"Damp" bathroom ceiling

[Robbox]

We recently moved into new house, well '60s house. The bathroom had a slight musty smell which I just put down to general humidity. But the I noticed one corner had small traces of black mould and the surface felt damp. So out came the meter and all around the perimeter the ceiling has above average moisture readings. Some corners read quite high. The ceiling has been artexed at some stage. I haven't had a chance to look in the loft to see what the top side looks like, because the loft is boarded out. But one thought I have, is it possible that moisture will become trapped in the space above the ceiling and cause the ceiling to be damp? Could this happen slowly over a long period?
If I find the top side is black, does that mean I need to replace the ceiling?

 

[^woody^]

It's condensation from the room, not damp from the loft.

You need to insulate above the ceiling, possibly ventilate the room, brush the mould, treat the ceiling with a biocidal wash, then seal it with a stain block before painting.

You might need to take the loft boarding up or raise it, or use a different insulation product to fit in the space available.

 

[Robbox]

or use a different insulation product to fit in the space available.

It's rock wool and only 100mm, was thinking of replacing with 100mm Celotex. Thoughts?
At some point we want to renovate the bathroom, I think I would do two things on top the usual items. One is fit a through the wall heat recovery unit and two is replace the ceiling with moisture resistance boards.

 

[^woody^]

The problem with rigid board on a ceiling is getting a good fit with no gaps. Bear that in mind.

Heat recovery is a decision for you. It won't help condensation any more than a normal fan.

Moisture resistant board is of no value in this situation

 

[Robbox]

You don't think heat recovery will keep the room humidity at a constant low, then shower humidity will be less overall?
What about using foil backed PB or maybe insulation backed PB? Im assuming that the ceiling is 'cold' and allows moisture to condense.

 

[^woody^]

I'm not a big fan of the concept of heat recovery in our climate - the temperature differential is not great enough. It won't do a great deal to reduce condensation issues anyway. You just need to extract moist air from a bathroom.

Foil backed plasterboard is used to stop vapour getting out of the room. That is not your problem.

A ceiling is cold if it's not insulated. Insulating it deals with that. It's no use using insulated plasterboard and lowering the room height of you can insulate in the loft. The insulation needs to go over the joists too.

 

[Robbox]

Thanks. I think I have to pull up some boards in the loft. I think I will find inadequate insulation cover right in the corners.

 

[Robbox]

Been in the loft and found the answer to the damp corner. The first ceiling joist from the outside wall is the usual couple of inches off the wall and that gap is not filled with any insulation. Unfortunately, the whole loft is boarded out with T&G chipboard, so getting rockwall insulation into the gap is nigh on impossible.
I see two possible solutions, drill holes at regular intervals through the boards and fill with loose insulation or squirt PU foam in through holes.
One other solution would be to cut the floor-board away to give full access and fill with insulation. But even with a good jigsaw i can see problems of maintaining a clean cut, plus the gap becomes a nice muck trap!
Any thoughts?

 

[JohnD]

you are unlikely to stand within two inches of the edge, so you can lift that board. If it was me I might use a floorboard saw or circular saw to cut through the tongues on that section, and screw it back afterwards, perhaps with a new nog.

If your bathroom is often full of steam and water vapour (which is lighter than air so rises to the ceiling) move it outside. An extractor fan will do that.

 

[Robbox]

Extractor fan is installed and over-runs. Small top light is always open. RH goes from 65% to 75%.
Maybe if I use jigsaw to cut that strip out, then fill with 100mm celotex, the cut out strip could just sit back on top....