Loft Condensation - to dehumidify or not?

Cola133

We moved into our house December 2024, by January we noticed that the felt in the loft was absolutely dripping with condensation, almost like it was raining in the loft. We've installed loads of lap vents and the previous owners installed 2 tile vents. I can see daylight in the eaves areas so these aren't blocked. The roof is fine through warmer temps, but as soon as it gets really cold, it becomes terribly wet and the main structural beams begin to get mouldy, which is a huge worry.

Every roofer we've had out wants to of course replace the whole roof. The house was built in 1980. I can't afford 10k at the moment and from what i've seen online a lot of people who have replaced their roof, still have the same problem.

We do have a bathroom extractor fan that goes up into the loft and out through the eaves, but all looks fine. 

In preparation for the coming Autumn/Winter 2025, I'm wondering what more can be done. I was thinking about a dehumidifier, but I've seen the comments about dehumidifying the outside world too, but I wonder if anyone has actually tried it in their damp oft and if it made a difference?

Thanks for any help.

moneysaver1978

do you have any insulation in the loft? Your bathroom extractor should be vented to outside of the house not into the loft.

WIAWSNB

Cola133 said:

We moved into our house December 2024, by January we noticed that the felt in the loft was absolutely dripping with condensation, almost like it was raining in the loft. We've installed loads of lap vents and the previous owners installed 2 tile vents. I can see daylight in the eaves areas so these aren't blocked. The roof is fine through warmer temps, but as soon as it gets really cold, it becomes terribly wet and the main structural beams begin to get mouldy, which is a huge worry.
Every roofer we've had out wants to of course replace the whole roof. The house was built in 1980. I can't afford 10k at the moment and from what i've seen online a lot of people who have replaced their roof, still have the same problem.
We do have a bathroom extractor fan that goes up into the loft and out through the eaves, but all looks fine. 
In preparation for the coming Autumn/Winter 2025, I'm wondering what more can be done. I was thinking about a dehumidifier, but I've seen the comments about dehumidifying the outside world too, but I wonder if anyone has actually tried it in their damp oft and if it made a difference?
Thanks for any help.

That level of condensation is very strange, especially when both you and previous owners have taken the correct steps to try and control it. Some condie is pretty normal in the coldest and least windy days, but yours sounds waaay excessive.

Two things generally cause visible condensation in a loft space. The first is lack of ventilation, and this should be 'through', with a noticeable draft being felt on breezy days up there. So, confirm that there are good vents fitted at both ends to provide this crossflow. 

The other cause is an excessive amount of moisture getting up from the house to the loft space, such as you'd get if, say, the hatch was left open, or was very leaky. Normal plasterboard ceilings may appear to be watertight, but like many building materials, they do allow some vapour to pass through. So, absolutely ensure that you have good quality loft insulation up there to reduce this. 

Double check that any extractor fan does not leak, but you reckon yours is ok? 

Apart from that, I'm struggling.

I'm not sure what folk would recommend as the best way to vapour-barrier the ceiling - I wonder if there is a way that won't cause other issues? 

FreeBear

WIAWSNB said: I'm not sure what folk would recommend as the best way to vapour-barrier the ceiling - I wonder if there is a way that won't cause other issues?

A vapour barrier is not usually installed unless you have high humidity and extremely low external temperatures. Retrofitting one in a 1980s build would be a very costly undertaking.

I do wonder if @Cola133 has an impermeable felt under the tiles - This would be prone to condensation unlike the breathable "felt" in current use. Stripping the roof to fit a new membrane is also going to be expensive (£5K-10K), and probably not recommended unless the original felt is breaking up.

grumpy_codger

moneysaver1978 said:

do you have any insulation in the loft? Your bathroom extractor should be vented to outside of the house not into the loft.

Cola133 said:

...

We do have a bathroom extractor fan that goes up into the loft and out through the eaves, but all looks fine. 

...

.....................

grumpy_codger

WIAWSNB said:

Cola133 said:

We moved into our house December 2024, by January we noticed that the felt in the loft was absolutely dripping with condensation, almost like it was raining in the loft. We've installed loads of lap vents and the previous owners installed 2 tile vents. I can see daylight in the eaves areas so these aren't blocked. The roof is fine through warmer temps, but as soon as it gets really cold, it becomes terribly wet and the main structural beams begin to get mouldy, which is a huge worry.
Every roofer we've had out wants to of course replace the whole roof. The house was built in 1980. I can't afford 10k at the moment and from what i've seen online a lot of people who have replaced their roof, still have the same problem.
We do have a bathroom extractor fan that goes up into the loft and out through the eaves, but all looks fine. 
In preparation for the coming Autumn/Winter 2025, I'm wondering what more can be done. I was thinking about a dehumidifier, but I've seen the comments about dehumidifying the outside world too, but I wonder if anyone has actually tried it in their damp oft and if it made a difference?
Thanks for any help.

...

The other cause is an excessive amount of moisture getting up from the house to the loft space, such as you'd get if, say, the hatch was left open, or was very leaky. Normal plasterboard ceilings may appear to be watertight, but like many building materials, they do allow some vapour to pass through. So, absolutely ensure that you have good quality loft insulation up there to reduce this. ...

Another way for the humid air to the loft are light fitting.

And I don't think that soft loft insulation stops vapour, quite the opposite. It makes the loft colder that increases condensation if humid air finds its way to the loft.

Albermarle

FreeBear said:

WIAWSNB said: I'm not sure what folk would recommend as the best way to vapour-barrier the ceiling - I wonder if there is a way that won't cause other issues?

A vapour barrier is not usually installed unless you have high humidity and extremely low external temperatures. Retrofitting one in a 1980s build would be a very costly undertaking.

I do wonder if @Cola133 has an impermeable felt under the tiles - This would be prone to condensation unlike the breathable "felt" in current use. Stripping the roof to fit a new membrane is also going to be expensive (£5K-10K), and probably not recommended unless the original felt is breaking up.

As many older houses manage ok with out any felt or membrane under the tiles at all.
I wonder whether just removing the current felt might be a cheaper option?
Not an expert, just a suggestion.

grumpy_codger

My house is with felt and the loft is very dry, although insulation is not the best - just about 10cm between the joists under good floorboards. The hatch and the light fittings are reliably sealed.

Felt or membrane is there for a reason - it reduces the risk of small (and big) leaks.

ETA: kitchen extractor fan is worth checking too. If installed by some cowboy it can pump air partly into the cavity (and ultimately to the loft).

WIAWSNB

My mention of the vapour barrier was a pondering of whether something like that could be used directly against the ceiling p'board - on the loft side - before insulation is then returned on top? Ie, a method of preventing or reducing moisture vapour from percolating through into the loft from the house in the first place?

But, would this instead cause issues of damp at the p'board level - would it trap moisture there? No idea.

Bottom line - if there is this huge amount of condie, the moisture is coming from somewhere. The only two places I can think of are, from the house below, or from a roof leak. 

Assuming it isn't just a case of ineffectual ventilation - there may be all these 'vents' fitted, but if they ain't providing the right flow, then they won't work. 

Ventilating is the key; if you removed a row of slates from each side, then that loft would be dry (if it wasn't raining...)

rob7475

You need the stop the heat from your home hitting the underside of your roof. It sounds like you have enough ventilation up there so I'd be checking the insulation is sufficient, especially around the edges and on the loft hatch etc. When the weather is cold outside, it doesn't take much warm air hitting the underside of your roof to create a lot of condensation.

Do you have lots of condensation on your windows during winter? You may need to address the humidity in the home as well as the loft space. If that's the case, look at getting a dehumidifier for the house or something like MVHR / PIV

Cola133

@moneysaver1978 The bathroom extractor is in the bathroom ceiling with a tube that goes up into the loft and out through the eaves. I can see the grill vent outside my bathroom window. 

@WIAWSNB @FreeBear @grumpy_codger

The roof has the original impenetrable felt membrane. It looks in good condition.  We've put in lap vents in almost all the places where the felt overlaps, right down to the eaves. The original owners had 2 roof tile vents put in.

@rob7475 I do have a lot of condensation in the house during the winter. The previous owner installed a PIV in the loft to the landing ceiling, but even on full power it doesn't stop the condensation on the windows. The bathroom is fitted with recessed spotlights and, what looks like, a pvc ceiling. The loft hatch is wooden board with no insulation. We have a metal loft ladder which overhangs the door a bit, so I'm not sure about putting insulation on it?

Condensation in the house and loft was obviously a problem for the previous owners. The loft is only part boarded, in front of the gas boiler which we've just had put up there, in front of solar meters and just a little down the centre.

The insulation does look a little old, so maybe I should get that all redone? What type of tradesman would do that so it's done properly? I fear a general builder will just put the insulation down as quick as possible without thinking about ventilation.

Thank you everyone for your ongoing help and advice.