- Original article
- Open Access
Comparison of correction factor for both dynamic total thermal insulation and evaporative resistance between ISO 7933 and ISO 9920
Journal of Physiological Anthropology volume 39, Article number: 23 (2020)
Thermal insulation and evaporative resistance of clothing are the physical parameters to quantify heat transfer and evaporative dissipation from the human body to the environment, respectively. Wind and body movement decrease thermal insulation and evaporative resistance of clothing, which is represented as correction factors for dynamic total thermal insulation (CFi) and evaporative resistance (CFe), respectively. Then, CFi and CFe are parts of the key parameters to predict heat strain of workers by computer simulation. The objective of this study was to elucidate the difference of CFi and CFe between ISO 7933 and ISO 9920 and compare the difference of predicted rectal temperature, water loss, and exposure time limit calculated by using each correction factor.
CFi of ISO 7933 (CFi7933) and ISO 9920 (CFi9920), and CFe of ISO 7933 (CFe7933) and two kinds of CFe of ISO 9920 (CFe9920a, CFe9920b) were compared in terms of relative air velocity, walking speed for three kinds of thermal insulation of clothing. Next, two modified predicted heat strain (PHS) models were developed: modified PHS integrated with CFi9920 and CFe9920a (PHSmA) and modified PHS integrated with CFi9920 and CFe9920b (PHSmB). We calculated the rectal temperature, water loss, and exposure time limit by PHS, PHSmA, and PHSmB and compared the results.
CFi7933 and CFi9920 were almost similar in terms of Var and walking speed, while CFe9920a and CFe9920b were larger than CFe7933 when Var was more than 1.0 m·s−1. Intrinsic clothing insulation (Icl) diminished the effects of Var on CFi7933, CFi9920, CFe7933, and CFe9920b. However, CFe9920a was not influenced by Icl. The predicted rectal temperature and water loss difference were larger between PHS and PHSmA as CFe difference got larger. The duration time when limit of rectal temperature of 38 °C was reached (DlimTre38) calculated by PHS was significantly longer than PHSmA, PHSmB at higher Var.
Precise correction factors for evaporative resistance are required to predict rectal temperature, water loss, and work-time limits.
Clothing decreases heat transfer between the human body and the environment through convection, conduction, radiation, and evaporation. The thermal characteristics of clothing are mainly represented by total thermal insulation (IT) and evaporative resistance (ReT). Thermal insulation and the evaporative resistance of clothing depend not only on the clothing itself (fabric and design, such as apertures or folds in the clothing), but also on several conditions, such as the wearer’s activity level, relative air velocity (Var) applied to the wearer, and posture [1,2,3,4,5]. Air flow promotes heat transfer by increased air permeation through the clothing fabric and openings inside the clothing. Human subject experiment using an ergometer showed that forced draft decreased body core temperature and sweat rate by increasing evaporative heat loss [6, 7]. A wearer’s activity also increases heat transfer by the exchange of air between the inside of the clothing and the environment by “pumping effects”  through openings in the clothing. The walking movement of thermal manikin facilitated heat dissipation by convection and evaporation. Experimental results of mean skin and core temperature of a walking manikin were closer to a walking human trial data than a standing manikin . When sweat absorbs heat from the skin, it turns into vapor. Thereafter, vapor is transferred to the environment to a greater extent through higher ventilation due to air flow or a wearer’s activity. Increased vapor transfer to the environment decreases the microenvironmental humidity inside the clothing, which promotes evaporation on wetted skin, leading to increased heat transfer between the body and the environment. In a hot environment, the avenue of heat transfer from the human body to the environment is mainly through the evaporation of sweat due to decreased heat transfer by convection from decreased temperature difference between the skin and the environment. When heat loss by sweating is suppressed by vapor-impermeable clothing, though it serves to protect the human body from hazardous materials, thermal physiological strain increases [10,11,12].
IT and ReT in dynamic conditions are required to calculate heat transfer to the environment and to predict core temperature, water loss, or skin temperature, etc., in thermal models. However, the limited availability of climate chamber, sweating thermal manikin with active simulation, and laminar air flow make it difficult to measure thermal insulation and evaporative resistance of clothing under various specific conditions for air velocity or activity. Then, to numerically predict IT and ReT under wind or active conditions from static conditions, some studies on correction factors for dynamic total thermal insulation (CFi) [1, 13,14,15,16,17,18,19,20] and evaporative resistance (CFe) [2, 15, 16, 21] have been carried out.
Havenith et al.  investigated the effects of both walking and wind on the insulation value of clothing ensembles by conducting human subject experiment. Nilsson and Holmér  evaluated the total insulation in the wind and by walking via a thermal manikin. The equations for CFi and CFe in ISO 7933  were developed as part of the BIOMED EU-project led by Malchaire. Three papers [14, 15, 21] were produced in the project. By utilizing the data of Havenith et al.  and Nilsson and Holmér , Holmér et al.  proposed CFi for over 0.6 clo of clothing insulation (Eq. 30 in ) and nude (Eq. 29 in ) under walking and wind. CFi for nude in  was later changed to Eq. 3 in  by Havenith et al. Havenith and Nilsson  proposed the equations for CFe from the empirical relation of change in im with change in heat resistance. Parsons et al.  summarized the results and proposed the computer code. Equation 30  and Eq. 3  were included in the present ISO 7933 . In the revision of ISO 7933 , predicted heat strain (PHS) model was proposed by Malchaire et al.  and adopted in the International Organization for Standardization (ISO) as ISO 7933 . PHS was validated using laboratory (672 experiments) and field (237 experiments) data . Following the publication of ISO 7933 , the ISO committee ISO TC159/SC5/WG1 started the ISO 9920  revision. For this work, Havenith [17, 18] reanalyzed more data from a cooperation between Nilsson and Havenith, from Kim and McCullough , and from Nilsson et al.  on CFi in addition to Holmér et al. . CFe in ISO 9920  was also changed from that of ISO 7933 . By analyzing a more extensive data from Havenith laboratory compared to what was used in ISO 7933 , it was found that CFe of ISO 7933  seemed to have overestimated the effects of movement and wind on vapor resistance. After discussions in ISO TC159/SC5/WG1 and presentation of the data, CFe was revised to that of the present ISO 9920 . For static conditions, the Lewis relation and static moisture permeability index (imst) were used to estimate evaporative resistance from thermal insulation for both ISO 7933  and ISO 9920 . For dynamic conditions, CFe was derived from the empirical relation between CFi and dynamic moisture permeability index (imdyn) in ISO 7933 . On the other hand, in ISO 9920  two equations were provided to calculate CFe: one was an empirical equation including relative air velocity and walking speed, and the other was an empirical relation including CFi.
The key predictions of PHS are rectal temperature and sweat rate. Some researchers evaluated the predictions of PHS by comparing with physiological data. Kampmann et al. showed a pronounced underestimation of rectal temperature and correct estimation of sweat rate in moderate activity . Parsons  pointed out that applying PHS in the assessment of rapidly changing environments and short exposures was not possible. Lundgren et al. also showed that intermittent work exposure challenged the accuracy of the PHS model . They provided the data on the overestimation of PHS simulation on rectal temperature in heavy activity and cooling effect of sweating in recovery . Wang et al.  demonstrated that rectal temperature and sweat rate predicted by PHS were higher than those of human subject data when wearing higher thermal insulating or higher evaporative resistance clothing than the scope of PHS model. From the perspective of occupational hygiene in terms of thermal environment, it is important to determine the maximum allowable exposure duration. In ISO 7933 , a maximum allowable exposure time is provided based on rectal temperature reaching 38 °C or a cumulative sweat loss limit based on acclimation state. In the determination of the duration time limit, environmental conditions, metabolic rates, and thermal characteristics of clothing should be inputted as important factors. CFi and CFe also play an important role in predicting heat strain under dynamic conditions. It was reported that CFe of ISO 7933  and ISO 9920  were different, and the CFe difference predicted the duration time limit difference for exposure . To predict a suitable CFe, it was necessary to further compare among ISO 7933 , two kinds of ISO 9920 , and experimental data.
The purpose of this paper was to compare the two kinds of CFi or three kinds of CFe for ISO 7933  and ISO 9920  with experimental results and to study the effect of differences in CFi or CFe on predicted rectal temperature, cumulative water loss, and duration time limit of exposure under constant conditions.
Two kinds of CFi (CFi7933: Eq. 1.1–3 and CFi9920: Eq. 2.1–3) and three kinds of CFe (CFe7933: Eq. 3, CFe9920a: Eq. 4, and CFe9920b: Eq. 5) were summarized in Table 1. There were three functions of CFi in terms of walking speed and relative air velocity according to the intrinsic thermal insulation of clothing (Icl). Relative air velocity changes depending on conditions in ISO 7933 : When the data on walking speed and walking direction to wind are provided, relative air velocity is calculated as the VECTOR difference between air velocity and walking speed. When walking direction is unknown, relative air velocity is air velocity if air velocity is larger than walking speed and otherwise walking speed. When both the direction of walking and walking speed were not known, relative air velocity is supposed as air velocity. To avoid complexity in the calculation of relative air velocity from air velocity or walking speed, relative air velocity was directly used to calculate CFi or CFe in this paper. To predict static total water vapor resistance (ReT) from static total thermal insulation (IT), imst and Lewis relation were applied in ISO 7933  and ISO 9920 . In our calculation, imst of 0.38 was used as normal clothing. Here, walking included only the effect of body movement by walking, excluding the effect of air velocity due to walking. First, we compared the independent effects of relative air velocity and walking speed between CFi7933 and CFi9920. Table 2 shows the validity of ISO 7933  and ISO 9920  concerning relative air velocity, walking speed, and Icl. First, when we calculated the effect of relative air velocity on CFi, walking speed was fixed at 0.01 m·s−1 and relative air velocity varied from 0.0 to 3.0 m·s−1. For walking speed effect, relative air velocity was fixed at 0.15 m·s−1 and walking speed varied from 0.0 to 1.2 m·s−1. Second, we similarly compared the independent effect of relative air velocity and walking speed among CFe7933, CFe9920a, and CFe9929b. Third, we compared the combined effects of relative air velocity and walking speed on CFi or CFe. The relative value of CFi9920 to CFi7933, CFe9920a to CFe7933, and CFe9920b to CFe7933 was calculated for a two-dimensional area of relative air velocity from 0.0 to 3.0 m·s−1 and walking speed from 0.0 to 1.2 m·s−1.
Integration of CFi9920 and CFe9920a or CFe9920b to PHS
To investigate the effect of different correction factors on PHS, two modified PHS (PHSmA, PHSmB) were developed: PHSmA with CFi9920 and CFe9920a, and PHSmB with CFi9920 and CFe9920b. First, we calculated the difference of final rectal temperature and water loss calculated between PHS and PHSmA under condition A, condition B, condition C, and condition D (Table 3) with a maximum calculation time of 1 h. In this study, we supposed continuous work for 1 h without taking a break. Other calculation conditions are presented in Table 3: moderate metabolic rate (145 W·m−2), height (1.70 m), weight (65 kg), drink available, acclimated, imst (0.38), relative humidity (RH) (from 0 to 100%), and ambient temperature (from 30 to 40 °C). Moderate metabolic rate corresponds to sustained hand and arm work (hammering nails, filing) and arm and leg work (off-road operation of lorries, tractors, or construction equipment) . Furthermore, 1.0 clo (1 clo, 0.155 m2·°C·W−1) of Icl was used because 1.0 clo was the maximum thermal insulation of clothing, as shown in Table 2. Next, duration time when limit of rectal temperature of 38 °C was reached (DlimTre38) was compared between PHSmA and PHS and between PHSmA and PHSmB at four conditions of relative air velocity and walking speed (conditions A, B, C, D) with a maximum calculation time of 8 h. Paired t test was used to test the significant difference between each model. Here, we supposed a worst-case scenario: continuous work for 8 h without taking a break. The total number of plots was 847, where RH varied from 0 to 100% at 10% intervals, ambient temperature from 30 to 40 °C at 1 °C intervals and mean radiant temperature from ambient temperature to ambient temperature + 60 °C at 10 °C intervals. The other calculation conditions of metabolic rate, height, weight, heat acclimation, imst value, and drink availability are the same as listed in Table 3.
CFi7933 and CFi9920 decreased similarly with relative air velocity and walking speed in nude and under the clothing thermal insulation of 0.3 clo and more than 0.6 clo (Fig. 1a). The reduction rates of CFi7933 and CFi9920 for relative air velocity were largest in nude, second in 0.3 clo, and least in larger than 0.6 clo. However, CFi7933 and CFi9920 did not differ in nude, the clothing thermal insulation of 0.3 clo and more than 0.6 clo in terms of walking speed (Fig. 1b). CFe7933 was smaller than CFe9920a and CFe9920b in both for relative air velocity and walking speed. For a nude condition at 3.0 m·s−1 of relative air velocity, CFe9920a was larger than CFe7933 by more than three times. For larger than 0.6 clo, CFe9920a and CFe9920b were almost the same and were about two times larger than CFe7933 (Fig. 2a). For the effect of walking speed, CFe9920a and CFe9920b were also as large as CFe7933 (Fig. 2b). We compared the combined effect of relative air velocity and walking speed on CFi7933 and CFi9920, CFe7933 and CFe9920a, and CFe7933 and CFe9920b in Fig. 3. For CFi, the ratio of CFi9920 to CFi7933 was almost 1 in the large part of the calculated scope (relative air velocity 0.15–3.0 m·s−1, walking speed 0.01–1.2 m·s−1), indicating that CFi9920 and CFi7933 were almost the same (Fig. 3a–c). For CFe, both the contours of the ratios of CFe9920a and CFe9920b to CFe7933 were almost parallel to the y-axis, which means that both CFe9920a and CFe9920b change similarly in terms of walking speed (Fig. 3d–i). The ratio of CFe9920a to CFe7933 was the largest in nude (Fig. 3d), next in 0.3 clo (Fig. 3e), and the smallest in more than 0.6 clo (Fig. 3f). The ratio of CFe9920b to CFe7933 was not different in terms of clothing thermal insulation in the calculated scope (Fig. 3g–i).
Figure 4 provides the predicted rectal temperature (Fig. 4a–d) and water loss difference (Fig. 4e–h) between PHS and PHSmA for a 1-h exposure with a scope of ambient temperature from 30 to 40 °C and RH from 0 to 100% in four conditions (conditions A–D). Under condition A, in high ambient temperature and RH region, the predicted rectal temperature by PHSmA was higher than that by PHS. The maximum rectal temperature difference in the scope was about 1.4 °C (Fig. 4a). And the maximum water loss difference was about 400 ml (Fig. 4e). The region where the predicted water loss differed was almost the same area as that of the rectal temperature difference (Fig. 4e). Under condition B (Fig. 4b), the region where predicted rectal temperature and water loss differed was similar to condition A. The maximum rectal temperature difference was about 1.0 °C (Fig. 4b) and maximum water loss difference (Fig. 4f) was about 300 ml. Under condition C or D, both rectal temperature and water loss between PHS and PHSmA did not differ as much as condition A or B.
The differences in DlimTre38 between PHSmA and PHS and between PHSmA and PHSmB are shown in Fig. 5 for the four conditions (conditions A, B, C, and D). DlimTre38 differences between PHS and PHSmA and between PHSmA and PHSmB were the largest under condition A and second largest under condition B. The largest DlimTre38 differences between PHS and PHSmA were 454, 444, 377, and 309 min under conditions A, B, C, and D, respectively. The largest DlimTre38 differences between PHSmA and PHSmB were 434, 310, 89, and 71 min under conditions A, B, C, and D, respectively. DlimTre38 values of PHS were significantly larger than PHSmA and PHSmB by paired t test under the four tested conditions (P < 0.001). DlimTre38 values of PHSmB were significantly larger than PHSmA by paired t test under the four tested conditions (P < 0.001).
In this study, we compared CFi and CFe between ISO 7933  and ISO 9920 . The vapor resistance value was reduced less in ISO9920 than in ISO7933. CFi was close to each other, but CFe9920a and CFe9920b were larger than CFe7933. To the best of our knowledge, there is no other study to compare correction factors of ISO 7933  and ISO 9920  in detail in terms of relative air velocity and walking speed. Next, we investigated the effect of CFe differences on predicted rectal temperature and water loss under warm and humid conditions. When the difference between CFe7933 and CFe9920a was large, the predicted rectal temperature and water loss were higher in PHSmA than PHS at high ambient temperature and RH. DlimTre38 values by PHS were significantly larger than those of PHSmA and PHSmB.
Many researchers [1, 2, 14, 20, 21, 34,35,36,37,38,39,40] demonstrated the dependence of CFi or CFe on relative air velocity and walking speed mainly with human subject study or thermal manikin (Fig. 6a–d). In this paper, CFi and CFe of ISO 7933  and ISO 9920  were compared with experimental data including the recent research published after the issuance of ISO 9920 . Concerning CFi, CFi7933 and CFi9920 were close to experimental results both in nude and clothing conditions (Fig. 6a). CFi of Qian (cloth) , Morrissey (garment zip fastened)  were close to CFi7933 (≥ 0.6 clo). Morrissey and Rossi  showed that CFi with relative air velocity was lowered in an unfastened garment zip. Thus, how one wears clothing could also influence CFi with relative air velocity. CFi of Lu et al. (nude)  was close to those of CFi7933 (nude) and CFi9920 (nude). CFi of Lu et al. (moderate)  was also close to CFi9920 (≥ 0.6 clo).
CFi7933 and CFi9920 in nude conditions decreased with relative air velocity more than in clothing conditions. Qian and Fan [34, 35] and Lu et al.  also showed that CFi of a nude body was smaller than a clothed body under the same relative air velocity (Fig. 6a). However, with walking speed, CFi in nude conditions and clothing conditions were almost the same for CFi7933 and CFi9920 (Fig. 6c). The experimental results also showed that CFi dependence on walking speed was not related to Icl (Fig. 6c).
CFe of experimental data, CFe7933, CFe9920a, and CFe9920b, were different in two ways. First, CFe9920b was larger than CFe7933 in both nude and more than 0.6 clo (Fig. 6b, d). CFe9920a was also larger than CFe7933. Experiment data of CFe [2, 34, 35, 38, 39] which the standards should be based on were not consistent (Fig. 6b, d). The differences in experimental conditions or calculation methods of ReT in the study of thermal manikin could explain the ReT difference . Second, the CFe9920a did not depend on IT. This is contrary to experimental results showing that CFe in nude decreased with relative air velocity to a greater extent than that in clothing. To resolve these discrepancies, more experimental study regarding CFe dependence on relative air velocity and walking speed would be needed.
For the combined effect of relative air velocity and walking speed on CFi, Eq. 1.1, 1.2, 2.1, and 2.3 in Table 1 showed that relative air velocity and walking speed independently affected CFi. Heat exchange increased in the front trunk (chest, abdomen, pelvis) with a frontal wind. Meanwhile, heat exchange increases more in the arm and foot than the front trunk when walking in nude or light clothing . In a combined condition of wind and walking, the effect of relative air velocity on IT was larger than that of walking speed and affected that of walking speed . To explain these effects, the interaction term of relative air velocity and walking speed could be needed for CFi equation.
A higher CFe induced a smaller maximum evaporation rate and higher wettedness in the skin, leading to a smaller evaporation efficiency. To correct a smaller evaporation efficiency and maintain a balance in heat transfer, the sweat rate increases. In our calculation, PHSmA predicted that water loss increases at a lower ambient temperature than PHS and reached a maximum sweat rate (SWmax) at lower ambient temperature. In PHS, PHSmA and PHSmB, SWmax is determined by metabolic rate and acclimation.
SWmax = (M − 32) × surface area of human body × factor of acclimation (6) where M stands for metabolic rate in W·m−2 . The surface area of human body was expressed in m2. Factor of acclimation was 1.25 for acclimated person and 1.0 for unacclimated person. Before sweat rate reached SWmax, the rectal temperature did not increase. But after reaching SWmax, the rectal temperature started to increase in PHS model. Then, the time when the predicted rectal temperature started to increase was almost equivalent to the time when the predicted water loss reached the maximum sweat rate. When predicted sweat rate by PHSmA reaches SWmax and by PHS did not, only the predicted rectal temperature by PHSmA increases. This relation explained that the differences in the rectal temperature between PHS and PHSmA were closely related to the differences in predicted water loss. Under every condition, the zone where rectal temperature differed almost overlapped the zone where sweat rate differed (Fig. 4). The difference in rectal temperature and water loss was larger under conditions A or B than conditions C or D. A larger difference in CFe (Table 4) would contribute to a larger difference in predicted rectal temperature and water loss.
Our study showed that differences of DlimTre38 between PHS and PHSmA were largest in condition A (Fig. 5). The largest difference in CFe among PHS, PHSmA, and PHSmB in condition A (Table 4) could result in DlimTre38 difference. The large amount of water loss due to a low evaporation efficiency by a high CFe increased the probability of reaching the maximum sweat rate and an elevated rectal temperature. Thus, at high CFe, rectal temperature increased in a lower heat stress environment than for a low CFe. Under heat stress conditions where body rectal temperature started to increase, an inaccuracy in CFe led to a large prediction error for DlimTre38 values. Originally, predicting heat strain under such boundary conditions was required to avoid heat disorders. As such, the prediction errors due to an inaccurate CFe should be lowered as much as possible under such boundary conditions. Since many kinds of clothing exist, it could be difficult to develop a CFe that covers all kinds of clothing. Thus, it would be necessary to derive a CFi or CFe that is specialized for work clothing to prevent work-related heat disorders. Many other factors, except for wind or walking activity, such as how clothes fit, posture, and openings, were reported to affect IT . Further study is needed to estimate precise correction factor considering other factors.
The clothing area factor (fcl), defined as the ratio of the clothing surface area to the body surface area, also plays an important role in the analysis of heat exchange between a clothed body and the environment. Though fcl is decided only by static clothing thermal insulation, it is used in both static and dynamic conditions in ISO 7933 . Since fcl was shown to depend on clothes’ fit or posture  and clothing shape was changed by wind , some corrections to fcl should be considered.
Moreover, the scope of relative air velocity is limited to 3.0 and 3.5 m·s−1 for ISO 7933  and ISO 9920 , respectively. When PHS is applied to outdoor work, the scope should be extended to an air velocity of more than 3.0 m·s−1.
The correction factors for dynamic total thermal insulation (CFi9920) and evaporative resistance (CFe9920a and CFe9920b) proposed in ISO 9920  were compared with those of ISO 7933 (CFi7933 and CFe7933) . The vapor resistance value was reduced less in ISO9920 than in ISO7933. CFi were close to each other; however, CFe of ISO 9920  was much larger than that of ISO 7933 . The relation of one CFe in ISO 9920  on relative air velocity was not influenced by the intrinsic thermal insulation of clothing. The difference in CFe lead to a difference in predicted rectal temperature and water loss in the critical region of ambient temperature and RH where predicted sweat rate reached maximum sweat rate. Duration time when limit of rectal temperature of 38 °C is reached (DlimTre38) was different according to the CFe used in the calculation. A larger difference in CFe results in a larger difference in DlimTre38 value. The development of a correct CFe is required to predict appropriate work time limits in hot working environments.
Availability of data and materials
The datasets used and/or software during the current study are available from the corresponding author on reasonable request.
- DlimTre38 :
Duration time when limit of rectal temperature of 38 °C is reached (minutes)
- CFe :
Correction factor for dynamic total evaporative resistance (dimensionless)
- CFe7933 :
CFe of ISO 7933 (dimensionless)
- CFe9920a :
CFe of ISO 9920 using relative air velocity and walking speed (dimensionless)
- CFe9920b :
CFe of ISO 9920 using CFi9920 (dimensionless)
- CFi :
Correction factor for dynamic total thermal insulation (dimensionless)
- CFi7933 :
CFi of ISO 7933 (dimensionless)
- CFi9920 :
CFi of ISO 9920 (dimensionless)
International organization for standardization
- IT :
Total thermal insulation (m2·K·W−1)
Predicted heat strain
- PHSmA :
Modified PHS integrated with CFi9920 and CFe9920a
- PHSmB :
Modified PHS integrated with CFi9920 and CFe9920b
- ReT :
Total evaporative resistance (m2·kPa·W−1)
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The author expresses a deep appreciation to Prof. Bernard in South Florida University for discussing about correction factors of clothing and PHS.
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The author declares that I have no competing interests.
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Ueno, S. Comparison of correction factor for both dynamic total thermal insulation and evaporative resistance between ISO 7933 and ISO 9920. J Physiol Anthropol 39, 23 (2020). https://doi.org/10.1186/s40101-020-00235-9
- Predicted heat strain (PHS)
- Thermal insulation
- Evaporative resistance
- Correction factor