Mould in Loft. Recently insulated

[mcprinter]

1930s Semi
Went into the loft today.
Smelt a bit musty. Some of the rafters have mold on them.
Half the roof is just slate. The other is slate with felt (80's extension)
Most of the nails through the slate/felt had drips of water on. Some of the felt looks a bit wet.

Condensation?
I took a thermometer up with me but only for a few mins as was a bit peed off.
16ish deg and 50-60% humidity. Is the temp high?

I added loft insulation a few months ago. Some original 100 fibreglass. 200mm knauf
I went to the rafters but not touching the slates/felt.
Do I need to pull it back a bit?

Not had any problems in the past 20 years. Got rid of our carboard moving boxes when I did the insulation and all were dry.

More recent work is chimney taken down below roof level. Left open. I have sort of blocked the breast in the lounge but doubt it is air tight.

 

[flameport]

16C is far too high for a loft - in the winter it should be a similar temperature as outside.
Loft insulation should be at least 300mm in all areas.

The condensation is caused by warm air from the house below entering the loft. You need to find and seal all possible routes for air from the house, including around the loft hatch, light fittings, cables/pipes and so on.
Additional loft ventilation is probably needed as well, via soffit vents, ridge vents or in the gable walls if those are external.

 

[mcprinter]

Thanks for the reply.

It has been fine moisture/mold wise for years.

The only recent changes are
Topping up insulation to 300mm (Small boarded area with 270mm loft legs and chipboard)
Loft hatch insulated with kingspan. Was previously just ply.
Stack removal and roof slated where it used to be.

No external gable wall

Hot water fortic cylinder is in the loft and the jacket is a bit rubbish

 

[big-all]

you should have airflow at the eaves perhaps a 4" gap all round

 

[mcprinter]

16C is far too high for a loft - in the winter it should be a similar temperature as outside.
Loft insulation should be at least 300mm in all areas.

The condensation is caused by warm air from the house below entering the loft. You need to find and seal all possible routes for air from the house, including around the loft hatch, light fittings, cables/pipes and so on.
Additional loft ventilation is probably needed as well, via soffit vents, ridge vents or in the gable walls if those are external.

About 3 Weeks ago, I knocked blown plaster off of the chimney breast in an upstairs room.
There is a half inch 3 foot long gap in the ceiling so some air would be able to get into the loft.
Would that be enough?

 

[flameport]

Yes, a gap that size will allow a massive amount of air through. You need to fill it in.

It's the same area as a round hole 5 inches across in the ceiling.

 

[mcprinter]

Yes, a gap that size will allow a massive amount of air through. You need to fill it in.

It's the same area as a round hole 5 inches across in the ceiling.

Thanks

I have put some gaffer tape over it for now. Waiting for my plasterer to sort that and skim the room.
It is not completely open. Joist is half inch from chimney and has laths

 

[Eccles]

Not a solution but it helps to think carefully: Mould spores are always in the UK atmosphere but can only grow where they find moisture and little or no airflow that's why for example houses with suspended wooden floors always have a few air bricks judiciously placed.

 

[mcprinter]

Yes, a gap that size will allow a massive amount of air through. You need to fill it in.

It's the same area as a round hole 5 inches across in the ceiling.

That seems to be it. Taped over the gap last night
Loft 7 deg this morning (2 outside). 80% humidity - I guess it needs to dry out.
Checked the insulation. Pulled a couple of bits away from the eaves but most already have a good gap

Chimney stack removed in August
Loft insulated in September
Plaster removed from breast about 3 weeks ago. I went into the loft at the time and didn't notice any damp or smells.

Thank You Flameport. I would have never come to that conclusion.
Will keep an eye on it.

 

[JohnD]

See also https://www.diynot.com/diy/threads/condensation-in-loft-answers.160032/