Analysis of air gap volume in structural firefighter turnout suit constructions in relation to heat loss

Abstract

Air layers in multi-layer firefighter clothing ensembles resist heat transfer from the body to the environment. By reducing the volume of air between clothing layers, heat loss may be improved throughout the multi-layer firefighter turnout suit clothing system, potentially leading to reduced heat strain for the wearer. This research utilized a systems-level approach to the methodology in order to measure the effects of fabric properties and garment air gap dimensions on clothing system heat loss through specially configured turnout suit constructions. One experimental configuration incorporated a tight fitting stretchable moisture barrier garment. Another construction used thermal knit underwear to represent a closer fitting thermal liner. Air gap surface area, volume, and thickness were estimated using three-dimensional body scanning. This study showed the significant impact of fabric air permeability and clothing air gap volume on heat loss through structural firefighter suits. Tested individually, the tighter fitting moisture barrier construction permitted greater heat loss in comparison to the traditional fit moisture barrier. Heat loss differences associated with moisture barrier fit were not observed when the moisture barriers were configured in the three-layer turnout clothing system. This research showed that microclimate air gap volume is strongly correlated with total heat loss. It confirmed the significant impact of clothing air layers on heat loss through firefighter turnout systems.

Keywords

clothing insulation air gap protective clothing firefighter heat stress

The amount of heat and mass transferred through a clothing system is directly affected by the air gap thickness created between multiple clothing layers.^1,2 Air layers are inherent in clothing construction. They function to resist heat and moisture transfer between the skin and environment,² hampering the amount of heat that may potentially be dissipated. There is a difference between the still air enclosed within the fabric and the enclosed air between clothing layers. For most clothing ensembles, the volume of air enclosed in the fabric layers exceeds the volume of the material itself, indicating that thermal insulation is a function of the enclosed still air layer.³ The compression of fabric in different locations throughout the garment must also be considered.

Evaporative resistance of moisture through the garment is also heavily dependent on the thickness of the enclosed still air layers. A reduction in air gap volume by using thinner, tighter (closer) fitting materials may lower the overall resistance to heat transfer. In that case, however, material components begin to play a more important role.³ Material properties, including air permeability, coatings, and membranes, directly impact evaporative resistance.^3,4 When applied to structural firefighter turnout suits, the moisture barrier (MB) layer may limit evaporative heat loss due to its semi-impermeability.

The study of air gap thickness and how it impacts heat transfer on the material level has been widely investigated on sweating guarded hot plates for comfort^5,6 and on Thermal Protective Performance (TPP) testers for heat protection.^7–9 There have been limited studies, however, with a full clothing ensemble.¹⁰ When considering garments, air layers between the clothing become even more important.³ Each clothing layer has a still air layer attached to its outer surface. Outside of this area, the air is not bound and will move due to temperature gradients³ and forced convection of air. When multi-layer clothing ensembles are considered, such as a structural firefighter turnout suit, the total garment insulation will be much greater than the insulation of the material layers alone.

It is also important to consider design and fit, as they determine how closely the garment drapes around the body,¹¹ affecting the volume of the air layers. A looser fit garment increases air gap volume thickness and affects heat transfer. Multiple layer clothing systems, such as structural firefighter turnouts, create multiple air gaps within the ensemble between each clothing layer. For firefighters, this ensemble first includes the base layers (stationwear) worn underneath the turnout suit. The structural firefighter turnout suit consists of a three-layer base composite that includes the outer shell (OS), which is the firefighter’s first line of defense against thermal hazards; the middle, semi-permeable MB layer; and the thermal liner (TL) layer, which is the thickest for thermal protection and worn closest to the skin.

Clothing design, body shape, and fit do not allow clothing layers to be separated enough to enclose the same constant volume in air layers over the entire surface area of the body. Instead, air layers should be characterized in relation to garment fit (tight or loose) rather than size.¹ With a tighter fit garment, less air is included compared to a looser fit, hence the justification to explore tighter fitting materials in structural firefighter turnouts.

In addition, the movement of the wearer and the external air speed also play an important role in influencing the air gap volume between clothing layers. Air movement through wind disturbs the outer air layers of the ensemble as it enters through clothing openings or through the fabric itself.³ Body motion of the wearer also creates movement in the air layers, but in a different manner. Wearer activity works to pump air between different clothing compartments within the ensemble. This forces the exchange of air, known as “pumping” or the “bellows effect.” Wind mostly affects the surrounding air layer and the layer directly underneath the outer garment, whereas body motion affects both the enclosed and surrounding air layers.^3,12

In summary, air gap volume in structural firefighter turnouts might play as significant a role as the clothing materials themselves in determining the heat loss properties of the system. In the past, bench-level material studies have been conducted^13–15 to examine air gap thickness, but there is less knowledge regarding the impact of air gap volume on the three-dimensional (3D) human body when worn as a garment. Previous research has found that differences among fabric properties were overstated compared to differences between corresponding garments when tested for physiological comfort, highlighting the issues of relying on fabric-level test methods alone.¹⁰ It is important to gain a better understanding of how air gaps impact overall heat loss in structural firefighter turnout suits. Further, it is vital that modifications to the garment design, such as tighter fitting materials, be explored to improve the physiological comfort of the firefighter in order to reduce incidents of heat strain.

It is important to note that such a reduction in air gap volume may also lead to a reduction in thermal protection. Firefighters, however, only spend 5–20% of their working time in or around a scenario where the threat of heat and flame exposure exists.^16,17 Thermal protection provided by the base layers worn underneath the turnout suit is not currently considered by the National Fire Protection Association (NFPA) as part of the protective ensemble. Physiological comfort may be improved when air gap volume is reduced without sacrificing thermal protection, if base layers are considered. Additional research on base layers, outside the scope of the current study, should be conducted if a reduction in air gap volume provides a significant heat loss improvement on the ensemble level.

The purpose of this research was to determine if a reduction in the volume of air layers in multi-layer clothing systems, such as structural firefighter turnout suits, leads to significant improvements in heat loss. This research addressed the following questions.
How do fabric thickness, air permeability, and air gap volume relate to the thermal insulation, evaporative resistance, and heat loss capabilities of each individual garment layer (OS, MB, and TL) in structural firefighter turnout suits?

Does the reduction of air gap volume through the implementation of closer fitting garment layers improve heat loss through the multi-layer firefighter clothing system?

Methods

To determine the impact of air gap volume in structural firefighter turnout ensembles, a combination of fabric- and garment-level methods were used. To assess the heat loss capability of the individual base composite layers, the thermal and evaporative resistance of each garment was measured, using a sweating thermal manikin, according to ASTM F 1291 and ASTM F 2370, respectively.^18,19 These measurements were then combined to determine the overall predicted manikin total heat loss (MTHL) in a 25℃, 65% relative humidity (RH) environment, which are the standard test conditions when evaluating structural firefighter turnout suit materials at the fabric level, according to the NFPA 1971 Standard on Protective Ensembles for Structural Fire Fighting.²⁰ By predicting the MTHL in the same environmental conditions as the materials, comparisons may be made between the MTHL and material-level heat loss measurements conducted on a sweating guarded hot plate, or plate total heat loss (PTHL). PTHL measures heat loss through the fabric on a flat surface while MTHL measures the systems-level heat loss of a garment ensemble on a thermal manikin that captures the impact of air gaps and drape on the human form. Further analysis was conducted to determine how fabric thickness, air permeability, and garment air gap volume related to MTHL.

Materials

The OS, MB, and TL layers that typically make up the three-layer base composite of the turnout suit were individually separated and evaluated for garment-level MTHL, fabric thickness, fabric weight, air permeability, and garment air gap volume. The OS consisted of a polybenzimidazole (PBI®) and para-aramid blend, the MB was a polytetrafluoroethylene (PTFE) laminated to a woven aramid, and the TL was two layers of a nonwoven aramid batting quilted to a woven aramid facecloth. Material details and fabric properties for each individual base composite layer can be found in Table 1.
Table 1.
Material structure and fabric properties of individual base composite layer garments

Garment layer Material structure Fabric weight (g/m²) Thickness (mm) Air permeability (cm³/s/cm²)

Outer shell (OS) PBI Matrix, 60% para-aramid/40% PBI 242.8 0.40 5.8

Moisture barrier (MB) Crosstech black, PTFE laminated to an aramid fabric 162.7 0.34 0.0

Thermal liner (TL) Caldura SL 2-Layer E89 100% aramid quilt 255.0 1.23 48.2

PBI: polybenzimidazole; PTFE: polytetrafluoroethylene.

The previous literature suggests that the introduction of stretch materials for a tighter fit reduces the air gap volume of the clothing system.¹ To determine how a reduction in air gap volume, by using a tighter fitting MB, would impact heat loss and comfort, a proprietary, stretch membrane material was fabricated into a full garment including a front-zip jacket and pants. Seam seal tape was used in the construction of the garment to ensure no liquid penetration could occur. Due to the extreme tight fit of this MB garment, it was implemented as a next-to-skin concept, which necessitated the re-arrangement of the base composite layers. The tight fit MB was tested on the sweating thermal manikin as a single independent layer (Tight Fit MB), with the full ensemble (OS + TL + Tight Fit MB), and with a knit thermal underwear layer as a replacement for the traditional TL (OS + Knit TL + Tight Fit MB). A 100% soft-knit Nomex® constructed thermal underwear set was chosen as a thinner, closer fitting substitute for the TL. The same configurations were also assessed on the 3D body scanner to quantify the air gap volume.

Procedures

Fabric thickness and weight were measured for each individual base composite layer (OS, MB, and TL). Fabric thickness was measured according to ASTM D 1777, test option one. Three specimens (50.8 cm × 50.8 cm) were measured with a thickness gauge (mm) at an applied pressure of 4136.85 Pa at various locations of the fabric to determine an overall average thickness.²¹ Fabric weight was measured according to ASTM D 3776, small swatch option. Three specimens (50.8 cm × 50.8 cm) were weighed on an analytical balance and the weight was calculated in mass per unit area (g/m²).²²

The air permeability of each individual base composite layer was measured according to ASTM D 737, using a Frazier Air Permeability tester. Ten measurements were taken on each specimen with a test area of 38 cm² and test pressure of 125 Pa (Pascal).²³ The unit of measurement was cm³/s (cm³/s/cm²), which represents the air flow through a fabric at a constant pressure drop. The test orifice diameter was adjusted for each fabric sample in order to obtain reliable and reproducible measurements for each fabric type. The average air permeability (cm³/s) was calculated from the 10 measurements for each fabric. The results are presented in Table 1 along with fabric thickness and weight.

Manikin total heat loss

A Newton 34 zone model sweating thermal manikin was used to measure both dry and wet heat loss for each clothing ensemble under specific environmental test conditions. Evaluating MTHL allows for the entire ensemble to be assessed, including pockets, accessories, boots, gloves, and helmet. This method takes into account the amount of body surface area covered by different materials and various numbers of layers, the fit of the garment, and the increased surface area for heat loss.^24,25

Environmental conditions, including air temperature and humidity, can have a significant effect on the heat and moisture transfer through a garment. If air temperature is equal to or greater than skin temperature (35℃), convective heat loss is no longer possible and the body relies on the evaporation of sweat to remove heat from the body. To measure the convective versus evaporative heat loss of the clothing system, two separate tests must be conducted at the manikin level that require different ambient conditions. Thermal insulation (R_t) was measured in a non-isothermal condition, per the ASTM F1291-10 standard test method, at 20℃ (at least 13℃ below skin temperature) and 50% RH.¹⁸ Evaporative resistance (R_et) was measured in an isothermal condition, per ASTM F2370-10, at 35℃ and 40% RH.¹⁹ Because manikin THL calculations are based upon measurements taken in two different environments, the heat loss values are predictive rather than actual measurements.²⁴ MTHL(predicted) was calculated according to the following equation (Equation (1)) at 25℃/65% RH using the resistance data gathered from both ASTM test methods.²⁴
$\begin{matrix} {MTHL}_{(predicted, T, RH)} = \frac{T_{s} - T_{a}}{R_{t}} + \frac{P_{s} - P_{a}}{R_{et}} \end{matrix}$
(1)
where MTHL₍ _predicted _, _T _, _RH ₎ is the predicted manikin THL for specified environmental conditions (W/m²), T is the specified temperature condition (℃), RH is the specified RH (%), T_s is the specified temperature at the manikin surface (℃), T_a is the specified temperature of the local environment (℃), P_s is the calculated water vapor pressure at the surface of the manikin (kPa), P_a is the calculated water vapor pressure in the specified local environment (kPa), R_t is the total thermal resistance of the clothing ensemble and surface air layer (℃·m²/W), and R_et is the total evaporative resistance of the test ensemble and surface air layer (kPa·m²/W).^24,26

This calculation ignores such effects as condensation, absorption, and fabric type differences in various environmental conditions, but is useful for direct comparisons between garment configurations as it considers the entire body surface area, construction, and design of the 3D garment, and fit on the human form.

The OS, MB, and TL garment layers were each assessed individually for MTHL as single layers (Figure 1). The tight fit MB was then evaluated for predicted MTHL alone (single layer) and in two configurations with the full turnout ensemble (OS + TL + TightMB and OS + Knit TL + TightMB). In total, six different ensembles were assessed. All garments were with boots, gloves, and a helmet. The manikin was dressed in a cotton T-shirt, athletic shorts, and socks as base layers, except for the tight MB configurations, which were compared back to control ensembles tested without base layers. Each garment was tested three times for dry and wet heat loss in two conditions: static (standing with still air speed at 0.4 m/s) and dynamic (walking with wind at 2 m/s).
Figure 1.
Outer shell, moisture barrier, and thermal liner garments tested for manikin total heat loss on a sweating thermal manikin in a normal working condition scenario.

Three-dimensional body scanning

A 3D body scanner was used to determine the thickness and volume of the air gap between the body and the clothing layer. In this study, a Size Stream (SS) 3D body scanning booth and desktop application were used to measure the air gap between the base layer and each individual base composite garment layer, including the OS, MB, and TL layers. The SS 14 ED body scanner had a small footprint (1.65 m × 1.07 m × 2.16 m) and performed three consecutive scans (6 s each) to provide a comprehensive avatar of the human subject wearing the garment. In addition, the tight fit MB was measured alone, with a traditional fit TL, with the knit TL, and with the full ensemble configurations (OS + TL + Tight Fit MB and OS + Knit TL + Tight Fit MB).

Each garment was scanned once, which is an average of three consecutive scans conducted by the body scanner, from which the average surface area and volume were calculated. Previous research has determined the accuracy of 3D scans to be less than 1 mm and has proven that using such technology to investigate air gap thickness is reliable.^27,28 Sensor scanning specifications for measurement included 14 sensors placed at six angles around the body at seven different heights. The volume of the air gap was calculated by subtracting the volume of the base layers, or nude scan, from the volume of each individual base composite layer, or tight fit MB garment, respectively. Garments were measured on a human subject model who closely fit the measurements of the sweating thermal manikin. It should be noted that fit of the garment on the manikin and the model are not identical. Fit does play a role in the air gap volume measured and, therefore, the amount of resistance provided by the air gap toward heat transfer.

The air gap volume and surface of each scanned suit was calculated by subtracting the volume, or surface area, of the base layer scan (T-shirt and athletic shorts) from each individual layer garment volume, or surface area. For the tight fit MB, the nude scan was subtracted as base layers could not be worn due to the tight fit. The average air gap distance (mm) was calculated according to the following equation (Equation (2)).
$\begin{matrix} A_{gap} = \frac{V_{airgao}}{{SA}_{nude}} \end{matrix}$
(2)
where A_gap is the average distance between the base layer/nude body and the clothing layer over the surface area of the body (mm), V_airgap is the volume of the clothing layer minus the volume of the base layer/nude body (cm³), and SA_nude is the surface area of the nude human subject model (with underwear) (cm²).

Statistical analysis

To determine the statistical significance of the measured differences in predicted MTHL, compared to the baseline control, two-sample t-tests, assuming equal variances, were performed. All data was tested for normalcy and normal distributions were confirmed through a probability plot and the Anderson–Darling test statistic. A one-way analysis of variance (ANOVA) was conducted with each data set (for each test condition) to determine if significant differences were present. Pearson’s correlation coefficients were used to determine if significant relationships existed between MTHL and other factors, including air gap volume, air gap surface area, air gap distance, fabric thickness, and air permeability. A p-value less than 0.05 was chosen to indicate a significant difference.

Results and discussion

Single base composite layers

Manikin thermal resistance (R_t), evaporative resistance (R_et), and MTHL results of the individual garment layers have been previously published in a preceding article.²⁹ This data is shown in Table 2. The TL had the highest MTHL on the sweating manikin in both the static (117.5 W/m²) and dynamic (265.1 W/m²) test conditions. Predicted manikin heat loss results demonstrated statistically significant differences between all three individual layer garments in the static condition, with the MB having the lowest heat loss, followed by the OS and TL. In the dynamic test condition, differences between suits were more pronounced but not statistically significant. The OS had the lowest heat loss in this condition, followed by the MB and TL.
Table 2.
Individual layer garment thermal resistance, evaporative resistance, and manikin total heat loss (MTHL)

Garment Static manikin data
Dynamic manikin data

R_t (m²℃/W) R_et (m²Pa/W) MTHL (W/m²) R_t (m²℃/W) R_et (m²Pa/W) MTHL (W/m²)

Outer shell 0.250 (0.011) 0.050 (0.000) 111.9 (0.4) 0.141 (0.000) 0.022 (0.002) 233.1 (13.3)

Moisture barrier 0.234 (0.001) 0.053 (0.001) 110.1 (0.6) 0.121 (0.001) 0.021 (0.001) 256.4 (9.1)

Thermal liner 0.263 (0.001) 0.045 (0.001) 117.5 (1.7) 0.138 (0.001) 0.019 (0.001) 265.1 (13.1)

The TL’s low thermal resistance (R_t) and high MTHL values may be explained by inherent fabric properties, such as thickness and air permeability, or by air gap volume, as it is worn as the closest layer to the body. Table 3 depicts the surface area, air gap volume, and air gap distance calculated from the body scanning measurements for each single-layer garment.
Table 3.
Three-dimensional body scanning results of each single-layer base composite garment

Garment Surface area (m²) Air gap volume (m³) Air gap distance (cm)

Outer shell 1.07 0.086 4.78

Moisture barrier 0.90 0.067 3.72

Thermal liner 0.89 0.072 3.97

The OS had the largest surface area, air gap volume, and air gap distance, followed by the TL and MB. These findings are not surprising, as the individual layers were separated from a traditional structural firefighter turnout suit with the OS constructed to be the outermost layer, reflecting the largest surface area and air gap volume. The TL is a thicker material contributing to its lack of conformity to the body and the larger air gap distance, compared to the MB. The MB is thinner and lighter weight, allowing it to drape closer to the body. The MB layer had the smallest air gap volume and thickness.

The TL had the highest predicted MTHL despite its thickness and fabric weight. To further analyze the air gap volume data and determine how fabric properties relate to MTHL, Pearson’s correlation coefficient was determined. Each factor (fabric thickness, air permeability, and air gap distance) was correlated to the MTHL of the individual base composite garment layers in both test conditions.

The analysis of the air gap distance with MTHL showed little to no correlation between air gap and MTHL in the static (r = –0.06) or dynamic test condition (r = 0.53). The relationship between MTHL and fabric thickness, however, demonstrated a strong, positive correlation (r = 0.987; p < 0.1) for the static MTHL. Surprisingly, the data indicated that as material thickness increased, so did MTHL. Fabric thickness is cited in the literature as a determinant of insulation,^12,30 and thermal resistance (R_t) has also been primarily determined by fabric thickness, independent of fiber type³¹; however, the relationship discovered in the analysis above was not expected. These results do not follow clothing insulation principles, which demonstrate the opposite, that as fabric thickness increases, heat loss decreases. Therefore, fabric air permeability was analyzed as an additional indicator in the context of this study.

Pearson’s correlation coefficient showed a strong (r = 0.991; p < 0.01) correlation between the predicted static MTHL of individual garment layers and fabric air permeability. These results indicate that air permeability significantly influences garment heat loss, more so than air gap distance or fabric thickness, for the individual base composite layers at the manikin level in this study. This finding can be further explained by the large contributing role the evaporative heat loss component plays. Air permeability is directly related to the moisture vapor transfer through the garment as opposed to fabric thickness and air gap distance, which are directly related to the thermal resistance, or convective component of heat loss, which plays a smaller role in overall garment MTHL. Individual correlations between thermal resistance (R_t) and evaporative resistance (R_et) further confirm this conclusion. Pearson’s correlation coefficient showed a stronger relationship between air permeability and R_et (r = −0.95) than R_t (r = 0.89). While both demonstrate strong, significant relationships, air permeability has a greater effect on the evaporative heat loss component than the convective, dry heat loss component. The TL had the greatest air permeability, which corresponded with the lowest R_et and largest MTHL value.

Tighter fitting structural turnout ensemble

The implementation of tighter fit as a design modification for heat loss improvement was explored through the use of a stretch membrane garment and knit thermal underwear as potential substitutes for the MB and TL layers, respectively. The tight fit MB was tested by itself (Tight Fit MB), in conjunction with a traditional turnout ensemble (OS + TL + Tight Fit MB), and with a TL substitute (OS + Knit TL + Tight Fit MB). Figure 2 illustrates the comparison of MTHL results between the traditional fit MB and the tighter fitting stretch membrane.
Figure 2.
Predicted manikin total heat loss (MTHL) comparison between traditional and tight fitting moisture barriers.

In both the static and dynamic test conditions, the stretch MB with a tighter fit had significantly (p < 0.05) higher MTHL compared to the traditional MB. An increase of 84.3 W/m² was measured in the static condition alone. A reduction in thermal resistance (R_t) of 0.1025 (m²℃/W) and evaporative resistance (R_et) of 0.0228 (m²Pa/W) was measured when wearing the stretch MB garment compared to the traditional MB in the static condition. In the dynamic condition, an improvement of 33 W/m² was detected for MTHL and reductions in R_t and R_et of 0.0328 and 0.0023 were measured, respectively, for the stretch MB garment. The statistically significant improvements in heat loss may be due to implementing a tighter fit and reducing the air gap volume by 0.030 m³ (MB = 0.062 m³; tight fit MB = 0.033 m³).

It is important, however, to first consider the inherent material property differences between the two MBs. The traditional MB and the stretch MB were tested separately, as single-layer fabrics, on a sweating guarded hot plate for fabric (PTHL). These results are given in Table 4 (with standard deviations) showing that the stretch MB has lower evaporative resistance and higher PTHL at the fabric level than the traditional MB. Therefore, it cannot be concluded that the single-layer MTHL results indicate a positive benefit due to tighter fit alone, as the materials have inherently different breathability properties, as well.
Table 4.
Fabric thermal resistance, evaporative resistance, and plate total heat loss (PTHL) of traditional and stretch moisture barriers (with standard deviation)

Base composite layer Thermal resistance (R_ct) (m²℃/W) Apparent evaporative resistance (AR_et) (m²Pa/W) PTHL (Q_t) (W/m²)

Moisture barrier 0.080 (0.001) 0.010 (0.000) 649.7 (3.70)

Stretch moisture barrier 0.086 (0.001) 0.009 (0.000) 691.82 (16.62)

The tight fit MB was then tested in two full turnout suit configurations: first with the traditional TL and OS layers; second with a tighter fitting knit TL and traditional OS layer. These results, compared to the traditional control (OS + MB + TL), are shown in Figure 3.
Figure 3.
Predicted manikin total heat loss (MTHL) comparisons of tight fitting suit configurations. OS: outer shell; MB: moisture barrier; TL: thermal liner.

Even though significant increases in MTHL were found between the tight fit MB and the traditional MB when tested as single layers, the same benefits in multi-layer clothing ensembles were not measured. Realistically, there were negligible differences between the three configurations in the static condition.

In the dynamic test condition, however, greater differences between suits were measured. The OS + TL + Tight MB had significantly lower MTHL than the traditional control and OS + Knit TL + Tight MB. There was no significant difference between the control and the OS + Knit TL + Tight MB, meaning, with the materials used in this experiment, there was no benefit of incorporating tighter fitting stretch materials in the turnout ensemble. A limitation of this research, however, was the traditional OS layer. The OS was not substituted with a tighter fitting material, therefore, it quite possibly eliminated any benefits of the reduced air gaps of the inner layers.

To determine how air gap volume played a role in the heat loss data, 3D body scanning was conducted to quantify the reduction in air gap for the tighter fitting garments. The tight fit MB was scanned alone, with the traditional TL, and with the knit TL. Additional scans were then conducted with the traditional OS layer on top of both TL configurations with the tight fit MB. The surface area, air gap volume, and air gap distance for each tight fit MB configuration are given in Table 5.
Table 5.
Three-dimensional body scanning results of tight fit moisture barrier configurations

Garment Surface area (m²) Air gap volume (m³) Air gap distance (cm)

Moisture barrier 0.903 0.065 3.59

Tight fit moisture barrier 0.044 0.002 0.11

TL + Tight Fit MB 0.905 0.070 3.85

Knit TL + Tight Fit MB 0.209 0.014 0.77

OS + TL + Tight Fit MB 1.05 0.092 5.08

OS + Knit TL + Tight Fit MB 1.07 0.082 4.53

OS: outer shell; MB: moisture barrier; TL: thermal liner.

The results in Table 5 illustrate the significant reduction in air gap volume with the stretch MB. Air gap distance was reduced by 3.48 cm when wearing the tight fit MB versus the traditional fit. This reduction in air gap supports the statistically significant improvements in predicted MTHL illustrated in Figure 2. When the traditional OS was worn, however, the difference in air gap distance between configurations was much smaller (0.55 cm) regardless of whether the tight fit MB was paired with a traditional TL or knit TL. This may reflect the lack of significant improvement in MTHL when the traditional OS was worn.

Pearson’s correlation coefficient was determined for further analysis between MTHL results and air gap volume data for the tight fit garment configurations. Results of the body scanning analysis, correlated to MTHL, demonstrate an inverse relationship between surface area and predicted heat loss in the static condition (r = −0.98; p < 0.05). As surface area increased, MTHL in the static test condition decreased. The same strong, inverse (r = −0.99; p < 0.05) relationship was determined for air gap volume and distance (Figure 4) when correlated with static MTHL. As the air gap size increased, the MTHL significantly decreased in the static test condition. For the dynamic test condition, no strong, significant relationships were established between MTHL and air gap surface area or size.
Figure 4.
Correlation between the predicted manikin total heat loss (MTHL) in the static condition and the average air gap distance.

Conclusions

The findings from this research have direct implications for designers, product developers, and manufacturers of functional performance clothing. A limited sample of specific material constructions and engineered garment designs were assessed within the context of this study meaning the conclusions presented here within can only be applied to the specific samples measured. Findings from the individual garment layer MTHL demonstrated the MB had the lowest heat loss and the TL had the highest MTHL when predicted in a 25℃/65% RH environment. This was surprising as the TL was also the thickest material, yet it had the greatest MTHL.³¹ Although the TL consists of a thick, multi-layer construction, the MB had a higher evaporative resistance (R_et) on the garment level, which contributed to its overall lower heat loss within the context of this particular study’s predicted test conditions and specific garment materials.

The results suggest that air permeability of individual fabric layers is the most influential material factor, of those considered in this study, when determining garment-level heat loss performance of single-layer systems. This contradicts previous literature that found fabric thickness to be the primary predictor of thermal insulation; however, the previous study assessed fabric-level test methods only.³¹ Previous research also found garments with higher permeability to have lower thermal resistance through natural convection.³² In this study, however, opposite results were found as the thick TL had the greatest air permeability and overall MTHL but did not have the lowest thermal resistance. It did, however, have the lowest evaporative resistance. These conclusions reflect the larger role evaporative heat loss has on garment heat transfer compared to the convective component.

The analysis of tighter fitting layers in the multi-layer protective clothing system did not result in significant improvements in MTHL within this study. A limitation of this study was the orientation of the stretch MB as its tight fit necessitated that it be placed closest to the skin, followed by the TL and OS. This arrangement is different to that of the traditional turnout gear in which the MB is the middle layer between the OS and TL. The significant reduction in MTHL, compared to the control, when incorporating the stretch MB into the clothing system next to the skin may be due to the difference in temperature gradients created by placing the air impermeable material closer to the body.

Body scanning data provided information regarding the surface area, air gap volume, and calculated air gap distance (mm) of each clothing ensemble. It was determined for the static MTHL that there were strong, inverse relationships between air gap size and heat loss through the ensemble. As air gap surface area, volume, and thickness increased, the MTHL of the clothing system decreased, specifically when the traditional TL and OS layers were added to the tight fit MB.

This research indicates that a reduction in air gap volume by using tighter fitting garments may improve heat loss through the clothing system when evaluating natural convection in a static test condition. However, these results may have been due to inherent material properties as the stretch MB had a significantly greater PTHL. Future research should include implementation of stretch materials and a tighter fit in all layers of the clothing system to determine if an overall air gap reduction in the entire ensemble leads to significant heat loss improvements.

Garment layer	Material structure	Fabric weight (g/m²)	Thickness (mm)	Air permeability (cm³/s/cm²)
Outer shell (OS)	PBI Matrix, 60% para-aramid/40% PBI	242.8	0.40	5.8
Moisture barrier (MB)	Crosstech black, PTFE laminated to an aramid fabric	162.7	0.34	0.0
Thermal liner (TL)	Caldura SL 2-Layer E89 100% aramid quilt	255.0	1.23	48.2

Garment	Static manikin data	Dynamic manikin data
Outer shell	0.250 (0.011)	0.050 (0.000)	111.9 (0.4)	0.141 (0.000)	0.022 (0.002)	233.1 (13.3)
Moisture barrier	0.234 (0.001)	0.053 (0.001)	110.1 (0.6)	0.121 (0.001)	0.021 (0.001)	256.4 (9.1)
Thermal liner	0.263 (0.001)	0.045 (0.001)	117.5 (1.7)	0.138 (0.001)	0.019 (0.001)	265.1 (13.1)

Garment	Surface area (m²)	Air gap volume (m³)	Air gap distance (cm)
Outer shell	1.07	0.086	4.78
Moisture barrier	0.90	0.067	3.72
Thermal liner	0.89	0.072	3.97

Base composite layer	Thermal resistance (R_ct) (m²℃/W)	Apparent evaporative resistance (AR_et) (m²Pa/W)	PTHL (Q_t) (W/m²)
Moisture barrier	0.080 (0.001)	0.010 (0.000)	649.7 (3.70)
Stretch moisture barrier	0.086 (0.001)	0.009 (0.000)	691.82 (16.62)

Garment	Surface area (m²)	Air gap volume (m³)	Air gap distance (cm)
Moisture barrier	0.903	0.065	3.59
Tight fit moisture barrier	0.044	0.002	0.11
TL + Tight Fit MB	0.905	0.070	3.85
Knit TL + Tight Fit MB	0.209	0.014	0.77
OS + TL + Tight Fit MB	1.05	0.092	5.08
OS + Knit TL + Tight Fit MB	1.07	0.082	4.53

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Department of Homeland Security, Federal Emergency Management Agency, Fiscal Year 2012, Assistance to Firefighters Grant program, fire prevention and safety (grant number EMW-2012-FP-01185).

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Garment	Static manikin data			Dynamic manikin data
Garment	R_t (m²℃/W)	R_et (m²Pa/W)	MTHL (W/m²)	R_t (m²℃/W)	R_et (m²Pa/W)	MTHL (W/m²)
Outer shell	0.250 (0.011)	0.050 (0.000)	111.9 (0.4)	0.141 (0.000)	0.022 (0.002)	233.1 (13.3)
Moisture barrier	0.234 (0.001)	0.053 (0.001)	110.1 (0.6)	0.121 (0.001)	0.021 (0.001)	256.4 (9.1)
Thermal liner	0.263 (0.001)	0.045 (0.001)	117.5 (1.7)	0.138 (0.001)	0.019 (0.001)	265.1 (13.1)