Poor insulation = mold!

Hello,
I spend a lot of time in a small weekend house (30 m2) that is located in nature. The house was built 30 years ago and consists of wood and stone wool. Recently, I installed new windows (triple glazing). Floor structure: concrete foundation - wooden slats - OSB boards - blue linoleum. I heat with a Rowi gas heater but only when I am on site. The roof is not insulated. Now I have a mold problem due to condensation everywhere in the corners and on the walls. How could I insulate the house so that I can stop the mold problem? I can renew the insulation and flooring, but I do not know how to solve the whole problem.
Thanks for any help.
Luis

If you do not heat, the insulation problem will only get worse. Either ventilate or heat (preferably both).

A gas heater causes a significant amount of moisture to be released. This will then also penetrate into the wood, [möbel] and walls. You won't get rid of it with a short airing before you leave.

I believe the most pragmatic option would be to run a dehumidifier with hose connection and integrated hygrometer when absent. It should only run if room humidity >50%. Condensate drain hose into a drain or somehow out, and done. Alternatively, always leave two opposite windows open, secured against burglary with bars.

A gas heater causes a significant moisture input. This will then also penetrate into the wood, furniture, and walls. You won't get rid of it by briefly airing out before you leave.

Exactly that. In the past, this was probably not a problem because the windows continuously allowed air exchange. The new windows are now tighter, and the moisture remains inside.

I’m not trying to scare you now. But to me it doesn’t look like mold. And I don’t think it is mold either. Mold wouldn’t just be at the bottom but also in corners, etc.

It’s also not due to insufficient heating but rather penetrating moisture. This is a fungus. If you’re unlucky it’s dry rot. Then demolition is necessary. The boards get moisture from below, from the back or wherever. You can clearly see that.

If the boards have any contact with the ground or there is a missing, defective moisture barrier, then the boards really draw the water up. It’s just starting. Then lack of ventilation and it begins.

You’ll have to open the wall and air needs to get in as soon as possible. Otherwise the fungus grows and you might have to tear down the whole house. The spores of the fungus are in the entire wood. And in every spot where no direct air reaches, the fungus grows. It might even be much worse on the back side.

But usually only between 10 and 25 degrees. So heating while simultaneously supplying moisture from below might accelerate this process. So the colder the better.

Wait until spring and it will feel really comfortable. Very dangerous.