Loft Condensation, humidity rises overnight

[mikey90]

Hi,

The felt in my loft is wet and condensated. This happened last winter and I thought we solved it by having our gutters cleared out but it must have just been as we were coming out of winter so coincidentally went away when it warmed up.

This year it's back and wet in the loft again. We had condensated windows in the morning throughout the house so we obviously had too much humidity indoors. I fixed this by allowing lots of air to flow through by opening all windows and now I always open windows when using bathroom etc. The humidity of both up and down stairs is now around 50% and we no longer have any window condensation at all.

The loft is still wet and is around 80-90 humidity. I realised some of the insulation was covering the soffit vents blocking outside air flow which from my understanding is very important for preventing condensation in the loft. After moving these I decided to leave the loft hatch open and open all the windows in the house. After a few hours the humidity up there dropped to around 78% and the temperature rose from 5 degrees to about 12 which I assumed was from the heating in the house rising up there. So I thought maybe this is going to fix it. However after closing the loft hatch and windows and going to bed later that day, the next morning I checked and the humidity in the loft had gone straight back up to 88%.

I'm just confused as to where that rise in humidity is coming from. From reading other threads, blog and watching videos as far as I know you are supposed to try to prevent the hot air from your house flowing up into the loft as that can cause it to become humid up there. However when I had the hatch open and the temperature rose in the loft the humidity actually decreased and then over night when it was closed and no heating was on it shot back up. Everytime I think I'm making progress on tackling the condensation and humidity up there I just seem to be back to square one!

I know it's a long one but any other suggestions would be greatly appreciated!

 

[Why Not Indeed]

However when I had the hatch open and the temperature rose in the loft the humidity actually decreased and then over night when it was closed and no heating was on it shot back up.

You're talking about relative humidity here. The colder the air, the less water it can hold before condensation happens. Even if the moisture content of the air in your loft stayed exactly the same, a fall in temperature would cause the relative humidity to rise. Think of relative humidity as "how close am I to condensation happening?". As you get closer to 100%, the chances of condensation appearing increases.

There are really only two approaches:
- Stop moisture getting in: close up gaps between the warm living space and the cold loft, make sure the loft hatch fits tightly, cover over open water tanks, check condition of bathroom extractor fan ducting
- Increase air flow: unblock soffit vents, insert lap vents, tile vents, install more soffit vents

 

[KenGMac]

mikey90, good evening.

As "Why not indeed" above, we are talking relative humidity, that is a fairly big topic, it is all about temperature and the ability of warm or cold air to keep the [unseen] water vapour in suspension in the air.

If you consider it, the house [below the loft] is warm and has a high but very, very variable relative humidity, have a shower and at least some of the moist air will escape into the house, boil a kettle, Potatoes, or pasta, and the relative humidity will rise.

There is no barrier to [water] vapour entering the roof void, as above make sure the loft hatch fits well. but the vapour will permeate into the loft space and it gets through the plaster, or plasterboard,

The loft is insulated meaning very, very little heat to get into the loft, the loft is then "cold" in "cold" air water MUST precipitate out and will form condensation on any surface.

Every time you open the loft hatch, warm air will flood into a cold space, warm air rises, quickly, plus moisture, result is that the Relative humidity rises in the loft, and the cycle runs and runs,

Plenty of ventilation as "Why not Indeed" above.

Ken.

 

[mikey90]

Thank you both. I have ordered some lap vents and will see how it goes.

 

[mad.phase]

Thank you both. I have ordered some lap vents and will see how it goes.

Appreciate this is from many-moons ago @mikey90 , but did your vents work? I have recently 'serviced' my loft, as it needed deeper insulation to help keep the house warm / reduce heat rising into it ... and I have added lap vents to the felt, but the sensor still says it's around 80% humidity!

Did your similar approach help reduce the humidity like you hoped?

And would anyone have any suggestions as to why mine is still at a high humidity?

Thanks for any replies.

 

[JohnD]

And would anyone have any suggestions

I'm glad you asked that.

CONDENSATION IN LOFT (ANSWERS)

This seems to be the time of year for condensation in lofts, so instead of replying to lots of queries I though I would write this "one-size-fits-all" post. Please also see //www.diynot.com/wiki/Building%3ACondensation-in-Houses Condensation in lofts occurs when the amount of water...

www.diynot.com

Why is there an extractor and duct seemingly going nowhere?

 

[Why Not Indeed]

That could be my loft. Same construction, same felt under the tiles, same grey block gable wall, same insulation. 11°C, 89% RH.
Do you actually have a problem with damp/mould in there?
I assume that extractor vents to the outside wall?
If you have a problem with moisture condensing on the inside of the extractor tubing and running down into the bathroom, lag it.

 

[mad.phase]

I'm glad you asked that.

CONDENSATION IN LOFT (ANSWERS)

This seems to be the time of year for condensation in lofts, so instead of replying to lots of queries I though I would write this "one-size-fits-all" post. Please also see //www.diynot.com/wiki/Building%3ACondensation-in-Houses Condensation in lofts occurs when the amount of water...

www.diynot.com

Why is there an extractor and duct seemingly going nowhere?

Thanks for that link - I’ll give it a read. It’s connected to the bathroom, then out to the side of the house. I had a stronger fan fitted, because of issues in the bathroom

 

[mad.phase]

That could be my loft. Same construction, same felt under the tiles, same grey block gable wall, same insulation. 11°C, 89% RH.
Do you actually have a problem with damp/mould in there?
I assume that extractor vents to the outside wall?
If you have a problem with moisture condensing on the inside of the extractor tubing and running down into the bathroom, lag it.

There are a few oddities up there … maybe it is yours? That side, no I currently don’t - but am looking to prevent that, as I have read people’s lofts have seen damp due to humidity

 

[Owen74]

Mine is very similar to yours @mad.phase - Often hits 90% RH with temp around 11 C.

Yet even with those readings I can sometimes find the loft dry without any condensation. Other days there's loads of condensation (mostly around plastic roof valleys). It's very random. The condensation turned to light mould on some of the woodwork too.

I have loads of lap vents and plenty of ventilation, and insulation pulled back from soffits. Done everything I can think of. The only thing I have left is to install a vapour barrier and reinsulate the whole ceiling.

Not sure if it makes a difference but my condensation situation got worse after we had an extension built, which made the loft an 'L' shape. Don't know if maybe certain shape lofts are worse for this than others, in terms of air flow.

 

[mad.phase]

Mine is very similar to yours @mad.phase - Often hits 90% RH with temp around 11 C.

Yet even with those readings I can sometimes find the loft dry without any condensation. Other days there's loads of condensation (mostly around plastic roof valleys). It's very random. The condensation turned to light mould on some of the woodwork too.

I have loads of lap vents and plenty of ventilation, and insulation pulled back from soffits. Done everything I can think of. The only thing I have left is to install a vapour barrier and reinsulate the whole ceiling.

Not sure if it makes a difference but my condensation situation got worse after we had an extension built, which made the loft an 'L' shape. Don't know if maybe certain shape lofts are worse for this than others, in terms of air flow.

Thanks for the reply. Sounds very similar to mine. If I get chance at the weekend I’m going up to double check everything f again. At the mo’ just checking the figures shown on my phone. Annoying is the word!