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After reading this question: Does a bathroom exhaust fan have to vent to the outside? I realised that the house I live in now, and everywhere else I have ever lived, everything just vents into the roof cavity, and the roof cavity has "whirly birds" on the roof to circulate fresh air through the roof cavity.

This includes two bathroom exhaust fans and the range hood in the kitchen.

I would say that the bathroom exhaust fans are original, or close to original, to the house, which is mid 1970's. This means that there's been 40 years of venting into the ceiling cavity. The rangehood is perhaps only 5-7 years old.

I'm no master builder, but I do not see any signs of mold or rotting in the ceiling at all, and when we purchased the property 3 years ago the building inspector explicitly stated that they found no moisture related damage in the roof space.

In the apartment that I used to own, we asked strata for permission to install an exhaust fan, and we were told we could do it on on the explicit rule that it only vented into the roof cavity, and did not exhaust outside (because that involved modifying common property). So until now, I never thought anything of it.

Is this something that I should spend money on changing?

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    If there's good ventilation in the attic space, you may never have a problem. The practice of venting to the outside, is to reduce the probability of a problem. Even if you don't vent the exhaust properly, you might never have a problem. You are, however, increasing your risk by not venting properly.
    – Tester101
    Commented Jul 16, 2014 at 2:13
  • In a drier temperate climate such as Sydney Australia, condensation from the bathroom exhaust is likely to be less than in other places around the world and more likely to evaporate before mold gains a foothold. Sound construction practices are generally empirically based and often local.
    – user23752
    Commented Jul 16, 2014 at 15:00
  • @benrudgers good point
    – Mark Henderson
    Commented Jul 16, 2014 at 22:06

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In an apartment building they may have deliberately designed that area to function as a plenum space. That design would -- one hopes -- include selecting/arranging building materials that could tolerate the moisture these fans would pump into that zone.

In your house, it's less likely that folks made the appropriate effort.

Rot isn't the only issue. At best, more moisture in your insulation will mean it insulates less well than it should. I don't know enough to have any valid opinion on whether the fact that you have the spinners to help actively encourage airflow will make up for that.

Personally, I'd say either fix these to exhaust to outside, or at least spend a bit of money to get an expert opinion on whether you can get away with not doing so.

(One of my bathroom vents exhausted into the attic. My inspector, an old-timer, grumbled but didn't actively recommend fixing it. My insulation contractor took one look at it and said "I'm fixing this.")

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