The roofers came round to have a look.
They said it couldn't be a conventional leak as the roof is solid, and I guess as I agree as it hasn't been raining.
They speculated that since it's been very cold, that water vapour in the air had found a cold spot and condensed.
This must've happened on the underside of the breathable membrane,
then dripped onto the foil covered insulation and run down to where the roof meets the wall and caused the damp patch.
They also said this sort of thing is more likely to happen in an unheated space (the room is unheated at the moment while interior diy is happening)
they said when we turn the heating on this shouldn't happen again, and implied that they don't want to/won't do anything about this.
Does anyone know if this could be an explanation of what happened?
1) I thought condensation happened when warm air meets a cold surface, so saying that it's more likely to happen in an unheated room sounds wrong to me?
2) Surely the double layer of insulation should be stopping warm air from escaping, I know it's not going to stop all warmth escaping, but surely it should stop enough so that this doesn't happen?
3) I also thought the membrane was breathable, so that water could exit without causing problems like this
4) If it is water condensing on the underside and running down to where the roof meets the wall, then surely there should be a mechanism so that it doesn't just empty onto the wall and soak into it? If there is a possibility that this can happen surely there should be an outlet for condsensed water? Have the roofers constructed something incorrectly? It seems crazy that they can come along and say "condsendation happens", without having built something into the roof design to deal with this condensation.
5) It seems like it's too much water to be caused by condensation, how much water would cause this amount of damp patch?
6) If it is condensation then why has it only appeared in these 2 places, wouldn't condensation be more likely to cause a wet line running along where the roof meets the wall? There's also no damp patches in the other room at the back of the house that's also unheated. Why would the condensation form in 2 particular places and not elsewhere?
7) If it's not condensation running down, does anyone have any other theories on where this damp could have come from?
8) We've just spend a load of money replacing the roof and are unhappy at the roofers just washing their hands of this and not wanting to do anything about it. I know the damp patches have disappeared but how do I know they won't come back and be worse?
The chimney is at the other end of the room, so it's not being caused by a chimney problem.
The stepped skirting (see the picture linked above) is a saniflow toilet outlet which comes from the next room, through this room and to the outside. So that's got nothing to do with the leak either.
This is one of the classic problems when a room below a roof has been replastered, there is so much moisture the it will get into the roof space and condense on the underfelt.
Assuming the roof is cross ventilated just let the drying out take its course.
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