[Cocktail]

Do you have any issues with mold after air tightening the house? Here in Finland pretty much every house needs constant airflow/intake and the houses built to breathe to fight against mold. Im curious if youve had any issues, especially due to living closer to the tropical zone.

[Bluestrike2]

The problem is solved, barring serious complications with your home's envelope, with mechanical ventilation systems that give you control over how and where air is being exchanged in your house, while filtering it as well. That's assuming your building envelope doesn't have any serious issues; if water is leaking into wall assembly because of a poorly installed roof or gutter system, for example, no amount of ventilation is going to keep things from rotting away in a few years.

Anyhow, although leaky houses can help deal with moisture, that's pretty much an incidental effect. Leaky homes also comes with significant expense in the form of heat loss, worsened indoor air quality, pathways for bugs to enter the home, etc. There's a reason why we've been moving towards ever-tighter houses, even with the upfront costs and effort required for air sealing, insulation, and mechanical ventilation.

In the GP's tropical climate, I'd expect that humid air leaking into the house does very little to actually dry out the wall cavities; if anything, there's a good chance that those leaks would do the opposite and support any mold growth.

[regularfry]

When I got my basement damp-proofed against a nasty mould problem there were three options for how it could be done. In increasing order if effectiveness, they were: 1) they could re-plaster with a sealant; 2) they could mount plastic stand-off sheets to the wall with plasterboard covering; 3) they could air-tighten and install a positive pressure system.

So PPSes are sold as a damp prevention mechanism; as long as you install one when you air-tighten it's doing the job that natural airflow would otherwise take care of.

Also worth noting: each successive option added a zero to the price.

[valenterry]

He won't because it's Thailand. The air inside the house won't be significantly warmer than outside for a prolonged period of time. Not so in Finland.

Your best option is a centralized or decentralized automatic ventilation system with heat exchanging and humidity control. On top of that, the better the insulation the less the air is cooling down near walls/windows and the less likely the humidity is condensing into water causing mold. And then last, the less humid the air inside the house is, the more it can cool down before condensation is happening - so keep the humidity low not only with automatic ventilation but also by making sure to not cause humidity by cooking, showering (and breathing, haha) and if you do, air out for just a few minutes.

[2rsf]

What do you mean by humidity control? a central one?

I am not saying you don't need one, but here in Sweden modern houses do not use humidity controls and are still air tight and mold free, maybe at the cost of letting more air and energy be swapped. Older houses have local vents at the baths which are humidity sensitive and will blow out excess humidity quickly, again at the cost of wasted energy.

I suppose that controlling humidity centrally will somewhat save energy, but at what cost and complexity?

[valenterry]

Well, unless you stop breating, showering, cooking, keeping plants indoors, etc. you will have to somehow moderate/control the humidity as it will grow over time until the dew point close to walls/windows or other cold surfaces is reached.

The reason why houses in Sweden can be mold free is because people get taught that they need to air out their house on a regular basis. And if you look it up, it tells you to air out not during noon (even though it's warmer so you lose less energy) but in the morning and late evening - specifically for humidity control.

So...

> maybe at the cost of letting more air and energy be swapped

Yes, exactly that. But since we are in the context of air pollution, opening windows to air out isn't a great idea. That's why I mentioned an automatic ventilation system (which pretty much always contains filters).

> Older houses have local vents at the baths which are humidity sensitive and will blow out excess humidity quickly, again at the cost of wasted energy.

Not only that, fresh air will also be sucked in from somewhere else outside - which then again brings the air pollution inside.

[2rsf]

All new houses here have mechanical ventilation, usually with heat recovery and heat pumps but without filtering (excluding the heat pump's coarse filter for dust).

I got curious and read a little, it is generally accepted here that higher pressure in the house can actually lead to mold, maybe because there is no moisture control and it is expected that some of the moisture generated in the house will be driven into the walls, probably assuming the walls are not perfectly sealed. On the other hand air pollution in Sweden is a non-issue, except maybe for a couple of streets in Stockholm.

[valenterry]

Yeah that makes sense - if you have clean air then why would you need to filter. :)

[davemp]

Ideally you would have a an air tight house with a mix of interior and exterior insulation based on your min/max temperature deltas and their dew points to keep the sheathing from being too cold

[valenterry]

That alone is insufficient. Think about the bathroom: the air will be hot and very humid. The walls would have to be way too hot to have a dew point that isn't reached when then hot&humid air cools down at the wall.

So you are left with reducing the humdity by airing out in some way (with windows, with a very non-airtight building, or using automatic ventilation). Or, of course, you can use a dehumidifier.