DIY Doctor UK DIY Forums

HI.
My daughter has recently moved to an in excess of 100 year old cottage.
There is very little roof space and part of the bedroom ceilings slope in line with the roof. With the recent heavy frost she noticed that condensation formed on the sloping part of the ceiling. This occured after leaving the window on security opening all night and when heating came on the condensation appeared. Bearing in minde that the space between ceiling and roof tiles is only the depth of the timbers used in the roof construction and that the window and radiator are situated immediately below this section. My proposed solution is to remove the sloping secttion of ceiling and insert insulating material as far as possible, fix a vapour barrier to beams and plasterboard. Is this OK if so what type of insulation and vapour barrier would you use and what type of plasterboard. Also all fireplaces have been blocked off and chimney capped but no ventilation has ben fixed. JC

Your proposal is sound.
Pull down the ceiling and see whats there.

You do not mention what the ceiling is made of?
I would expect the window to be streaming with condensation before a plaster ceiling shows signs of damp.
Has the ceiling been painted with gloss paint?
A typical plaster ceiling will absorb lots of humidity before it looks damp, in fact a lot of humidity will pass through plaster, about two pints a day..

Current practice is to leave a space of two inches under a roof to circulate air and help keep the timbers dry.

Having said that, you need to fill the remaining space with polystyrene. A one inch thick sheet of polystyrene will save roughly 90% of the heat lost through the ceiling, once over 5 inches and the return in heat saved will take years to recover the cost.
Probably, 2 inches of polystyrene sheet between the joists, will do the trick. With another one inch sheet under the joists to avoid ghosting. Even so, when it is very cold you may be able to pick out the nail heads.

About, 40% of heat lost from a property, goes out through gaps in the structure. Placing a damp proof/water proof membrane under the joists and above the polystyrene sheet will stop this movement through the ceiling.

It is a matter of concern that there is no ventilation. In many ways condensation on single glazed windows, is a useful reminder to do something about it. Modern double glazed units require lower temperatures and higher humidities to indicate a problem.

The instant solution is open a window, a more comfortable one is to keep the indoors temperature and humidity steady. A drop in overnight temperature outside coupled with a more than 2 degree drop indoors will more often than not lead to condensation problems - keep in mind that humidity is a very fine invisible gas.

[quote="Perry525"]Your proposal is sound.
Pull down the ceiling and see whats there.

You do not mention what the ceiling is made of?
I would expect the window to be streaming with condensation before a plaster ceiling shows signs of damp.
Has the ceiling been painted with gloss paint?
A typical plaster ceiling will absorb lots of humidity before it looks damp, in fact a lot of humidity will pass through plaster, about two pints a day..

Current practice is to leave a space of two inches under a roof to circulate air and help keep the timbers dry.

Having said that, you need to fill the remaining space with polystyrene. A one inch thick sheet of polystyrene will save roughly 90% of the heat lost through the ceiling, once over 5 inches and the return in heat saved will take years to recover the cost.
Probably, 2 inches of polystyrene sheet between the joists, will do the trick. With another one inch sheet under the joists to avoid ghosting. Even so, when it is very cold you may be able to pick out the nail heads.

About, 40% of heat lost from a property, goes out through gaps in the structure. Placing a damp proof/water proof membrane under the joists and above the polystyrene sheet will stop this movement through the ceiling.

It is a matter of concern that there is no ventilation. In many ways condensation on single glazed windows, is a useful reminder to do something about it. Modern double glazed units require lower temperatures and higher humidities to indicate a problem.

The instant solution is open a window, a more comfortable one is to keep the indoors temperature and humidity steady. A drop in overnight temperature outside coupled with a more than 2 degree drop indoors will more often than not lead to condensation problems - keep in mind that humidity is a very fine invisible gas.[/quote]

Hi
Thanks for prompt reply.
When installing one inch polystyrene under beams how do I fix and what type of vapour barrier do I use and does vapour barrier go between joists and polystyrene or on top of the polystyrene. I presume that I then fix plasterboard directly on top.
Thanks.
JC

Fix the waterproof plastic sheet under the beams using a staple gun.
Fix the inch thick polystyrene under the plastic sheet using a few galvanized clout nails.
Fix the plasterboard using more clout nails.