Should a tanked cellar have low humidity?

Hennymore

Previous owner had the cellar tanked (coating put on inside walls and presumably floor, not to the full height of the wall) after getting a small amount of water in there during major flooding in the area. The tanking might still be under warranty.

The cellar is just one unfinished room. It has a sump and an air brick to the outside.

I put our humidity sensor in the cellar a while back and saw a number around 80% relative humidity. The rest of the house is typically 50-60. Cardboard left on the cellar floor gets mouldy.

Should a properly done tanking job get the humidity level down, or should I only think of it as protection against actual water seeping in? Which I’ve never seen.

I’ll go through my papers and see if I can find a specific guarantee, but it would still be useful to know what to expect from tanking.

shortcrust

I don't think it's actually tanked from what you've said. Tanking is sort of like building a waterproof shell inside the room, and often involves pumping away the water that forms between the layers (I think!). It's costs thousands. A property tanked cellar should be a dry room.

The humidity and cardboard make it sound like a typical damp cellar. I don't think a 'coating' of something is going to change that so I wonder what promises the contractor made for the work.

Doozergirl

Cellrs are supposed to be damp.

Cellars are even allowed to flood on occasion. It's underground. It's perfectly normal.

Half tanking a cellar is just rubbish. They've panicked with the 'flooding' and had a 'damp proof specialist' around who knows beggar all about what people think they're employing them to know about. They've bought nothing useful at all. Half tanking a wall means there's nothing to even guarantee.

The Victorians (or whoever before them) weren't stupid. They weren't painting bitumen all over cellar walls, they knew exactly what they were for, so they're better off left that way unless you spend the £00,000s making it properly dry, which is never worth the effort outside London.

Yes, your cellar should have higher humidity levels but it should also be really well ventilated. I wouldn't be surprised at all at a cardboard box going mouldy - it is not dry storage. What is important is lots of air bricks and through-ventilation to keep your floor joists dry. They shouldn't be covered over either unless there is good airflow between them.

Damp spaces should be allowed to breathe.

Jackmydad

Too many variables to say. You'd need to pay a real independent expert to look at it.
I'm not talking about someone who will sell you some sort of wonder cure.
Depends on the humidity outside in a well ventilated space.
For instance the humidity outside here at the moment is 68%. That's fairly near the sea.
Air temperature outside is 23 C
I'm showing 65% humidity on the gauge in the room here with all the windows open.
Weather site gives the dew point as 17 C
So if I had a nice cool cellar (which I don't) the floor could well get damp from condensation just from the air coming in.
Not saying that your situation is the same. Relative humidity is pretty complicated. I'm certainly no expert.
Anyway. No cheap way to cure the damp I'd say.