Help! Condensation issue - advice needed

Hi, I'm looking for advice in this situation I discovered in my house.

In my kitchen which is on the upper storey of my house, about half of the floor area is an overhang, like a cantilever, and you can walk under it when you are outside. It's about 2.7 metres above the ground outside. There is fibreglass batt insulation under the floor (2.6), and then the underside of the joists are covered over with cement board. Its been about 5 years this way, and this year the kitchen vinyl feels distinctly damp where it is the overhang, and dry where it is not. I found black mould on the vinyl under a piece of furniture. I am assuming I have a damp sealed cavity. The overhang area is about 5 metres wide, by 2 metres deep.

I've seen these cement board overhangs before, so there must be a way of having them and not have this problem. Can some folks advise me on how this would be done properly? If I put those little round plastic soffit vents in, will this work as ventilation on an underfloor like this?

Why are you thinking there's a moisture problem under the floor because of mould on top of the vinyl?

Originally, I thought maybe it was just surface mould from something spilled under that furniture. But when I walk across the floor, the whole floor area above the overhang part is kind of dampish and sticky feeling even the part that doesn't have anything covering it. And it suddenly changes to dry feeling at exactly the line where it transitions to not being overhang.

Because of a temperature difference? What's under the rest of the floor?
Vinyl is waterproof.

Yeah, it's odd. But in the previous years I've never had this happen. Under the rest of the floor is just the downstairs entryway and closets. I insulated that floor too, even though it's inside. And it has the GIB on it, as the ceiling of the downstairs area.

I just went into the bathroom where my son just took a shower. The vinyl in there is not actually wet, but has that same damp sticky feeling. The upstairs is heated with a free standing radiant type woodfire, which should make the upstairs pretty dry.

What is the humidity level in % inside the house and the kitchen area in particular. It may be that it is high and the overhung floor vinyl surface is cold thus moisture laden air is condensing on the surface of the vinyl. What is the material under the vinyl ? Is it timber based or fibrous cement sheet or something else. Although you have insulated under the overhang, how airtight is it. You may need to revisit the quality / method / sealing of the insulation that is installed.

sounds to me you are getting condensation on top of the tiles the same as forms on a window,cold air on the outside and warm air on the inside equals condensation. How do you fix it? that could be any bodies guess and apart from spending heaps of money you could run a dehumidifier,don't dry clothes inside and put a bowl over the sink hole to stop moisture coming out. bottom line is you probably won't stop it as long as you have the vinyl. Best of luck

Under the vinyl is H3 flooring plywood, on standard wood joists.

This is interesting. So, the expectation is that the condensation is not seeping up from the underfloor, but is forming on the surface of the vinyl. And that this requires a good temperature differential. And it didn't happen in previous years, and the overhang floor used to be a bit warmer, I notice.

This info gives me an idea. I think there's a good chance that the joists have shrunk a bit, since they were put in 5 years ago. That could leave insulation gaps, ruining the R value, and making the floor colder. Thus, condensation.

There's a light fixture outside, in the center of the overhang. I think I'll take it off and see if I can get a look in, check out if there's any dampness evident. If not, then I'll decide if/when to rip down and reinsulate or just live with it. Thanks!

Yep - think you are heading down the right track. Also check how well the over hang portion has no air movement through it as well as the insulation is a tight fit . Prevent any air movement between the overhang section and the house.