Introducing heating into re-erected traditional buildings

Introduction

Changing lifestyles, the development of more efficient insulation materials, combined with the rising cost of fossil fuels, has led to a growing awareness of the potential benefits of using the building itself, and in particular the floor slab, (acting as a heat reservoir), as a potentially efficient and cost-effective means of heating houses.  The purpose of this paper is to investigate the potential benefits of introducing under-floor heating into re-erected traditional buildings within a museum context, and more specifically at St Fagans: National History Museum.

Background

The purpose of any building is to provide accommodation and shelter for its occupants, livestock, belongings, machinery or crops. In the case of human occupants it is important to provide not just shelter (from wind & rain), but also, if possible, a degree of comfort. Traditionally, this generally took the form of a fire, which not only heated the room, but could also be used for cooking, preserving meat & fish (by smoking), for keeping valuable commodities (such as salt) dry, or for drying out the dog if he’d just fallen into the mill pond!

The adoption of the chimney, during & after Middle Ages, meant not only the more efficient removal of smoke from rooms, but also enabled the insertion of a second floor, something that was not possible with earlier open-hall arrangements.  The heat from the fire could now be used to heat not only the room in which the fire was located, but also the room above. If the chimney stack was built of brick or stone, it also served as a means of holding and transferring heat, or at least warmth, into the rooms above.  Where there was a central stack, heat could be distributed through even more of the building.

Fuel for fires came from whatever was available locally – wood, peat, coal or culm (a mixture of coal dust and clay). Traditionally, the fire was never allowed to go out.  Last thing at night the coal, peat or culm would be built up into a mound and then covered with a thin layer of clay.  Using a poker, 1 or 2 holes would be made in the top – to act as smoke vents.  A kettle of cold water was then hung above the fire.  By daybreak, when the first occupants woke up, heat from the enclosed fire had boiled the water.  The clay covering was cracked open and the fire was built up once again.

The Museum Context

Moving a traditional building into an open-air Museum setting such as St Fagans provides the opportunity to show how it would have looked in the past, how it was furnished, and decorated, and how people lived and worked there. Wherever possible, fires are lit in the houses, in order to show not only how people heated their homes and cooked in the past, but also as a means of keeping warding staff warm and to try and achieve the best environmental conditions for both the building structure and display items such as furniture.

However, being a Museum, recreating the past can give rise to certain problems. For instance, it is not usually possible to keep fires lit for 24 hrs a day as would have been done originally, because of security and health & safety considerations. Consequently, fires are lit just before the Museum opens to the public at 10am, and are allowed to go out by the time it closes at 5pm.

Not keeping a building warm can create its own problems when, for instance, it comes to lighting a fire. It has been found that in some of the cottages, when fires are lit in the morning, when the buildings are cold, the rooms fill with smoke. This situation can remain for several hours in wintertime, until, that is, the chimney has warmed up and smoke begins to be drawn up it more efficiently.

If a graph were to be drawn showing the temperature and humidity levels in one of these houses, we would find that they would fluctuate quite considerably during the course of 24 hours. When the fires are lit in the mornings, the inside temperature gradually builds up and the humidity levels gradually drop.  By the afternoon, the buildings are comfortably warm.  Then, from about 3pm, by which time the houses have reached their optimum in terms of warmth and RH, the fires are allowed to go out. Our graph would show temperatures falling and humidity levels creeping up again. These pendulum-like fluctuations mean that it can be difficult to control moulds, rot and dampness in some buildings. 

It is therefore important to try and achieve the best balance. Too high a level of RH can result in mould growth and rot; too dry, and there is a danger of the buildings, and especially timbers, drying out too quickly, resulting in shrinkage, cracking, twisting and splitting. Keeping a fire in the house overnight would serve to maintain more balanced conditions inside.

However, even if it were possible to keep the fire alight after the Museum closed to the public, one would still be faced with another problem. Whereas, traditionally, the fire was prepared last thing at night, and could be kept going until the family woke at daybreak, this represents a maximum period of about 10 hours. The Museum scenario of leaving the fire unattended from 5pm to 10am would mean keeping it alive for 16 – 17 hrs, which would be virtually impossible.

Case Study 1: St Fagans Castle (Elizabethan gentry house)

One possible solution that has been explored at St Fagans, is the introduction of some form of under-floor heating into some of the buildings. This was first tried in 1991 when major refurbishment works were carried being out at St Fagans Castle and in particular the kitchen, which was to be re-displayed to its late Victorian appearance.  There had long been a problem with this room.  The floor level is lower than the remainder of the ground floor, and the flagstone floor was always cold and damp, with moisture condensing on the surface. This had obvious implications as far as furniture was concerned, as it gave rise to a situation where contact with the moist floor surface created ideal conditions for moulds.

The decision was taken to install electric cables under the floor, with the flagstones serving as a heat store. This was controlled by a thermostat located about 3m above floor level.  The results were encouraging.  The problem of condensation was removed and, as long as the thermostat was not interfered with, it maintained a reasonable temperature inside and much improved levels of RH.

Case Study 2: Fron Haul (slate-workers' houses)

In 1998 the Museum was asked to supervise the removal and re-erection of a small terrace of four houses from Tanygrisiau, near Blaenau Ffestiniog, Gwynedd, to the National Slate Museum, Llanberis.  As with St Fagans Castle, it was decided to install under-floor heating.  Once again, an electrical system was chosen, the objective being to provide low-level background heat, effectively to keep the interiors from ‘chilling’, especially at nights and in winter.

The cables were laid in zig-zag lines across the floor, with several circuits in each room, arranged in such a way that two circuits occupied each area or zone.  Thus, if one circuit failed, there was always a second circuit to maintain warmth.  Had all the circuits been kept separate, if one failed, unheated 'cold spots' would have resulted within the room. 

The installation has worked successfully for the past seven years, and, even though it is known that one section of wiring has failed, the second circuit has enabled the room in question to stay warm.  The fact that the floors were covered with slate flags obviously helped, as these were able to act as ‘storage heaters’.

Case Study 3: The ‘Green’ House

In 2001, the Museum hosted an architectural competition to design a ‘House for the Future’. The winning design, the ‘Green’ House, by architects Jesticoe & Whiles, was conceived as an energy-efficient, sustainable building.  It incorporates many elements that hark back to traditional constructional techniques,- the use of oak-framing, slate roofing and clay block walls for instance.  All windows are double-glazed and there is a strong emphasis on high insulation specification.

The building can be heated in several ways.  Large areas of glazing on the south side mean that the interior can quickly be warmed by direct sunlight.  A ground source heat pump linked to a 35m deep borehole circulates heated water via a compressor unit.  A constant temperature of 10ºc underground is raised three-fold in the heat pump and this is then fed under the floor, which acts as a giant heat reservoir. 

There is also a small stove in the main living space downstairs which burns pellets made from recycled sawdust.  However, the building is so efficient that the stove has only been used on very rare occasions.

Case Study 4: Aluminium Prefab

Post-war prefabs were notorious for their poor insulation qualities.  Former residents often spoke about how they would bake in summer and freeze in winter.  When the Museum was offered such a building in 1998 the decision was taken to see whether matters could be improved somewhat.  Although there was a coal fire in the living room, with a primitive form of central heating where cold air drawn from the hallway was heated and fed through ceiling grilles into the two bedrooms, this was never a particularly successful design. So the Museum opted for an under-floor solution again.  Sheets of wired ‘foils’ - similar to like large toaster elements, - were stapled to the underside of the timber joists under the floors. Extra insulation was added in order to improve performance.

The system has not been a total success.  This is partly due to the fact that the outside doors are kept open most of the time when the building is open to the public, resulting in considerable loss of any heat that has built up.  It has also resulted in the floor boards drying out and shrinking, so that gaps have started to appear between the boards.  That said, at least at nights it is now possible to maintain a degree of low-level background heating.

Case Study 5: St Teilo’s Church

The major rebuilding project currently under way at St Fagans is the re-erection of St Teilo’s church and its refurbishment to its possible appearance about 1520.  The building is built using masonry and lime mortar.  It incorporates a lot of original timberwork in the roofs of the porch, south aisle and north chapel and a substantial amount of new (green) oak in the nave and chancel as well as the carved Rood screen which would have separated the congregation from the high altar in the chancel.

It was realised quite early on that there could be advantages in incorporating under-floor heating within the building, not only to maintain comfortable working conditions during  winter, but also as a means of controlling RH levels. 

Using the experience gained of the various systems already tried out at Museum sites, it was decided to use a ground source heat pump (GSHP).  This time, instead of drilling down 35m as had been done at the ‘Green’ House, coils of pipework (slinkies) were laid the 2m down in 6 trenches extending some 50m each.  Water pumped through these coils was then fed back through the heat pump (which effectively acts like a fridge in reverse) and into the church, then via a set of manifolds through pipework laid under the floor. 

The floor itself was specially constructed with a geotextile membrane laid first, then 150mm of insulation, another layer of geotextile, followed by 150mm of limecrete.  The pipes were then laid out and fixed onto the limecrete base before being covered by 50 – 75mm of lime screed and finally flagstones. 

The system here is controlled not by thermostats, but rather by humidistats which measure the RH.  Again, the intention is not so much to heat the building, but rather to provide a degree of low-level background warmth and to enable a RH level of around 70% to be maintained.  The system is still being monitored and refined, but results to-date appear very promising. 

For instance, data collected during the period 27 November to 12 December 205, shows that the outside temperature fluctuated around freezing point for about 5 days, then hovered around 7ºC for about 4 days before fluctuating again above and below freezing point. Such temperatures would normally have meant that no building work, especially plastering, using lime, would have been possible for most of that time, as 7ºC  is considered the critical point, below which fresh lime plaster would probably fail. 

However, thanks to the use of the GSHP, the inside temperature actually remained above this level for most of the time, enabling work to continue and our deadlines to be met.  The plasterwork did not suffer from any of the shrinkage or crazing that would normally have been expected when working at that time of year.  

Equally critically, the new timberwork has not suffered from having heating in the building, because the system is controlled by a humidistat rather than a thermostat (& RH levels did not fall below 60%).  The system installed in the church is not intended as a means of heating the building, though it can be increased above its present level, and will certainly take the chill off the temperatures on a bleak January morning.

Summary

Is under-floor heating therefore the answer for maintaining levels of background warmth and RH levels in re-erected traditional buildings in Museums?  In some cases, as we have seen, the answer will almost certainly be yes. 

However, one must err on the side of caution, and recognise that, as with the aluminium prefab, there may be better or more appropriate methods available for some buildings.  Buildings with beaten earth floors, for instance, may also be a problem, as introducing heat into such floors may well cause them to dry out, become brittle and break up. 

But where floors are of solid mass-construction and where greater control of RH is important, then the use of some form of under-floor heating may well be the answer.