A lack of ventilation in the voids beneath suspended timber floors (the sub-floors) can allow the relative humidity to rise to high levels, and the dangers this causes can be two-fold: firstly it can place the floor joists in danger from rot; and secondly, although there may well be an effective damp proofing course in the house walls, definite symptoms of rising dampness can be recorded. These latter may simply be high moisture meter readings or in severe cases, distress to plaster finishes, skirting boards and décor etc.

Modern building regulations ensure that sub-floor ventilation is adequate in most houses built after 1960. However, houses built before then are a different matter, and in those built between the Georgian period and the Second World War, it can be almost non-existent.

In addition, many properties built before 1940 did not have damp proofing membranes or oversite concrete; the builders simply levelled the earthen floor, or solum, and then covered it with the floorboards. And in properties such as these, moisture emitted from the damp solum evaporates into the enclosed area beneath the floors, where it can dramatically increase the relative humidity, often resulting in levels of 80% or more.

Timber, which is kept in a relative humidity that is constantly above 75% is susceptible to rot.  In most cases this doesn’t actually happen, but in such conditions it takes only a slight increase in the timber’s moisture content to instigate fungal activity, usually in the form of one of the wet rots, although dry-rot is also a possibility.

And this is most likely to happen where any of floor joists rest upon masonry walls, regardless of whether there’s a damp proofing course installed in those walls, or not.

This happens because masonry has a much greater temperature lag than fresh air, and if it gets cold, it tends to stay cold for long periods.  Thus condensation is more likely to occur where timber meets masonry than on a piece that’s surrounded by air, i.e. in the middle of the joist.

I often go underneath floors and see droplets of water on the lower faces of joists where they rest on the walls; it can often be brushed off with my hand. The only thing keeping these joists safe is the fungicide treatment they received before installation; and funnily enough, this (anti-fungal treatment) is more common in Victorian and Edwardian properties than those built in the middle of the last century. Georgian houses often have oak joists, and these are generally less susceptible to rot than softwood ones.

Another problem caused by high sub-floor relative humidity is that of ‘rising dampness’, which affects even those properties that have excellent damp proofing courses already installed; and here I’m talking about original dpc’s, not retro-fit ones.

This occurs because of the layout of the brickwork and the dpc beneath the floor of the house; this can be seen in the diagram below:

Depending on the brick's ability to absorb moisture this can be quite significant, and the problem is often made worse because these bricks, which are not meant to be seen, are often of lesser quality than ‘show’ bricks. Thus their glaze is inferior and they absorb moisture more easily.

Over the years, the moisture becomes resident in the brickwork and eventually starts to creep upwards.  Although I suppose that technically this is rising dampness, it isn’t caused by ground water, which rises because of a failed or absent damp proofing course.

Naturally, this dampness, which in practice is often only minor, will send the average moisture meter and its operator into paroxysms of activity: lights flash, buzzers sound and teeth are noisily sucked. Meter says ‘Yes’; injection dpc required; end of story.

But of course it isn’t, or rather it shouldn’t be.  A more logical approach to this problem is to increase sub-floor ventilation: either by unblocking years of dirt and removing vegetation from the existing airbricks; replacing old terracotta or masonry ones with modern ones (unfortunately modern are usually plastic); or installing extra ones. Not only will this bring the ‘rising dampness’ under control, it will also improve the environment for any timber within the sub-floor void, which by itself is a very effective way of reducing the risk of dry rot.

The installation of an injection damp proofing system will not attend to these latter issues, although it may reduce the effects of dampness in the lower reaches of the walls.

Of course, it could already be too late for some timbers, and their ends may need to be replaced, or augmented with parallel joists.  If that’s the case, don’t simply attend to the timber; address the issue of ventilation at the same time.