Condensation in loft

[Wintermonster]

Can anyone give any helpful suggestions of how to prevent condensation in the loft when it gets really cold?

Once a year I have to go up there and clean off the mould that's grown.

It only seems to be this time of year.

We don't extract into the loft space, we don't dry clothes indoors, we run dehumidifiers in the kitchen (which actually is in an extension anyway so not under the loft).

The loft has been boarded and painted as previous owners used it as an office space.

We just use it as a loft store and keep the hatch closed. Luckily everything up there is stored in plastic boxes but I'd like to get it under control.

The loft is ventilated but also has a window to the side.
When I noticed the condensation a few weeks ago I cracked the window open slightly to help with airing it. Would this be the right thing to do?
I haven't had the time to go back up there an check if it's making a difference yet.

it's OK to leave the window slightly open, provided no rain or wildlife can get in. A loft should be cold and draughty.

Are the loft slopes felted, boarded or plastered?

Are there any holes in the ceiling below, for example for downlighters or pipes?

are there water tanks in the loft? Are they insulated, with tightly-fitting plastic lids? Is the water in them cold or warm? Do they leak?

Does the bathroom have an extractor fan that works and is used?

I see the loft is boarded and painted. Presumably it was not done to building regulations? Do you suppose there is insulation behind the boarding? Do you think there are unseen leaks behind the boarding?

Thank you @PigletJohn

I am 100% sure of no leaks as roofers have been up there to check and they say no leaks as that was my first worry.

The worst of the condensation is on the wall adjoining next door (we are attached one side only).

The boarding and decorating of the loft space was definitely not done to regulations and about 20 or maybe as much as 30 years ago.

There is a very deep reveal between the ceiling and the loft boarding but what that actually means in real terms I don't know.

When we bought the house the loft hatch was open and a staircase running up to it.

We took the stairs down and closed the hatch.

It's not a heated room.

No holes in ceilings other than standard ceiling rose light fitting to each bedroom (2 bedrooms only).

The bathroom has spot lights but the bathroom itself has its own loft space and isn't situated under the main loft (typical 2 bed Victorian terrace)

It probably needs more ventilation. The warm air from the house is getting in there and condensing on the cold walls. Building regs normally says 25mm continuous ventilation at the eaves and the ridge. It's very unlikely you have achieved this. I would get your roofers back to add a number more vent tiles, and like Piglet John says, leave the window cracked open if its safe to do so. And make sure the hatch is well sealed.

let's guess then, that warm air and water vapour are rising up into the loft from the house below. Unless it is something to do with loft plumbing.

Water vapour is lighter than air so it rises until it either escapes or finds something cool to condense on.

It often rises into a loft from a steamy bathroom but will rise up even from a leaky pipe under the floor. have you got a water meter?

if you have eaves on the side of the house pull back the loft insulation just enough that you can see daylight through the gap. This will allow airflow. You can cut a slope on it with big scissors. If you only have eaves on one side you will not get much air current, but you could have an airbrick or other ventilator put in the wall at each end. Builders will find it easier to make a 100mm or 150mm round hole using a core drill, as used for extractor fans. it is a bit noisy and dusty but not as bad as chiselling a gap out. It can have a cowl vent fitted on the outside to keep the rain out, it doesn't need a flap (which would rattle annoyingly). It would be possible to fit a fan but I don't think that is necessary as long as you have openings on at least two sides, natural air current will blow through. The higher the openings, the better.

Definitely no loft plumbing

It's eves on both sides. Previous owners had taped up the eves doors.

We have taken tape off but DH has stacked quite a few boxes up there so it's as good as covered again (he didn't believe me when I told him that could possibly be the cause of our issue)

The tiles are concrete and I can't see any vents when looking on the outside of my house...