts. Don't just get a building inspection company. Get an actual expert who specialises in moisture issues. Can't think of what that sort of company would be. But I guess a start could be getting a older builder in who has been building for years, who does consultancy work that that area. . The big problem with newer houses, is that the house is so well sealed, that moisture just can't vent out.unlike older houses which were air leaky, so wasn't as much of a problem. Mould equals high humidity, and often it is warm moist air. You can often tell if a house has this problem just by walking into it. Some humidity sensors around the house may also help to find where the problem is. A shower type of dome makes lottle difference for this type of problem, as that moisture is still contained in the house.
When living in Dunedin, I would leave a couple of windows open in the winter when everyone was out at work. Much cheaper and easier to heat a dry home than a damp one. Same practice to avoid condensation on windows and other damp issues. Like others have said, a house needs to breath.
Many people don't do that these days, possibility due to security. But you can instead install mechanical ventilation including a heat exchanger.
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