Experimental research on the indoor environment control of natural insulation materials: Carbonized rice hull and straw bales

Abstract

The purpose of this research was to assess the performance of straw bales and carbonized rice hulls when used as natural insulation to regulate the indoor environment. Toward that end, specimens of straw bales and carbonized rice hulls were tested in the Korea Conformity Laboratories. Based on the results, real-scale mockup rooms insulated with these natural materials were constructed for cost analysis and performance evaluation. The cost analysis results showed that carbonized rice hulls are relatively cost effective and economically feasible. The thermal conductivity of carbonized rice hulls is lower than that of straw bales, and no difference in thermal performance was shown in relation to the different construction method used for each material. With regard to humidity, both mockup rooms built with each of the natural materials exhibited stable variations compared to the outside weather, and both materials sustained a general humidity within a comfortable 40–60% range. The straw bales mockup room was shown to produce a higher CO₂ emission possibly due to the bio-metabolism (anaerobic fermentation), suggesting that this material should be used with caution. Carbonized rice hulls were proven to be a good natural insulation material and a good regulator of indoor humidity and would not yield CO₂ due to bio-metabolism.

Keywords

Natural insulation material Indoor environment Carbonized rice hull Straw bales Mockup

Introduction

National and international efforts have recently increased in the field of architecture to reduce adverse energy and environmental loads. As quality of life for most families in Korea has risen, the demand for acceptable residential environments with comfortable and energy efficient indoor environment has increased as well. As a result, there have been various studies with an aim of creating energy-saving features and amenities for the residential environment. In this context, architectural methods using natural materials have become a focus of attention. In today’s building industry, inorganic insulating materials are predominantly being used, although there is now an increasing interest in natural fibre insulation.¹

Using natural materials can reduce embodied CO₂ emissions and energy consumption during production and transportation. In terms of life cycle, these materials would ultimately degrade naturally without producing pollutants.² This fact was verified by the UK Department of Trade and Industry through a research project on construction materials from crops.³ Among these natural materials, straw bales and carbonized rice hulls were deemed harmless to the human body. As rice farming is common in Korea (with a rice self-sufficiency rate of 104.6% in 2010 and production: 4,295,000 t⁴), the supply of the raw materials is readily available and economically feasible after harvest. As a result, their use as architectural material has increased in the private sector and rural areas.

In the UK, the average amount of recoverable cereal straw ranges from 2.75 to 4 t/ha,¹ and the energy input needed to bail the straw has been calculated as 2970 MJ/ha (0.75–1.09 MJ/kg),⁵ which indicates a lower energy consumption than is used for general insulation materials.

The insulation performance of straw bales is dependent on a number of variables. According to the results of a test conducted in compliance with the standard of the American Society of Testing Materials (ASTM C-177-85),⁶ the heat conductivity of straw bales with an average density of 134 kg/m³ was 0.0457 W/m·K when heat flow was perceived to be perpendicular to the straw orientation, and 0.0606 W/m·K when the heat flow was perceived to be parallel to the straw orientation.⁷ Further test results confirmed this tendency.^8,9 Also, the heat conductivity was verified as 0.052 W/m·K (perpendicular to heat flow) and 0.057 W/m·K (parallel to heat flow) with a density of 75 kg/m³, and 0.056 W/m·K (perpendicular to heat flow) and 0.060 W/m·K (parallel to heat flow) with a density of 90 kg/m³, illustrating that the heat conductivity value is correlated positively with the density of straw bales. Humidity is another variable that could affect heat conductivity.⁸ Related research^9,10 showed that heat conductivity value could be increased by 20% as the relative humidity increases from the dry stage to 80% at 23℃. These results were supported by other similar study.¹¹

For building elements with straw bales, humidity changes were monitored at the top and bottom of the straw bales wall; with a relatively high humidity being maintained at the bottom.² An equation to predict the timing of straw bales decomposition was established and verified through a chamber experiment on humidity monitoring.¹² Further, the effects of humidity on the decomposition of straw bales were verified by simulation¹³ and confirmed that long-term infiltration of water is the major cause of decomposition. Similar research demonstrated through monitoring that the outer top of the straw bales wall in winter and the inner bottom in summer would have the highest humidity readings, and that moulds could proliferate in these sections.¹⁴ In related research,¹⁵ straw bale walls were monitored to predict the time and conditions of decomposition. The conditions for straw bale decomposition were verified, but the establishment of an accurate prediction model was limited due to the lack of weather data. Further, the same research predicted a rise in CO₂ concentration due to decomposition. However, CO₂ concentration was not monitored, and indoor CO₂ concentration changes that could affect human body were not verified in the related researches, thus additional monitoring and verification studies are required.

Carbonized rice hulls are different from rice husk ash, which is burned, crushed and used as a concrete admixture. Nam-young Nonghyup (Agriculture Cooperative Society of the Nam-young region, Jeollanam-do, Korea) has the only mass-production facility in Korea. However, self-produced for fertilizer is available in farmhouses, and small production machines are currently on the market; therefore, carbonized rice hulls are easily obtainable in rural areas.

Carbonized rice hull retains more than 50% of its form after carbonization. The density between particles is low, and hence breathability is high.^16,17 The physical–chemical properties of carbonized rice hull are shown in Table 1.¹⁷ The carbonized rice hull is weakly alkaline; generally, it has a low-material density and water retention capacity.¹⁷

Table 1.

Physico-chemical properties of carbonized rice hull.

pH		8.45	Bulk density (g/mL)	0.103
Water soluble cations (ppm)	K	4747	Particle density (g/mL)	0.730
Water soluble cations (ppm)	Ca	27	Porosity (%)	85.9
	Mg	32	Water holding capacity (%, W/DW)	248.5
SiO₂ (%)		28.4

Prior research on carbonized rice hulls focussed on using them as a horticultural and hydroponic medium in agriculture because of their properties.^17–22 Further, they were proven to be highly absorptive for oil,²³ ammonia²⁴ and heavy metals in industrial waste water.²⁵ However, research on carbonized rice hulls as an architectural material is not available. As a result, they have been used as insulation in privately initiated construction projects without performance verification, and there are many baseless arguments in the construction field. The positive capacities of carbonized rice hulls were proven, including indoor humidity control, indoor pollutant absorption (due to good permeability), water holding capacity and absorption capacity. Thus, verification research is required to illustrate the potential of carbonized rice hulls as an architectural material.

The purpose of this article is to illustrate the benefits of using natural insulation materials. To do so, the performance of straw bales and carbonized rice hulls was assessed, when used as insulation to control the indoor environment. The high-economic feasibility of using these natural insulating materials in rural areas of Korea was also investigated. Real-scale mockups of a demonstration rooms insulated with these natural materials were constructed, and the construction costs were analysed. The monitoring experiments were carried out from winter to summer, and changes to the indoor environment were verified for each type of insulation material. In particular, CO₂ concentration in the straw bales was monitored to provide an indication of the possible impact on indoor air quality.

Materials

Heat conductivity

In preparation for the mockup rooms, three specimens of carbonized rice hulls and straw bales were constructed, and a performance analysis was conducted by the Korea Conformity Laboratories to measure their fundamental properties. The test method was according to KS L 9016: 2010 of the Korean Agency for Technology and Standards under the Ministry of Knowledge Economy. The results of the analysis showed that the heat conductivity was 0.069 W/m·K (density: 106.9 g/m³) for carbonized rice hulls and 0.092 W/m·K (Density: 207.5 g/m³) for straw bales, proving that the carbonized rice hulls insulation performed better. For straw bales, the value appeared higher than the results of former studies.^7,8 This was examined because of the high density of the specimen compared to former studies, which verified the result⁸ that density correlates positively with heat conductivity value.

Mockup rooms

For this research, real-scale mockup rooms insulated with carbonized rice hulls and straw bales were constructed. A comparative cost analysis and monitoring experiments were carried out. The mockup plans of the rooms are shown in Table 2.

Table 2.

Mockup rooms and monitoring sensor plan.

The mockups were planned as two 8.5 m² rooms, and the direction, window size and heat transmission coefficient were matched to establish identical monitoring conditions. Straw bales, roughly 900 mm × 450 mm × 360 mm were produced. The construction efficiency could be reduced significantly if the thickness was adjusted by cutting; thus, a straw bale wall with a 450-mm basic thickness was constructed. The thickness of the wall insulated with carbonized rice hull was 315 mm in order to determine the heat transmission coefficient of the wall.

Indoor areas of both mockup rooms were constructed to be identical in order to create matching indoor environmental conditions, and both mockups faced south to capture the optimum sunshine. Mockups were constructed with lightweight timber structures, and a double-stud 2 × 4 wall system was used to take into account the thickness of the wall. Glass wool R19 24 K was used as the roof insulation material, and XPS (Extruded Polystyrene) served as the floor insulation. Windows made exclusively for timber houses with a heat transmission coefficient: 1.80 W/m²·K were used, and the inside and outside surfaces of the walls were finished with 30-mm soil plaster.

Monitoring system

To assess the environmental control performance of the insulation; temperature, humidity, formaldehyde and CO₂ sensors were installed with a data acquisition system. Weather data on the site were measured within 20 m from the mockup rooms. Sensors and instruments for the indoor environment were set up at the centre of the room and 1 m above the floor, and a Lab view-based monitoring system was programmed and used for the experiment. The sensor placement is also shown in Table 2 and the accuracy and range of the measurement devices are shown in Table 3.

Table 3.

Accuracy and range of the measurement devices.

Monitoring	Range	Accuracy
Temperature	−20℃ to 70℃	±0.6℃ at 25℃
Humidity	0–100% Rh	±2% (20–80%), ±3% (<20%, >80%)
CO₂	0–5000 ppm	±50 ppm, ±3% of reading

Methodology

Monitoring was done continuously from January 2012 in order to analyse seasonal indoor environmental changes due to the insulation. Other than the initial error values, the data from 12 February to 29 July 2012 were used for the analyses and conclusions.

Data were collected and saved at each minute by an automated monitoring program. Before monitoring started, the windows and doors were all opened; after 1 h of ventilation, the monitoring began. In February (the initial stage of monitoring), due to the limitations of the measurement intervals, three repeated experiments were conducted over nine days, and after an instrument upgrade, continuous monitoring proceeded. A heating experiment was carried out from 3 March to 16 March to verify the indoor environment control performance under heating circumstances. Aside from this period, extra heating, cooling and ventilation were not supplied in order to measure basic performance. Further, data were not recorded during 17–22 March and 23–30 May due to a power supply malfunction of the monitoring system, and these days were thus excluded from the analysis. Additionally, formaldehyde concentration was monitored along with CO₂ concentration to determine indoor air quality. However, the values were measured below 0.08 ppm during the experiment (indoor formaldehyde concentration standard of WHO), so the data were excluded from the results of the analysis.

Results

Construction cost analysis

Cost analysis results for the mockup constructions are shown in Table 4. For the comparative cost analysis, construction costs for the lightweight timber mockup insulated with glass wool (identical indoor area and heat transmission coefficient) were estimated and compared. The costs for the straw bales and carbonized rice hull mockups were analysed based upon an actual construction process, and the construction cost for the lightweight timber frame mockup was estimated by a timber house construction company. To ensure the analysis was accurate, all material costs were compared with costs from the National Cost Information magazine, which is published by a National Cost Investigation Organization under the Ministry of Strategy and Finance. The differences were within ± 10%, so the construction cost analysis results are reliable in the market. The results of the cost analysis showed that straw bales had the highest construction cost, followed by carbonized rice hull and lightweight timber, in that order. Material cost was more than 60% of the overall construction cost in all three, and windows, doors and roofing made up a large part of those costs. Carbonized rice hull was the most expensive insulation material of the three, and labour costs were higher for straw bales than the others.

Table 4.

Mock-up construction cost analysis results.

Category		Carbonized rice hull		Straw bale		L.W. timber frame
Category		US$	%	US$	%	US$	%
Materials	Structure	1216	16.9	1146	15.6	1357	21.6
	Wall insulation	975	13.5	708	9.6	131	2.1
	Roof/floor insulation	119	1.7	119	1.6	119	1.9
	Finishing (inside /outside)	675	9.4	675	9.2	675	10.8
	Roofing /door/windows/etc	1837	25.5	1798	24.5	1798	28.6
	Sub-total	4822	67.0	4446	60.5	4079	65.0
Labour	Carpentry	1056	14.7	1056	14.4	1056	16.8
	Labour work	1232	17.1	1760	23.9	1056	16.8
	Sub-total	2288	31.8	2816	38.3	2112	33.6
Others	Transport	88	1.2	88	1.2	88	1.4
Others	Total	7198	100	7350	100	6279	100

Note: 1000 Korean won = US$ 0.88 (Currency rate: 2013.06.02.)

Temperature monitoring results

The temperature monitoring results are shown in Table 5. For outdoor temperature, the average temperature increased gradually with seasonal changes from 1.0℃ in February to 28.0℃ in July. The temperature variations occurred larger in March and April than in February, and they were smaller after May. The difference between indoor and outdoor temperatures was 3–4℃ based on the average, and it lessened as summer approached. For indoor temperature, no difference was exhibited between mockups.

Table 5.

Temperature monitoring results.

		February (20)	March (26)	April (30)	May (24)	June (30)	July (29)
Mean temperature (℃)	S	4.11	15.92	15.76	22.19	26.36	28.19
	C	4.46	16.16	16.02	21.83	26.34	28.36
	O	1.02	5.93	14.46	19.53	26.41	28.02
Standard deviation	S	2.31	6.39	4.65	2.26	1.84	2.31
	C	2.26	6.13	4.41	2.12	1.81	2.14
	O	4.77	4.92	5.97	4.69	3.92	2.89
Variation (℃)	S	11.3	18.9	18.8	10.3	10.6	10
	C	11.7	18.8	17.2	10.7	11	9
	O	22.7	26	27.4	21.3	16.5	12.9
Maximum (℃)	S	9	23	25.2	26.7	32	34
	C	10	23	23.9	26.7	32.4	33.5
	O	12.4	19.7	27.5	32.4	34.9	34.5
Minimum (℃)	S	−2.3	4.1	6.4	16.4	21.4	24
	C	−1.7	4.2	6.7	16	21.4	24.5
	O	−10.3	−6.3	0.1	11.1	18.4	21.6

S: straw bale; C: charred rice hull; O: outside.

In February, which is winter in Korea, three repeated experiments were conducted in three-day intervals to check and set the instruments. After the third experiment, instruments were replaced and data proceeded to be measured continually. The outdoor temperature was 10.3–12.4℃, which was typical winter weather until mid/late February, and the temperature rose on 29 February and 1 March. The results of temperature monitoring indicated that both mockup rooms exhibited stable temperature transitions compared to the outdoor temperature. When the outdoor temperature displayed a variation of 22.7℃ (−10.3℃ to 12.4℃), the straw bales mockup room had a variation of 11.3℃ and the carbonized rice hull room had a variation of 11.7℃, indicating relatively stable temperature changes, and the temperature was influenced relatively little by repeated experiments. Due to the accuracy of the temperature sensor (±0.6℃), the small differences in the monitoring temperature of straw bale and carbonized rice hull rooms do not suggest any significant difference in their relative performance.

To confirm the performance of mockup rooms in a heating environment, a heating experiment was carried out for two weeks, during 3–16 March. Heating was supplied by an electric heating film on the floor, which was programmed to supply heat at the same time at 20℃, so both mockup rooms were supplied with the same heat energy. As a result of the heating experiment, the temperature transition of both mockup rooms displayed a nearly identical graph, demonstrating that the temperature control performances of both mockup rooms were similar.

In April and May (spring season), the average temperature in the straw bales mockup room was 15.8℃ and 16.0℃ in the carbonized rice hull mockup room, showing that the temperature of the mockup rooms exhibited no significant difference, and similar results were shown in May.

In June and July (which is early summer and monsoon season), the average temperature of the straw bales and carbonized rice hull mockup rooms exhibited results similar to the outdoor temperature transitions. The difference between the mockup rooms was deemed insignificant.

Humidity monitoring results

The humidity monitoring results are shown in Table 6. Humidity gradually increased from the dry winter of February to monsoon season of July, and on rainy days and during the monsoon season, humidity values increased rapidly to 80%. Both mockup rooms sustained a 50%–60% relative humidity generally, but occasionally crept over 60% during monsoon season, and indoor humidity remained low during early March throughout the heating experiment.

Table 6.

Humidity monitoring results.

		February (20)	March (26)	April (30)	May (24)	June (30)	July (29)
Humidity Average (%)	S	58.54	46.80	53.09	56.85	56.57	59.39
	C	47.08	41.43	50.10	53.34	53.02	59.11
	O	44.43	53.15	48.11	50.91	51.04	64.00
Standard deviation	S	2.49	5.28	2.39	1.53	0.89	1.63
	C	5.33	6.04	3.41	1.66	0.69	2.96
	O	10.70	14.96	16.27	14.83	11.36	9.99
Variation (%)	S	17.8	23.3	13.3	6.9	4.4	6
	C	24.2	24.7	13	7.1	3.3	10.2
	O	45.8	60.7	60.2	65.2	57.9	63.8
Maximum (%)	S	56.4	59.8	87.6	89.7	57.8	62.2
	C	54.8	54.9	55.8	56.8	53.9	63.4
	O	69.9	80.5	82.4	83	76.9	84.8
Minimum (%)	S	38.6	36.5	44.3	52.8	53.4	56.2
	C	30.6	30.2	42.8	49.7	50.6	53.2
	O	24.1	19.8	22.2	17.8	19	21
Rainy day (#)		4	10	12	8	6	12

S: straw bale; C: charred rice hull; O: outside.

In February, which is generally dry in Korea, humidity was low, with an outdoor average of 44.4%, except on 25 February when rained. The humidity result was typical of a dry winter. In early February, humidity values were gathered in three-day intervals by three repeated experiments, and the mockup rooms exhibited 17.8% and 24.2% variations, which was relatively stable compared to the outdoor values. The straw bales mockup room maintained an average humidity of 52.8% and a variation of 17.8%, so its humidity performance was more stable than the carbonized rice hull mockup room (average: 47.1%, variation: 24.2%), and generally a comfortable humidity range of 40%–60% in all periods was sustained. The carbonized rice hulls mockup room maintained a relative humidity condition under 40% at some points during the three repeated experiments and was shown to maintain a relatively low humidity level compared to the straw bales. However, humidity transitions were generally sustained in the comfort zone.

In the heating experiment, transition patterns of humidity for both mockup rooms were identical, and the humidity of the straw bales mockup room was about 5% higher on average. Notably, as heating continued, the humidity was reduced gradually in both mockup rooms; and both maintained a relative humidity over 30% during the whole experiment period.

From April to June, which is spring to early summer in Korea, the average outdoor humidity was 48.1%–51%, and carbonized rice hulls rooms exhibited a 3.0%–5.4% higher average humidity than rice hulls rooms. Both mockup rooms sustained a comfortable humidity level of 50–60%, and a similar variation occurred in both mockup rooms, so there was no significant difference in humidity stability.

In July, there were 12 rainy days caused by the monsoon according to the weather station. Notably, there was continuous rain during the 13–19 July, so the indoor environmental control performance of the mockup rooms were monitored and comparatively analysed during the monsoon season. The average outdoor humidity was high at over 84.3% during this time due to the rain, and the humidity variation in outdoors was as high as 63.8%, depending on the rain. Under these circumstances, the humidity range of the straw bales mockup room was 56.2%–62.2% and the carbonized rice hull mockup room was 53.2%–63.4%. The relative humidity in both these mockup rooms was largely stable compared to the outdoors.

The humidity of the carbonized rice hull mockup room was about 2% lower than the straw bales mockup room in early July, but as the monsoon continued, humidity was 1%–2% higher. However, the differences in measurements were not significant as the accuracy of humidity sensors was ±2%.

CO₂ monitoring results

CO₂ monitoring results are shown in Table 7. The average outdoor CO₂ concentration was 434.3 ppm with a measurement range of 300–500 ppm regardless of the season. The carbonized rice hull mockup room had a slightly higher CO₂ concentration at 491.7 ppm than the outdoors; but taking into consideration of the instrument accuracy (±50 ppm), the difference was not significant. However, the CO₂ concentration in the straw bales mockup room was very high with an average concentration of 1049.1 ppm; therefore, the CO₂ emission resource was likely from the straw bales.

Table 7.

CO₂ monitoring result.

		February (20)	March (26)	April (30)	May (24)	June (30)	July (29)
CO₂. average (ppm)	S	536.5	680.9	1200.0	1472.2	897.5	1338.2
	C	333.8	409.7	459.8	496.7	522.2	661.7
	O	478.9	443.5	472.7	404.6	402.2	417.2
Standard deviation	S	57.6	83.1	728.5	650.1	93.7	563.5
	C	29.7	41.0	68.6	47.3	32.1	124.1
	O	46.7	47.4	39.8	37.4	26.3	30.8
Variation (ppm)	S	370	501	3132	3115	479	2482
	C	226	246	396	322	242	625
	O	293	223	233	222	154	201
Maximum (ppm)	S	757	985	3618	3835	1161	3144
	C	485	523	736	709	682	1065
	O	624	568	567	524	476	518
Minimum (ppm)	S	387	484	486	720	682	662
	C	259	277	340	387	440	440
	O	331	345	334	302	322	317

S: straw bale; C: charred rice hull; O: outside.

High-CO₂ concentration was constantly monitored in the straw bales mockup room compared to the outdoors during February to July. In February, when the temperature and humidity were low, the CO₂ concentration in the straw bales mockup room was not very different to that measured in outdoors. However, as the temperature rose during March, the CO₂ concentration also rose and over 1000 ppm was measured after April.

During the heavy rains in April and July, CO₂ concentration in the straw bales mockup room increased rapidly. This could be due to the infiltrated humidity through the wall which had triggered the bio-metabolism of straw bales.

Discussion

The result of the cost analysis showed that both mockup rooms constructed with natural materials were more expensive than the estimated cost for construction of a general glass wool-insulated mockup room. The straw bales mockup room had the highest cost for construction. Glass wool is mass-produced, and carbonized rice hulls cost 37% more than straw bales due to the manufacturing process for carbonization. However, rice hull and straw bales can be obtained easily after harvest in rural areas, and if a farmhouse can self-produce carbonized rice hulls and straw bales, material and transport costs can be reduced and an equally competitive position can be achieved compared to the synthetic insulation materials. With regard to labour cost, carpentry for the structure and roofing have the same costs, but insulating and finishing work have different costs, so that carbonized rice hulls appeared to be the most cost effective. The straw bales mockup room required relatively more time to seal the gap between the bales and the frame for air tightness; therefore, there was 6%–7% higher labour costs compared to the mockup room built with carbonized rice hulls. Thus, the carbonized rice hulls would be more cost effective than the straw bales, and if precondition of self-production is satisfied, it can be equally competitive with synthetic glass wool insulation materials.

Temperature monitoring showed that there were slight differences in some periods, and these were significant based on a t-test (t: −4.14, p < 0.01). However, the insulation performance of both mockup rooms appeared to be the same considering the accuracy of the instrument. Therefore, when the heat transmission is identical, the difference in the construction method between straw bales (masonry and plastering) and carbonized rice hull (pouring and plastering) would not affect thermal performance of the constructed structure.

For precise humidity control analysis, absolute humidity was calculated based on temperature, relative humidity and altitude of the site, and the results are shown in Figure 1. The outdoor absolute humidity increases gradually because of the seasonal changes in the summer. Due to the lack of indoor humidifiers or any other resources, the reason for higher absolute humidity in the mockup rooms compared to the outdoor conditions was likely due to the desorption effect of straw bales and carbonized rice hulls. In July, during the monsoon season in Korea, the opposite result occurred, where indoor absolute humidity appeared low compared to the outdoor conditions and again this was likely due to the absorption effect, demonstrating the effectiveness of the natural materials for maintenance of indoor humidity. The carbonized rice hulls can hold 2.5 times the water of its dead load,¹⁷ thus the mockup room insulated with carbonized rice hulls demonstrated the capacity to maintain a 3%–10% lower in relative humidity. Both carbonized rice hulls and straw bales exhibited an effective humidity control performance during the general monitoring periods.

The comparative results of outdoor humidity, CO₂ concentration and the amount of precipitation are shown in Figure 2. CO₂ concentration increased in accordance with increasing humidity due to precipitation. Notably, in late April through to early May and monsoon season, very high CO₂ concentrations were measured compared to other periods and were above 2500 ppm. Therefore, ventilation within the rooms is crucial during the rainy season. Generally, an indoor CO₂ concentration of 2000–3000 ppm would start to pose a negative effect on the human body. Headaches and dizziness can occur over 3000 ppm, and it is hazardous if this concentration is maintained.²⁶ In Korea, there is no building code to regulate CO₂ concentration in residential buildings. However, CO₂ concentration is regulated below 1000 ppm in the Standard of Indoor Air Quality for Multiple Use Facilities.²⁶ The amount of time people spend in residential buildings is a lot longer than multiple use facilities, so these results indicate that counter solutions for high-CO₂ concentration are required.

The ideal moisture content of a straw bales wall is 14%, and biological activities do not occur under this ratio.¹⁶ Also, the moulds and bacteria are activated from 20℃ to 65℃.²⁹ According to the weather station data, the amount of precipitation on 21 April was 49 mm, which was the highest rate during March to May, and a lot of rain fell on 6 and 15 July: 62.5 and 66.5 mm, respectively. Additionally, the temperature of the period was over 20℃ when there was a rise in CO₂ concentration, which satisfies the temperature requirement for biological activities.

With regards to metabolism, the fermentation refers to an energy generating process where organic compounds acts under anaerobic conditions.²⁷ The metabolic reactions are represented by the formulas as follows²⁸

(Anaerobic fermentation)

C_{6} H_{12} O_{6} (glucose) \to {2 C}_{2} H_{5} OH (alcohol) + {2 CO}_{2} + 2 ATP

(1)

(Acetic fermentation)

C_{2} H_{5} OH (alcohol) + O_{2} \to {CH}_{3} COOH (aceticacid) + H_{2} O + 8 ATP

(2)

{CH}_{3} COOH (aceticacid) + O_{2} \to {2 CO}_{2} + {2 H}_{2} O

(3)

It can be inferred that the anaerobic fermentation started due to the satisfaction of temperature and humidity for biological activity as test results revealed. According to formula (1), the glucose (C₆H₁₂O₆) in straw bales changed to alcohol, and it produced CO₂ and energy in the fermentation process. In addition, the alcohol was fermented to the acetic acid (CH₃COOH) by formula (2), and the acetic acid produced more CO₂ in the continuous process (3). The experimental researcher reported light acid scent in the straw bales mockup room, thus the acetic fermentation could be confirmed. Therefore, this proves that fermentation of straw bales caused a CO₂ concentration increase in late April through early May and monsoon season.

As a result, during the spring and monsoon seasons, which have a lot of rain comparatively, CO₂ concentration can increase to a critical level that could negatively affect the residents’ health due to the fermentation of a straw bale. Therefore, straw bales should be used with caution for the safety of indoor air quality. In addition, thorough plans and precise construction are required to prevent fermentation, including sufficient eave length, water proofing of the lower wall surface and moisture-proofing.

Conclusion

In this study, real-scale mockup rooms insulated with straw bales and carbonized rice hulls were constructed, and construction costs were analysed. Further, monitoring was carried out to assess the indoor environment control performance of natural insulation materials, and the deduced results are as follows:

The heat conductivity of carbonized rice hull was 0.069 W/m·K (density: 106.9 g/m³), and this is better than that of straw bales: 0.092 W/m·K (density: 207.5 g/m³).

The cost analysis has illustrated that carbonized rice hulls are more cost effective compared to straw bales, and if self-production is satisfied, it can be competitive with general insulation materials.

The temperature monitoring has shown that the differences between the mockup rooms built with the natural materials were insignificant. Thus, there is no difference in insulation performance of straw bales room built with masonry method compared to the carbonized rice hulls room built with pouring method.

The humidity monitoring has shown that both mockup rooms could sustain a comfortable indoor humidity of 40%–60% due to the desorption effect of materials during winter to spring and the absorption effect of materials in summer. Thus, verifying that both straw bales and carbonized rice hulls are effective for controlling indoor humidity.

The CO₂ monitoring has shown that CO₂ concentration in the straw bales mockup room could rise above 3000 ppm and could lead to the bio-metabolism of straw bales and initiate anaerobic fermentation which could produce CO₂ emission, and this could pose an adverse effect on residents’ health. Therefore, thorough planning and careful construction including water-proofing would be needed for architectural use of straw bales.

This study has verified the heat conductivity, economic feasibility and indoor environmental control performance of using carbonized rice hulls as a natural insulation material for building construction. Significantly, the study has illustrated that the carbonized rice hulls could maintain a steadily low-indoor CO₂ concentration.

The results of this study could provide a useful reference for selection of materials based on economic feasibility and environmental performance of natural materials. For future study, additional verifications of long-term performances of carbonized rice hull and durability are required.

Footnotes

Authors' contribution

All authors contributed equally in the preparation of this manuscript.

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

This work was supported by the National research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2010-0029453) and (NRF-2011-0018116).

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Experimental research on the indoor environment control of natural insulation materials: Carbonized rice hull and straw bales

Abstract

Keywords

Introduction

Materials

Heat conductivity

Mockup rooms

Monitoring system

Methodology

Results

Construction cost analysis

Temperature monitoring results

Humidity monitoring results

CO2 monitoring results

Discussion

Conclusion

Footnotes

Authors' contribution

Declaration of conflicting interests

Funding

References

CO₂ monitoring results