Interaction of historic building fabric and atmosphere

architrave

3000 words – 15 min

The examples given in this section are for illustrative purpose, many variations exist.

Hygrothermal materials

Matching the colour and texture of mortar is of obvious aesthetic concern during conservation and restoration. However, the purpose and behaviour of the constituent parts must also be understood to avoid damage, degradation and ultimately failure.

Bricks and mortar

A commonly understood example is the re-pointing of soft fired vapour ‘open’ clay brick – with a hard vapour ‘closed’ cement pointing. It is not specifically the use of cement that damages the brick. The constituent parts of lime mortar can create a mismatch to the masonry. Equally a ‘closed’ masonry can be mismatched to an ‘open’ mortar.

[photo Edinburgh – NHL5]

The composition of the mortar affects the workability, strength, durability, flexibility, setting times and ability to self-repair. The characteristics are determined by particle size and distribution, round or sharp, vapour open or closed and whether pozzolans are present or added. Some materials are locally abundant, naturally or as waste, others are well travelled and prized.

A myriad of ingredients alter the properties of the mortar: silica sand, stone dust, calcite, chitin, keratin, horse hair, cow dung and blood. Instead of understanding how this alchemy is achieved, we ‘refer to local knowledge’, an empirical mix, for which the why has long been lost.

Plentiful local masonry, whether brick or stone, can be used for the entire build, or supplemented with materials brought in from further afield. Each with characteristics that are prized and prioritised for specific details. Some applications require clay bricks to be ‘closed’, hard fired in the extreme heat of the kiln. Others need to remain ‘open’ and are soft fired at a more modest temperature or simply baked dry in the sun. Similarly, the stone may be selected to be either more ‘open’ or ‘closed’ as well as for its ability to weather change.

How the stone is quarried also influences how it performs in use. Selected and cut; either along the direction of the bed, perpendicular to the direction of pressure or selected as ‘free’, each piece has a purpose whether for constructing a head, sill, mullion or jamb. Different cuts lend themselves to a variety of structural applications and yet. Through variations in composition, there would also appear to be some logic to how the building junctures relate to each other.

As cooled air falls away from a cold surface, warm moisture-laden air is drawn in to replace it. ‘Closed’ materials are slow to respond. Moisture vapour is concentrated on the adjacent ‘open’ materials, particularly when in proximity to denser materials that are capable of increasing thermal lag. The density and composition of a hygroscopic material affect how it and the adjacent materials engage and respond to the changes of external weather as well as how they engage and respond to each other.

More open materials benefit by responding to warm moist air and are warmed by moisture transfer. The warmer surface reduces the risk of moisture condensing as well as slowing the convection current delivering moisture.

Hygroscopically, ‘open’ material respond quickly to the moisture vapour present. Some have a good capacity for variation in vapour and resist failure. Other materials can quickly reach saturation, increasing the risk of degradation and inevitable failure. Choice materials are selected to buffer the moisture storms created by the difference in thermal mass at junctures. These buffer zones need to respond quickly without failure, moving moisture away from the point of attack and distributing it widely throughout the component and onto the adjacent. Distributing more widely and in all directions, prevent saturation at a single point and assists the passage to evaporation. The density and composition, as well as the direction of the bed, would appear to influence these characteristics.

More than, igneous, sedimentary, metamorphic or man-made; the density, composition and structure of both the mortar and masonry all influence and respond to the materials as well as the environment around them. Their capacity for moisture vapour and speed of response acts as a buffer, a reservoir that prevents vapour from ‘dropping out’. If detailed correctly, the structure is protected from degradation, diffusing moisture to the slower materials around them or back out into the air once the peak has passed.

By careful positioning appropriately materials, traditional buildings would appear to order them in a hierarchy to remove discord and create harmony. These patterns can be observed both inside and out.

Hygrothermal exteriors

The building envelope can in simple terms be described as a box, yet its simplicity conceals variations in hygrothermal behaviours that need to be managed. The heavy base of the walls have large thermal mass and additionally are in contact with the ground, further increasing their thermal inertia. Party walls add additional mass to the façade, whereas corners have a greater exposed surface area to volume, likewise the details around windows and doors. Diurnal temperature and moisture fluctuations set up repetitive air current as different elements of the building warm up and cool down at different rates.

Many factors influence where cold spots attract moisture vapour to the outside of a building and none more than the evaporative cooling of water. Buildings are detailed to shed water, rooftops and coping stones channel water away from the façades. Canopies shelter doors and windows, even the modest overhangs of a window sill reduces exposure to rain as well as breaking up the repetitive air currents that bring moisture vapour to drop out on the section of the wall beneath the window.

The effects of repetitive air currents on thermal mass can be managed by both the use of choice materials as well as the design of their detailing. By managing these influences the thermal envelope’s ability to cope with both external and internal weather is improved.

The plinth, like a bell cast in the render, alleviating the misunderstood symptoms of rising damp, by breaking up the air as it falls down the façade, reducing moisture drop out where, because of its contact with the ground, the base of the wall is cold. Quoin stones at corners manage the lower mass to exposed area, exposure to wind and the increased evaporative heat loss. Whereas inversely, at junctions with party walls, they buffer the increase in thermal mass, where internally moisture is also attracted to the corners.

Choice stone is often ornamented to maximise the opportunity to stay dry and diffuse moisture effectively. They are also positioned to correspond with specific internal elements. Dentils and architraves keep roof rafters and bearer plates dry, horizontal banding corresponds to the location of the floor joist and the entablatures, above windows, ensure that the timber lintels behind are kept moisture free. Keeping dry from the outside, as well as ensuring the diffusion of moisture vapour that arrives from within.

When warm moisture-laden air is conveyed to the cold surface of a window, the closed glass and gloss paint, protecting the timber, directs the convection of cooling air to the wall beneath. The sill, cut from choice stone, passes right through the wall aiding good diffusion of moisture at this dew point and the overhang of the sill shelterers the masonry below. The spandrel panel, below the window, is often embellished to increase surface area and the opportunity to evaporate, again keeping them dry outside and enabling the diffusion of moisture that arrives from within. If this section of wall is finished outside in render, removal will more often reveal a more ‘open’ material than the rest of the build, often open clay brick is used and may also extend around the whole aperture, with a brick arch to the top and brick jambs to the sides.

The detailing around a window provides a critical buffer zone. Constructed from choice materials that are quick to respond and have good capacity to diffuse moisture, so reducing the risk of failure. A sill cut from ‘freestone’ is suited to diffusing moisture through the wall beneath the window. However, it is too soft to form a lintel. Therefore the head, cut from stone with a stratified bed is laid with its laminations in a vertical plane along its length from side to side to improve its strength. The jambs at the sides, have a less pronounced bed but still carry laminations along its length now from top to bottom, its strength is supported by the adjoining wall. The mullion is an unsupported central column. Cut perpendicular to bed, the laminations are horizontal, stacked like pancakes and provide great resistance to shear.

Where the different orientations of bed meet at the juncture of mullion and head, the mass of the lintel and the wall it supports are slower to respond than that of the mullion. Hygrothermal conflict will occur and with time degradation, at this point of load, will lead to failure. This detailing can be seen in the classical assembly of, architrave, abacus, capital, column and plinth.

Choice materials need to be selected. The architrave, chief beam, is laminated for spanning the load and the abacus, a padstone, is used to bridge the division of the architrave. A capital of ‘freestone’ is placed between the abacus and the top of the column. If stone, ‘free’ from the bed, is used for the full length of the column, it would be susceptible to shear. However, this short section of, ‘freestone’, has the correct composition to quickly diffuse moisture, furthermore if ornamented, then the pull of repetitive air currents are broken up, whilst also increasing the surface area that improves evaporation. Similarly, at the base, the plinth is resilient to moisture vapour whilst also being detailed to break up repetitive air currents. The materials chosen may change, but the principles of this detailing can be seen in examples from cultures across the world.

Hygrothermal interiors

An interpretation of this classical assembly can also be seen inside, unravelled around the simple box of a room. There are great differences in thermal mass, as well as the response time to vapour, that need to be managed between the different components. As warms air fills the room, from top to bottom, convection pulls the warm air from the ceiling, towards the mass of the walls and then onward down towards the floor. Hygrothermal behaviour needs to be considered in the selection of materials as well as the detailing for them to be resilient to change but also opportunistic for buffering moisture and energy transfer.

Ornamented plaster ceilings have been used for many centuries all over the world, including, Greece, Roman, Moorish, Islamic, and Indian architecture. In pre-Tudor Britain, ceilings were simply the underside of floors, not a practical solution for dirt, noise and most importantly warmth. As with the arrival of any new technology, the early adopters of plastered ceilings were more likely the wealthy. This might lead to the assumption that rather than for practical comfort a richly ornamented ceiling was solely for the purpose of displaying one’s wealth.

However, the choice of constituent parts and the order of application would suggest a need for the material to respond quickly to moisture with reduced risk of failure and in doing so, capture, not just the moisture, but also the warmth into its mass.

As one example. The lath and plaster ceiling is made from thick riven lath, perhaps of sweet chestnut, it is known for its resilience to moisture, pests and decay. It is timber of choice, not forestry waste. The plaster applied is made from the burning of chalk and then mixed with fine unfired chalk to produce a smooth fine ‘open’ plaster. The base coat has coarse calcitic aggregate and plenty of keratin, as horse, cow or goat hair. This thicker layer adds thermal inertia and good capacity for moisture buffering. The thinner layer of topcoat, although limited in capacity, with smaller particle size, gives greater surface area to the aggregate, responds more quickly and is then able to diffuse this to the coarse layer beneath. The addition of distemper, aids this process further, the fine chalk is mixed with animal glue creating a thin but highly responsive outer layer.  

The surface area and mass can be further increased with ornamentation, for improved hygrothermal performance. Ornamentation also breaks up air currents, creating turbulence, assisting a steady transition of moisture. Ornamented or not, not all the moisture and energy contained within the air is captured by the ceiling on its way to the walls.

cornice

Where the ceiling meets the walls, another potential storm needs to be managed. The walls have greater mass than the ceiling and will take a little longer to warm. Here the cornice performs an important function. Made from good hygroscopic materials the cornice also breaks up the air that would become trapped, ‘eddy’ in this corner, concentrating potential moisture drop out at the top of the wall. The greater the mass of the wall the, ground floor walls are greater, and higher the ceiling, ground floor ceilings are higher, the greater and grander the need for the cornice. This cornice can be extended in part down the wall and ornamented again to increase surface area, create turbulence and finished with a distemper of chalk and bone glue.

Now the air, with its moisture reduced and broken with turbulence, meets the walls. The walls may be finished with distemper or preferably papered as their first line of defence. Wallpaper continues the tradition of hanging fabric on walls such as tapestries. Good wallpaper is made from linen, known for its moisture buffering characteristics and beneath the linen, lining paper should also be used.

The cellular structure of wallpaper affects how it responds to moisture. Having been pulped, the material’s fibres open and responds well to vapour. Wallpaper paste is soaked into the paper. Made from methylcellulose, wallpaper paste is hydrophilic, used for sizing paper, it reduces the papers ability to absorb water by capillary action without reducing its ability to absorb moisture vapour. Before Papering, the surface of the plaster is also prepared with glue-size, also hydrophilic, inhibiting the acceptance of water whilst improving acceptance of vapour.

The combination rapidly responds to moisture vapour when it arrives, diffusing through the paper, then each layer of plaster, as previously describes, until it engages with masonry behind. Within a room, the walls have varying thermal mass. Internal walls may be lath and plaster on timber stud, brick or a timber frame infilled with brick or other material. Where brick is used internally, it is soft fired and more open to moisture. These can also be used on the inside of external walls or perhaps prioritised for chimneys or under windows.

skirting

Further down, the remaining moisture vapour being conveyed by the air reaches the base of the wall. Even in an upstairs room, the base of the wall will inevitably be the coldest part; a skirting will be required. Like the cornice above, the skirting boards protect the base of the wall from the eddy current that assists and concentrates the delivery of moisture vapour to the corner. The skirting is an insulated vapour control layer, resisting peaks in moisture when they arrive. If the plaster came down to the base of the wall, it would absorb excessive moisture and be prone to failure.

Skirting performs the same function as the plinth at the base of a column, preventing airborne moisture vapour from dropping out or condensing where the increased thermal mass makes the base of the wall cold. The top edge of the plinth or board is profiled, ’removing the arris’ removes the sharp corner, this inhibits degradation at the tip where a small volume of stone or timber will receive moisture from both sides. The running of for example a torus mould removes the arris and when undercut with a grove, further distributing the area to volume, add a cincture on top to assist the sweep away from the wall and the skirting becomes an ‘Ogee’. The cincture and torus can also be seen at the base of the column where it meets the plinth.

Downstairs, walls are thicker and in contact with the ground, increasing thermal mass. Raised off the ground, suspended floors lessen this effect, whereas solid floors or those that sit below ground will have additional thermal mass. The higher the mass of the wall, the larger the skirting board required. It is sometimes extended up the wall with panelling. This insulated vapour control layer prevents airborne moisture vapour from condensing in as well as on the base of a wall.  Symptom misinterpreted as ‘rising damp’ can be resolved by understanding it as condensation.

Skirting is mounted on twisted timber pegs, separating it from the cold base of the wall. The outer face of timber is prepared with a lime-rich primer. Penetrating in, it reduces the timbers ability to absorb water by capillary while improving the capacity to diffuse vapour. Further, more oily undercoats are applied, the surface is now prepared to bind to a hard finish coat. This glossy vapour control layer, containing metals such as white lead, minimises the migration of moisture. Appropriately detailed, gloss painted joinery is used at junctures with a high-risk of condensation. Skirting, architrave, doors and windows are all protected at the dew point.

Conclusions

The historic building fabric was designed and detailed to manage moisture vapour by successfully interacting with the atmosphere. Before modifying the original, the function of the original form should first be fully understood.

This lack of understanding has resulted in renovation maintenance and improvement works that have caused unacceptable negative consequences. Simply by understanding whether a particular building element was intended to be moisture vapour open or vapour closed would improve the approach. Form without function is dysfunctional.

The modern renovation process covers or replaces vapour open materials with a bricolage of modern vapour closed alternatives. The principles of ornamentation no longer apply, the wallpaper is stripped away from protecting the lime plastered walls. It is an unwanted old fashion for which the skills to replace are no longer valued. The lime plaster exposed is seen as old and needing to be replaced. Polyvinyl acetate and cement are applied, followed by gypsum plaster then sealed with washable paint. The joinery is stripped of its protective layer of gloss to expose the beauty of the wood, the purpose of the door is now warped. Open-plan living distributes the vapour generated by activities throughout the house; the laws of thermodynamics no longer apply. Where once heavy linen curtains buffered moisture and cold, now excessive damp gatherers, damaging the health of both the home and its occupants.

The design of our traditional dwellings encapsulates the collective wisdom over time. It is now possible to reverse engineer the knowledge of the past. Researching how the original fabric interrelates with a new product must not present a conflict of interest. Such research will require double-loop learning and real-world collaboration.

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