1       Indoor user comfort

In cooperation with UCD

1.1       General principles of comfort

Perception of comfort is subjective and it is almost impossible to satisfy the needs of each individual. Instead, designers should aim at conditions that are acceptable to a majority of users.

Following text is divided in 4 parts that describes indoor user comfort: thermal comfort, visual comfort, indoor air quality (IAQ) and acoustics.

 

1.2       Thermal comfort

Thermal comfort is the sense of well-being with respect to temperature depending on the balance between the heat produced by the body and the loss of heat to the surroundings. This balance is influenced by seven parameters: metabolism, clothing and skin temperature are related to the individual, while the air temperature, relative humidity, mean radiant temperature and air speed to the surrounding environment [UCD99]. Skin temperature is not an independent parameter: it is a function of metabolism, clothing and room temperature.

Metabolism is the sum of chemical reactions that occur in the body to keep body temperature at a constant temperature of 36.7 °C and to compensate for heat lost to the surroundings. Produced metabolic energy depends on the level of physical activity.

§  Clothing provides thermal resistance to the exchange of heat between the surface of the skin and the surrounding atmosphere.

§  Air/room temperature affects heat losses from the human body by convection and evaporation and is a very important parameter in the sensation of thermal comfort. The optimal indoor temperature depends on the region, climate and season. A temperature of 18-22 °C in winter and 24-26 °C in summer are good starting points, but this is also influenced by the activity of users and the adaptive behaviour. Optimal temperature at knee height (0.5 m) should be ensured.

§  Relative humidity is the amount of moisture in the air as a percentage of the moisture as if it were saturated at the same temperature and pressure, which affects the heat loss through the rate of evaporation. In normal circumstances, its influence on the sensation of thermal comfort is relatively small. People can tolerate a wide range of humidity between 40-70 %, depending also on the temperature. 

§  Mean radiant temperature is the average surface temperature of the elements enclosing a space. It influences the heat losses by radiation and by conduction where the body is in contact with surfaces. As a rule of thumb, mean radiant temperature should be less than 3 °C below the air temperature. Asymmetric radiation might also cause local thermal discomfort.

§  Air velocity results in a cooling sensation through heat loss by convection and increased evaporation. No excessive air movement is allowed indoors (0.1-0.15 m/s in winter and 0.25 in summer).

Specific local conditions, such as incident solar radiation, body weight or other subjective factors also affect the perception of comfort. The perception will be significantly improved if people can adapt their environment according to their needs, e.g. by controllable shading devices, possibility to open windows, changing clothing or position. The possibility to adapt the heating or cooling setpoint also improves the perception of thermal comfort.

It is impossible to specify precise values for the six main parameters due to the interactions between the parameters and the differences between individuals. Several thermal indices exist to describe the perception of thermal comfort, e.g. optimal operative temperature, comfort zones, predicted mean vote and predicted percentage of dissatisfied. The comfort equation developed by Fanger is the most commonly adopted. He also suggested easy-to-use comfort charts as design aids, which are widely used today [FAN82].

In historic buildings, the preservation of art objects and furnishing can be sometimes more demanding than the comfort requirements of users, especially regarding humidity levels. 

Energy efficiency and thermal comfort

Historic buildings usually are badly insulated, have single glazing and leaky windows, high rooms with cold downdraughts, which lead to thermal discomfort and very high energy consumption. Poor insulation results in colder internal surfaces, which then have to be compensated by higher air temperatures in order to reach the same thermal comfort.

Additional insulation will increase the mean radiant temperature of the room and allow for lower air temperatures, still resulting in the same sensation of comfort. This contributes to additional savings, as 1 °C reduction in the design air temperature can save 5-10 % in heating energy consumption depending on the climate and the building.

The high thermal mass of historic buildings is generally advantageous regarding thermal comfort, as the internal climate is relatively stable. It can result in discomfort, however, if intermittent heating is applied, since the heating up of the large mass requires a long time.

 

1.3       Indoor Air Quality (IAQ)

The air quality depends on the air quality outside the building, pollutant emissions within the building, the ventilation rate and maintenance of mechanical systems, etc. [UCD99]. These parameters do not only influence the sensation of comfort, but also the health of occupants.

Indoor pollution, e.g. constant exposure to low level emissions such as organic solvents, volatile organic compounds (VOC) and cleaning agents, can cause asthma, allergies and even more severe health problems. Historic buildings usually have very low solvent and VOC levels, but it is important that sources of pollution are also minimised during retrofit. There are international standards which define the allowable concentrations, e.g. MAC (Maximum Allowable Concentration) for work spaces, ME (Maximum Environment value) and AIC (Acceptable Indoor Concentration).

In airtight buildings, adequate ventilation rates and efficient air distribution must be provided to maintain a good IAQ. If the outdoor air is of acceptable quality, sufficient ventilation can be the solution to problems of stuffiness and odour. Sick building syndrome (SBS) is, however, observed almost exclusively in mechanically ventilated buildings. SBS occurs when occupants experience acute health and/or discomfort effects that are apparently linked to time spent in a building, while no specific illness or cause of these effects can be identified. This is not typical in historic buildings, as they were naturally ventilated. But if airtightness measures are taken and mechanical ventilation is introduced, correct installation and proper maintenance are essential to avoid such problems.

Much research has been and is still being carried out in this field, as many unknown contaminants remain, and there are also uncertainties regarding their health effects.

 

1.4       Visual comfort

Visual comfort depends on the quantity, distribution and quality of available light [UCD99]. The source of light can be natural, artificial or a combination of both. Windows are important not only as a source of light, but also their psychological effect is invaluable.

§  The required light level (quantity) is determined by the particular activity. These values are fixed in standards.

§  Distribution is often more important than the quantity in itself. Perception of brightness is influenced by the evenness of lighting levels and also depends on the reflectivity of walls and other surfaces. Contrast is the difference between the appearance of an object against that of its immediate background. Glare is an excessive contrast by the introduction of an intense light source (sun, interior light sources) into a visual field, causing discomfort or fatigue.

§  Quality of light is hard to define. It includes the direction of light, colour and variation over time. Daylight has excellent quality. The light spectrum of artificial light should be as close as possible to that of natural light.

§  In historic buildings, the glazing to façade ratio is low, but the enlargement of windows is usually not good practice. In deep-plan buildings, the installation of a lightpipe can be a way of providing adequate daylighting for internal rooms or basements. Here, daylight enters the pipe through a roof opening for example, then reflected and intensified along a mirror-finish tube. Diffuse light enters the rooms through a clear, polycarbonate dome in the room’s ceiling.

§  Grand central staircases and atria are often originally roof-lit or will allow the introduction of roof-level glazing. In the roofspace, the addition of dormers or rooflights may be acceptable. In Mediterranean climates special care must be taken to minimise the risk of overheating (e.g. solar-control glazing, openings). Rooflights were traditionally covered with whitewash in the spring, this was gradually washed away before winter and prevented the entry of light and heat in summer [FEI03].

§  In historic buildings, too much light may also cause problems, as its UV component fades textiles and paintings and embrittles leather. Blinds or shutters might need to be added for preservation.

 

1.5       Acoustic comfort

Source of noise can be external (e.g. traffic), internal (loud or disruptive noises generated by activities within the building), building construction and finishes (impact noise from hard finishes) and building services (e.g. mechanical ventilation) [UCD99].

Air-borne sound transmission can be minimised by eliminating gaps in the external envelope and internal partitions. External noise may compromise the possibility for natural ventilation through open windows. Ventilation grilles between interior spaces should be provided with sound baffles and adjacent spaces should be of similar use. Sound transmission can also be reduced by increasing the mass of structural building elements. Historic buildings, being made of heavy materials, can usually resist intrusive noise, but there are some weak points. Badly fitting windows and doors need edge sealing, while ceilings might need extra insulation. For windows with thin glass, insertion of a heavy secondary glazing can be a solution.

When retrofitting a historic building, its acoustic characteristics have to be measured and the interventions planned according to the requirements. Problems may arise if the modern function will differ from the original. Long narrow historic buildings with linked volumes either side give discontinuities in reverberation. Cube and double-cube rooms, domes and barrel vaults have problems regarding reflections [FEI03]. Historic buildings are usually built with hard finishes with low sound absorption. Reverberation times in rooms are therefore typically long. Carpeting and similar finishes will increase sound absorption, while floating floors and suspended ceilings reduce impact noise transmission. In this case other sound-absorbing elements should be applied (see Figure 61).

As far as possible, building services should be located in unoccupied spaces. Drainpipes should not be carried in ducts next to living rooms or bedrooms. Ventilation fans should be as large as possible so as to run at the lowest possible speed.

Figure 61: Sound absorbing panels in an office

 

1.6       References

[FAN82]          Fanger P.O. Thermal comfort, analysis and applications in environmental engineering, Florida, Robert E. Kreiger Publishing Co., 1982.

[FEI03]            Feilden B. M. Conservation of Historic Buildings, Architectural Press, Elsevier, Oxford, 2003.

[UCD99]         UCD Energy Research Group, Dublin. A Green Vitruvius, James and James, 1999.