Loft humidity after insulation and vents added

[mrrusty]

93% at 9.9 degrees in the loft and 84% outside at 7 degrees indicates to me a lack of ventilation

You might be overlooking the fact that to actually take the readings you have to open the loft hatch, which lets a load of nice warm humid house air rise up in to the loft, and the cold air in the loft descend down in to the house.

With decent ventilation and good insulation the internal and external air temp should be the same.

It could be, but the danger is then thermal inertia. If the loft and outside has tended to equilibrium, and if that is a highly saturated situation because it is raining outside, if the outside suddenly cools, it is possible that surfaces will cool below the dewpoint faster than the ventilation can completely reduce the water vapour back down to external conditions

In ships holds, where much of my knowledge on this topic originates, you use forced/controlled ventilation to keep both surfaces and cargo away from the dewpoint. What we see here is akin to "ships sweat" where a ship travels in to a cold sea and the hold air touching the internal sides of the ship potentially gets cooled below the dewpoint. Of course, it's a different kettle of poisson now that most cargo is containerised, but it's still an issue.

 

[Cockneyuk]

You might be overlooking the fact that to actually take the readings you have to open the loft hatch, which lets a load of nice warm humid house air rise up in to the loft, and the cold air in the loft descend down in to the house.

no it’s a dedicated sensor up there, I read it remotely.

 

[Cockneyuk]

93% at 9.9 degrees in the loft and 84% outside at 7 degrees indicates to me a lack of ventilation, hence the higher temperature in the loft. With decent ventilation and good insulation the internal and external air temp should be the same.

The higher internal humidity may indicate a water vapour leak. Is there any chance a bathroom extractor hose could have been damaged or disconnected?

No pipes venting up there. Two old disconnected lead pipes but I’ve cut them and just need to bring them down. One went through the ceiling and down through a small bedroom and the others went nowhere.

It’s only changed from a safe dry storage space to this since the insulation and vents were added.

This house has been neglected for possibility forty years. The old contract of the last tenant was sparse and he never let anyone in to do anything except when the combi was put in, 2012 I believe. We took it on and are finding all the issues he ignored.

 

[Cockneyuk]

(As an amateur) I'm confused - is this a ventilated cold loft or an unventilated warm loft? It looks like a weird mixture of the two at the moment ..

As am I. Which I why I’m asking for help to understand as I will have to move if they have turned what was our storage space into a cold loft where everything will get ruined as soon as it gets cold, that is why I need to understand as the loft hatch was moved specifically for us to access and store stuff.

 

[Cockneyuk]

(As an amateur) I'm confused - is this a ventilated cold loft or an unventilated warm loft? It looks like a weird mixture of the two at the moment ..

As am I. Which I why I’m asking for help to understand as I will have to move if they have turned what was our storage space into a cold loft where everything will get ruined as soon as it gets cold, that is why I need to understand as the loft hatch was moved specifically for us to access and

What vapour barrier?

The off white fabric like stuff laid across the rafters. If that’s not a vapour barrier that’s on me but that’s what it’s been called I’m just repeating what was discussed by the people who were up there.

 

[Cockneyuk]

so call loft temp 10 degrees. saturated vapour density at 10 degrees = 9.21. X 93% = 8.56 which is the saturated vapour density at about 8 degrees. Any surface colder than that will have liquid condensation.

If it's 7 degrees outside, quite likely the inner roof surface is approaching 8 degrees, because a heat gradient of only 9.9 down to 7 isn't going to keep it much above the outside temp. Inner roof will be at the dew point for the air in the loft = internal condensation.

Thank you, I worked out the Vapour m3 yesterday but that’s a handy table.
So am I right in thinking they have taken what was a warm unvented loft and sort of changed it to a cold vented loft ?