3 Jul 2007

How should we Handle Humidity?

I still hear the old canard that 80% of our body weight is made up of water. I remember hearing it back at school and I’ve had it repeated to me by my children, so it feels like it’s still doing the rounds. Actually, that fount of all knowledge, Wikipedia, reckons it’s rather less, around 55% for men and 51% for women (more fat). I am not going to argue. It’s perhaps not 80% but it’s still a hell of a lot of water.

What you don’t so often hear — in fact just about never — is what proportion of your home is made up of water. I am not talking about water in the pipes and in the hot water cylinder and various tanks: I am referring to water bound up in the fabric of the house. There’s a surprising amount: nothing like the 50-odd% that exists in the human body but very possibly around 10% of the mass of a house will be made up of bound-water. Timber, for instance, has a moisture content which varies from as little as 5% (incredibly dry) up to over 30% (dripping wet), but often tends to settle down in a centrally heated house at around 10%. Masonry materials, even concrete, are hygroscopic to a surprising extent and can take on and release moisture according to conditions. You only have to drop a brick in a bucket of water to see the effect it has on its weight.

The fact is that a detached house, which can weigh anything between 50 and 200 tonnes, depending on size and construction methods (and that’s excluding all the foundations), could be holding as much as 10 tonnes of bound water within the walls, floors and roofing, and a lot more within the fixtures and fittings as well.

What does 10 tonnes of water look like? Due to the miracle of metricated measurements, 10 tonnes of water turns out to be 10m3 in volume, about the size of a small bedroom, or perhaps 70 bathfulls, if you prefer that. It also turns out to be 10,000litres, which is the amount of water a typical household uses in about two weeks, or around 1500 flushes on a 6lt low flush toilet.

All this bound-water doesn’t have to stay bound. When conditions dictate otherwise, it can either absorb more water or it can release water via evaporation. This does rather depend on the surfaces surrounding the materials: some are very water permeable, others are not. Exactly how much water transfers between the solids in the house and the air in and around it is unknown - it’s never been measured, as far as I know – but it’s likely to be fairly substantial. If it amounted to a change of just 1% of water by weight, we would be talking about 100 litres of water.

Now on a typical day in the life of a detached house, there is usually between 2 and 4 litres of water vapour suspended in the internal air. Take a house with a floor area of 160m2 (comfortable 4-bedroomed job): it will have around 400m3 of conditioned (heated) air space within. In winter, this will be kept at something like 20°C and 50% relative humidity, which should hold around 9gms water vapour/m3. That equates to 3.6kg or 3.6litres for the whole house. If the relative humidity increases above 75%, you start to notice the atmosphere becoming uncomfortably fuggy and if it gets much higher than this you start to see condensation on hard, cold surfaces, typically glass in windows. Indeed, persistently high relative humidity levels at whatever temperature tend to cause all manner of nasty problems connected with condensation.

A typical household will also be producing water vapour during their day-to-day activities. Humans will give off between 0.5 and 1lt per day each, animals slightly less. Cooking, washing and showering also contribute. In fact you might expect around 5 to 10 litres of water vapour a day to emanate from human activity in a typical working house. It’s still a pretty paltry amount, but it has to be dissipated because if it’s allowed to build up without check, the relative humidity will reach dangerous levels. In fact, our domestic ventilation strategies are predicated on the fact that the No 1 enemy is water vapour. If water vapour is kept at reasonable levels, then all the other concerns like cooking smells, off gassing from solvents, body odour and carbon monoxide poisoning will take care of themselves.

This is the way your winter ventilation strategy is worked out. The external air (let’s say it’s 5°C and 90% relative humidity, it often is) holds around half the amount of water vapour per m3 than the internal air (20°C and 50% RH). The maths is a little complex because warm air takes up far more water vapour than cold air and though the relative humidity levels make it look like there is less humidity in the warm house, the absolute humidity levels tell a different story.

Because the water vapour is floating around in the air, if you swap the internal air for some external air, then you will also be swapping the water vapour levels as well. You are basically sucking water vapour out of the house.

Diffusion is ignored in all this. The fact that there is over a thousand times more water bound up inside the building than there is floating around in its airspace just doesn’t come into the equation, despite the fact that everyone agrees that moisture levels within a building are constantly changing. Doesn’t it seem strange that we should spend so much effort and energy expelling 10 litres of water vapour a day from our homes, when they are 10,000 litres of water already sitting in the fabric?

There are alternative water vapour management strategies out there. The best established one hails from Germany and it consists of fitting highly permeable materials to be used as a reservoir to store moisture, with a view to letting it be evaporated back inside when conditions allow. In Germany, it is seen as part of the Building Biology movement and they regard the use of humidity-buffering materials as one of their key principles. Needless to say, they regard the use of mechanical ventilation as an anathema. A lot of building scientists regard the Baubiologists as cranks, but there was some independent testing of their humidity buffering principles carried out in Canada in 1997 by Straube and Burnett which found that what they called the Dynamic Hydric Response of a wood wool board (Durisol) was excellent. They found that when the relative humidity in the room was increased from 30% to 70%, the Durisol responded by absorbing 7% of its dry mass weight. Straube and Burnett worked out that in a typical situation where humidity was rapidly increased from around 50% RH to 80% RH, the extra water vapour could easily be absorbed by the Durisol board.

It’s not a property exclusive to Durisol. They looked at the water vapour permeability of a whole range of products and estimated the following sorption ratings:

• Plasterboard painted with emulsion - score 40
• Concrete, unfinished – score 90
• Brick, natural finish – score 110
• Softwood, unfinished – score 150
• Strawbale behind lime plaster – score 240
• Durisol board behind lime plaster – score 250

They reckoned that anything with a score of 50 or higher would work as a humidity buffer. They also pointed out that such walls work automatically, don’t break down and require no energy to operate. Unlike mechanical ventilation, of course. Another issue with mechanical ventilation, which is seldom touched on, is that it won’t work consistently in all spaces: there are places within rooms — the classic one being ‘behind the wardrobe’ — which remain largely stagnant and simply never get ventilated. Humidity buffers work by diffusion and don’t require air movement to operate effectively.

The proponents of humidity buffering don’t suggest that it becomes a replacement for ventilation, rather that it’s much more efficient at removing excess humidity from the air inside a house and that this should in turn reduce the requirement for current ventilation strategies. Now take the process one stage further and reduce or remove materials from the house which off gas or contain solvents, and remove the threat of carbon monoxide by having no gas burning indoors, and you have a home where ventilation at half an air change per hour is simply uncalled for.

This way of building in fact brings about an alignment of the interests of the natural building materials crowd and the energy efficiency tyros, two standpoints which have been traditionally been divided over the issue of how to best ventilate a house.

On a personal note, I am hoping that the house we are planning to build next year with Baufritz will be able to push the boat out a little and to explore some of these humidity buffering strategies. Baufritz already use them in the homes they build in Germany but have so far run into a wall of blinkered red tape in the UK. “If it’s not in Part F, you can’t do it” seems to be the mentality here. Maybe with a little coaxing, they will allow us to experiment. I’d like to build a house without mechanical ventilation, without trickle vents and without extract fans, just to prove that it can be done and that indoor air quality won’t suffer and that energy efficiency won’t be compromised. Will it be possible? Watch this space.


  1. Hi Mark,

    great article. I found it searching for permeability properties of woodwool cement boards.

    I'm contemplating the idea building with a wall structure with woodwool boards on both exterior and interior and rice hulls as one foot thick insulation in between.
    (here-s a paper on rice hull insulating properties: http://www.thelaststraw.org/backissues/articles/Rice%20Hull%20House.pdf )
    Any opinions?

  2. Opinions? Not really. I like the idea of it, but have no idea how practical it may be.

  3. >>I’d like to build a house without mechanical ventilation, without trickle vents and without extract fans <<
    I would too, and we may build that way in our earth-plastered straw bale house, but I'm not convinced (yet) that you can keep CO2 levels within save limits if the building has been built (fairly) air-tight.