Text for lecture: Tim Padfield, Building to the new relaxed climate standards

[Speaker biography]

Tim Padfield is a conservation scientist specialising in the mutual interdependence of the museum building and the collection it houses. He has worked for the Victoria and Albert Museum in London, the Smithsonian Institution in Washington DC, USA and the National Museum of Denmark, with academic interludes at the University of Leeds and the Technical University of Denmark. He has an MA in Chemistry from Oxford University and a PhD in building physics from the Technical University of Denmark. He now works freelance, based in Devon, UK.

[Title page]

From the unsentimental viewpoint of a physicist, museum collections are stuff that is, physically, past its best, but has by chance or sentiment avoided being thrown away.

Given the infinite variety of delicacy of actual objects in collections I ask you to look at the matter from an entirely opposite view point: instead of debating the acceptable climate range of generic types of object, see what the container can achieve in the way of a good climate, then test this against the standards based on material susceptibility to damage.

This is my market town, Totnes, in south west England. Its houses use the best method yet devised for keeping walls dry in a windy and wet climate - slate hung vertically. But these are from the sixteenth century. Let us see how we protect ourselves against the weather nowadays.

[Royal Ontario Museum]

For decades, museum architecture has been influenced, some might say distorted, by strict environmental standards which force the use of air conditioning. In the Royal Ontario museum the designers bave even abandoned trying to air condition the building and have retreated to air conditioning the individual showcases with piped supplies.

[ROM interior]

[Queen Victoria's bathing machine]

Now we can celebrate the relaxation of standards for conservation climate, as shown in this recently constructed canopy for Queen Victoria's bathing machine at Osborne House on the Isle of Wight. Keeping off rain and direct sunlight may provide 95% of the protection afforded by full air conditioning. Many of us will also celebrate the relaxation in bathing etiquette which has made this vehicle an amusing period piece.

[Flaking wall painting]

But have the new standards relaxed enough from the period pieces which were their predecessors? Or by an amount which allows real change in the design of museums, their stores and archives?

I take as my theme the British and European quasi-standard for archives PD5454:2012. The now obsolete BS5454:2000 has been enormously influential in the museum world, worldwide, because it was firmly, though ambiguously, numerical, in a way that gave confidence to architects and engineers writng, and responding to, technical contracts.

Now the stringency has abated. But is the logical and scientific content more convincing?

[Ribe temperature]

Now it's time to talk about the weather, a topic popular in general conversation but avoided by nearly all museum standards. Here is the temperature record for Ribe on the west coast of Denmark. A typical northern European weather. It consists of a combination of daily cycles, shown expanded in the inset, superimposed on an annual cycle. Unpredictable hot and cold spells scarcely spoil the simple pattern of these two superimposed natural cycles.

It is a trivial matter entirely to suppress the daily cycle in a museum store or archive, even in a museum gallery.

[Quote 6.3.1 from PD5454]

PD5454 is firm in its advice. It advocates thermal inertia to ensure stability, without precise information on how much.

[Temperature evolution through a brick wall]

Here is how thermal inertia works. As heat from the daytime solar radiation is absorbed at the surface, it passes into the wall but is absorbed as molecular vibrations in the substance of the wall, then partly re-emitted so that the heat wave from the daytime solar radiation is delayed and diminished on passage through the wall. The perspective section through the wall on the right, shows how the temperature cycle within the wall diminishes as one approaches the inside surface. The inside temperature peaks about 12 hours after the outside maximum. Note that this suppression of the influence of the daily cycle requires only a modest thickness of brick, or any other dense material. Please reserve also for reference in two slides time the large difference between the air temperature and the surface temperature. This indicates high heat flow.

[Koeln archive ruin]

One disadvantage of such a heavily built construction was tragically revealed by the 2009 collapse of the Cologne City Archive. The massive rubble of the walls pinned down the crumpled archive documents so it took more than a year to rescue them. The walls were unnecessarily thick to suppress the daily cycle, not thick enough significantly to influence the annual cycle.

[temperature through insulation]

There is another way to moderate the daily temperature cycle: insulate the wall instead of loading it with heat absorbing mass. Here is the performance of an insulated wall whose thickness has been adjusted down to give exactly the same temperature variation at the inside surface as the brick wall. So a deeper analysis of the building physics suggests that a lightweight, insulated archive is both cheaper and equally effective, and less burdensome when disaster strikes. Notice the smaller difference between the air temperature and the surface temperature. This wall is resisting heat flow rather than absorbing its energy. This difference only matters if there are heat sources within the building, in which case the insulated space will gradually warm up, over the average outside, while the brick wall will not. If retaining heat within the building is advantageous, as in northern Sweden - use insulation. If the daily average temperature is comfortable, as in the Mediterranean region, use a massive uninsulated wall. Study the local climate rather than use international building practice.

[Underground temperature]

However, PPD5454 is not entirely wrong in its advice, just imprecise. thermal mass is capable of moderating even the annual temperature cycle. Walls would have to be about four metres thick to achieve the degree of buffering shown for the daily cycle, but there is plenty of heat absorbent material under the building, so an uninsulated floor laid directly on the ground will greatly reduce the annual temperature cycle within the building above it.

[Temperature in Ribe store]

Using the ground as a heat sink reduces the annual temperature cycle within the building to around 8 degrees. These data are from a museum store in Ribe, Denmark.

This swing from 16C to 8C is very modest, surely not likely to damage, and being below human comfort temperature, it contributes to preserve the collection by reducing the chemical decay rate.

Notice also that even though the heat sink is at the bottom of the building, even in summer the temperature difference between the floor and the six metre high ceiling is less than two degrees. Using a cluttered floor for year-round temperature buffering is entirely practical.

[PD5454 quote 2]

So how does this temperature cycle fit with PD5454? Not well. Although purely paper stores are allowed to sink to 5C, a general collection is limited to 13C minimum. The reason given is the evidence from a single published paper that beeswax seals precipitate natural components below this temperature. This is a good case for risk analysis enthusiasts to chew on: the welfare of a few wax seals that have never before been below 13C against the benefit to most other unstable materials from a lower average temperature. Other reasons given for the temperature limit are operational and deferential (meaning not too different from BS5454 to avoid embarrassment).

The bottom line of this quote introduces the other major preoccupation of conservators - the relative humidity. It turns out that the permitted range from 35 to 60% is unnecessarily generous. In archives and stores we can do much better than that, with very simple, low energy means.

[Suffolk record office climate]

The relative humidity within the Suffolk record office, built on the same lines as the Cologne city archive, stays within a 5% span all the year round. There is no explicit humidity control. This stability is achieved by winter heating alone, to a fixed 14C, which conveniently brings it in line with PD5454 temperature requirements. This RH stability is achieved through buffering by the abundant paper documents, which release water vapour during the winter, so that the RH declines by only a few percentage points. In the summer, the inside temperature is often below the outside temperature, risking a high inside RH, but the paper absorbs moisture from the infiltrating air so the RH only rises a few percentage points. The green trace shows the magnitude of the imbalance between inside and outside water vapour in the air.

At this point I will anticipate a frequently voiced objection to humidity control by irreplaceable national treasures. The water molecules sorbed and desorbed by paper at moderate RH are very loosely bound, with a very short residence time before they hop off and are replaced from the vapour in the surrounding space. The object does not own its water molecules - they are rapidly exchanged with the environment, regardless of how that environment is controlled, even when instruments show a constant climate. It is the RH stability that matters, not how it is achieved.

Sadly, the temperature span in the Suffolk record office exceeded the range permitted by the previous BS5454:2000, so it was air conditioned, a year before the successor document permitted its temperature cycle as OK for anything. Both these documents describe themselves as advisory, but many decision makers lack the confidence to boldly go beyond the advice of the British Standards Institution, or CEN.

[bricks in corner]

Many museum objects are not moisture absorbent but nevertheless susceptible to damage from RH extremes. It is possible to augment the buffer capacity of a museum store with low air infiltration by lining the walls with unfired clay bricks. This is the cheapest available humidity buffer. readily snatched from the brick works production line just before entering the final firing.

[climate in the clay room with calculated inert RH]

The measured RH in this experimental empty room is shown in this diagram, together with the orange line showing the expected RH if the room surfaces were entirely inert to water vapour. The buffer does something good, but can we quantify it for design purposes? Nothing ever gets built without at least the illusion that the consequences of decisions have been quantified.

[climate in the clay room with calculated buffered RH]

Here is the calculated buffered RH, using a simple method described in an article by Padfield and Jensen which is on the website shown at the end. Their simulated prediction is the red line. The spiky real climate record is caused by sunlight entering the room and temporarily overwhelming the reaction rate, to both heat and moisture, of the clay lining the walls.

One can conclude that designing a museum store is quite simple: it should be a lightweight superstructure on an uninsulated floor, with winter heating to reduce the annual average RH and accord with PD5454, or summer dehumidification to keep the annual average temperature lower and thus minimise the rate of chemical decay, except maybe for beeswax seals. Economically, dehumidification of a nearly airtight building with limited movement of objects and people is very cheap. It is only needed in summer when solar energy is abundant.

[Birkehoej section]

I finish by pointing out that before the twentieth century abandonment of designing buildings for naturally achieved comfort, there was a fine tradition stretching back at least 5000 years of designing for durable storage of valued relics. This is a cross section of a grave mound excavated in Denmark, showing the subtlety of its construction, designed to keep the interior dry and the temperature even. Its two metre thick capping puts it firmly in agreement with PD5454, but the only available lightweight insulation was sheeps wool and maybe the builders were not confident of its durability over thousands of years. Our task is constantly to re-evaluate building design in the light of new materials and techniques, but don't abandon using basic physics to test current fashion and sales pressure for their true merit.

[Acknowledgements]

I thank my colleagues in the museum climate group at the National Museum of Denmark, and others who have contributed data and drawings.

This lecture, and an associated, more formal article, are free to download from my website.

Lecture: http://www.conservationphysics.org/cpw/uploads/Storage/padfield-parislecture-v01.pdf
Article: http://www.conservationphysics.org/cpw/uploads/Storage/building-phys-for-museum-stores.pdf