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Thermal summer comfort behind glass. Are the classic comfort indicators sufficient?

Cold radiation from glass surfaces is controlled by low-emissivity coatings and triple glazing. Bu what about summer comfort behind glass?

The phenomenon is well known. On a summer day we feel the scorching heat of the sun through the car window on our bare arm, despite air conditioning being on maximum. The same phenomenon exists in buildings. Thermal discomfort can occur in the summer when a person is in the vicinity of a window, even though the building is cooled and is equipped with 'solar control' glass. [Arens, 2015]

Thermal summer comfort

Comfort modelling

This discomfort phenomenon is not detected with current comfort analysis methods, not even with dynamic indoor comfort simulations. Recent standards evaluating comfort, such as the American ASHRAE 55, ISO 7730 and the European CEN-16798 (the successor to the EN 15251 standard), completely ignore the effect of short wave radiation, which is typical for direct solar radiation, on the human body. [Arens, 2015]

Conventional comfort indicators such as PMV (Predicted Mean Vote) and PPD (Predicted Percentage of Dissatisfied), defined by the Danish professor Povl Ole Fanger in 1970, simplify the boundary conditions of the comfort analysis and make an abstraction of both direct solar irradiation on a person and the physical position of the person in space. In these models, only the average radiation temperature of the surfaces is taken into account (Mean Radiant Temperature), thus ignoring some of the most important factors of the comfort experience. [Hoffman, 2012]

To compensate this, more advanced comfort models have been developed. These comfort models take into account the radiation temperatures of the surrounding surfaces individually and in function of the physical position of the person (directed operative temperature), so that radiation asymmetry can be determined. This method only eliminates part of the possible causes of discomfort. For instance, the effects of direct solar radiation (short wave radiation) on the person’s body are not taken into account with this approach. [EQUA, 2018]

The methods described above are therefore inadequate for the evaluation of thermal comfort in the vicinity of a glazed facade. In order to mathematically describe this phenomenon, new innovative methods have been developed. The Berkeley Comfort Model interprets comfort experience through the effect of various environmental influences on the human body, such as convection, long wave radiation, short wave radiation, diffuse radiation, and direct radiation. For this, the human body is divided into 18 zones and metabolic reactions are also taken into account. [Hoffman, 2012]

In a practical application of this model for a hospital room in Denver, with a peak radiation of 985 W/m² (for reference: peak radiation in Belgium is 750 W/m²) and assuming 3 combinations of glazing/sun protection: 1) a clear glazing without external sun protection and 2) a glazing with a g-value of 0.33 (33% of the incident solar energy is transmitted; classical clear glazing allows 60% to 70% of the solar radiation) once without and 3) once with external sun protection, the following conclusions were reached [Hoffman, 2012]:

The study shows that only the latter combination offers sufficient comfort. Although glazing with a g-value of 33% lets in significantly less solar energy, it appears that a low g-value contributes only to a limited extent to the thermal comfort of the human body in the vicinity of the window. The g-value therefore does not appear to be a sufficient indicator to guarantee thermal comfort. Outdoor sun protection, on the other hand, appears to significantly improve thermal comfort.

Arens E. et al. (2014) have developed a simplified comfort model in which direct solar irradiance on a person is taken into account and converted into an increase in the Mean Radiant Temperature (MRT), which is a parameter in the Predicted Mean Vote (PMV) comfort index. This method has been validated on the basis of test set-ups with test subjects. Also in this model the importance of low direct shortwave radiation becomes apparent. A maximum total transmission of 15% (glass + shading, as in the 3rd test set-up by Hoffman (2012)) is assumed to avoid an excessive increase in subjective thermal perception [Arens E. et al, 2014]

The calculation model by Arens et al. also makes it possible to visualize the impact of direct solar radiation on the Mean Radiant Temperature (MRT) in a room.

local comfort/discomfort of different parts of the body in the case of solar control glass without additional solar protection

The impact of indoor sun protection

Indoor shading is commonly known to be less efficient in controlling incoming solar energy than a outdoor shading. Using spectrum analysis in the simulation software EQUA IDA ESBO 2.3, which calculates the transmissions under the different spectra and the different angles of the solar radiation on the basis of a full spectrum analysis, it was investigated which combined g-values ​​(glass + sun protection, defined as g(tot) can be achieved and what other phenomena can occur when using interior screens to improve thermal comfort in the vicinity of glazed facade parts. The following combinations have been investigated: 1) glass with a g-value of 0.45 and inner sun protection in white cloth, 2) glass with a g-value of 0.45 and a highly reflective inner sun protection, type Verosol Enviroscreen, 3) glass with a g-value 0.33 and a highly reflective inner sun protection, type Verosol Enviroscreen. As a comparison, a test case has been added with glass g-value 0.33 and light gray outer screen.

The combination of a conventional interior sun shading system with solar control glass with a g-value of 0.45 results in a g(tot) value of 0.34. Only the combination of glass with a g-value of 0.33 and a highly reflective inner sun protection results in a g(tot) value of 0.15, which Arens quotes as the upper limit to sufficiently restrain the effect of short-wave radiation. However, we notice that a second effect occurs, namely the heating of the interior sun protection fabric. This heating leads to long wave radiation (infra-red), which in turn leads to discomfort.

visualization of the virtual increase in MRT due to direct solar radiation with the CBE MRT visualizer (Center for the Built Environment Berkeley)

Quality of daylight

Reducing the g-value of glass by means of coatings and/or tinted glass has two related side effects. Initially, the light transmission (LTA) decreases because the coatings and/or tinted glass transmit less light. Secondly, the color fastness of the light decreases as only part of the visible sunlight spectrum is transmitted. This light spectrum has a particular role in controlling the hormonal cycle of the human body, and thus in the physical and emotional well-being of the users of the building.

Subjective comfort feeling

Comfort feeling is partially dependent on the person and partially on objective circumstances. Extensive research has shown that with the same objective comfort conditions (temperature, humidity, clothing, etc.), the comfort levels reported by the users are significantly influenced by the extent to which the user can intervene on his environment. It has been established that having control over, among other things, sun protection and the opening of windows significantly increases the subjective feeling of comfort. [Schweiker, 2013]

Conclusion

Although not all studies and models have been completed, it can be stated that:

– Current comfort indicators do not guarantee good thermal comfort in areas with close proximity to glazing because short wave radiation is not taken into account.

– Short wave radiation can cause discomfort for the human body in the vicinity of glazing.

– Even with low g-value glazing, too much short wave radiation reaches the body so that the discomfort experience can be significant.

– Sun protection is necessary to get discomfort effects under maximum control. Research recommends a maximum short wave transmission of 15% (glass + (indoor) sun protection).

– The application of an interior sunshade reduces short-wave radiation. However, long-wave radiation is caused by the heating of the fabric.

– Offering the possibility to the user of the building to actively intervene in the amount of solar radiation contributes significantly to the subjective feeling of comfort.

In buildings where the users stay in the vicinity of glazed facade elements for a short or longer period of time, the provision of solar shading is therefore a necessary measure from a comfort-technical point of view.

References

Arens Edward et al, Modeling the comfort effects of short-wave solar radiation indoors, Building and Environment, 2014

EQUA Simulations AB, IDA ICE 4.8 Reference Manual, 2018

Hoffmann Sabine et al., Assessing thermal comfort near glass facades with new tools, BEST 3 building enclosure science and technology conference Atlanta, 2012

Hoffmann Sabine et al., Balancing daylight, glare, and energy-efficiency goals: An evaluation of exterior coplanar shading systems using complex fenestration modelling tools, Energy and Buildings, 2015

Subramamiam S., Employing Radiance in thermal comfort simulations involving complex fenestrations, conference presentation, 2018

Schweiker et al, Explaining the individual processes leading to adaptive comfort: Exploring physiological, behavioural and psychological reactions to thermal stimuli, Journal of Building Physics, 2013

Tzempelikos A. et al., Indoor thermal environmental conditions near glazed facades with shading devices – part II: Thermal comfort simulation and impact of glazing and shading properties, Building and Environment, 2010

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