hi folks. i’m hoping for some advice on two issues:
- high humidity inside the house (i’ve lost the data, but think it was 70-80% over winter)
- condensation in the cold loft space in the winter
i don’t know where to start, so any questions welcome!
Here are a few starter questions I’m sure would be useful for people to know.
Humidity:
How is the house constructed? What ventilation is there (Room vents, Trickle vents on windows etc)? Is it a longstanding problem or occured recently? Is it just high humidity readings or do you get problems related to humidity like mould or condensation? What are the likely sources of the humidity (showering, bathing, cooking, laundry are all pretty standard but any chance of water ingress or leaking) Do you have and use extractor fans in bathroom and kitchen? Do you dry clothes inside in the winter?
Loft space:
What is the construction of the roof/ loft? What’s the roof covering? Is there felt under the covering? Are there any roof vents? Is there insulation? Where and what type? Where is condensation appearing? Anything in the loft apart from insulation and stored suitcases?
In general relative humidity is a function of water content and temperature, condensation is just where the relative humidity exceeds 100% and water deposits on surfaces. Fixes will be to do with ventilation to carry away water vapour, reducing the sources of water vapour or increasing the Temperature in some fashion.
Re condensation the loft space - we had the same problem. Our house is a Victorian solid brick and slate construction semi. The roof liner is non breathable plastic.
Sorting the loft out felt like a marathon.
Our loft insulation project started with a rip out - it was a cold loft that had been boarded out as a makeshift office and had no ventilation.
After the rip out of plaster and timber board, then a good clear out of rubble and the removal of the completely, insufficient insulation, we were ready to reinsulate.
We chose sheepwool and laid 20cm - the changes in the loft were amazing - it all started to dry out and the climate up there felt completely different. This was in summer.
Roll on winter and getting the Christmas decorations down, I was filled with horror to see condensation pouring down the roof liner. 
The resolution!!
We had ensured ventilation at the eaves, but we had a problem with a stud wall in the bathroom that was open at the top in the loft under the insulation - this wall had the hot water pipes for the central heating and boiler in it . It was creating a 'stack effect ’ the heat from the central heating was rising up and hitting the cold air in the loft and condensating. We had humidity monitors up in the loft too - they read 93%. The temperature outside was about minus 5 degrees.
We had previously been advised to ensure the eaves ventilation and take the insulation up to the brickwork, but not over, then stuff the gaps and fill the cracks when insulating the loft. We ensured the ventilation , but we hadn’t done the latter .
Realising the stack effect from the stud wall we stuffed insulation as far down that as we could and sealed the top opening in the loft with PIR board and builders foam to prevent the heat escaping up into the loft to meet the cold air and condensate.
Additionally, we found that we needed to fill around the steel lintel support we have supporting the chimney, as well as the loft hatch and stuffed the insulation right up to the gable end wall and the adjoining wall with next door. The loft hatch was a particular problem too - we stuffed insulation around all the frame, used draught sealer tape to prevent heat from below getting through the ill fitting hatch, then bought a loft insulation pillow to sit on top. All of this seems to have prevented any stack effect and warm air escaping into the loft from the house below - we didn’t see any more condensation in the loft after we completed the work mentioned.
We have also been advised to top up the insulation by another 10cm and put a wind-shear breathable layer on top of the insulation. We will be fitting that soon and monitoring the loft again.
One final situation we are aware could be another factor to set up condensation in our loft is from next doors loft - we have a few gaps on the party wall between us - their house had a water tank with a broken thermostat - we could hear the tank bubbling away when we were up in the loft . This will have been creating heat and perhaps steam which may have come into our loft and condensated there -We understand the solution to this would be to have ridge vents - to allow any hot air to rise up and be pushed out to prevent the condensation setting up.
I hope sharing our experience helps you solve your loft problems at least. We felt the whole situation was a nightmare and were it not for the community here we wouldn’t have had a clue where, or how to solve this ourselves. The forum and expertise and experience of members here, has already saved us a lot of money.
Best Wishes
Carla
thanks for your detailed reply!
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You’re welcome. I know how much this community has helped me, so just passing it forward 
Quick update on this: we had the roof felt replaced from the old, cracked, unbreathable stuff, to much mor air permeable material. Seems to have done the job during the cold snaps!
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You bring me great hope!! We’re about to get our roof refelted too, for the same wet roof reasons.
What new roof liner did you installed?
I have Tvvek in my 2005 warm roof.
A problem with these non-woven felts is that they entangle bats so if you have, or may get, bats roosting in your roof, see
How about the Cromar Vent 3 classic roof felt ? We already have a couple of rolls of this.
I’m no expert on roofing felts but if it’s old type it should be ok. You could try brushing a sample with a stiff or wire brush to see what happens.
You could ask
About whether you need to take precautions and if so about the felt.
I remember that you live in a Victorian house so if you are in a bat area you may need to take precautions.
sorry, i don’t know what brand/product it is - its not printed on it either!
It’s not a question I ever imagined I’d be asking to be honest. I have no idea what I’m going to do with all the information I have gleaned about managing wet roof spaces!! It just shows you how much we go out of our way to learn when driven by a problem!! 
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The breather membrane should have a bit of writing to identify it and possibly indicate the British standard it complies with or performs to ( BS5534)