Loft condensation

[The Natural Philosopher]

Oh dear...well you CAN but then you get..wet rafters..

I think we know know why...the real discussion issue is 'what is the simplest way to fix it?

.

[george [dicegeorge]]

easiest - a lot more ventilation in the loft, and kitchen extractor fixed.

Are there indoor plants adding to the moisture of cooking, washing and breathing?

[g]

[Tim Streater]

Well I don't because I've never done it. I never investigated what the builders put under the 4" they installed 15 years ago in our old house, or what there is under the 10" in the new extension here. What should I be expecting to find?

[Andrew Gabriel]

Yes, but at two levels.

The first is what can I do quickly (like in half a day) to dry out the loft and allow the rafters to start drying. I won't be able to do a long-term fix until later in the year.

I might try something with the dehumidifier, such as having it upstairs and discharging into the loft, but I need to do some measurements to make sure it extracts enough moisture so it doesn't make the situation worse, and I didn't have a humidity meter on me when I was there last time. (Unfortunately it won't work in the loft as it's too cold for it.)

Second is what's a proper long term fix (which isn't too much work of course;-) This requires proper diagnosis, since fixing the wrong problem won't do any good.

It looks like moisture through the lath and plaster might be significant. So, what's the best way to do this? I could probably roll back the insulation to get to the laths (it runs between the joists, and then another layer across the top of the joists).

Could I simply buy some of the thinner damp proof membrane and lay this over the laths, or is there a better material? I have a large roll of non-breathable sarking which might work for this.

Can it go up and over each ceiling joist? Cutting it to fit between the joists would leave long unsealed stretches along the joist edges, defeating the object of trying to seal. However, if it goes up and over each joist, the joist tops will be sealed cold spots (with only one layer of insulation over).

Another option is replacing the ceilings, but taking down two large lath and plaster ceilings is not something I like the idea of, and overboarding always seems to me like a bit of a bodge.

[Hugo Nebula]

The BS for ventilation suggests that there should be more ventilation at the lower level of the roof than high level (25mm & 5mm respectively), otherwise a'stack effect' can occur, dragging in more moist air.

You say you have installed vents to the gable; are these at a higher level? If so, try blocking these up to see if that works.

Wood will survive seasonal moisture, provided it can evapourate away. Dry rot needs a moisture content of >20% for a long period to develop, so provided there is ventilation, it will dry out in the summer. If it IS a problem, you will need to consider a vapour barrier to prevent moisture from the dwelling entering the loft. Even if there are no openings into the loft, vapour can pass through a lath & plaster ceiling.

Is there anything else about the usage of the house that we're missing? You say that "there's pretty much no moisture source in the house anyway (far less than there would normally be due to lifestyle of the occupant)". Is there central heating in the house? Is it heated to a 'normal' level? Is it occupied full-time? These things can have a bearing.

[The Natural Philosopher]

they might have laid some old tesco bags under it for starters.:-)

[The Natural Philosopher]

Open the loft hatch mebbe and let some warmth in there, the ventilation will do the rest.

Somehow you need a vapour barrier in there if the insulation is as good as it seems to be.

And it really is old lah and plaster.

A lot depends on the rooms below. A simple fix is to reline below over existing with foil backed plaster board, and skim or use lining paper to finish off. and repaint..

I am not sure about fixing from above, but lifting insulation and laying plastic sheet over the lot and re laying insulation might work as well

Or even strips of plastic laid between the joists: its not to make it watertight just to cut it down till the existing ventilation can cope.

Sounds ok to me..see what others say.

As I said. major reduction is the game. not 'sealing' ..think of it as a path with a leaky plug and a dripping tap..as long as you are getting rid of it faster than its coming in, it wont end up in the rafters.

Anything will do. celotex is foil baked as is fooiled plasterboard. Could cut strips of that and simply flop them down between joists.. betweem

It isn't really. I've got bits of doubled up plasterboard here..its standard for extra sound insulation in many places.

>

[robgraham]

I know this is going slightly off topic, but why, Andrew, do you see overboarding as a bodge ? I have an L & P ceiling that is cracked, has been for the 40 years we've been here, and I'm about to strap it with suitable timber framing and board it. What's wrong with that? Rob

[Andrew Gabriel]

Large one is half way up, and central. Small one is right at the bottom in one corner (at the eves), where I thought there might be a risk of stagnant air. There are also vents formed where the sarking droops between the rafters in places, but they're mostly half way up too. It would be very easy to create some more lower down just by proping a gap in the sarking layers.

Does anyone know how to convert a resistance reading into a moisture percentage? I hammered two nails into what looked like the wettest rafter, and measured 750kohm. I hammered them well in to get as low resistance reading as possible (wanted it well below the meter's max of

2Mohm). The idea is I can check back again later to see if anything I've done changes the timber moisture level.

Yes, this does appear to be the case.

Occupant doesn't use shower or bath (although I did after being in the loft;-), doesn't cook on the hob or oven, and washing is taken away to be done by one of us and returned dry.

It has full central heating, which I monitor and control remotely. Occupant often stays with other members of the family for long periods, and it was empty for a long period over winter. When not occupied, central heating drops back to 11C. I think all the windows were shut too, whereas in previous years, I left a few in the venting position, so it may be the house was also more humid than before due to lack of cross ventilation through the rooms. There are several places where the subfloor can vent into the house, such as a grille for gas fire ventilation, and lots of gaps in floorboards in unoccupied areas (e.g. under stairs), and the subfloor being bare earth is likely to be a source of humidity - the earth is always slightly damp.

[Andrew Gabriel]

I guess I'm just too much of an anal perfectionist.

The existing L&P ceiling seems to be a bit heavy for the joist size/span. From memory, I think they're 3x2's and probably about 4m long. It sags now, but not noticably when you're in the room. I would be a bit concerned how much sag would be added.

[robgraham]

OK, that's reasonable. My joists are 15 ft 8x3's. Don't think I've a problem. Sorry to go OT :>/

Rob

[robgraham]

I keep on thinking I should be able to add something useful to this discussion because I too had a problem with mould on the roof timbers at one point in this property's recycling to modern usage. The trouble is that it was so long ago I can't remember at what point it went away and the timbers have been admirably dry ever since - that is the point, I didn't actually do anything specific to address it, just that something in the redevelopment cleared the problem.

This is an old stone farm cottage near Edinburgh so damp is endemic. I don't have any specific roof ventilation; I do have 8" plus of insulation laid over L & P ceilings - the family live in it, shower, bath and cook and we have CH of 20C in the evenings and 15C during the day.

One thought - it isn't possible that there's a pipe somewhere that has developed a minute leak and is doing a good job of 'greenhouse' mist ventilation - you aren't growing something-you-aren't-allowed-to up there are you Andrew ?

The other thought now with the input on occupancy, is that the lack of air movement due to doors opening and shutting, etc is having something to do with it. Rob

[Roger Chapman]

ISTR that one of our contributors once said that he had persuaded a BCO that wedged gaps in sarking could provide adequate ventilation for boxed in rafters.

[<me9]

How about painting the ceilings with an oil based impermiable paint. When I did my extension the drylined surfaces were painted with gyproc drywall topcoat as a vapour check coat. Don't know how effective it is, haven't had any damp in the loft other than when the whole roof was iced up externally and then thawed, leaking through nail holes in the sarking. Sometime soon I'm going to have to strip off one quadrant of the roof to find the leak. Not this weather though.

[Andrew Gabriel]

To follow up...

I did two things - more ventilation in the house, and some heating in the loft to keep it 2C warmer than outdoors. (It was dropping below outside temperature on some nights).

Within 10 days, the resistance of the rafters had increased to above what my test meter could read, so they dried out quite quickly. Some ~20 days later, I bought a damp meter, and that's showing the timber moisture content at 17-18%, which seems to be a pretty normal reading ffrom what I read. The other thing that's happened and surprised me a bit, is that the fine surface mold which had appeared has also started disappearing; I imagined that would stay even if it didn't grow anymore, but seems not. No damp smell anymore either. Timber is all still rock-solid.

Not yet done anything to change loft ventilation or provide a moisture barrier at ceiling level, but at least one or other of the short-term measures seem to have had an instant impact.