The published Research Report [‘Report’] is based initially on data obtained in the laboratory concerning air movement in cavities. Whilst it is appreciated that the report may be interim, there appears to be an unusually high proportion of flawed extrapolations to cavities and voids in real buildings. Although air movement is considered in the extrapolations there is virtually no coverage of the very important factor of atmospheric moisture in voids and its origins.

The Report also refers to dry rot in several places but does not, in the case of voids, identify how it can originate. The paper appears to concentrate mostly on airflow as a drying agent rather than evaluating the importance of atmospheric moisture as part of this process. Whilst airflow is important to achieve ventilation, its primary function is to replace internal moisture laden air with drier air but this is dependent upon where the replacement air is coming from. In the case of many biological organisms such as moulds, dust mites, bacteria, etc, not only is the airflow a part of the process but also relative humidity.


The level of atmospheric moisture in a domestic inhabited building is the sum of two components (1) moisture generated internally by occupation, plus (2) external atmospheric moisture. Thus there is more water vapour internally in an occupied building than externally certainly during the colder months of the year when house ventilation is far more limited due to closed windows, etc.

Water vapour exerts pressure-vapour pressure; this is directly related to the actual amount of water vapour retained in the air. Water vapour moves down its pressure gradient ie, from the internal environment (high vapour pressure) to the external environment (lower vapour pressure). In an inhabited building the difference between the internal and external vapour pressures is known as the ‘differential vapour pressure’ or ‘excess vapour pressure’. The movement is away from the areas in which moisture is generated to other areas in the building which are at a lower vapour pressure; this includes into subfloor voids and roof spaces..

In relation to water vapour, the warmer the air the more water vapour it can retain. The ratio of the actual water vapour retained at a given temperature to the maximum it can retain at the same temperature is known as the relative humidity. It is a measure of saturation, not ‘amount’. So air at 80% relative humidity at 20°C retains 80% of the maximum possible water vapour at that temperature.

Unlike vapour pressure relative humidity is temperature dependent. For example 60% relative humidity at 20°C would be 83% relative humidity at 15°C. Furthermore the ‘drier’ the air (low relative humidity) the greater its capacity to take up moisture so at any given temperature the better drying regime is, say at 50% humidity as opposed to 80% humidity. In the case of vapour pressure, 1.20kPa equates to 0.74 g/kg dry air irrespective of the temperature

Measured external relative humidities for the colder months of the year are averagely high-80% plus. Longer terms of 85-90+% also occur.

British Standard 5250:2011, ‘Condensation ‘ records monthly mean relative humidities which are used to calculate, for example, interstitial condensation. In the British Standard for Heathrow, October to March, these are given as 77-86% relative humidity at air temperatures of 4.7-11.2°C, the highest humidities being between October to February.

Furthermore, based on these figures, the British Standard gives the average ground temperatures for the same months to be 7.7-12.8°C; note all are greater than the external air temperatures. However that Standard identifies that these temperatures will be even greater for timber suspended floors, very approximately, between internal and external temperatures. Clearly heat is added from the inhabited part of the property.

From BS 5250:2011


Section 2: ‘Why do we think that air movement protects timbers?’

This section quotes, ” The idea that ventilation is beneficial for the occupants of the building is not in dispute. But the notion that it [ventilation] prevents timber decay is more questionable”.

The first part of the above is certainly true in that the greatest water vapour burden is in the occupied area (Figure 1).

Rot from subfloor condensation

In relation to subfloor and roof voids where ventilation is more limited, it is going to depend on what source of moisture causes the rot. If it is caused by timbers being embedded in continuously damp masonry or set directly on a persistently damp oversite then it is unlikely that ventilation (airflow) will have much impact on the initiation of the decay.

If, however, the cause of the decay is long-term subfloor condensation then clearly this will have an impact on the decay which was initiated by this mechanism (photo opposite).

High relative humidities alone are not sufficient to initiate rots: levels of moisture in wood above the fibre saturation point are necessary for this to occur.

The major factor, therefore, still to be taken into account for moisture conditions within cavities and voids, is atmospheric moisture and its origin.

Section 3: ‘Ventilating a small cavities into rooms: laboratory assessment’.

The results from the evaluation of a small scale laboratory experiment where a 50 mm cavity was constructed show that airflow is limited, with stagnant areas, vortices and turbulence. However, the small scale laboratory set up is unlikely to reflect the situation found in inhabited buildings where cavities and especially voids are significantly larger and ventilated in assorted different manners. Nevertheless, it does illustrate patterns in the movement of air in that particular case.

Unfortunately, from this air movement data the author of the Report has extrapolated as to the drying effect in cavities. For example, reference is made to cavity drain membranes in section 2 of the Research Report about air movement behind such systems. In reality this is irrelevant in that the membranes (dry linings) are provided to isolate the internal surface of the room from a damp underlying substrate.

The Report states, “The general perceived reason for ventilation is to keep a cavity dry –“. Cavities formed as a result of lining techniques are almost always to isolate a damp substrate from the internal surface. Any ventilation of cavities, eg, subfloor voids, roof spaces, is to attempt to control atmospheric moisture levels, not drying the masonry/timber structures.

In the assessment no consideration is given to the importance of atmospheric moisture or its origin as part of ventilation, the main potential problem in cavities and voids.

Section 4.2: ‘Sub-floors:

The section again deals with airflow and fundamentally fails to appreciate that air for ventilation carries water vapour and the conditions which actually exist in subfloor voids and roof spaces.

Spread of rot due to extremely high rh.

As stated above, it is highly unlikely that lack of ventilation will have little or any effect against rots which initiate where timbers are in contact with/embedded in persistently damp masonry. However where atmospheric moisture is concerned this can and does cause rot to timbers by subfloor condensation and also condenses forming on cold masonry where the timbers may be in direct contact. Atmospheric moisture levels can support the spread of decay once initiated and allow mould growth in some situations (see photo above).

Section 5: ‘Conclusions ‘:

It is this section, unfortunately, that reveals a number of significant flaws in the Research Report. This is fundamentally due to a limited understanding and simple background knowledge in relation to atmospheric moisture (internal and external), its origin and distribution, especially in relation to real world ventilation.

Of “The objective of ventilation is to remove excess moisture, frequently condensation, resulting from building usage. This may be valid in modern impervious building constructions, but is it relevant to historic buildings?”

There are no valid reasons why ventilation of subfloor/roof voids should be any different between modern inhabited building and historic inhabited building. Indeed, ventilation is designed to remove excess atmospheric moisture but it depends from where it is to be removed and the air that is being introduced to remove such excess moisture.

In the occupied living area atmospheric moisture is usually significantly greater in the colder months that externally and therefore ventilation is required by one mechanism or another.

Of “Timber buffers humidity so that a traditional enclosed roof space (for example) may produce a very stable environment. The large surface area of wood can easily absorb any excess moisture rising from the rooms below, even if there is a new kitchen or bathroom.”

The above is now solely considering the effect of atmospheric moisture on timber not embedded in or set on damp materials.

Wood is a hygroscopic material; it will adsorb moisture from the atmosphere. The amount of moisture adsorption is dependent on the relative humidity although there is a small temperature factor; the greater the relative humidity the greater the moisture that is adsorbed.

As with any hygroscopic material the absorption will cease once it has come into equilibrium with the particular relative humidity. For more water to be adsorbed the relative humidity must be increased. Conversely if the relative humidity declines then water will be released from the hygroscopic material back into the atmosphere. So if we have a natural material such as wood in a given humidity already, say 75%-90%, the levels typically found in subfloor voids and roof spaces, then clearly the wood will come into equilibrium with those relative humidities. Should one increase the relative humidity even by an extra 5% – 6% then it is evident there is going to be very limited amount of extra water vapour adsorbed due to the wood being quite ‘full’ at 75-90%.

So even considering perfect ventilation (100% efficient) from the outside, ie, complete replacement of internal air with external air then the relative humidities within the roof spaces would be and remain very high because they are the same as the relative humidities externally. So effectively the timber would be fully buffered at the external humidities which remains mostly greater than 80%, certainly during the colder months of the year.

The Report states that timbers buffer the humidity in an enclosed roof space may produce a very stable environment. As described above the flaw in this is statement is that the amount of buffering by wood in a roof is going to be very limited due to the natural conditions already present. Furthermore, the passage of water vapour through wood is relatively slow so it is only going to be the surfaces that initially afford water adsorption from the atmosphere; this significantly limits wood to take up moisture rapidly or lose it should the relative humidity decline. Thus the buffering effect of wood is going to be very limited where the standing relative humidity is high, and almost certainly under these conditions be of no practical value to stabilise the environment.

Below are the humidities recorded in a roof space during the colder months of the year: this is typical of a number examined. Note the very high relative humidity – almost identical to the external relative humidity. As described above, the capacity for timbers to adsorb more water should the relative humidity rise further is extremely limited.

Similarly for a subfloor, if ventilation was absolute (100% efficient), the air in the subfloor void would be continuously replaced fully by external air. Therefore the relative humidity would be very high due to the high external air relative humidity and certainly much higher than internal occupied areas relative humidity (note: relative humidity is temperature dependent). Indeed, the external relative humidity data shown above and below for both subfloors and roof voids are very close to that recorded in BS 5250:2011 (see previously.)

The question therefore remains how much more is going to be ‘buffered’ by the addition of any other source of moisture from the kitchens/bathrooms or rooms below? Effectively very little and thus buffering by timber is of very little consequence. In roofs and subfloor voids wood buffering is very limited and is of very little value in controlling the environment where relative humidity is persistently naturally high. To rely on this statement reference the buffering effect of wood in the Report is probably very unwise.

Subfloor condensation

As stated previously, the internal occupied environment retains more water vapour than external levels. As such it has greater vapour pressure the results of which some water vapour passes down its pressure gradient into the subfloor and roof voids. If this excess water vapour is not swept away even by the humid air external (lower vapour pressure) this excess leads to condensation in these areas. Thus ventilation is still very important.

Of: “Ventilation to the exterior, however, will introduce moist air into the structure for most of the year.”

This can only at best be described as nonsense. It clearly reflects a distinct basic lack of understanding of atmospheric moisture and its movement, all of which is already well-established in the public domain.

The figure below is an example demonstrating that the external air is drier than internal air in inhabited areas, certainly through the colder months of the year (less ventilation). Furthermore the movement of air within an inhabited building is from inside to out; this is all well documented in the British Standard 5250:2011. The simple reason is that the total internal moisture atmospheric is the sum of the external level of moisture vapour plus the level of moisture vapour generated by habitation (figure 1).

As the internal vapour pressure is greater than the external vapour pressure the internal atmospheric moisture therefore moves down its pressure gradient, ie, from inside to external. (Also note the influence of the external vapour pressure has on the internal levels as shown in the above – this is typical of the influence of external conditions.)

It is not, as suggested, the other way round that moist air enters the building from outside-moisture vapour movement is from inside to out and not outside to in as written in the Report.

Thus to say, “Ventilation to the exterior, however, will introduce moist air into the structure for most of the year” is totally incorrect and complete reverse of well documented facts.

It is also interesting to note that the statement made in this section of the Report that “Ventilation to the exterior, however, will introduce moist air into the structure for most of the year” is a complete contradiction to the comment made lower down on that page that, “Damp air within the cavity can be exchanged for drier air outside the cavity”.

Of, “If the structure is actually damp than the results may be equally unsatisfactory. For example if the outside air with an average humidity of 60% – 70% percolate into a damp subfloor void then it will cool and its relative humidity will rise as its temperature drops.”

Again, as above, this is incorrect and further reinforces the absence of basic understanding and information of conditions within occupied environment.

The air externally has a higher relative humidity to start with (cold). Furthermore, the subfloor void is almost always warmer during the colder months of the year than external air and external ground temperatures; this especially includes subfloor ground temperatures. This is further shown in BS 5250:2011.

So should outside air percolate/ventilate it into a subfloor void it will warm, not cool. And therefore, based on the above statement in the Report, the relative humidity will lower, not rise. This assumes no moisture passing into the voids from the inhabited areas.

However, in relation to both subfloor voids and roof spaces there is the factor of water vapour being generated internally by occupation and this has a greater water vapour pressure than external levels. Some of this water vapour will pass down its pressure gradient into the roof void and the subfloor void; this adds to the roof and subfloor void levels of moisture.

One therefore finds that in inhabited buildings the subfloor and roof space water vapour levels – not relative humidity levels – are somewhere between the those present in the internal and external environments (see figures below). What this demonstrates is that the subfloor void and roof spaces are ventilated by one mechanism or another. If this were not the case then water vapour levels would reach those in the occupied part of the building.

 G.R.Coleman B.Sc(Hons), M.R.S.B.,C.Biol.,A.I.M.M.M

Posted by Complete Preservation


  1. very informative. A great blog. Thank you.


    1. Glad you liked it Jacki. Graham doesn’t miss a trick!


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