Condensation in loft - over fascia vents

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slaval

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Hi, would be grateful for some advice. Noticed recently that the north facing side of the roof has quite a bit of condensation on the underside of the waterproof membrane. I've installed lap vents the other day and but so far thats not cured the problem. I did notice that there is maybe too much overlap of the membrane plus it is also pretty taught so I don't know if the lap vents will be doing their job properly. Some of the loft insulation closest to the eaves is quite damp so I am not sure if I should just replace that at this stage. We do have over fascia vents coupled with felt support trays and I don't know if this set up is obstructing the air flow. The support trays have a curved profile on the outer edge that come around over the vents. Having said that, I think its been installed correctly... just don't seem to be getting good air flow. I could install soffit vents also.. is this good idea seeing as I already have the over fascia vents?

Any advice would be appreciated. Thanks.
 
How thick is the insulation?
Do you have any extractor vents or soil vents going in to the loft area?
Do you have down lighters in to the rooms below?
 
Vents either end to get air flow.
More insulation.
Warm moist air hitting the membrane so maybe you have poor sealing loft hatch or not enough insulation.
 
the insulation is about 30 cm - depth of rafters plus another layer on top, although I have pulled some back near the eaves for now. The eaves ventilation is not obstructed though but I am curious about the felt support tray and the way it curves around the vent - seems like that would obstruct the air flow. The roof was redone just 2 years ago and I think the membrane is taughter than it should be which isn't helping.
 
www.diynot.com

CONDENSATION IN LOFT (ANSWERS)

This seems to be the time of year for condensation in lofts, so instead of replying to lots of queries I though I would write this "one-size-fits-all" post. Please also see //www.diynot.com/wiki/Building%3ACondensation-in-Houses Condensation in lofts occurs when the amount of water...
www.diynot.com www.diynot.com
 
I have a nasty feeling that the over eaves vents have been installed wrongly. I have just been up in the loft, as close to the eaves as I can get, turned the lights out and I can't see any daylight at all... surely I should be able to. I am thinking that I will just go around and install soffit ventilation. A shame because the over eaves is more aesthetic, but I don't see any alternative short of removing tiles and fixing properly. Better that the timbers don't rot. In theory though, is there any issue with having over fascia vents AND soffit vents?
 
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This time of year, the "outside air" is 90%+ humidity so a rise of just a few degrees will create substantial moisture release. You can't stop the outside air so you must control the inside air flow. Allowing more air means higher volumes of cold damp air so unless you stop the heat loss, increasing airflow just adds to the problem.
 
old duffer said:
This time of year, the "outside air" is 90%+ humidity so a rise of just a few degrees will create substantial moisture release. You can't stop the outside air so you must control the inside air flow. Allowing more air means higher volumes of cold damp air so unless you stop the heat loss, increasing airflow just adds to the problem.
Click to expand...

Ah.

Sorry mate you have that AaF.

Increasing the temperature of cold damp air turns it into warm dry air.

Relative humidity percentage is not a measure of amount of water content.
 
JohnD said:
Ah.

Sorry mate you have that AaF.

Increasing the temperature of cold damp air turns it into warm dry air.

Relative humidity percentage is not a measure of amount of water content.
Click to expand...
So can you explain why warm air hitting cold air seems to release water if in actual fact its removing the water...and then where does the water go..I'm confused now

So my Amazon sensors that show temp and humidity don't actually provide any benefit in helping me tackle condensation because they don't measure the right kind of humidity? What is the right kind of humidity to measure and how do I do that?

Right now, an outdoor sensor is showing 6.7 degrees and 96% humidity...what does that actually mean then or does it not mean anything?
Help appreciated on this, I've been getting this wrong for years..I feel so stupid now
 
You could see water condensing from the warm air when it hits the cold air - same way a cold window fogs up when you breathe on it.

See attached chart.
99% relative humidity at 5 degrees would be barely 40% relative humidity at 20 degrees.
It's the same amount of water in the air by volume, but warm air can hold more of it before it condenses.

This is why it's hard to prevent condensation forming on a very cold surface - such as a single-glazed window. If the window pane is (say) a 5 degree surface, you'd need to reduce the interior humidity below 40% if the interior air temperature were 20 degrees - which is quite difficult.

Here's a dew point calculator:
waterleak.co.uk

Dew Point Calculator - Simple And Fast Version

Our simple Dew Point Calculator will help you understand what the surface temperature is at which condensation will form on surfaces in your home.
waterleak.co.uk waterleak.co.uk
 

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