Last winter I added a load of extra loft insulation. Made sure not to block the soffit vents.At the same time as adding insulation I had the cavity walls filled. Went up to the loft last night to find damp and condensation dripping off the inside of the roof (on the felt) . This seems to happen when the outdoor temperature goes sub zero, and we have the heating on. Does anyone have any solutions? I thought about adding more vents in the loft, which ones are best. would prefer to avoid a dehumidifier (noisy and expensive to run). What about adding a waterproof membrane underneah the insulation?

Are there any water tanks up there?

What about adding a waterproof membrane underneah the insulation?

Don’t do this! (edit: I mean don’t do just this) more air vents is what you need.

You’ve got three options for more air vents.
i) Ones that fit in the soffit boards
ii) Replace a couple of tiles with tile vents
iii) Ventilation bricks, by drilling a whole through an outside wall.

What is best depends on your house and situation.

What about adding a waterproof membrane underneah the insulation?

I suspect the issue isn’t humid air travelling through the insulation and then condensing inside the roof, it’s more humid air already int he roof or coming in from other sources.

You can get the dessicant type dehumidifiers which don’#t have compressors so are very quiet and only cost pennies a week to run.

I’d be wary about further sealing the roof void with waterproof membranes – maybe look at some extra ventilation (extractor fan on a timer that’s vented to outside and comes on for 10 minutes an hour?)

Nowt to do with all that lovely hot air you just let into the space ?

Hi. – thanks for advice, there’s no water tank up there. The answer does seem to be ventilation, the damp only appears when there’s no breeze outside to push air through the loft. This makes me wonder whether adding more vents will do the job?

It’s a general rule that if adding more insulation you’ll need more vents to stop condensation. – I’ve not heard of anyone fitting a powered extractor though I’m not a builder (just experienced DIY level 🙂 )

If you can fit them yourselves then it’s reasonably cheap, so maybe try it and see?

1. Increase ventilation to roof space.
2. Vapour barrier under insulation.
3. If you have extract fans to you bathroom/kitchen using them helps a lot.

The priority is probably in the order 3, 1, 2

Most people under estimate the quantities of water vapour that everyday living/breathing produce.

Ridge vents will help a lot what ever sunlight there is will help drive convection within the space.

We bought an old terrace house where the loft had been boarded out.
The building regs guy from the council came and told us to remove the boarding and insulation from the roof spars as no tile vents had been fitted and could cause damp in the roof.With the boarding the loft was draught free ,now on a windy day you can feel the draughts through the loft.