UKH

When we bought our house the home report mentioned condensation in the loft, so we were keeping an eye on it. We got the loft boarded and upon inspection everything was fine and dry. The loft was already partially boarded when we bought, the new boards were just central (not complete coverage) and put on stilts so the insulation should be even less compressed than it was, plus a new insulated loft hatch. We are very careful with humidity in the house (normally in the low 50%'s). 

We went up there late last year during the cold snap and the roof rafters were soaking wet (literally), dripping on all our stuff which had gone mouldy. Humidity around 99%. 

We got 4 vents put in and ran a dehumidifier for several weeks straight, pulling out litre after litre of water. The boards dried for the most part, largely due to the increased air flow, but the humidity never dropped below 70ish%. We have a wireless sensor which outputs graphs of all the temp/humidity data, very interesting!

We've decided to close it up, turn off the dehumidifier and see what happens. The humidity seems to have gone up to around 80%, but the thing is outside it's pouring with rain and about 85% humidity. I wouldn't expect that air blowing in through the vents to have much of a drying effect. 

The question is, what is a standard winter humidity/temp for the loft? With the vents in place, is 80% ok if the temperature equalises with the outside temp? Not sure what the target should be. 

Conditions outside: 5.0C/85%

Loft Conditions: 6.5C/82.3%

Home Conditions: 19.3C/53%

Should be ventilation all around the eaves leading in above insulation. Don't ram insulation into the eaves. You can get special things that hold the insulation down.

If you have sarking most water will drip back down on to that and run out if roof built properly. 

Also stop moisture going up there. Extractors in kitchen and bathroom. Seal ceiling cracks and if redoing any new ceilings on first floor use foil backed plaster board.

The science is basically warm air contains more moisture vapour than cold. Moisture vapour escapes, goes through pretty much anything other than aluminium and asphalt, into cold of loft and condenses into moisture on cold surfaces. 

Good ventilation, meaning a flow of air through the loft, removes the moisture. 

Others more expert will doubtless chime in, but one thing to consider straight away is that your humidity measurements are almost certainly relative humidity, and the point about RH is that for the same amount of moisture in the area, the RH rises rapidly as temp falls. Hence the dew point. 

So the air inside and outside the house could well have the same amount of moisture - it's just the RH rises as temp drops. 

Paul 

In reply to MG:

One thing to consider is where the moisture is coming from. Is hot moist (in total terms rather than RH) air from the house (bathroom, kitchen, bedroom etc) collecting in the now cold loft and condensing on everything?

Make quite sure the rain is not coming in.

Out of interest, current conditions sampled with me (not using calibrated devices though) - 

Outside 96% RH, 4.1°C, up from 92% earlier in day when 5.2° in sheltered spot protected from direct rain (heavy rain currently and been wet on and off all day).

Loft this evening 59% RH, 5.9°C, and for comparison outside unheated store is now 88% up from 82% day/early evening with a temp of 4.1° down from 5.6° earlier.

Can’t help on normal expectations for lofts in homes - been too long since I used to be involved in work relevant. Your indoors RH is fine - mine is currently ave of 42% today which I think is a bit low.

Are you a family of 6 all coming home and having hot showers in a bathroom directly under the loft space? Do you have a high extraction rate extractor in the bathroom? Do you open the window to vent the room when using the shower? Could be part of the issue.

There were some days during that cold snap when my well insulated and ventilated bouldering wall built in my garage was sopping with condensation running down the vapour barriers in the roof. I haven’t had that in the ten years we’ve owned the house.  

Two people, shower downstairs, everything extracted directly outside. Combi boiler so no water tank. 

Some of the houses on our estate had the wrong roof felt fitted. It was non breathable and caused the problems you describe. Sounds like you have the non breathable type.

https://www.amazon.co.uk/Vents-Prevents-Condensation-Attic-Ventilation/dp/B...

Try these, cheap and simple. Worked for me, though got 2 packs. 

These also worked in my house. I also bought an Evac 2650 dehumidifier which takes an unreal amount of moisture out the air. I still get condensation in the bedroom on a north facing gable end wall

Where is the water coming from?

• Is it warmer (and damper) air getting into the loft and cooling and condensing
• Is it a very minor leak which is dampening the roof "felt" (membrane) which evaporates during the warm and condenses when the loft cools

As far as I can see there are only 4 tactics

• Remove all loft insulation to warm the loft (obviously this is the "bad" non-option)
• Find and fix minor leaks that are making dampness in the "felt" that feeds condensation
• Get a roofer to check for leaks (like cracked tiles, ridge issues, whatever) and fix on find and while they are there add some vents. I had this done recently and by adding vents that freed up tiles to use to fix a few cracked ones, so on an old roof these go hand in hand
• Meticulously check for any drafts of warm air getting into the loft (loft hatch?) and draft exclude them somehow

Since you've already done some vents, it only leaves draft excluding or minor roof leaks?

I wouldn't worry about the specific number for humidity percentage in loft as much seeing if condensation is occurring?   If you slide hand up an overlap in "felt" do you feel dampness, are there any damp patches on loft insulation/boarding, is any roof timber damp?  If not then good.  Or... maybe your roof is elderly with many tiny leaks (like mine) and probably needs replacing     I'd love a new roof with modern membrane, correct number of vents and covered with Cwt y Bugail Welsh slate. Still waiting for that EuroMillions result

We had some condensation (and leak) issues when we moved in so put in a few vent tiles to help keep it dry ish. Unless it's super still outside giving no through flow then RH in the closed up attic will pretty much track the outside world (often 90+% this time of year). In reality we rarely get totally still weather and there will be leakage of heat (good) and water vapour (bad) from below. Mine has seen a lot of warm-wet air leakage from below through various ceiling holes/gaps over the last few years but in reality the increased ventilation has been sufficient to manage it adequately. In practical terms the vents have pretty much fixed it. I've never instrumented the attic but my garden RH meter is never below 85% at the moment. The air in the attic is the air outside (assuming well controlled leakage), if the surfaces cool fast as the weather changes they will condense, short of active control (heat/dehumidifier) that's just life, it will dry out so long as those conditions are the exception not the norm and you have adequate ventilation.

Check your bathroom fan is actually exhausting to the outside, it can shift a lot of water into a loft! I had a look at a mate's place during the cold snap, he had water running down a hallway wall, it was condensed shower steam getting through a missing bathroom light into the attic then condensing on Velux flashing and before tracking down. He hadn't been using the fan (noisy!).

jk

interested in this as similar issue in my house - we recently had roof replaced from leaky and draughty roof (Victorian and slate), we now has a roof that isn't leaking but also lacks any draughts so we now get loadsa condensation.

builders came back and put a dozen or so of the felt lap vents mentioned above in which seem to be helping albeit very very slowly. good first call rather than expense of doing anything from the outside straight away though.

i have draft proofed and insulated loft hatch much as i can, but interested in other folk's solutions to this. mine is a standard chipboard hatch, i stuck two layers of 50mm insulation board to mine (https://www.diy.com/departments/recticel-instafit-polyurethane-insulation-b...) and some draught excluder sticky stuff round the edges also.

any other tips on loft hatch insulation welcome!

for those using dehumidifiers up there - are you running extension cords up there? or did you get mains sockets installed? I'd like to avoid leaving the hatch open for prolonged periods to run a cord up as rising warm air is the problem as others have said, but a faff/cost involved to get a socket installed.

if the felt lap vents don't do the job in next few months then apparently next cheapest option is putting some holes in either the fascias or soffits (forget which is which).

really the builders should have vent tiles in when they replaced it all

as others have said i am very wary about causes within the house and use extractor fans and windows in shower rooms fastidiously.