Very Bad Loft Condensation - ideas on how to solve

[iamrcb]

So, it got cold; my annual soaking wet loft problem has returned.



When the temperature drops, one side of my roof, which faces north-west, gets severe condensation, which drips constantly and makes the insulation and stuff in the loft wet.

The roof covering is about 15 years old, the tiles in very good condition and it apparently has a non breathable roof membrane.

There is zero eaves ventilation, due to the construction of the roof and fascias there is no way air could get in. I have six roof tile vents, midway up the roof, three each side. Its a random rubble stone walled house, former chimneys both ends.

The humidity in the house is now under control and is normally at 50%. The shower is vented directly out to a roof tile vent and the flexible duct is covered in insulation.

Everytime this happens I've got to put a dehumidifier up there to dry it out.

How do I solve this?

Reduce the source of moisture? I've solved a previous condensation mould problem and now have a well ventilated house and humidity readings of 50% throughout. I did think about a damp proof membrane running between and over the joists with the loft insulation on top.

More ventilation? This will require roofing work, probably at least a couple of hundred pounds. Eaves/Ridge/Gable Ends?

Forced ventilation? Install a small fan to draw air through, turning it on when there is a risk of this happening (automatically?) - seems a bit OTT but it would at least guarantee some air movement.

I've got to get this sorted this year!

Edited by iamrcb on Sunday 18th December 21:18

Edited by iamrcb on Sunday 18th December 21:30

[iamrcb]

williamp said:

I believe its due to lack of through-flow of car. You say you have vents, but do they allow a flow of air? Static air will cause condensation as it will warm up, and condensate when it touches the cold tiles. having the air cold, with good insulation will stop this

The 6 x 80mm diameter vents are all obscured by a close-by purlin so they are not ideal. Generally the wind blows up the valley where I live, so the airflow from the front to back of the house is probably not as much.

I have around 200mm of the foil wrapped space blanket stuff. The loft is always pretty cold.

[iamrcb]

OldSkoolRS said:

I'm suffering similar, though not as bad as yours it seems. I've already added eaves ventilation (which did improve it some), plus I cleared the area below the condensation to help with air flow. I only get this issue in one quarter of my loft which is a North facing area and unfortunately the area above the bathroom. My latest course of action is to replace the 10 downlights in the bathroom as they allow moisture to rise into the loft above. I've just bought some IP44 rated ones that are completely sealed, but I don't need these in all positions, so I'm going to order some suggested in another thread I started recently that are air tight (even tilting lights). I hope to do this over the Christmas break as llike you I want to sort the problem out.

When getting the decorations down today I also had a look at my loft hatch and realised that this is far from air tight, so even if the bathroom door is opened after use to help disipate the moisture, it will pass up into the loft via the hatch, as would any moisture from the rest of the house. I'm going to change the loft catch from the type you push up to unlock, to one that you turn 180 degrees to open, then I can add a seal all the way round the hatch to make it airtight when it is closed.

Might be worth looking at your hatch and any downlights: I bought some fire rated ones from Screfix this week and found that they weren't airtight: I understand they work by some material expanding in the case of a fire, but that doesn't help stop the moisture issue, so I returned them to buy the ones from this link:

http://www.tlc-direct.co.uk/Type/_Downlights.Mains...

I've just got "normal" lights, plastic jobbies. I did put some rubber seal around the loft hatch, although I can't imagine that it needs to be air tight considering the scale of the problem

[iamrcb]

Simpo Two said:

That wall bottom left - looks like stone - is it cavity? Just wondered if moisture is getting through from outside.

How about insulating and lining? That would help to keep cold away from warm.

Its a solid (random rubble) stone exterior wall with an old, blocked off chimney in it. I used to fill the gap between top of wall and underside of roof with insulation, but pulled it out to try and get some airflow going, although there is no way in which air from outside coukd get in, so I may put it back.

Insulating between the rafters and lining (i.e. warm roof) would be expensive as its a fairly big roof.

Edited by iamrcb on Sunday 18th December 22:21

[iamrcb]

TooLateForAName said:

That felt looks quite tight?

I slipped lengths of plastic pipe between the layers of felt - that helped the ventilation quite a bit.

Too tight? Is it supposed to be loose? I didn't think so.

I will give the small diameter plastic pipe thing a go on one part of the loft to see if it makes a difference

This problem happens without fail every time it gets very cold outside.

I was surprised to read the comment about leaving this problem be. It just doesn't seem right. My ceilings don't get stained and my insulation is covered in plastic so it doesn't get sodden. I'd be much happier if it didn't happen.

[iamrcb]

I've been in the house 5 years. It's happened every year.

I have one third of the loft space boarded, which is mostly full of plastic boxes up to 750mm off the floor.

The worst part of the loft for condensation has no boxes in.

[iamrcb]

I'll post some photos and cross a section through the eaves detail. According to the chap that fitted the 6 roof tile vents the membrane is the very last of the non-breatheable type and was put in about 15 years ago.

Things to definitely do:

- Move all the boxes to the gable ends, leaving the whole floor space free
- Insert short lengths of small diameter plastic tube between the laps of the membrane to get some air in/out
- Check first floor ceilings for tiny cracks at the wall joints and cable penetrations (at lights)
- Insert a sheet of DPM between under the insulation on one third of the loft (as a test)
- Move the insulation away from the eaves a bit further (about 0.5m back at present)
- Improve sealing around the loft hatch (again)
- Seal up the joints around the shower extractor duct (even more)

Options to consider if these don't help: Gable end vents, ridge vents, eaves vents, more roof tile vents

I need to find out if these could be contributing to moisture in the loft space:

- Short, horizontal combi boiler vents to the outside, steam blows up towards the tiles on the gable end
- Top of an old (chopped down) chimney is slightly open to the loft. One upstairs fireplace is open to this chimney. I did stuff insulation in the gaps between the top of the wall and the underside of roof
- A wall extractor vents through the random rubble stone wall but the duct is not well sealed, so moist air may be able enter permeate through the loose stone fill and up to the loft space (doubtful)

This may sound like a daft idea, but a 20W extractor fan is only £2 a month to run constantly. Considering that this is only a problem for two months a year its a very small cost and saves me running the dehumidifier to dry it out when it gets soaked. I could just switch it on when required. I'm not happy to let it just get wet for a few weeks per year. If such an extractor fan moves 85 cubic metres of air per hour, with the loft being around 64 cubic metres this is over an air changes per hour, or if my theory is wrong, then its a lot more than zero air movement. With mechanical ventilation becoming normal in new-builds it can't be that silly an idea.

Edited by iamrcb on Tuesday 20th December 21:38

[iamrcb]

Thanks for the advice JR and everyone else.

I will try to further prevent moist air entering the roof space where possible (hatch, penetrations, former chimney, shower fan duct) and then add roof tile ventilation

There is no roof overhang at all, so nowhere to place soffit vents.

Edited by iamrcb on Wednesday 21st December 13:30

[iamrcb]

herewego said:

Why is the insulation pulled back from the eaves if there is no eaves ventilation?

just in case "some" ventilation was happening