condensation in loft - 1817 design fault perhaps

[michael]

I’ve just been up in one of my lofts and there’s lots of condensation droplets on the underside of the roofing felt which are obviously dripping as my newly installed 18mm ply board ‘crawl way’ is very damp.

This loft is above what is now a ground floor, single story bedroom, corridor and bathroom and consists of a single elevation ‘lean to’ felted and slated roof i.e. the roof leans against a solid brick wall sealed at the top with lead flashing.

The loft space is about 10meters long by 2 meters wide with a maximum height of around 1.5 meters (against the lean to wall) tapering down to nothing towards the gutter line.

There are about three ventilation points along the length of the roof. I don’t know what they’re called but they are slate sized plastic panels with a vented mound on top.

The loft floor (lath /plaster ceiling of the bedroom, bathroom and corridor below) is insulated with rock wool

This is an original structure from about 1817. Interestingly there is a cavity wall along the back elevation (the wall which the roof leans against) from ground to ceiling level. At the bottom of this wall (at floor level) there is a series of original terracotta grates or large air bricks which allows an air flow from the rooms, up through the cavity and into the loft space. At the moment the loft is very cold and so the cold air from the loft is coming down the cavity and through the grates into the rooms. Previous owners had installed hit & miss grilles over these air grates to stop the cold air from the loft entering the rooms. I have in the main left these grilles open as the original grates must be there for some reason.

Not sure what I should do here to stop the condensation in the loft. Should I allow the flow of air between the room and the loft to continue or should I seal it off. As mentioned above when opening the hatch to climb into the loft, it’s noticeably very cold up there.What were these original grates in the wall for? and would it be detrimental to close them off? the rooms in question are efectively below ground level as on the other side of the lean to wall, the pavement and road align approximately with ceiling level (the building is on the side of a hill)

My guess is that its warm air from the rooms condensing against the cold roofing felt in the loft space so perhaps the terracotta grilles in the cavity wall which allow warm air in the rooms to travel up into the loft space was an 1817 design blunder?

 

[chuckey]

If I understand your description properly, the outer skin of this cavity wall is underground? So the question is what happens to any water that migrates through this wall. I would remove one of the "air bricks" and make sure that the cavity is open and extends right to your foundation level, so any drips coming in through the wall can run right down to the earth. Then I would fill the cavity right up with loose pour polystyrene beads. These will reduce the air flow and warm up the room. when you do this stuff glassfibre insulation into the top of the cavity to stop the stuff being blown out and likewise around the air vents (or keep them closed). There will still be a small air flow but not so much that you'll notice.
Once the cavity is partially sealed as above, in the autumn look in the loft to see if the sarking is still sweating, if it is, put an air brick into the end wall of the loft to increase the ventilation.
Frank

 

[middi]

The sure way to stop your condesation is to allow free air flow circulation in the roof void, perhaps by installing a netted grid at each end of the gables. The floor must be well insulated .

If you need the space for dry storage or semi-living aerea then you need to insulate the roof aerea correctly, which probably means getting rid of the existing membrane and start from scratch.

 

[PC]

Common problem, Improve ventilation!

PC

 

[MdB]

Common problem, Improve ventilation!

Not in this case. The problem is due to warm moist air from the rooms traveling to the cold loft and condensing. This flow wants to be reduced and not increased.

My solution would be to reduce this airflow by blocking off the vents. I would then make sure there was plenty of ventilation in the loft (preferably from one side to the other). Depending on the structure of the cavity wall, I would then make sure that there was some limited ventilation to the cavity - probably to the outside so that there was a small amount of airflow within it.

 

[Nemesis]

I suspect PC meant improve ventilation to the loft.

 

[michael]

Unfortunately there are no gable ends as such, so I cant vent the loft from each end. This is a single story structure where the back elevation (the wall on which the roof leans) is below ground and the front elevation looks out on to a courtyard. In other words it follows the contours of a very steep hill, the high part of the hill at the back of the structure and the low part of the hill at the front.

Another part of my property, which is two storeys high, adjoins one of the gable ends of this single storey structure so there is a vertical brickwork wall going up above the roof level. If I were to punch a hole from within the loft through this wall, I would be able to look through into the first floor of my adjoining property. The other gable end adjoins my neighbour’s three storey property so the same applies. If I punched a hole through that wall from within the loft I would end up in my neighbour’s front room.

There’s really no sign of water coming through the back wall. This is probably due to the fact that there is a small tarmac surfaced road on the other side complete with York stone pavement and granite kerb stones.This road is approximately in alignment with my ceiling level .Beyond the road there is a thick stone retaining wall about eight feet high with drainage points spilling out onto an old but well maintained (by the council) stone gully. In other words water off the hill is well managed and is directed away from my property.

The loft seems to be fairly well ventilated with its three (imitation slate) vents. You can certainly feel the draught when the wind blows.

My conclusion, rightly or wrongly, is that I should cover up the original air bricks on the internal skin of the back cavity wall during the winter or when the heating in the rooms is turned on. This will prevent the warm air from the rooms travelling up to the loft space and condensing. During the spring/summer when the heating in the rooms is turned off and there is more of an equilibrium between the loft and room temperatures, open the air bricks up for maximum ventilation.

I’ve recently bought some new 9”sq brass hit and miss grilles to fix over the air bricks for this purpose. Hope this works.

 

[Pford75]

You've not mentioned what sort of 'felt' you have under the roof. Most roofs are now done with breathable membrane...not felt. One elevation of our roof was 'felted' in the past by the previous owners and this suffered from the problem you describe, when it was re-slated we used breathable membrane and the problem disappeared.

You might find it challenging to stop all warm air entering into the loft space, so you might also want to consider insulating between the rafters to increase the temperature above condensation point.

 

[michael]

Unfortunately, the felt is the old bitumen type sheeting and not the more modern breathable membrane sheeting. If all else fails, I might try Pford75’s idea of insulating between the rafters, this should in theory isolate the cold felt surface and reduce condensation. I’m guessing that it’s the ‘conduction’ of cold from outside through the slates that causes the felt surface to get cold enough for condensation and not the cold air in the loft. Would I be correct in this assumption?

What type of insulation would you use between the rafters? And how do you hold it in place?
If my assumption above is wrong however and it is in fact the cold air within the loft that lowers the temperature of the felt then foil coated insulation board between the rafters would no doubt create the same problem i.e. warm air condensing on cold foil, or am I talking rubbish here?

One further thought. If I insulate between the rafters and eliminate the very cold surfaces, would it then be safe to allow a stream of warm air up into the loft?

Sorry if I'm babbling, it’s all getting a bit confusing.