I have a Victorian terrace house with a nice cellar. When I bought the house the surveyor said the timbers in the cellar were a bit damp and I should ventilate the cellar to let them dry out. Over the years I have fixed a drainage issue which occasionally let water into a corner of the cellar and a bit at the front of the house which did the same.

The cellar window has been open for years as advised but it makes the cellar and the kitchen above very cold in winter and the dryer hose was poking out of the window which was a bit shoddy. The window was also rotten from being half underground in the cellar well and being of poor quality.

I recently put a new window in the cellar which I made myself out of pressure treated timber. All sealed up ok with a proper vent in it to let some air in/out, but obviously not as much as an open window which the new window doesn’t have.

I am a bit paranoid that the wood beams in the cellar will now get damp (despite me knowing I’ve stopped the water ingress) so I’ve bought a moisture meter to keep an eye on things. Only a cheap one but it’s better than nothing and it’ll help me track changes at the very least.

So what %age moisture is ok? Any ideas or any other thoughts? I don’t want my cellar ceiling collapsing really. Will the wood just get damp anyway even if there is no visible water ingress? The cellar does have heating to keep off the chill – plumbed into central heating.

20% is normal for graded timber

http://www.mbmfp.co.uk/graded_carcassing.php

Even if you’ve stopped the visible leaks, the cellar is going to get a bit damp just through moisture ingress through the walls – as a Victorian property it’s not going to be insulated or properly waterproofed down there.
However, even if you keep it warm you’re still going to need to ventilate it or the humidity will be high. I assume your new window has a trickle vent – I wouldn’t have thought this is enough so would suggest you look at fitting an intermittent extract fan on a timer or as a last resort, use a dehumidifier down there on a regular basis.

As above, keep the air moving and it will prevent spores from gaining a foothold.

You might find you’ll be emptying a dehumidifier a lot if you put one down there, so go for the tried and tested of adequate ventilation. And you’re correct, a trickle vent will not be enough, whether a bathroom type extractor would suffice would be a case of fit one and monitor it, although you’ll want to keep it running most of the time. Alternatively, you could put in one of the plastic twirly window vents and ideally, with either of those options, something at the other end of your cellar for air to enter.

20% is about right. IMO it will be what it will be down there, its been happy for the last 100+ years, so now you’ve almost sealed it up, it probably won’t be any longer.