Slight condensation in loft

James South · 2 Jan 2021

I've just been in the loft to put something in there and while up, I noticed between two rafters light condensation, along with some across the ridge of the roof, between the rafters. No where else seemed affected. Other times I have been up there has been nothing, although this tends to be in Spring and Autumn.

It was difficult to tell by eye if it was water or simply the torch reflecting off the material however upon touching it, it felt wet.

To me, what is odd, is that it is only affecting one part of the roof and not the others, and only on one side. Is this something to be overly concerned about or simply the roof doing its job? There was no sign of mould either.

The humidity in the hall is usually around 55%. We have a fan from the bathroom passing through the loft to an external wall, however, this seems to be working ok. The loft had extra insulation added a prior to us buying it so I believe it's double insulated. When I went up the house was around 20c and outside maybe 0c.

Thanks

Jim

catlad · 2 Jan 2021

A bit of condensation at this time of year is nothing to worry about.

James South · 3 Jan 2021

catlad said:
A bit of condensation at this time of year is nothing to worry about.

Thanks. I've been back up today and little difference. Left the humidity meter but it's around 45%. The insulation isn't over the soffit with the air vents in it and no condensation that low down. A few drops on the insulation but nothing too bad. We do have a bathroom fan that vents through the loft by about a meter or so but the insulation around that is dry.

It's only affecting the west side of the roof so presumably no direct could be a factor. The East side with solar panels appears fine. One or two of the beams looked like they may have a wet line next to the black felt but again, only on parts. A few dodgy images attached.

James South · 3 Jan 2021

Update. Humidistat was reading 58% after 30 min in the loft and around 7.5c. Outside it's probably 1-3c. Turned the torch off and from one of the soffit vents I can see daylight so I know some fresh air is getting in.

catlad · 3 Jan 2021

You can open the laps in the felt if you think its a problem, that way as it dribbles down it can exit out of the roof space. Something like plastic clothes pegs should work to keep the laps open.

James South · 4 Jan 2021

Thank you, slightly unsure about playing around in the roofspace myself. Would something like this increase the airflow?

https://www.amazon.co.uk/Vents-Prev...eywords=vent+flaps+lift&qid=1609761223&sr=8-8

I'm just unsure about tampering with the felt and structure of the roof space.

Our inside humidity is 48% at present so it's not as though the air entering the loft is overly humid from downstairs but presumably because it is cold it is condensating. Also odd, to me, that it's only on one side.

Why Not Indeed · 4 Jan 2021

Sounds a bit like my roof. One side faces east-north-east, is shaded from the south by a tree and gets very little sun at this time of year. This side has quite a bit of condensation on it. The other side faces west-south-west, isn't shaded, and gets quite a bit less condensation on the inside.

I wouldn't call lap vents "tampering", it's more "tweaking"

Adding them in is very easy to reverse if you think that they're causing a problem.

James South · 4 Jan 2021

Why Not Indeed said:
Sounds a bit like my roof. One side faces east-north-east, is shaded from the south by a tree and gets very little sun at this time of year. This side has quite a bit of condensation on it. The other side faces west-south-west, isn't shaded, and gets quite a bit less condensation on the inside.

I wouldn't call lap vents "tampering", it's more "tweaking" Adding them in is very easy to reverse if you think that they're causing a problem.

It's odd but hopefully not uncommon.
Tampering may have been the wrong choice of word. Basically doing something I've no experience in with something that's pretty fundamental to a house!

Have you overcome your issue and if so, how? We have solar panels on our non condensation side too so maybe they are helping in some way.

JimDogweed · 4 Jan 2021

Sorry to jump on your thread, but I have a similar issue.

I've been up in to our loft today - putting away Xmas stuff. We've only lived here since March so I'm not familiar with the normal behaviour of the house during winter - it's old terrace house, built around 1910.

I noticed some condensation on the felt that I've not noticed before. There are some cardboard boxes etc there which don't appear to be damp. It looks like it's mainly on the North side of of the roof (which is above a bedroom), and towards the edge. There is some insulation between ceiling joists (it doesn't look like it's blocking but I will double check this) but I can feel a slight breeze while I'm in there.

I've been monitoring the humidity in the loft today with a sensor I have and it's been just under 90% - BBC weather says it's around 75% outside.

Is this something I need to worry about or is it just "one of those things" to keep an eye on? Are Felt Lap Vents worth looking at?

Thanks in advance.

Edit: The temperature in the loft has been ~7 degrees, similar to outside.

catlad · 4 Jan 2021

Lap vents are simple and cheap to fit so worth trying yourself.

Why Not Indeed · 4 Jan 2021

James South said:
Have you overcome your issue and if so, how?

No. I bought 10 of the lap vents and fitted them but it wasn't enough to solve the problem and I thought they were pretty expensive for what they are. I cut up a length of PVC pipe to open a few more of the laps and then kind of got bored of the problem. That's where we stand now, it's 7° and 89% relative humidity up there. I suppose I should carry on with it as it's the cheapest possible fix I can do myself.

James South said:
We have solar panels on our non condensation side too so maybe they are helping in some way.

If they're increasing the temperature of that side of the roof by reducing the loss of heat through it, then that's going to reduce the likelihood of water condensing on the underside. It would presumably also reduce how much much sun hits it.

JimDogweed · 4 Jan 2021

Why Not Indeed said:
That's where we stand now, it's 7° and 89% relative humidity up there. I suppose I should carry on with it as it's the cheapest possible fix I can do myself.

That matches my loft - Hope you don't mind me asking, how long have you had the condensation and has it caused any major issue? How is you insulation up there?

I'm still on the fence about using the lap vents - I read they could cause an issue with "wind uplift load" but not sure if it's been an issue for anyone.

Why Not Indeed · 4 Jan 2021

I've only been here a couple of years, so the most I can say is that it's been a problem for a least a couple of years. It's not drafty at all up there, no soffit vents or the like. My roof is unusual in that it's a trussed roof built over a flat roof [https://www.diynot.com/diy/threads/internal-loading-on-roof-trusses.519833/] so don't draw too many conclusions from it. I've insulated about 75% of the space between the flat roof and the trusses with Ecose.

foxhole · 4 Jan 2021

The time spent with the loft hatch open is enough to allow enough moisture from the home into loft and produce a little condensation.

James South · 5 Jan 2021

Thanks all, we've decreased our inside living humidity to around 47% at the moment, so hopefully, that might help. It has been as high as early 60's. We had condensation on windows etc. in the mornings but that has cleared up since leaving them slightly open, there are no trickle vents.
It's supposed to get warmer down here by around 8-10c next week so it will be interesting to see if it clears up a little. I have a dehumidifier I can pop up there if it still doesn't clear and maybe get a specialist over when the restrictions clear. I'm assuming nothing catastrophic will happen in the immediate short term since I only noticed it because I went up there and otherwise wouldn't have spotted it. It might be more vents are needed in the soffits or something on the ridge.

Edit - We have the dehumidifier for drying clothes inside when we need to. With two small kids needs must, especially during winter and wet weather.

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