The comfort of a home, an office, is a key requirement in designing places and environments, because it aims to improve the quality of living and ensure high levels of well-being for all users.
It is our task as designers to know, evaluate, and design the best solutions that take into account the many parameters that affect environmental well-being.
To know means to gather all the information about the features of the location (climate, dominant winds, irradiation …), the destination of the premises, the building materials, the habits of the users. Based on these indications, the positive aspects to be used and the negative aspects to be addressed through an integrated design are subsequently evaluated.
In order to evaluate the levels of optimal comfort refer to the following welfare classes:
- physical-thermo-hygrometric well-being: the state in which the individual within an environment expresses a state of thermal neutrality
- acoustic Well Being: a psychophysical condition in which an individual, in the presence of a noise field, declares to be in a state of well-being, taking into account also the particular activity he is doing
- visual well-being: sthe state in which the individual can best perform the various tasks he is called upon to fulfill
- respiratory well-being and air quality: an individual’s satisfaction state in relation to the breathing air, where there are no pollutants at concentrations considered to be harmful to human health
The feeling of environmental well-being depends on parameters related to the individual and on factors related to the environment itself. The welfare state can be expressed as a state of thermal neutrality in which the person is not affected by cold or hot feelings and can be obtained through certain environmental parameters including:
- average ambient air temperature detectable at the center of the environment at a height of 1.5 m using a dry bulb thermometer shielded by solar and infrared radiation.
- average radiant temperature defined as the average of the temperatures of the individual surrounding surfaces, weighted according to their area
- air velocity, defined as the average relative air velocity respect to a person still in the environment. It produces an increase or decrease in evaporative and convective cooling of the human body resulting in heat dissipation.
- air humidity, defined as the relationship between the partial pressure of the water steam contained in ambient air and the saturation pressure at the same dry bulb temperature of the air
- clothing (1 clo = 0.155 m2 ° C / W) defined by the overall thermal resistance of the clothing to heat transfer through them
Kind of activity performed (1 met = 58.2 W / m2) defined as the energy produced by body mass metabolism during each activity
The human body continuously exchanges heat with the surrounding environment by convection, radiation and evapotranspiration. In particular, heat exchange by radiation takes on important importance for environmental well-being as small differences in temperature between the body surface and the surfaces of the environment involve considerable quantities of exchanged energy that can cause discomfort (remember that the Physics law describing heat exchanges by irradiation Qirr = ε * σ * (T14 – T24), temperature differences are elevated to fourth power). Consequently, in order to ensure levels of comfort it is essential to avoid phenomena related to local discomfort such as:
- asymmetric radiative exchanges
- localized air currents (draughts)
- high vertical temperature gradient (warm head and cold feet)
- contact with cold surfaces (uninsulated wall)
|Results from some surveys conducted on a number of people, revealed that a 3 ° C difference in air temperature between the head and the feet causes a percentage of 5% of dissatisfied persons. UNI-EN-ISO 7730 rates as a level of acceptability for a sedentary person performing a sedentary activity, a vertical air temperature difference of 3 ° C between 1.1 and 0.1 m from the floor (head to ankle level). Direct contact with a floor with a temperature that is too high or too low causes a localized discomfort.
Among the welfare requirements, for a sedentary activity in winter conditions, the standard recommends a surface temperature of 19 ° C to 26 ° C, with the possibility of designing floor heating systems at 29 ° C. Indeed surface temperatures limit of the floor for barefoot people is slightly different: in a bathroom the optimum temperature is 29 ° C for a marble floor and 26 ° C for a wooden floor.
The asymmetry of radiant temperature due to windows or cold vertical surfaces should be less than 10 ° C and the relative humidity between 30% and 70%. To avoid one of the most common causes of local discomfort such as air draughts, it is advisable to keep the air speed within certain values depending on the air temperature.
The UNI EN ISO 7730 standard defines a method for assessing the thermal sensations experienced by a sample of people under certain environmental conditions and their degree of discomfort (thermal dissatisfaction).
By combining the various parameters (temperature, humidity, …), it is possible to assess the comfort level of an environment according to the judgment expressed by a number of people by computing two indices: the PMV index ( “Predicted Mean Vote”), which represents the average value of the votes of the sample of people who express, according to a seven-point scale ranging from +3 = very hot to -3 = cold, their own thermal sensation inside of a certain environment , and the PPD index (“Predicted Percentage of Dissatisfied”) that provides information on thermal discomfort or on heat mood, predicting the percentage of people who would feel too hot or too cold in a certain environment.
The PMV index can be determined when are estimated the clothing (thermal resistance), the activity performed and are measured the environmental parameters (air temperature, average radiant temperature, air velocity, partial water vapor pressure) . The PPD index provides a quantitative forecast of the number of people who are insensitive from the thermal point of view. The relationship between the PMV and PPD indexes is based on a research conducted on the thermal sensation expressed by a sample of 1300 subjects dressed in lightweight clothes exposed for three consecutive hours in an environment
|From the relationship between PMV and PPD described in the graph it is shown that at a PMV = 0 there is still a 5% unsatisfied person and that the PMV range of tolerance between 0.5 and +0.5 corresponds to a value of PPD of 10%. ASHRAE 55 accepts a 20% for PPD, that is a PMV between +0.85 and -0.85.
The regulations describe variations of the PMV index based on parameters such as activity level, hygrometric degree, and clothing resistance index.
The protection of environments from outside noise or from adjacent environments is becoming more and more important either for the requirement of a regulatory certification added to the current law (DPCM 5/12/97) and for the increasing sensitivity of people to noise pollution issue.
Once the external or internal noise sources are defined, the sound waves propagation modes are then evaluated within the various environments of a building that can be divided into:
- aerial noise generated by an external source (traffic, airplanes, …)
- aerial noise generated by the interior source of the building (technical spaces, elevators, commercial activities, recreational …)
- rumore di tipo impattivo (calpestio solaio)
- impact noise (floor clomping)
- noise generated by plants or people’s activities
Aerial noise propagates in two ways: by direct air, where the sound source is in the same environment as the listener who is reached directly by the sound waves without them undergoing any attenuation (with the exception of the absorption and reflection phenomena depending on the geometry and surface coatings of the environment) due to the presence of obstacles such as walls, floors.
By indirect air when the sound wave undergoes attenuation by the presence of obstacles (partition walls) in its propagation path. The type of air noise is closely related to the acoustic performance of the building structures that make up the housing, building skin (vertical walls, roofs, floors). Another mode of propagation is the structural one where sound waves are generated by vibrations of impacted structures (floor clomping noise). This type of noise depends on the acoustic characteristics of the structures (floors, partition walls), and of finishes (plaster, floors) inside the building. The acoustic welfare is evaluated in terms of acoustic quantities such as:
- sound pressure level
- level of sound intensity
- sound power level
Air propagation occurs conventionally in two ways: by direct air, that is when the sound source is contained in the room where the listener is present. In this case the sound waves that developed from the source reaches the listener without encountering obstacles and do not encounter walls to cross but only surfaces from which they are reflected. The phenomena involved are therefore substantially reflection and absorption. By air, but through partition walls, that is, when the sound wave, in its propagation path, encounters a wall to cross so that, before reaching the listener, it attenuates the amplitude according to the specific modes that we will see below.
With reference to the physical laws regulating aerial noise transmission, a shell of the building made up of different sub-systems is the element with the worst acoustic performance to affect overall acoustic behavior, so on a facade the window system and the wall attachment node represent the weaknesses to which more attention is to be given.