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Moisture behaving badly

Richard Renouf elaborates on the updated British Standard code of practice for the management of moisture in buildings – BS 5250 – which was released in October.

I MUST have learned a lot at school but I can’t recall many actual lessons. But there are two specific ones that have stuck in my mind. The chemistry teacher was late – or so we thought. On the front bench was a tripod with a balloon tied to it. The classroom door opened, but instead of ‘sir’, a lit candle appeared and the room quickly fell silent.

The candle, attached to a long pole, moved slowly across the front of the classroom until it reached the balloon and there was a huge flash and a very large explosion, then the teacher appeared at the other end of the pole with an enormous grin on his face.

‘Don’t worry,’ he said. ‘I warned the teachers upstairs.’ Then he gave us a lesson on the reaction between the hydrogen in the balloon and the oxygen in the air combining to make water. It’s a principle that’s now being used to drive hydrogen-fuelled cars.

The second was a different chemistry teacher who took an uninflated balloon (is there a pattern here?) and weighed it on some very accurate scales. He then inflated the balloon and re-weighed it. Of course, I knew it wouldn’t change weight because the amount of air in and above the balloon was exactly the same.

But I was wrong.

The balloon had gained weight and the lesson that followed included calculating the weight of air. I can’t remember the answer, but I’ve just googled it and it works out at 1.22kg for every cubic metre at 20deg C, but the warmer the air is, the less the weight for any given volume.

That’s the reason relative humidity is relative: it varies according to the temperature of the air. Warmer air can hold more water vapour without it condensing. So if air at 2deg C has a relative humidity of 90%, then gets heated to 20deg C the relative humidity will drop to about 24%.

If you turn this on its head, air at 20deg C with a relative humidity of 65% will become fully saturated at about 13deg C – and below this water will start to condense onto cold surfaces. This is the reason why even on a warm day, condensation will form on the outside of a cold drink.

The behaviour of moisture, not just as condensation, is critical in buildings. Within many of our lifetimes, heating has gone from open fires to central heating and even to underfloor heating, windows have gone from single- to double- and even triple-glazing, and floors have incorporated structural damp-proof membranes, insulation, and often anhydrite screeds.

The updated British Standard Code of practice for the management of moisture in buildings – BS 5250 – was released in October and it’s a significant revision because the changes in building materials and regulations over recent years has forced a thorough review of moisture issues in floors, walls, and roofs while also taking into account the requirements for energy savings.

It’s not a standard that gets quoted in many flooring documents or installation instructions. Most of what we need to know is in the specific standards relating to floors, starting with BS 8204-1: 2003 for screeds and then in the standards for wood, laminate, resilient, and textile floorcoverings.

There’s a 17-page section (Section 10) which covers solid and suspended floors, including the need to ensure new floors laid next to old floors are adequately protected from moisture wicking from the old to the new, a problem which I’ve come across many times, and for adequate drying times for newly laid solid floors especially above a damp-proof membrane.

The standard highlights that modern flooring structures can encourage interstitial condensation – the formation of condensation between the layers of the slab where a moisture-laden material meets a colder surface above or below causing de-bonding issues.

Laying LVT flooring in a property with unstable temperature conditions is unwise. This can be a problem close to outside walls where temperature differences can fluctuate more than towards the centre. The problem can be worsened by poor ventilation.

The introduction to the standard says previous methods of moisture risk assessment have dealt mainly with individual building elements and not the interactions between them or the effects on the whole building as a system.

Until this century the comparatively low standards of air tightness and insulation, even in new buildings, meant the moisture risks and consequences of internal high humidity were relatively low. Things are clearly changing, and the revision to the standard is intended to encourage consideration of the whole-building impact of any individual element.

No doubt this approach will begin to influence many of the standards on which we rely, but even before this happens, we need to ensure that the processes and materials we use protect our installations from issues caused by other elements of the building and protect them from our work, too.
www.richard-renouf.com

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