Very Bad Loft Condensation - ideas on how to solve

[JR]

iamrcb said:

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.

I think that this is your problem. Can you post a cross section through the eaves as I cannot imagine a detail that is impossible to vent

[cirks]

condensation = warm air against cold surface (which, of course you know).
However, your loft shouldn't be warm if the insulation on and between the rafters/joists is any good. Most tiled roofs should have breathable membranes. If you want to see what happens to joists that continually get wet like yours appear to, I can probably find some from the roofs down the road from us that (although older than yours) basically fell apart with rot.
I'd suggest either getting some more vent tiles on that side of the roof or, as another poster said, just cut some slits in the membrane. You could always cut some out and put breathable in its place.

[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

[Flanders.]

Honnestly before you go and do all that get some Pipe insulation, slice it into Wedges and slide it between the layers of Felt. 90% of the time it works.

[van cleef]

you need more ventilation as I come across this problem all the time.

I would go with a row of roof tile vents along the bottom of both sides of the roof then a row along the top at one side . This will let the warm air rise and escape out the top vents and it will then draw cold through the bottom vents.Sometimes if your going into your loft a lot as your opening your loft hatch you will leak warm air in to the loft which will condensate immediately which will appear on the felt and any exposed nails first.

Don't cut holes in the felt or stick pipes through the felt as it will start to condensate on the under side of the roof tiles as the warm air hits them and the tile battons won't put up to a large volume of moisture.The moisture needs to escape to the outside of your roof

Edited by van cleef on Wednesday 21st December 07:19

[JR]

iamrcb said:

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)

Yes, and put up shelves on the gable walls
No, as per post above
Petty unless it's been done in a hamfisted way
Test of what?
I'd do the opposite. The insulation should be the full width of the floor. You're letting warmth up... You need eaves vents: http://www.thediyworld.co.uk/roof_space.htmlhttp://www.manthorpe.co.uk/Building/Products/Roof-... The best documement is this but I don't know how to link it: Mineral Fibre Loft Insulation, Cold Bridging, Ventilation & Heat Loss ... www.kingspaninsulation.co.uk/.../White-paper--Loft... File Format: PDF/Adobe Acrobat - Quick View

Often a problem. Overlap the hath in the loft by 6" all round and have 8" of insulation stuck to it.
Is it a particularly bad seal at the moment?

[JR]

iamrcb said:

- 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)

All of those need to be addressed.

[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

[JR]

iamrcb said:

I will try to further prevent moist air entering the roof space where possible ( former chimney,

This should be considered carefully. If you simply seal up the chimney this can lead to damp problems in the house. I think that you need to vent to the outside somehow, whether directly or via the loft space.

[JR]

iamrcb said:

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

but you could still do facia or top of external leaf vents which I think are easier to satisfactorily install than tile vents near to the bottom of the roof. (Whichever; there's more than one way to skin a cat.)

[herewego]

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

[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

[Spudler]

van cleef said:

Need ventilation.

A row of roof tile vents along the bottom of both sides of the roof then a row along the top at one side .

Don't cut holes in the felt or stick pipes through the felt. The moisture needs to escape to the outside of your roof.

This ^^, plain and simple.