How do roofs work?

[headgear]

One possible source of wet air in your loft is a central heating header with an overpressure release pipe feeding into it. I have seen these steaming away in lofts....
If you don't have such a thing then that's that.

How much of the roof was replaced in the 90's ? any original hardwood timbers left? If so the water may well be activating any dormant fungal spores. As for leaving that alone have a read through your insurance policy regarding fungal attack. :-(

Lots of lofts are experiencing moisture issues with sustained freezing temperatures and little wind to force ventilation; the cold air cannot support the same vapour as usual and has little opportunity to blow away, the roof structure, timber and felt are all at ambient cold and will readily gather condensation. In other words, yes you can have destructive levels of simple condensation, especially in the situation described with blocked vents, inadequate underfelt vent space, and ready pathways for warm moist air.

For now, seal the access traps as best you can. In the spring get some scaff up and strip the part of the roof which is at issue. Remove any rot from timbers or replace/repair if too far gone. Fit rigid foam insulation between rafters leaving 50mm gap below felt line,Breathable membrane, battens, tiles back. If your ceilings are plasterboard knock them out and add more insulation inside, then new ceiling. That should give you 75 to 100 mm of effective insulation without losing too much headroom inside. If your ceilings are lath and plaster you have to make a call on fabric conservation value vs thermal and moisture comfort ( ie just have a bit of insulation behind the ceiling).

If your structural timbers, wallplates and/trusses are rotten and wet in the spring you must get a suitably qualified and experienced person to examine them and advise on repair or replacement.

Sorry if this sounds a lot, but forewarned is....etc. and what I am describing is about as bad as it could get :-(

 

[vicky whitworth]

Hello, I'd still welcome any advice about how I might remove the insulation in the void between my sloping ceilings in the bedrooms and the roof so that I can insert solid board insulation and leave a 50mm gap, without having to take the tiles off?

Also does anyone know how much ventilation I should have in my loft? It's about 13m long by 5' max at the apex. Currently there is only the one air brick, and the blocked grills in the soffits.

Has anyone used lapvents, and if so are they any good?

The roof is definitely not leaking. The wet is all condensation, some of the roof timbers are spongey and have a mould growing on them. This is not looking good. Urgent action is going to have to be taken.

Thank you for all the advice so far. I do feel I understand the problem even if I don't grasp an effective solution.
Vicky

 

[vicky whitworth]

Hello, I feel I'm making some progress understanding what is going on in my loft causing the damp thanks to all your useful advice. I am now trying to understand how the gap between the roof and the walls was finished in vernacular cottages pre-Victorian. I'm assuming that they didn't use fascias and soffits, did they? Since it looks like I will have to have the tiles taken off to access the areas of the loft that need attention I would like to change from the current modern fascias and soffits (with grills in the soffits) to something more traditional. But I don't understand how birds, etc were prevented from getting in the gap between the roof tiles and the wall at the eaves - perhaps they weren't? I know I need a gap to allow air to access the roof for ventilation but I don't want to loft full of small critters.

Anyone know what used to go on at the eaves? Thanks, Vicky

 

[Flyfisher]

My understanding is that roofspace ventilation was generally adequate as a result of the construction of older properties and it was only as 'modern' materials and techniques were introduced that it became a problem, i.e. addition of roofing felt, insulation etc. I'm pretty certain our roof timbers are only in such good condition because of all the ventilation afforded by pantiles and reed/lime sarking. Airbricks in gable walls would also help. Of course, this all made for terrible thermal performance, but that was less of an issue in years past.

Ventilation grilles around the eaves are, I believe, a relatively recent introduction into the building regs., but the intention is clear. As is breatheable roofing felt. And, of course, the better insulated the attic space, the colder the air, so any moisture that does find its way up there (and centrally-heated warm air can hold far more water than the generally colder house air of the past) will very quickly condense. If the condensate then runs down behind tightly packed insulation material it may not evaporate too readily.

All amateur conjecture based on my readings and experience over the past few years, but it seems to me that ventilation and insulation are opposing forces in terms of 'property health' and, like most engineering challenges, people are constantly searching for the optimum trade-off between them.

 

[Penners]

Vicky - I fear that getting the packed-in mineral wool out from between the rafters of your roof is probably going to be tricky. But it all depends on how tightly it is packed, and how easily you can gain access to the cavities, either from the attic or by removing the soffits below.

If you can get a stright view down the cavities from above, then you might try making yourself some sort of raking tool on the end of a pole that will hoik the stuff out a bit at a time. But ultimately it's a matter of "suck it and see".

 

[worms]

While it doesn't seem to be the case in this instance, it is possibly worth mentioning that the junction between wall and coomb ceiling also seems to be a common location for internal condensation that might sometimes be confused with external damp/condensation. My understanding is that the cold air drops down behind the ceiling and if this can't keep on going down behind the wall or vented to the outside, you end up with a cold patch where the internal moisture condenses.

 

[Penners]

Sounds a logical enough explanation.

 

[skier-hughes]

I've had no joy getting mine out yet, tried rake, hoe and pitch fork, all too large to fit down gap. I've now ordered a long grab handle (litter picker) which should arrive tomorrow and I'll try again.

 

[Lime]

The problem appears to be that there is not enough space between the ceiling and the underside of the tiles and felt to insert any insulation that will do a worthwhile job.
(With respect I am overlooking the present problem of removal of old insulation... I'd use long tongs.. but that is because I have them)

The idea of inserting stiff board insulation into the space will not really be very effective.
Why?
Because air will circulate between the ceiling and the lower part of the insulation by both convection and draughts and take the heat away with it.
To insulate the ceiling with any success would require that any air circulation is prevented under the flat insulation.

Although I don't normally consider the stuff worthwhile, I think this is a good situation to install layers of thermal foil M/P insulation (B&Q do it).

 

[Johneds]

That little sloping section of roof is a common source of condensation problems. I've seen uninsulated ones that caused severe black mould staining to the ceiling below. The mould usually grows where there is a gap above but not where the rafter is. The effect is known as pattern staining. So it's not uncommon for people to stuff the gap with insulation which cures that particular problem - but unfortunately then causes general condensation in the roof space itself.

Removing the mineral wool and replacing with rigid board is a solution that I have specified many times, and I can tell you it does work quite well providing you can physically get the old wool out. I wouldn't worry too much about a 50mm gap. That's really only necessary where you are venting rafters the whole length of the roof pitch. You are only talking about 500mm or so here so a 20 or 25mm gap would be plenty good enough.

If you can't get the old stuff out from the inside an option is to remove the bottom couple of courses of tiles and remove from the outside. Althougn possibly only cost effective if you are single storey.

 

[worms]

Althougn possibly only cost effective if you are single storey...

...and in England. For a Scottish roof you would have proper timber sarking to deal with!

 

[vicky whitworth]

Thanks for all your thoughts. My husband is thinking about using the BBQ tongs to get the insulation out - don't know how long he thinks those tongs are but the sloping ceiling bit is about 6 foot!!! Last time we had a BBQ he must have had too much beer to remember the tongs being that long!!! He's not good at this sort of stuff - says he's an artist and not an artisan!

I'm starting to think that maybe we don't have insulation going all the way down that void as we do have those damp patches on the internal ceiling at the angle between the slope and the vertical wall. This needs further investigation.

I'm a bit confused about the advice on the solid board insulation - some think this is a good idea, others not. Anyone else like to chip in?

I'm not really bothered about the damp patches on the ceiling but I'm worried sick about the wet roof timbers. If it was just the damp patches I'd ignore them (that's what I've done for the last few years - slap a coat of paint over them each year and all seems fine for a while!) but I can't not address the wet roof problem. Don't normally get stressed by this sort of thing. Vicky

 

[skier-hughes]

I'll tell you how I get on with my long handled grippers when they arrive, here is my roof/ceiling
http://periodproperty.co.uk/forum/viewtopic.php?f=1&t=10907

 

[vicky whitworth]

Thanks Skier-Hughes - the link was great and sums up my issues nicely. Good luck with the grabber thingy. I now have the image of hundreds of us up and down the land lying in our lofts extracting insulation grab by grab and cursing those obsessives in the late 80s who insisted of stuffing insulation into gaps where ventilation would have been a good thing.
Vicky

 

[worms]

I'll tell you how I get on with my long handled grippers when they arrive

Yes and let us know how they deal with the wasps nests that you mentioned in the other thread! Wasps seem to love that bit of the roof space between sarking and ceiling!

 

[skier-hughes]

Wasps were all dealt with by a pest control company in the summer so they are all down and swept away now...... I hope

Grabbers just arrived in post, will have a go later if I get all my work out the way soon!!

 

[vicky whitworth]

Have had another builder look at the job today and he made the following comments:
• Breathable roofing membrane is a bit of an unknown and there is some evidence that it may deteriorate. Might be better to stick with the traditional roofing felt rather than replace with breathable.
• He thought he could get the old insulation out of the voids between the sloping ceiling and the roof but suggested that rather replace it with solid board insulation and a gap (bearing in mind it would have to be slid down between the rafters and wouldn't, therefore, be a good fit) why not use a more breathable insulation and stuff the voids with it - sheepwool he mentioned.

Any thoughts on this? I wondered how breathable anything stuffed down those voids would be. Seems pointless to replace it with something that might be no better than what's there.
Thanks.

 

[Penners]

I'm a bit confused about the advice on the solid board insulation - some think this is a good idea, others not. Anyone else like to chip in?

I'm not quite sure what is confusing you, but the solid board (sometimes called a "batt") stuff is like a rigid foam. Here's one well-known brand: http://www.celotex.co.uk/

To the best of my (limited) knowledge, this kind of material gives the very best insulation value for its thickness.

 

[skier-hughes]

Not tried getitng mine out yet, grabbers arrived broken, now waiting for replacements.

 

[AMc]

I'm a bit confused about the advice on the solid board insulation - some think this is a good idea, others not. Anyone else like to chip in?

I'm not quite sure what is confusing you, but the solid board (sometimes called a "batt") stuff is like a rigid foam. Here's one well-known brand: http://www.celotex.co.uk/

To the best of my (limited) knowledge, this kind of material gives the very best insulation value for its thickness.

The issue is with air tightness. Properly installed board is 3x better than wool/mineral fibre/fibreglass at the same thickness but it must be installed so that there is no opportunity for air to flow around it. As we old house owners know, ventilation heat loss a.k.a. drafts remove heat very efficiently. I was warned that failing to seal the edges of solid board insulation can reduce its effectiveness by 75%!

If you can't get the boards to tightly fit the space or fill the remaining voids with expanding foam (sorry) then you won't get the efficiency. As you will have issues getting into the roof space you are unlikely to get a good fit so you may see little benefit from solid board. The advantage of wool/mineral fibre/fibreglass solutions is you can stuff them into small spaces and they will expand to fill the gaps

For what it's worth I was under the impression that sheeps wool is vapour permeable and absorbs and releases moisture BUT I also believe the much cheaper, less 'environmentally friendly' options of mineral wool and fibre glass are also vapour permeable (but aren't absorbent) and will allow moisture to move.

I would be very weary of bridging the gap between the internal wall and roof with anything. I would consider removing the existing insulation and allowing the wall to dry out in the spring before you make a decision about what to do next.