I think your best quick fix to get the humidity down is going to be a dehumidifier. If the walls are very cold, you're going to get condensation unless you dry the air, especially if your bathroom extraction isn't great at the moment.

I run one for a few hours every evening (to deal with cooking) and morning (to deal with showers). No other extraction. There's a touch of mould still lurking but it was really getting out of control before I started doing this.

Is there any insulation in the walls at all? That's where the real solution lies.

Posted By: Viking HouseThe demumidifier is a good idea, I've not researched them much, how much energy do they use and is it more economical than heating fresh air with a fan heater?

They are better than just heating the air as you get to recover the latent heat of evaporation of the water that the dehumidifier removes - so the effective COP is > 1. The condensed water is useful too for those applications that require distilled water.

Paul in Montreal.

Sounds like not enough insulation in the walls. Airflow behind the furniture being enough to transport humid air in there but not enough to keep that bit of wall warm. Result is a little condensation which is feeding the mould.

And another thing ... the rapid rise of outside temperature is just what gives get lots of condensation on thermally massive surfaces before they can warm up. So it could be a short term/occasional problem.

Hope all this is upcheering.

Posted By: Gotanewlifestick in in-laws bedroom (here for another 2 weeks and the worst affected room) right now. Hate them of course as use lots of lecky but needs most certainly must.

Not very nice to say you hate the in-laws as they use lots of electricity ;)

Paul in Montreal.

Re condensation on windows: My rented house faces north/south, is built of slate, walls 40 cm thick with uninsulated wooden floor - huge void underneath and crittell windows, recessed into the wall so you can sit on the window cills. In order to stop draughts I used diy secondary glazing (its like strong cling film and attached with double sided tape) on some of the north facing windows in my open plan kitchen/sitting room about 40m2 Not only did temperature go up by 3-4 degrees and its now about 18 (it was really cold to begin with!) there is no condensation on the windows now, at all. I was very scathing about this stuff but am a convert now and its cheap and easily removed in summer. I was thinking your bedrooms are about the same temperature so maybe this would work for you.

Re damp/mould: I found loads of this about 8 years ago behind old layers of wallpaper in various places (some were vinyl/painted over etc ) and underneath waterproof paint (in bathroom). I've slowly managed to get rid of it by stripping it all off and washing off the mould and repainting with limewash. So far it has not reappeared.

Will keep watch on walls now! Although I've still got so much ventilation in this house I don't think mould/condensation gets a chance to sit very long.

Incidentally Gotanewlife I was re reading 'Cumulative Heat Loss with MVHR Ventilation Page 2 comments by Viking House and Rhamdu under General about opening bathroom doors to lessen condensation when taking bath/shower....doing this has helped alot in my bathroom which is an entire 'cold spot'. I wonder if opening your bedroom doors, that have mould, during day would help - and cheap until you can sort properly.

Mainly as an experiment I've just bought Argos' finest 425/0777 small portable dehumidifier (rated 220W, using 182W) to see if it makes any difference to visible condensation levels, etc, at the moment in the smallest bedroom. It's being powered by our PV at this moment! B^>

Slightly noisy, but not horrible.

Rgds

Damon

Hola GNL

Great song you should listen to when down with the mould blues - Mouldy old dough by Lieutenant Pigeon (showing my age here I know). Always raises a smile.

On a more practical note, others have mentioned insulation and dehumidification. Insulation essential of course. Have you considered positive ventilation? Practical Householder ran an article on this in the 80s. Nu-Aire and Vent-Axia, among ohters I suspect, make loft-mounted fans which filter air and then pump it into the house. No heat recovery that I am aware of, but it does reverse the usual 'chimney' effect of draughts going up and sitting warm under the ceiling. Cured my own and several other families' mould problems.

Runs at 40-80 watts from memory - which is a small fraction of a typical dehumidifier - and has a great side effect that it makes any woodstove or open fire draw better...guess why. :) Great thing, moving air.

Google Nuaire Drimaster and have a read.

All the best, Crusoe.

What are the walls made of? Tufa, or those cavity clay blocks (sorry, don't know the English name for them)?

If it's tufa, they wick water at a shocking rate, so you can get mould popping up in quite unexpected places. They also lose much of their ability to hold heat when damp.

We used to suffer from mould, and the only way we've found to avoid it is to air the house whenever possible - and that means all doors & windows open all day. A dehumidifier isn't an option for us because of the electricity consumption, although I'm considering putting a single-room MHRV in the bathroom at some point.

I suspect that you're getting it in the top corners because the warm air rises, and meets a cold wall/ceiling, but what Viking House says is the most sensible approach. As an indication, the RH here rarely drops below 85% in winter, so mould problems are likely to be more common than much of the UK.

BTW, those anti-muffa paints are universally useless. At least that seems to be the consensus around here. There is an additive for stucco that works quite well, but it's probably a bit late for that for you.

We open windows in bedrooms for an hour or so with doors closed, then close windows without necessarily immediately opening doors, allowing temperatures to gradually rise (and RH gradually drop I guess). But in any case the landing is not heated (directly) so the air outside the bedroom doors is rarely especially warm and moist when we open those doors sometime later.

Rgds

Damon

VH: for our smallest bedroom (~6m^3?) the 180W dehumidifier reduced RH from ~73% to ~50% in a couple of hours.

Rgds

Damon

PS. At RH 30% at a previous clients' building the static shocks were so common and fierce that I destroyed my last laptop...

Contadino, yeaaaa its the dreaded Tuffa! But sealed now well on the outside with paint. I cannot possible run the dehumidifier regularly either (especially now my one unit for one unit PV 'contract' was 're-issued' so that I now pay as much as 25cents/kWh but get back a flat rate of about 10 cents - not that they have actually said the rate the scumbags - but that is another story!!).

CWaters. I was getting mould in corners and edges of windows etc in open space – but obviously it was worse behind things (incl curtains). I guess the reason the opening windows/doors works is that with a massive house 20-30 minutes of fresh air changes the air for cool stuff but the heat in the room recovers very quickly from the warm mass.

I have found that the dehumidifier on its lowest setting 24/7 maintains the %RH in that room just about below that magic 70% (door shut) and with the TRV on the radiator I guess all the heat it makes saves me a tiny bit of wood. Also my dehumidifier is deigned to sit with its back against a wall and does a pretty good job of creating mixing airflows rather than chucking it out of one orifice. As a temporary measure whilst the in-laws are here it’s good enough. I have now looked for cold spots and cannot find any (nothing below 16 deg) but when the in-laws go and I turn the thing off we'll see.

Our bedroom has a big ceiling mounted fan and it is now on 24/7 low - it certainly feels enough to unify the temps in the room and no sign of mould coming back BUT it hasn't been long enough to tell and I haven’t gone looking for cold spots yet. Lights now off in the shower room to try and mitigate the extra energy consumption!

Contadino Ref Paint - we tried them all over a few years and all were useless - but then we found the latest one. It works differently to the others as the others seem to be intent on just sucking the condensation through to the wall; this one isn't as chalky but it contains anti-mould ingredients that clearly resist mould - when we used it we stopped short of moving one item of furniture and so we had a direct comparison and I tell you the effect is significant: about 75% less black mould growth and 95% white fluffy growth. The paint only comes in strong colours so maybe that is a clue as to its potent additional ingredients. I will look the name up if you're interested. (and can take the vivid colours).

Your 85% humidity is a real bitch, I feel for you; we never really get above 80% and at that point I'm just like you - every door and windows gets thrown wide unless rain is actually being blown in! I just wish I had time to fit the fan (it a Vent Axia Lo-Carbon Solo Plus SELV Fan + H/Stat & Timer.

“it's not quite as low wattage on trickle mode as the HR25 at 5.7watts, but it extracts 25% more air in trickle mode than it, and has a low boost function that extracts double the air at only 6.5 watts, and even it's peak mode draws less power than the HR25's peak. It also has both timer and humidistat functions” Gavin-A from previous thread (Ta again). HR just doesn’t add up for me financially in my particular situation (I know: “Heretic!)

Maybe looking back at the traditional winter lifestyle, and construction techniques, might help. Italian country homes were sparsely furnished, and heated only by an open fire, around which whole families would eat & sleep. The ceilings were traditionally 'volta stella' or 'volta botte', neither of which have any real ceiling corners to speak of.

So maybe long term the solution might be to round out those top corners with insulation material.

Homes were calce'd every year, inside and out, as it helps reduce/prevent mould from growing.

I'm not suggesting that you start living like a 60's 'vullanu', but it's easy to see how problems that have been endemic to an area since the dawn of time have been dealt with by people in the past.

Posted By: Viking House

blockquote>Posted By: evanCorrection - my dehumidifier is actually only taking about 250W on average. So, if you stop burning that 100W bulb 24 hours a day, you'd save enough energy to run the dehumidifer to make a big difference to the humidity levels.

Hi Evan, How long does it take your humidity levels to drop from say 70% to 50% using the dehumidifier? Do you leave it on constantly? I'm curious about the economics of using a dehumidifier versus a HRV unit to lower humidity levels, has anybody done the maths on it, how does a dehumidifier work? how many Watts are recouperated from latent heat?

Hi Viking, I can't answer your questions accurately at the moment as I don't actually have a humidity meter. In the recent cold/dry weather, it was taking 2-3 hours running in the evening for the humidistat on the unit to click off (dial set to half way) which is where condensation is manageable and the air feeling a bit warmer, so I guess it's getting close to 50%. I feel that's that's getting rid of the build up of moisture during the day from showers and cooking.
In wetter weather it could take all evening, 6 hours or so, to reach the same point, and obviously much more water collected.