Generally speaking, in architecture human comfort is dependent upon a number of factors:
Thermal Comfort
Maintaining the body at an acceptable temperature.
Air Quality
Ensuring the provision of clean air within enclosed spaces.
Illumination
Provision of adequate light levels.
Sound Quality
Clarity of communication and protection from noise pollution.
Sanitation
Providing for water distribution and waste disposal.
Subjective Perception and Adaptation
Human comfort is considered to be partially subjective, in that humans may perceive a room to be warmer or cooler than it actually is, e.g because of its color and materials. Variation in human comfort levels also results from evolutionary adaptations, i.e. where a body has adapted its specific physiology as a response to local environmental conditions.
Of these factors, however, it is a structure’s ability to provide the body with good air quality that is the most important of its functions. Therefore, this section will focus on the principles of thermal comfort and its provider, the sun.
Human comfort depends chiefly upon thermal comfort. Our core body temperature must remain at a constant 98.6°F. The body generates heat even while at rest. Indeed, the body must always be losing heat to maintain comfort because it produces more heat than it needs. In order to maintain equilibrium, the body must maintain a fairly steady rate of heat loss of around 300 Btu/hr when it is at rest and more when active. Lose too much heat and you feel cold, too little and you are hot.
Body heat is transferred through convection, conduction, radiation, and evaporation. When the ambient temperature is higher than skin temperature, the heat gained by radiation and conduction must be dispersed mainly through the evaporation of perspiration (see Passive Control/Evaporative Cooling, p. 88).
The body generates heat even while at rest. Indeed, the body must always be losing heat to maintain comfort because it produces more heat than it needs. When internal body temperature is insufficient the body starts to shiver, which in turn increases the production of body heat. In animals covered with fur or hair, “goose pimples” are created when muscles at the base of each hair contract and pull the hair erect as a response to cold. The erect hairs trap air to create a layer of insulation. As humans have lost most of their body hair, the reflex now serves little purpose.
The main factors influencing thermal comfort are:
Air Temperature
Air temperature is governed ultimately by solar radiation (see Solar Geometry, page 76).
Mean Radiant Temperature
Radiant temperature is governed both by the temperature of an object and its emissivity – its propensity to emit long-wave radiation. Mean radiant temperature (MRT) is the average radiant temperature of all objects within view of the subject and can vary significantly from air temperature (for example even a tightly sealed single pane window feels “drafty” on a cold winter day because its radiant temperature is much lower than other interior surfaces}. Because we lose a substantial amount of heat via radiation, MRT is as important a determinant of comfort as air temperature.
Air Movement
Air movement (a breeze or draught) is governed by air pressure. A breeze of around 20 in per second provides an equivalent temperature reduction of around 5.4°F.
Humidity
High humidity levels reduce evaporation rates. For human comfort, relative humidity should be between 40 per cent and 70 per cent. However, when relative humidity exceeds 60 per cent, our ability to cool is greatly reduced.*
*Relative humidity is an indication of the water content of air. It is measured as the percentage of the actual water vapour density to the saturation vapour density (both are measured as mass per unit volume). Saturation vapour density is the amount of water vapour needed to saturate the air, and varies according to temperature. For example, if the actual vapour density is 1⁄100 oz/ft³ at 68°F compared to the saturation vapour density (at that temperature) of 1⁄400 oz/ft³, then the relative humidity is 57.1 per cent.
1 “Goose pimples.”
2 A thermal image of the faculty of the thermal physics laboratory, Vanderbilt University, Nashville, USA.
Earth's Orbit
The earth orbits the sun in a counter-clockwise elliptical orbit once every 365.26 days. It spins counter-clockwise on its north–south axis once every day. (This accounts for the fact that the sun rises in the east and sets in the west.) This axis is tilted with respect to the plane of its orbit at an angle of about 23.4 degrees. The average distance from the earth to the sun is around 93 miles. In relative terms, if the earth was 1 inch in diameter, then the sun would be an 8 ft diameter disc around 984 ft away.
The Equinoxes
The equatorial plane divides the earth into halves – the northern and southern hemispheres. The intersection of the equatorial and ecliptic planes is called the line of equinoxes. One half of this line is the vernal (spring) equinox and the other half the autumnal equinox. At two points in the earth’s orbit this line intersects the sun, making the start of the fall or spring season. Perpendicular to the line of equinoxes is a line which contains the solstices. These are the points that start summer or winter when they cross the sun.
The Calculation of Solar Radiation
To calculate the position of the sun on any given day at a certain place on earth, two angles must be specified: the solar altitude and the solar azimuth. The altitude angle is the angle in a vertical plane between the sun’s rays and the horizontal projection of the sun’s rays. The azimuth angle is the angle on the horizontal plane measured from the north or south to the horizontal projection of the sun’s rays.
If we take a given location, e.g. Blackpool, UK, its location can be described as a pair of coordinates:
Latitude 53º 46’ 12’’ N
Longitude 03º 01’ 48’’ W
Using a sun-path diagram we can calculate that there is a maximum difference in altitude (how high the sun appears in the sky) between the winter and summer solstices of approximately 45 degrees and a maximum difference in azimuth (where on the horizontal plane the sun appears and disappears) between the summer solstice sunrise and sunset of approximately 270 degrees.
Summer Solstice Sun Position:
21 June 12:00 noon
altitude 59.73º, azimuth 180º (due south)
Winter Solstice Sun Position:
21 December 12:00 noon
altitude 12.73º, azimuth 180º (due south)
The hours of available (or potential) sunlight at the summer and winter solstices differ significantly. During the longest day (21 June) in Blackpool there is a potential 17 hours 6 minutes of sunlight from sunrise to sunset, whereas on the shortest day (21 December) there are a maximum of 7 hours 24 minutes.
Location-specific sun path-diagrams afford the designer a basic knowledge of where and when direct sunlight will fall on any given design. It is worth remembering that sunlight can be described graphically as parallel rays, due to the sheer size of the sun in relation to the earth. It may also be useful to note that moonlight is reflected sunlight.
For more detailed analysis of solar geometry, software packages such as ecotect can be used to model a specific sunlight condition in relation to a particular design. See Environmental Analysis, page 128.
The WMO (World Meteorological Organization) states the definition of normal sunlight as 120W/m2 or more shining on the earth’s surface. Sensors such as pyranometers are available to measure this threshold.
1 Sun-path diagram for Blackpool, UK, showing solar altitude and solar azimuth in relation to time and date. http://solardat.uoregon.edu/SunChartProgram.html.
2 Altitude and azimuth.
3 Multiple exposure photograph showing the “midnight sun” visible in high northern latitudes.
Climatic envelope: basic principles of thermal comfort as related to the building envelope under different climatic conditions.