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Hello and thank you for your site extremely interesting. I bought a house from 1930 with humidity problems (classic). In addition a false ceiling was made under the old ceiling, hence the difficulty of installing a VMC because the attics are almost inaccessible. So I thought of making ventilation openings with grilles on the low and high end of each room to circulate the air by difference in atmospheric pressure. But there is another option for which I would like to have your opinion: I discovered old chimney ducts that are walled and go on the roof. The former owner broke the homes to gain the surface. And on the roof there is always the top of chimneys. I thought to make a ventilation opening at the top of the wall where the old ducts pass so the stale air would be automatically exited to the top (roof) by atmospheric difference and the new air will be able to return by grids d ventilation at the bottom of the rooms (ventilation openings which give to the outside). So to summarize a passive breakdown and very cheap. Do you think that would work? Are there other similar experiences?

In many older homes, open fireplaces directly contribute to the ventilation of the habitat. One of my friends, a construction economist and therefore a "man of the art", opted for this solution and he is very satisfied with it. My answer is therefore very positive. Nevertheless, I advise you to place on the exits on the roof dynamic mechanical vacuum cleaners that will keep the ducts in depression.

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