First, the environmental data measured during the experiments are reported. Then, the results of the thermal and overall subjective perception ratings analysed with the linear mixed-models are described, followed by the results of the physiological responses. Finally, the comparison between PMV and asv is illustrated.

Indoor environmental conditions

Thermal environment

Indoor temperature was measured at a distance of 50 cm from each participant and at four heights (0.1 m, 0.6 m, 1.1 m and 1.6 m), corresponding to ankle, body, head (sitting person) and head (standing person) levels according to the EN ISO 7726 standard64. Both air temperature and globe temperature were recorded, so as to also capture the temperature resulting from the radiation of the adjacent surfaces, air temperature and air velocity. Operative temperature values, calculated from the measured air and globe temperature13,18, were used in the analysis. The resulting values did in fact not differ too much from either globe or air temperature due to the nature of the radiant heating and cooling system and the very low air velocity: the mean absolute difference between globe and air temperature was 0.19 °C, while the root mean squared difference between globe and air temperature was 0.24 °C. Differences were on average larger at the lowest height given the proximity of the sensor to the radiant floor. For each participant, an average operative temperature was calculated over the four measurement heights.

Figure 3 illustrates the average operative temperature for the three thermal conditions investigated, in each daylight exposure. The design temperature is indicated with a black dotted line to observe the trends of the measured temperatures in each daylight conditions. In fact, the test room allowed the setting of a specific temperature, but the different filters applied on the glazing did slightly affect the indoor thermal environment due to changes in solar heat gains (i.e., the solar transmission was higher when the visible transmittance was higher). As a result, at the end of each daylight exposure the operative temperature was higher compared to the beginning, and a slightly higher operative temperature resulted under the high illuminance level, followed by the medium and the low ones. Table 3 reports the average operative temperature values for each design temperature level measured at 20 minutes after the beginning of each daylight exposure, when the differences in temperature values across daylight levels were larger. As anticipated, these temperature variations across daylight levels were considered in the statistical analysis with the inclusion, together with the design temperature levels (i.e., 19, 23 and 27 °C considered as nominal levels), of the covariate ∆T. The latter was calculated as the difference from the design temperature level (e.g., 23 °C) and the measured operative temperature (e.g., 22.8 °C) at 20 minutes after the beginning of each daylight exposure as this time corresponded to the start of the questionnaire.

Figure 3 Average operative temperature in each temperature and daylight level combination. Full size image

Table 3 Average ± SD operative temperature in each temperature and daylight level combination (values in °C). Full size table

Other indoor parameters involved in the perception of the environment were constant across daylight and temperature levels (i.e., air velocity at 0.03 ± 0.005 m/s; CO 2 content at 1115.2 ± 183 ppm) or changed only across temperature levels but within a range considered as comfortable (i.e., relative humidity: 56.8 ± 6.6% at 19 °C, 44.6 ± 8.9% at 23 °C and 42.3 ± 4.4% at 27 °C).

Visual environment

The visual environment was constant within each experimental session as only sunny days were retained for the analysis (i.e., the daylight illuminance did not change). This was possible as experimental sessions with unpredicted changes in sky conditions and in illuminance levels were not included in the analysis, as already anticipated. On the other hand, illuminance conditions were not exactly the same across experimental sessions (i.e., in different days). The recorded horizontal illuminance (calculated as the average of two values measured at the desk of each participant, on their right and left) at each exposure level can be summarised as follow:

Low daylight illuminance: 136 ± 20 lx (min 90 lx, max 214 lx)

Medium daylight illuminance: 608 ± 90 lx (min 432 lx, max 796 lx)

High daylight illuminance: 1443 ± 183 lx (min 1049 lx, max 1929 lx)

The observed variations in daylight illuminance values, despite the inclusion of only sunny days in the analysis, are due to changes in the sun position across the year and the day, and the presence of haze and atmospheric turbidity65, conditions changing across experimental sessions (and not within them).

Subjective perception ratings

Results related to thermal perception are discussed first, followed by those regarding overall perception. For the thermal perception analysis, each question is analysed individually and the main effect of daylight illuminance and its interaction with temperature are first reported. To check the validity of the experiment and the questionnaire used, the main effect of temperature (intended as the design factor with three experimental levels) is also described. For the overall perception analysis, main effects of both daylight illuminance and temperature are described, together with their interaction. Figure 4 reports the graphical outcomes of the two types of subjective ratings at each temperature level and for the three daylight illuminance levels.

Figure 4 Subjective thermal and overall perception responses according to temperature and daylight illuminance levels. (a) Thermal sensation. (b) Thermal preference. (c) Thermal comfort. (d) Thermal acceptability. (e) Overall comfort (beginning of exposure). (f) Overall comfort (end of exposure). Significant effect of daylight levels with “*”p < 0.05, “**”p < 0.01, “***”p < 0.001. The same effect is reported on all the bars for all the temperature levels whenever there is a main effect of daylight, otherwise separate effects of daylight at each temperature level are indicated (i.e., thermal comfort). Significant effect of temperature levels with “#”p < 0.05, “##”p < 0.01, “###”p < 0.001. Full size image

Thermal perception

The analysis of thermal perception responses is divided according to the four questions investigated: thermal sensation (global and referring to the three body parts), thermal preference, thermal comfort and thermal acceptability.

Linear mixed model analyses showed that daylight illuminance did not influence thermal sensation, nor did interactions between temperature levels and daylight illuminance. The variations in thermal sensation votes across daylight illuminance levels that can be observed in Fig. 4a depended on the temperature differences across the illuminance levels, illustrated in Fig. 3. In the model, this difference is considered with the inclusion of ∆T, a factor that was shown as being significant for thermal sensation based on the statistical analysis (F(1,245) = 11.09, p = 0.001), with larger ∆T associated to higher or lower thermal sensation votes compared to those associated to smaller ∆T (i.e., the higher the measured temperature, the higher the thermal sensation vote; the lower the measured temperature, the lower thermal sensation vote). Nevertheless, it must be remarked that, although the average temperature differences between daylight illuminance levels were similar (see Fig. 3 and Table 3), the average differences in thermal sensation votes were not as constant, especially at 19 °C and at 27 °C. At 19 °C, the average thermal sensation votes were comparable under the medium and the high illuminance levels, but they were lower under the low illuminance one. At 27 °C, the average thermal sensation votes were comparable under low and medium illuminance levels, but they were higher under the high level. Although not significant, these results indicate a tendency of participants to feel cooler at low temperatures under low illuminance level compared to medium and high illuminances, and to feel warmer at high temperatures under high illuminance level compared to low and medium. As expected, temperature was also a significant factor for the determination of the thermal sensation (F(2,245) = 74.25, p < 0.001), with participants expressing to be between cool and slightly cool at 19 °C (M = −1.3, s.e.m. = 0.84), between slightly cool and neutral at 23 °C (M = −0.5, s.e.m. = 0.84) and between neutral and slightly warm at 27 °C (M = 0.5, s.e.m. = 0.75). These differences were shown to be significant following post-hoc pairwise comparisons of all possible combinations. The analysis performed for the thermal sensation of hand, feet and trunk reported the same results as for the global thermal sensation, confirming the results previously described.

Similarly to thermal sensation, thermal preference results (Fig. 4b) were not affected by daylight illuminance either, nor by its interaction with temperature. Only ∆T and temperature levels were shown to have significant effects (p = 0.001 and p < 0.001, respectively). Participants expressed a preference for a slightly warmer-warmer environment at 19 °C, no change-slightly warmer at 23 °C and no change-slightly cooler at 27 °C.

Results are different for both thermal comfort and thermal acceptability, with daylight illuminance significantly affecting such thermal evaluations. First of all, the interaction between daylight and temperature was a significant factor for thermal comfort, in particular when only the low and the high daylight illuminance conditions were investigated at 19 °C and 27 °C (F(1,107) = 6.73, p = 0.012), as well as at 23 °C and 27 °C (F(1,102) = 6.14, p = 0.016). As can be seen in Fig. 4c, and following analyses at each temperature level with only the low and high daylight illuminance levels, the thermal environment was less comfortable under the low illuminance compared to the high one at 19 °C (estimated difference of 0.48, p = 0.03 after post-hoc test), whereas it was less comfortable under the high illuminance level compared to the low one at 27 °C (estimated difference of 0.28, p = 0.03 after post-hoc test). Differences between low and high daylight illuminance levels were not significant at 23 °C, thermal condition in which the medium illuminance appears as the most comfortable one (Fig. 4c). Temperature resulted as a significant main factor for thermal comfort, with post-hoc analyses showing significant differences between 19 °C and 23 °C (p = 0.01), and between 19 °C and 27 °C (p = 0.01), with people being always less thermally comfortable under 19 °C.

The interaction term was not significant for thermal acceptability responses. On the other hand, daylight illuminance was a significant factor (F(2,247) = 6.4, p = 0.001), with a less acceptable thermal environment under the low illuminance level compared to both the medium and the high illuminances (p = 0.004 and p = 0.009, respectively). Despite the lack of interaction with temperature, it was possible to see that this result specifically occurred at 19 °C and 23 °C, temperature levels considered as cool-slightly cool and slightly cool-neutral, respectively (Fig. 4d). At 27 °C, thermal acceptability responses were equal under all the light levels. As for thermal comfort, thermal acceptability votes were affected by temperature (F(2,247) = 8.74, p < 0.001), with participants less accepting of the thermal environment under 19 °C compared to 23 °C (estimated difference of 19.01, p < 0.001 after post-hoc analyses) and under 19 °C compared to 27 °C (estimated difference of 14.37, p = 0.009 after post-hoc analyses).

∆T, calculated as the difference from the measured indoor temperatures and the three temperature levels (i.e., 19, 23 and 27 °C), had a considerable effect on all the thermal perception responses, except for thermal acceptability. As explained in more detail for the thermal sensation responses, the inclusion of the factor ∆T increased the accuracy of the evaluation as it followed the direction of the model (e.g., higher measured temperatures resulted in “more extreme” thermal sensations).

Sex and order of daylight illuminance levels were significant factors for thermal sensation and preference. The running mean outdoor temperature substantially affected thermal sensation and comfort. Thermal comfort was also influenced by participants’ BMI and morning and afternoon exposures.

Overall perception

The overall comfort perception was evaluated at the beginning (Fig. 4e) and at the end (Fig. 4f) of each daylight illuminance exposure. At the beginning of the exposure, overall comfort perception was significantly affected by daylight illuminance levels (F(2,247) = 7.40, p < 0.001) (Fig. 4e), with the low illuminance level resulting in the lowest overall comfort compared to the medium level (estimated difference of 0.27, p = 0.001 after post-hoc analyses) and to the high level (estimated difference of 0.23, p = 0.008 after post-hoc analyses). The main effect of temperature was only marginal at the beginning of the exposure (F(2,247) = 2.87, p = 0.06). On the other hand, at the end of the exposure (Fig. 4f), both daylight and temperature had a significant main effect (F(2,246) = 7.25, p < 0.001 and F(2,246) = 5.27, p = 0.006, respectively). The increasing effect of temperature on overall comfort at the end of the exposure can be seen especially for responses at 19 °C, which were significantly lower in comparison to those under the neutral and the high thermal conditions (estimated difference of 0.45, p = 0.001 and of 0.42, p = 0.002, respectively, after post-hoc analyses). Figure 5 reports the reasons of overall discomfort at the two exposure times. At the beginning (Fig. 5a), when the indoor temperature was still not affected by the incoming sun from the window (hence, no big real temperature differences were present across daylight levels), more complaints about the thermal environment were reported under the low illuminance level, followed by the medium and high levels, especially at the temperature levels considered as slightly cool (i.e., 19 °C and 23 °C). Complaints about the visual environment decreased at the end of the exposure compared to the beginning, as participants indicated the thermal environment as the reason of overall discomfort more often. This is an interesting fact because, even though overall comfort responses at the end of the exposure depended on both daylight and temperature levels as previously reported (Fig. 4f), participants mainly indicated “thermal” as the reason of their dissatisfaction, rather than both “thermal” and “visual”.

Figure 5 Reasons of overall discomfort at the beginning and at the end of each daylight exposure. Full size image

Physiological responses: skin temperature

Daylight was never a significant factor for the skin temperature measurements in any of the four body locations, considering the results within both the first five minutes and between fifteen and twenty minutes (the time right before the subjective perception questionnaire). As expected, measurements were mainly affected by temperature and by the baseline values (p < 0.001 in both cases).

PMV and thermal sensation vote comparison

Thermal sensation vote (asv = actual sensation vote) of participants were considered binned in each temperature level independently of the daylight illuminance level and were compared with the PMV values, calculated from the measured physical conditions (Fig. 6). Results show that the average values of asv differed significantly from the PMV ones, but only at the warmer thermal condition (27 °C) according to the Wilcoxon signed rank test (p < 0.001 and d = 0.61). At this temperature level, the asv vote was substantially lower (M = 0.52, SD = 0.75) compared to the PMV (M = 0.96, SD = 0.21). At 23 °C the difference resulted marginally significant (p = 0.053 and d = 0.37), whereas at 19 °C no significant difference was reported by the test (p = 0.42). The larger standard deviation of the reported asv votes compared to the calculated PMV values that can be observed in Fig. 6, was due to the inclusion of results in different daylight illuminance levels that, as reported before, led to changes in thermal perception ratings. A comparison of asv and PMV votes in each illuminance-temperature combination is illustrated in Fig. 7. In Fig. 7, it is possible to see that the overestimation of the PMV in comparison to asv at 27 °C is particularly visible under the low and the medium daylight illuminance levels. Under the high illuminance level at 27 °C, the difference between asv and PMV is smaller. This result further corroborates the effect of daylight illuminance levels on subjective thermal perceptions of people.

Figure 6 Comparison between actual thermal sensation vote and PMV at the three temperature levels (mean ± s.e.m.). Full size image