Condensation on underside of roof felt

setanta1984

I went up to the attic for something today, and noticed all of the north-facing side of my roof has a lot of condensation on the felt - very visible and wet to the touch. A few rafters looked to have wet patches on them too. The other, south facing side of the roof is fine. I presume this is due to the cold weather we have right now - I have never seen this before (in house 2 years, this is the first time we've had proper sub zero temperatures in that time) - everything looked fine taking decorations down last weekend. The north side of the house has not fully thawed out the last few days,remains frosted white all day, as it gets no direct sunlight.

I'm wondering, is this a common/normal thing to expect when the weather gets this cold and that part of roof doesn't thaw out? Or do I have a more serious problem with the roof in general? And if so what should I be doing? Thanks.

MicktheMan

It's a sign that perhaps the attic space ventilation may need some improvement. Depends on the roof type but in general if you go into the attic during the day with the light off you should be able to see some daylight at eves level on both sides. Check that insulation has not blocked the ventilation channel below the roof felt at the eves.

magicbastarder

yep, was going to suggest the same - i know something similar happened a colleague years ago when he rolled out some new insulation in his attic, and was telling us 'there was a gale blowing in at the corners so i stuffed them with insulation' - within a few months he had a pronounced condensation problem.

10-10-20

https://www.boards.ie/discussion/2058275452/condensation-on-underside-of-roof-felt

Was the attic hatch left open for a period while the decorations were taken down? If so, that's the likely source of much of the moisture. It's the long-term existence of moisture which is a problem rather than infrequent occurrences. You'd be best off checking the vents as per Mick's input and then re-check again in another cold period providing that the hatch wasn't open beforehand.

phelixoflaherty

Ya, ventilation at the eaves and better seal around the door

phelixoflaherty

Lots of big stuff up there might stop the cross flow of air too.

Banjo Carney

I noticed the same in my attic when I took down the decorations last week.

The hatch hasn't been open maybe since putting the decorations away last January.

The attic has vents also. Should I be concerned ?

Field east

https://www.boards.ie/discussion/2058275452/condensation-on-underside-of-roof-felt

I had that very problem . In my case it was mainly the rafters that were damp on the upper side. I got special roof vents fitted . The fitter just removed as slate and replaced it with a vent. Obviously , the bigger the roof the more vents you will fit

The Continental Op

I was in my parents old house last week starting a house clearance. The the inside of the roof felt on the north side in the mornings was wet, just the same as its always been for the last 45 years when the weathers been cold and the house hasn't fallen down yet.

10-10-20

I was watching the temperatures and humidity levels in mine recently.

Currently with the ceiling all insulated to 200mm it's at:

The Dewpoint is the critical thing there - that indicates that any surface at 0.7C or lower will start the formation of condensation. The external temperature is around 0C just now. Ideally I'd like my attic to be closer to 50% RH (and hence the dewpoint will also rise), but I think that's better than before I did some work up there on a bathroom vent and sealing some holes in the plasterboard last month.

Moisture is a problem when you see that grey staining at the junction of the rafters and the sarking, that's an indication of ongoing excessive moisture.

setanta1984

Thanks for all replies.

I can't really see the vent openings from inside the attic, but there is only 2 obvious vents in the soffit that I can see from outside, which probably feels too few anyway even if they aren't obstructed.

Just on this:

"Moisture is a problem when you see that grey staining at the junction of the rafters and the sarking, that's an indication of ongoing excessive moisture."

seeing as the rafters seem to my untrained eye pretty ok, and I've never noticed problems before now, that the condensation is purely down to the extremely cold outside temperatures we have currently, and is not entirely unexpected?

Seems like it has started to drip down off the felt too, as I noticed some carboard boxes had wet patches on them :/

Going by the replies here opening up the attic door regularly to be checking on it isn't the best idea with leaving warm air up there?

10-10-20

https://www.boards.ie/discussion/comment/119988277#Comment_119988277

seeing as the rafters seem to my untrained eye pretty ok, and I've never noticed problems before now, that the condensation is purely down to the extremely cold outside temperatures we have currently, and is not entirely unexpected?

In the absence of further indications during less cold weather and as you mentioned that this issue is prevalent on the North-facing side of the roof, I'd agree that this could be a transient issue. We have very little wind and that's not helping with that static pool of moisture already there. In more "normal" periods we'd have some wind and that moisture would move on. Is the South side free of condensation?

Seems like it has started to drip down off the felt too, as I noticed some carboard boxes had wet patches on them :/

Going by the replies here opening up the attic door regularly to be checking on it isn't the best idea with leaving warm air up there?

It's Schrödinger's Cat. If you monitor it by opening the attic hatch, the state changes. If you don't monitor it, you don't know the state.

Ridge-vents or an extra tiled vent (brought through the sarking!!) on the North side may benefit.