Loft humidity in Victorian house

2 Jan 2022
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United Kingdom
Hi there,

We moved into a Victorian house in the summer, however a few weeks ago when up in the loft I noticed some mold growing on some of the rafters against the roof, and the rafters themselves felt a bit damp to the touch.

The roof it fairly new (approx 10 years old), felted and not leaking. The loft space is very well insulated - for example during some recent cold weather the temperature in the house was around 20oC, in the loft it was ~7oC. The loft in general seems to stay at around 2 degrees higher than the outside temperature. The loft appears fairly well ventilated (when windy can feel air movement), and we have extractor fans for the 2 upstairs bathrooms. The rest of the house has a RH of around 55% (range of 50-60%).

I will admit, we were a bit foolish when we first moved in until recently - we were drying some washing indoors, and we were a bit lax in using the extractor fans in the bathrooms during showers, etc. This has been stopped now.

Now my questions are the following:

1) In a well ventilated loft, should the indoor humidity approximately match the outside humidity? Unless there is heavy rain, the loft humidity appears to be around 5-10% higher than outdoors. As mentioned above, temperature in the loft is typically around 2oC higher than the outside.

2) Does relative humidity really matter, or is dew point more relevant? I keep reading that the maximum RH should be no more than 60% else mold can grow. However, if the temperature is 12oC and the RH is 71% then the dew point is 7oC, so as long as the loft is higher than 7 (in that set of conditions), no condensation will occur?

3) Essentially i'm trying to work out if I have a bigger problem. I have put a desiccant dehumidifier in the loft, and that takes out plenty of water, and the rafters are now dry to the touch and no new mold has grown. Is this just a one off situation that has been rectified by our changed habits (washing indoors, extractor fans), or am I really aiming to get to a loft humidity of 60%?

4) Similarly, we have one room in the house (a 1960s extension) that has 3 external walls, and is always a bit colder than the rest of the house. It seems to sit almost constantly at 60% RH and around 18-19oC. I can use a dehumidifier and bring the RH down to 40-50%, but over the next 12 hours, it gets back to 60%. Is this a problem, if the dew point is calculated as 12oC?

Thanks for the advice!
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you say the roof is felted. Is it felt or porous membrane? I don't see you say how it is ventilated. Look at the eaves in the daytime. Can you see daylight through the gaps? Pull the insulation away from the eaves and fold the ends under.

Water vapour is lighter than air so will rise through the house until it either escapes, is blocked, or condenses on a cold surface.

If you have any holes in your ceilings, for example around pipes and downlighters, warm humid air will rise into the loft.

If the chimneystacks are wet, rainwater will enter the loft.

If you have any water tanks in the loft, check that they have close-fitting plastic lids and are not warm.

See also
are the fans in the house fully vented to the outside off the house directly rather than into the loft space hoping the air flow will cure the problem ??
To answer the questions:

- Roof is half felted half membrane (part of a newer extension from around 10 years ago). Seems they maybe reused the old felt when they replaced the roof as it seems in very good condition. We only have condensation issues around the felted area (not the newer membrane section of the loft/roof).
- Can't see any daylight during the daytime in the loft - will pull back more of the insulation and have another look.
- Chimney stacks are dry - no evidence of leaking roof.
- Water tanks are new (plastic) with tight lids and wrapped in insulation.
- There are uplighters in the 2 bathrooms. Used to be halogen but i've since changed them to cold LEDs.
- Extractor fans extract to outside (one directly through side wall, other is through a pipe that appears intact that goes through the roof to a vent in the undercloaking).
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is there anything to stop steam from the bathrooms gushing up through the holes in the ceilings?

In theory absolutely nothing - the LED lights do fit well, but yes, they are vented into the loft. I'm thinking that as they are no-longer halogen, if it might be possible to cover the vent holes?

Also, upon pulling the insulation back from the eaves, still no daylight - I tried in multiple places and it seems as though the eaves are sealed up completely. There was multiple layers of insulation I had to pull back - some of the stuff in the eaves looked pretty old...

So it would appear as though there is no ventilation at all from the outside - I must have mistaken the cold temperature for what seemed like fresh air. I assume I would need to put vents in under the eaves to aid ventilation? There roof itself has no gables - it's sloped in all 4 directions.

Edit - what do people think of lap vents? They seem fairly easy to install from a few videos i've seen online?

As a temporary measure I have the desiccant dehumidifier running in the loft again to dry things out, and also a normal dehumidifier that is used around the house (particularly in the bathrooms following showers/baths).
Last edited:
The warmer the air the more moisture it supports so inside can support higher humidity .
Hi all,

Just an update and a question. I've installed lapvents but I don't think they are doing enough, you can feel a very slight breeze and although the humidity has come down a bit it's still not great. I've also installed loft lids over the uplighters in the bathrooms and added more insulation. This also appears to have improved things slightly.

Next stop is installing soffit vents, as they don't seem to be present in the current fascias. We've waiting on a couple of quotes, but one of the roofers said that venting won't help matters. Because the current roof has a impermeable black felt condensation will always happen. Their suggestion would be to take the whole roof up, replace with a breathable membrane, then put the slate roof back on again. This to me seems like massive overkill, but like to see what people thought?

also a normal dehumidifier that is used around the house (particularly in the bathrooms following showers/baths).

That should not be necessary, have you not got a timed run extract fan in there? A simple fan extract to outside, should quickly deal with any moisture in a bathroom, making a dehumidifier unnecessary. Are there perhaps other sources of moisture - clothes drying in the house, boiling pans without lids.
Did you manage to solve this?


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