Roofs with condensation

[backwards]

Hi,

I believe the roof in the loft is suffering from condensation caused by moving a bathroom upstairs as it was fine before this was done. The give away clue is the water forming in droplets on the felt.

However it might be a leak, to be on the safe side, how can I tell? and how can I fix it?

Thank you,

 

[JohnD]

//www.diynot.com/forums/viewtopic.php?t=160032

 

[PerryOne]

Does your bathroom have a plasterboard ceiling?
Plasterboard is transparent to water vapour.
2.5 litres of water vapour can move through an 8 x 4 sheet of plasterboard in 24 hours at a temperature of 30 C and 100% humidity.
Water vapour condenses out of the air with a drop in temperature.
Do you keep your bathroom temperature steady?
Or do you turn the heating down or off at times?
Air can hold 30 cl of water per cubic metre at 30 C.
18 cl @ 20 C and only 5 cl @ zero C.
When you turn the heat down or off - that water vapour heads for the coldest surface usually a window, very often a loft space.
Do you have and use an extractor fan?
Do you leave the bathroom door open?
Does your loft access fit airtight?
Just a few details to consider.
When building a bathroom it is a good idea to line the ceiling with an almost water vapour proof plastic membrane before fitting the insulation and plasterboard.
Perry

 

[bobdabuilda]

Does your bathroom have a plasterboard ceiling?
Plasterboard is transparent to water vapour.
2.5 litres of water vapour can move through an 8 x 4 sheet of plasterboard in 24 hours at a temperature of 30 C and 100% humidity.
Water vapour condenses out of the air with a drop in temperature.
Do you keep your bathroom temperature steady?
Or do you turn the heating down or off at times?
Air can hold 30 cl of water per cubic metre at 30 C.
18 cl @ 20 C and only 5 cl @ zero C.
When you turn the heat down or off - that water vapour heads for the coldest surface usually a window, very often a loft space.
Do you have and use an extractor fan?
Do you leave the bathroom door open?
Does your loft access fit airtight?
Just a few details to consider.
When building a bathroom it is a good idea to line the ceiling with an almost water vapour proof plastic membrane before fitting the insulation and plasterboard.
Perry

Good advice in the first bit, but I would question the last paragraph-

What plastic membrane are you referring to or would recommend? Bog standard polythene is virtually impermeable to water vapour and unless it was chemically bonded to the plasterboard you could get water vapour condensing between it and the loft side of the board thereby making the boards damp. Have seen a similar problem with wooden floor laid over polythene covering a concrete floor- condensation build up over a year resulted in the boards swelling and buckling.

 

[kevnurse]

When building a bathroom it is a good idea to line the ceiling with an almost water vapour proof plastic membrane before fitting the insulation and plasterboard.

I'm not convinced thats a good idea. If the bathroom is not ventilated, the ceiling plaster will absorb the moisture (depending on the surface protection) and it will eventually become mush. Without the membrane, the bathroom and the void above it (inter storey void or roof void should be ventilated). Moisture must never be contained within a building...get rid of it.

 

[bobdabuilda]

I'm not convinced thats a good idea. If the bathroom is not ventilated, the ceiling plaster will absorb the moisture (depending on the surface protection) and it will eventually become mush. Without the membrane, the bathroom and the void above it (inter storey void or roof void should be ventilated). Moisture must never be contained within a building...get rid of it.

Exactly what I was thinking/referring to and opens up a new can of worms. I wouldn't put plastic membrane over pboard, just use the foil backed stuff if theres plenty of ventilation in the room or just leave it plain and rely on the loft ventilation to get rid of the moisture.

 

[JohnD]

rely on the extractor fan.

 

[bobdabuilda]

if its any good or exists!

 

[backwards]

Hi,

Thanks for the advice, The existing ceiling was simply skimmed by the builder we have been using for everything and then sealed with a bathroom ceiling paint and the edges with a bead of silicon.

The extractor fan seems to do a reasonable job of removing the steam from the shower. I leave it on for a bit longer to make sure the steam clears.

The loft access is unlikely to provide much of a barrier as it is simply a thin plywood board lying on top of some old insulating trim, the type you would normally see on a door.

The bathroom door had until recently been left open, amusingly to stop the bathroom getting too steamy, this is now being kept closed. I’ve also put some insulation roll on top of the access board. The problem appears to be lessening but there is still visible dampness on the felt.

How can I better insulate the loft access? When I was in the loft with the shower running I could feel a warm draw at this point.

Does the bathroom ceiling solution that we installed sound adequate?

Assuming the problem is now fixed will the loft space need drying out some how or will it sort its self out?


thank you

 

[PerryOne]

By 2016 all new houses will have to be built on the Passive House principle, which means they have to be airtight, with mechanical ventilation and instead of gas boilers and open fires external district heating using wood chip or electric resistance heating. The owners of the 27 million existing homes will no doubt wish to improve their insulation as well as the cost of heating goes up.
Perhaps now is the time to think about my solution, albeit, on a small scale, a nearly water vapour sheet, with polystyrene or similar insulation below the joists and skimmed plasterboard under.
Contrary to what other people have written, plasterboard is designed to be damp and as such is used for good purpose in fire resistance, the enclosed water taking time to boil off before the destruction of the plasterboard.

As a stop gap, you could do what we all did years ago and paint the ceiling with oil based paint, that is in itself water vapour resistent, we all had ceilings and walls with running condensation on bath night.

Once you keep the bathroom door closed all the time, and leave the fan running for twenty minutes or so afterwards, the arrival of spring and the warm weather will sort the problem for you.

During the summer find time to sort it properly.

Remember, the wind sucking you warm air out throught the holes in the fabric of your home costs you a lot of money.
Moving towards the Passive House principle is now promoted by the EU
across Europe.
Perry