Should the roof void of a flat roof extension be sealed off from the loft void in the bungalow, or should the two be continuous to allow free air exchange? I have a situation where the two voids are continuous and am having dreadful condensation problems at the junction between the two.
Flat Roof Extension to Bungalow
- Thread starter Palma8910
- Start date
Thanks. But I've been told the warmer air coming from the flat roof void is producing condensation as it comes into contact with the colder loft air.
if the flat roof is a 'warm deck' variety then there will be pockets of warm air within. whether this is substantial enough to cause problems to the adjoining loft is debatable.
why don't you stuff some insulation into the opening to each void as an experiment to see if it makes a difference?
why don't you stuff some insulation into the opening to each void as an experiment to see if it makes a difference?
If the flat roof has a void, then it is most likely needs it to allow through-air to evaporate any condensation which could form beneath the deck. If you block this up, then you could be creating a major problem within the flat roof in terms of condensation and rot
Whoever told you about warm air coming from the flat roof void is mistaken.
Where is the condensation forming exactly?
Whoever told you about warm air coming from the flat roof void is mistaken.
Where is the condensation forming exactly?
the air beneath a warm deck roof is likely to be warmer that that of the outside air though woods. 
Yes but the convection or movement of the air will make it very difficult for it to condense at the point where it meets the main loft.
It's not like warm air meeting a cold window for instance, where there is a sudden temperature change.
There will be a gradual temperature drop and constant movement within the two air voids and at the junction of them both.
Not impossible, but less likely than other possible causes
It's not like warm air meeting a cold window for instance, where there is a sudden temperature change.
There will be a gradual temperature drop and constant movement within the two air voids and at the junction of them both.
Not impossible, but less likely than other possible causes
This is a flat roofed 'orangery', i.e. there is a large glass lantern in the middle of the flat roof (which could be affecting air flow?). The orangery is 7.5m long by 3.5m wide and is attached to the bungalow on three sides. The front of the orangery has a continuous ventilation grill into the flat roof void, there are three ventilation mushrooms in the flat roof along the back (between the lantern and the start of the sloping bungalow roof, and there are 9 ventilation tiles in the bungalow roof about 0.5m above the flat roof (put in to try to resolve the problem).
There is a plywood layer board (?) fixed to the underside of the roof timbers just below these ventilation tiles, at the junction between the main roof felt and the flat roof felt (the original eaves having been cut back). The condensation is worst on the layer board and adjacent timbers and appears to run back into the flat roof void, where it soaks through to the surface of the ceiling plaster.
Putting a fan up there gets rid of the condensation, but of course does not deal with the cause!
if the flat roof is vented then it is not a warm deck roof. 
I did block up the openings for a few days. The condensation in the main loft disappeared - but the flat rood void was dripping wet (though of course, I did the blocking up when it was wet). I'm thinking of waiting till everything dries off (I have fan up there now) and retrying the blocking off.
It therefore appears that you have moist air coming from the flat roof and not merely warm air.
This will be either humid air from the room peculating through the ceiling, or warm and condensing, and then being carried through the void.
So blocking is not the answer. You need to either to either install a impermeable membrane to the ceiling or insulate the flat roof. Search for flat roof construction
But weigh up the cost of this against whether the condensation is actually a problem - many lofts get it on really cold days and it disappears without issue
This will be either humid air from the room peculating through the ceiling, or warm and condensing, and then being carried through the void.
So blocking is not the answer. You need to either to either install a impermeable membrane to the ceiling or insulate the flat roof. Search for flat roof construction
But weigh up the cost of this against whether the condensation is actually a problem - many lofts get it on really cold days and it disappears without issue
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