Will Anderson's Folly

Diary of an Eco-Builder?

I must admit that I now cringe when I hear someone describe themselves as an “Eco-Builder.” It’s just a tad too smug for the likes of me; a set-apart term that suggests that they are somehow a cut above common or garden builders. If you have to set yourself apart from the hoi polloi, then I prefer the term “Gentleman Builder” because it’s generally a lot more to do with class than it is to do with green politics. If you really want to be green, then don’t build: there is no such thing as an “Eco-Builder.”

The Independent doesn’t share my scruples. They are currently running a weekly column (Wednesday, Property) called Diary of an Eco Builder, penned by Will Anderson who is building a large new timber-framed house in South London and is writing about his experiences. Today’s column (3 August) is interesting because it deals with one of my pet subjects, moisture management. It’s a much misunderstood subject and Anderson perpetuates many of the misunderstandings.

Myth No 1: “Our walls must obviously be robust enough to stop water getting in” writes Will, “but this is a simple task compared with dealing with moisture going the other way.” Anderson might think keeping rainwater out of the house is a simple matter but despite thousands of years trying, we still haven’t mastered it. The huge bulk of problems we experience with both roofs and walls stem not from anything weird happening with water vapour, but from rainwater ingress.

Myth No 2: “The principal movement of moisture within walls is from the warm, humid interior to the cooler, drier exterior.” What he is referring to here is the phenomenon of vapour diffusion, which occurs at molecular level. Water vapour will tend to travel, albeit very slowly, from areas of high vapour pressure (i.e. indoors) to low ones (i.e. outdoors). That’s the perceived wisdom on walls, and has been since the 1940s. Unfortunately, more recent research has shown it to be largely irrelevant. There are far more important drivers governing the behaviour of moisture in walls, floors and roofs.

Myth No 3: “We generate huge amounts of water vapour in our homes, cooking, bathing, washing and breathing.” My quibble is with the use of the word “huge” in this context. A person living in a house produces about 1 tonne of water vapour a year: on its own that sounds like an enormous, or even a “huge” amount, but set it in context. A detached house weighs around 100 tonnes and as much as 10% of this is made up of water, bound in with the building materials. This water:solids ratio is not a steady state relationship; it’s in a constant state of flux, depending on conditions both indoors and outside. Odd litres here and there produced by the occupants – a tonne a year approximates to around 3 litres a day — hardly makes any difference to the overall behaviour of moisture within the house.

Myth No 4: “Condensation occurs whenever air cannot hold any more water.” I am indebted to William Rose’s excellent new book, Water in Buildings — an Architect’s Guide to Moisture and Mold* — for nailing this one. Condensation occurs when there is a suitably shiny, non-porous surface and the temperature is below somewhere around 12°C, but it doesn’t follow that condensation always forms whenever the air isn’t warm enough to hold the water vapour. The excess moisture is frequently taken up by the materials in the room without classic condensation taking place: carpet is very good at it, so is painted plasterboard and unsealed timber.

Myth No 5: Condensation is a “greater problem if it occurs in areas you can’t see, such as inside your walls or roof, where it can do everything from undermining your building fabric to rotting your building fabric.” It sounds absolutely terrifying, and it’s true to say that water can and does damage building fabrics. But to blame it solely, or even mainly, on our indoor activities is just plain wrong, although, to be fair to Anderson, this analysis is at the heart of BS5250, the standard which sets out the parameters for the Condensation Risk Analysis which underpins much of our design of timber frame housing and roof building. The Americans refer to it as the profile method and, according to Rose, using it inappropriately has led to “wildly exaggerated over-predictions of the likelihood of diffusion-related moisture problems.” Rose continues “A very informal survey among building science colleagues seems to indicate that diffusion-related moisture problems account for less than 1% of the moisture problems found in buildings. We have been seriously sidetracked by the profile method for the past 50 years.”

Does it matter that the likes of Anderson are regurgitating nonsense? Arguably, it’s harmless nonsense. We have lived with this largely incorrect analysis for 50 years and it hasn’t caused us much harm if only because of another, far more useful, practice we also have, which is maintaining a vented cavity behind the rainscreen or brick wall on timber framed houses. But there are many businesses out there making a good living playing on people’s fears of the “hidden menace of unseen condensation.”

Anderson finishes his article by running through his spec for his walls: he appears to be installing a heavy duty vapour barrier (the key weapon in the fight against condensation, according to the profile method) and a “breathing wall” routine as well, using Warmcel, the recycled newspaper insulation which sells itself as a natural alternative to plastic vapour barriers. I have never heard of anyone doing this before: it’s either plastic vapour barrier or breathing wall, not both combined. I would have serious reservations about Will Anderson’s design: the whole point about breathing walls is that they are designed to dry both outwards and inwards: placing a plastic vapour barrier inside his Warmcel may stop this happening. Heavens, it may even lead to the very thing he is trying so hard to prevent. Might our Eco-Builder be in the process of constructing one very soggy house?


*Water in Buildings — an Architect’s Guide to Moisture and Mold. Author William B Rose. Published by John Wiley and Son 2005. £31.15