Context 164 - May 2020

16 C O N T E X T 1 6 4 : M A Y 2 0 2 0 could compromise the building’s lifespan, that must also be considered. If the lifespan of a retrofit is short, and the carbon input high, this can easily overwhelm apparent carbon saving over the lifespan of the building. But in focusing on the mechanics of sealing and insulating buildings, are we missing another still more critical point? These risky and invasive measures are being taken for one principal rea- son: to prevent the loss of conditioned (heated or cooled) air from the interior. That in turn is because of a paradigm that the comfort of occu- pants – and therefore a building’s use – centres on the interior air temperature. We assume that we can only be comfortable in a very narrow air-temperature range, but this is a paradigm well overdue for questioning. Looking at history, this emphasis on air tem- perature is very recent, and arguably a conse- quence of the burning of fossil fuels. Before the 18th century there were no practical ther- mometers; and the development of the Rumford grate to burn coal (introduced at the end of the 1700s) was the first attempt in this country to heat the interior air since the Roman hypocausts. This quickly became fashionable, and over the course of the next 200 years, comfort became increasingly based on controlling temperature, and on installing equipment to do that. This proved carbon-hungry from the outset, and it had unintended consequences for comfort, such as greatly increased draughtiness. As the cost of temperature control rose, build- ings began to be sealed to trap the conditioned air. Researchers soon started to point out prob- lems with this: for example, that air temperature is a poor analogy for comfort (which varies from person to person, according to taste and level of activity); and that sealing the building leads to damp and health problems. What did people do to make their buildings comfortable before the introduction of the space heating? To understand traditional approaches, we need to revisit the underlying causes of thermal discomfort: essentially, an imbalance between a person and their surroundings. The body is an excellent thermal regulator, turn- ing energy from food into heat that circulates through the blood stream.To lose heat, the blood is sent to the skin, so that it can be transferred to the surroundings. Some two per cent is lost into still air, rising to 22 per cent if the air is moving and the skin is wet. A cold wet day can feel significantly colder than an icy dry day, despite being several degrees warmer, while it is often humidity that makes a hot day uncomfortable: as a marker of discomfort, humidity is extremely important. Some heat is lost by direct contact to surfaces. We know that feet are particularly susceptible: if your feet are cold, you will feel cold. Most heat (60–65%) is lost by radiation into the surrounding surfaces. These processes can be desirable if you are trying to lose heat; when exercising, for example. All this was well understood in the past by observation. Most actions to combat cold were designed to cut radiant heat loss: floors were covered with mats; cloths were hung on walls, and draped to make canopies that cut heat loss upwards. Contemporary paintings show that drapes occasionally covered entire walls, but more often they were simply hung behind where the person was sitting. In glazed buildings, such as chapels, they were hung across the bottom of the windows to trap the air chilled by the glass as it fell. In summer, when losing heat was desir- able, cloths could be taken down: tapestries were packed away over summer. Tapestries are now the best known radiant break, but much more popular in England, were painted cloths, which LIFESPAN CARBON OUTPUT Carbon without retrofit Carbon with retrofit: no repair or replacement Carbon with retrofit: regular repair Carbon with retrofit: regular repair and replacement Lifespan of retrofit measures