Damp Loft

[Painters10]

Hi

Since the cold spell has arrived I've noticed a rather worrying damp in the loft. I went up to put some boxes up there and touched a small wooden table I was storing. This has green fur on it. You can run your hand on the under side lining of the roof and it comes away soaking wet, dropping big puddles of water. The carboard boxes I have up there are now "soft".

We only moved into our house back in January and I've not spotted these issues before.

Rather than felt on the back of the roof it appears to have a plastic lining, grey on the down side and black on the tile side. The house was built in 1985 by a skinflint. Does anyone know what this type of lining is? I've only ever seen roofing felt where the plastic stuff is.

Other details: standard single apex roof, tiled outside, (clear) airbricks at each end of the house, some visible light through the eaves, about 100mm of insulation, partly boarded. There's no vapour barrier over the upstairs ceiling boards. There are no extractor fans in the bathrooms and the loft hatch is a poor fit.

I guess a lot of hot air is getting into the loft and then condensing with insufficient airflow to get rid of it.

Painters10

[Bob Mannix]

Condensation has three solutions:

ventilation, ventilation, ventilation.

Fit soffit vents front and back (and side if needs be) roughly equivalent in area to a 1cm slot all the way round.

That should be all that is necessary. If the loft hatch is in the bathroom (you don't say) seal it better as well.

[Andrew Gabriel]

Also, check for sources of high humidity up there, such as:

No lids on loft tanks. Pumping over of the central heating header tank (hot water coming out of the expansion pipe when pump is running). Holes though the ceiling into the loft, e.g. around pipes. These are particularly bad from high humidity rooms such as bathrooms, showers, kitchens. Poorly fitting lft hatch (not draftproofed). Extractor fans discharging into the loft. Rain leaking in (check for excessive dampness down gable iand party walls and chimney breasts -- elsewhere you'd notice it on the upstairs ceilings).

[stuart noble]

That about sums it up I think.

[Invisible Man]

Bear in mind that the added ventilation could result in lower loft temperatures. Make sure any plumbing in the loft is well insulated.

[Phil L]

You need more insulation, and insulate on top of the hatch, and more ventilation, obviously eaves vents would be better (if they aren't already installed), but for now you can just add some more air bricks at each end.

[BigWallop]

I'd say were correct in your diagnoses. The warm moist air in the house will always make its way upward, with the natural chimney effect of any structure, and using the bathroom and shower etc, is only filling the house with more moist warm air.

You say you can see day light at the eaves, so the loft isn't closed and air tight, but it might not have enough ventilation right at the top, on the ridge, where it would be most effective at removing all the wet stuff.

If you have an open gable on the end, then maybe a vent through at the top of the wall would help. One six inch vent would probably be adequate to keep changing the air in the loft.

Even sealing the loft air tight for the winter months, would help a lot. It would stop air movement into the loft, but it might also trap the moist air in the rest of the house.

Warm air rises. Cold air falls. The loft is catching all the warm moist air from the house and is trapping it right up the ridge of the roof. Open the ridge to let the moist air out and the problem should be solved.

[Ian_m]

A friend of mine had exactly the same problem "rain" in the loft in spring and mainly autumn. His house was new build 1998. Stuff just got flooded in the loft including stuff in plastic boxes, the water puddled and leaked in sending everying mouldy.

I think its call interstitial damp and is related to temperature and vapour pressure, basically the dew point is in the loft rather than somewhere less harmful. Adding insulation is very very bad as the dew point can then occur in the insulation, thus causing wet insulation.One cause is boarding the loft. Reducing loft insulation or loft boarding (thus moving dew point outside) is a tempory fix until cause is located.

My mate tried the following :-

- Extra ridge tile vents.

- Extra gable end vents

- circulation fans

- Humidy triggered fans (not good as noisy and tended to run at night).

- Removing loft insulation and taking up the boarding (this was most effective, but not a long term fix).

- Broke into plasterboard walls to verify the soil pipes had air admittance valves fitted. It is not unknown to air admittance valves not to be fitted (or faulty) and allow the sewer to vent into the wall cavity and into the loft space.

All made very little difference.

Eventually fitted an Amberdry (?) dehumidifier which was very successful. Uses silica gel + heaters so is quiet, but he did need to plumb it in as it kept filling its tank. Also works below 10°C which most compressor dehumidfiers don't. Warning, costs quite a bit to run, takes 500W when on, though obviously not on all the time.

However finally sorted it as obviously was damp coming in from the house. Basically.

- Every trickle vent in the house windows was blocked, at front by dropped cement in the cavity and back by insects.

- Fitted a humidy controlled (+temperature compensated) fan in main bathroom.

- Fitted a run on fan (as opposed to just on lights) in ensuite.

- Persuaded wife to keep kitchen door to hall shut when cooking.

- Swapped condensing tumble drier in kitchen to externally vented.

- Allways leaving a window ajar upstairs to allow ventilation.

Since doing the above, dehumidfier sold on e-bay not seen any damp in the loft.

[DM]

No - more insulation just lowers the effective loft temeprature even more and so can cause more condensation.

Insulating and sealing the hatch could help.

Eaves vents should be a lot easier to install that air bricks I imagine.

cheers

David

[Phil L]

It can't cause /more/ condensation, more condensation can only be achieved with more moist air getting into the loft to condense on the cold suface of the mono, which is what is happening now.

I don't think taking the gutters, fascias and eaves down on each elevation is easier than knocking out a few bricks. YMMV

The fact is, it doesn't matter if there is *no* insulation up there, water will still condense on the mono because it is the only thing seperating the loft from the outside air, unless you count the tiles, which are full of gaps and are open at the bottom anyway.

He needs to provide more ventilation and in the meantime, remove the water which is in the cycle of dripping, evapourating and condensing again, adding more insulation will reduce the evapouration bit, and more ventilation will take some of the moist air away, personally i would get a dehumidifier up there for a week or two after adding more vents and fibreglass.

[stuart noble]

Sort the insulation out and I'm sure there won't be any need for extra ventilation.

[George (dicegeorge)]

but isnt normal roofing felt permeable to water vapour, but the builder used some kind of cheap plastic which is not permeable, so the water vapour from the bathroom etc is going up and getting stuck at the roof level..

hence need ventilation in the bathroom, and more ventilation in the roof because of vapour from human breath, kettles etc etc and vapour from warm air downstairs going up to a colder loft and dew-pointing up there...

[g]

[ConfusedCarbuyer]

Dear Painters all posts are basically right more ventialtion (more insulation for heat retention) but none has mentioned the need to install a vapour check under the roof void This can be done by painting the ceilings with a vapour check such as an oil-based paint or the like chris

[Phil L]

The breathable sarking on the market nowadays is, but it's a fairly new invention, previous to that a product commonly called 'mono', which was in effect, polythene strengthened with filaments of something or other....it's not used these days due to the high condensation levels incurred, as described here by the OP. Previous to that, a bituminous roofing felt was used, and while this does have some of the problems of 'mono', it's not nearly as bad

It shouldn't be able to get up there though

[Phil L]

Which is ideal provided you like rain in the bedrooms

[stuart noble]

Well, exactly. If you loft is warmer than the garden, it must be getting up there somehow.

[DM]

As you lower the air temperature the air can hold less water vapour hence you get more condensation.

What you do is simply take a hole drill, drill holes about 20-30cm apart and then fit appropriate soffit vents in the holes. Takes about and hour to do the job.... I found this much easier than taking out any bricks and it also supplies a distributed air flow rather than concnetrated at one end.

cheers

David

[DM]

Sort the insulation out (improve it) and there is more need for ventilation.

cheers

David

[Bob Mannix]

Quite (as I said a while ago) and (ditto) the standard advice is to vent with a total area equivalent to a 1cm slot all the way round. If your total soffit length is, say, 20m, you need a total vent area of 2000 cm^2 or 14 vents 15x10cm in total (7 front 7 back). In such a case you can allow a bit for what's there and put say 5 front and 5 back. That's plenty of ventilation and a bit less would be a vast improvement and possibly OK but that's your choice. More for the smaller rounder ones as DM said above.

[stuart noble]

Assuming the house was built without any.