Condensation in loft - wall extractor poss cause?

[shorn]

Just to update the progress. Been up in the loft and cleared all the eaves. Insulation now stops at the top of the wall cavity and there is a clear view from the loft to the soffit vents. (Can see daylight all round). I was mistaken about the mice and chewed up insulation. It is actually cavity wall insulation that has spilled up over the top of the cavity. Felt a bit silly when I realised that. Anyhow, this has all been cleared from the soffits as well (it was obstructing the vents in some places). I've also cleared everything from the loft so it's empty bar a few items.

In addition to this, I've fitted 20 lap vents and have another 20 coming soon. I've put a dehumidifier up there to dry it out. It's doing a good job, have pulled out several litres of water and the underside of the felt is almost bone dry. I figure it will be best to dry it out completely before I can tell whether or not the lap vents are doing a job.
I've also removed 12 bin bags of loft insulation that was wet. The loft still has over 270mm of insulation, two layers, cross laid.

The plan is now get it dry and I'm going to place plastic dust sheets over the exposed insulation so if it does condensate it doesn't ruin what's there. I'll then see how the lap vents do. With a bit of luck they'll solve it, but if not, I'll then look to reroute the bathroom extractor to either a tile vent at least away from the soffit. Next would be to seal the hatch better. (Hatch is already insulated but could replace with a better sealed one).

After that I'd be looking at additional tile vents I guess. I'm wondering about airflow from outside in the loft. We have a very steep pitch, would that it less likely to get airflow in?

Also, wondering it it would be possible to rig a fan up there in order to help push air about?

 

[JohnD]

if the eaves are open both sides, this should generate a through flow of cold, dry, air that will be better than your fan.

if you have a throughflow, then the dehumidifier will be trying to dehumidify the world. They are only effective in a closed room.

The plastic sheeting may cause condensation if any warm, humid air from the house reaches it and cannot escape.

 

[shorn]

if the eaves are open both sides, this should generate a through flow of cold, dry, air that will be better than your fan.

if you have a throughflow, then the dehumidifier will be trying to dehumidify the world. They are only effective in a closed room.

The plastic sheeting may cause condensation if any warm, humid air from the house reaches it and cannot escape.

I did wonder whether the plastic sheeting might cause an issue. With regards to the throughflow, I'm just not that sure we get much. The pitch of our roof runs opposite direction to the two neighbours and I wonder if any decent throughflow is being cut down by their proximity.

I'm switching the dehumidifier off after today. Like I mentioned, the felt is now almost bone dry. Hopefully some of this will be down to the extra ventilation we now have and the dehumidifier has just helped speed it along.

 

[Leofric]

The plastic sheeting may cause condensation if any warm, humid air from the house reaches it and cannot escape.

Yes, any moist air from below should be allowed to escape into the ventilated roof space and not be trapped in the insulation to condense on the underside of cold plastic sheeting.

 

[plastic_peanut]

I recently insulated my loft using Knauf spaceblanket which is encapsulated in plastic.
It was rolled over the existing stuff at right angles.
Went in the loft over the weekend to return the Christmas decs and was astounded to see a large amount of condensation on the outside and inside of the plastic encapsulation. Some of the insulation was really saturated.

Anyhow, I removed all the plastic sheeting from the insulation and then cleared all the eaves of old insulation, wasps nests etc

The soffits don't appear to be vented, is it likely there will be sufficient air reaching the loft space? Should I consider adding vents?

The bathroom has an extractor with the duct running vertically to a roof tile but it doesn’t appear to be leaking. The house has down lighters so I wondered if they weren’t sealing against the ceiling.

The loft hatch is large and not insulated, this is something I will be addressing.

Any suggestions please?

 

[JohnD]

ventilate the loft better.

As there are holes in the ceilings, for example for downlighters, you have to prevent warm air from the house getting into the loft.

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

 

[plastic_peanut]

Hi John,
Thanks for the link, I have read that already but not hopefully helpful.
Bit of an idiots guide to the causes but doesn’t really provide detailed solutions.

The suggestion of sealing holes, are there any recommendations for ensuring the downlighters are sealed against the ceiling? Obviously they still need to be removed for bulb replacement etc

Any more detail on soffit vents, any recommendations of products would be helpful.

Many Thanks

 

[JohnD]

You have established that you have ceiling holes and no soffit vents. So the probable causes.

you can make a box out of plasterboard (which is fireproof, so safer than wood) and rest it on the ceiling from above the lamps. Or you can buy ready-made smokehoods for downlighters. They often have an intumescent lining.

You can drill holes in soffits with a hole-cutter or large spade drill.

Any holes in the bathroom ceiling (including holes for pipes) will be especially pernicious because of the warm steam.

You should be able to feel a cool breeze in the loft whenever there is any wind outside. it will blow in one side and out the other.

 

[plastic_peanut]

Thanks John, went up to check the loft today after a week since I did the improvements. No noticeable damp which is an improvement but certainly something to keep an eye on.

I think I was right to be apprehensive about the plastic on the encapsulated insulation. I am happier now it has been removed.
Will likely look at the soffit venting if the condensation returns.
Thanks again mate