Determining moisture levels in straw bale construction

The use of straw bales for the construction of buildings in the UK has to date generally been limited to the self-build fringe sector. In order to bring this form of construction into the mainstream sector, to benefit from its inherent low carbon and high insulation characteristics, it is necessary to guarantee the long-term durability of the straw. Maintaining low moisture levels is critical to the long-term resistance of straw to biological decomposition. This paper presents results from a study on moisture monitoring in straw bale construction, and includes the development of an empirical equation which relates straw moisture content to surrounding microclimate relative humidity and temperature. This knowledge allows continuous non-invasive condition monitoring of the straw in on-going research work and potentially as part of future managed maintenance of straw bale buildings.     

Development of a cost effective probe for the long term monitoring of straw bale buildings

This paper reviews current methodologies for measuring the moisture content of straw bale walls in buildings. It discusses the development of an affordable and accurate moisture probe that has been designed to be easily assembled by the builder or owner of a straw bale building from items readily available in the United Kingdom (UK). The probe uses a timber block inserted into the wall, relying upon the measurable moisture variances of the timber and relating this to the surrounding straw. The probes are designed to be used in pairs of varying length, taking measurements at different depths to give an estimate of the moisture gradient through the wall. In order to properly calibrate this device, a full set of sorption and desorption isotherms were established for both Oat and Wheat straw and three different timber species. The results from an environmental chamber have been compared to readings from specimens of the new probe installed in a straw bale house in the south west of the UK. The results were found to be consistent, to within 2%, with the laboratory findings.     

Auto-ignition in hygroscopic,organic materials - especially forest products - as initiated by moisture absorption from the ambient atmosphere

This paper summarizes and quantifies experience concerning self-heating and the risk of auto-ignition, due to the absorption of moisture from the surrounding air by low-density bales of hygroscopic cellulosic material such as textiles, waste-paper, and insulating boards during storage. Such risks do exist, especially after a drying operation, and they have apparently led to a number of fires.As a starting-point, the equilibrium moisture content of ligno-cellulosic material over a range of relative humidities and temperatures is presented as well as the heat release accompanying the absorption of water and the condensation of water vapour from ambient air. Using data on the specific heat of cellulosic material, the possible adiabatic temperature rise is calculated. The kinetics of moisture absorption into the bale and the rate of heat transport out of the bale are discussed as a function of the density of the material.It is shown that the heat released by an increase of a few percent. in the moisture content can raise the temperature of low-density hygroscopic material into a range where oxidation rates are sufficient to lead to the onset of carbonization in the centre of the bale. Thereafter, smouldering may lead to auto-ignition. Bales which are not yet ignited often show this carbonization zone in the centre.Preventive measures — such as a controlled remoistening after drying — are summarized.     

被动式太阳能集热蓄热墙对室内湿度调节作用的研究

在两间分别采用被动式太阳能集热蓄热墙体和普通保温节能墙体的同实体大的实验房屋中,利用多点温湿度及风速的计算机巡回检测系统,在2004年供暖期对室内外温湿度、风速、太阳辐射照度及墙体内温度等参数进行了实测。结果表明,未采用被动式太阳能集热蓄热墙体的对比房内相对湿度比太阳房高20%左右,并因湿度较高在窗户上出现结露现象。分析了两间实验房屋的热湿传递特性以及对一些物性参数如室内相对湿度、墙体的含湿量、人的舒适性等的影响。     

草砖建筑墙体的新材料——棉花秸秆应用研究

新疆是我国的产棉大省,每年有大量的废弃棉花秸秆被焚烧或掩埋。研究了以棉花秸秆为原料,加工制成草砖,作为新疆抗震安居住房的墙体材料。在此基础上分析了棉花秸秆草砖建房的优点,不仅增加农村住房工程墙体的新材料和新结构型式,使棉花秸杆得到重新利用,减轻农民建房的经济负担。因此,这种新型材料是一种具有较好的理论研究意义和应用推广前景。     

不同密度的麦秸砖墙导热系数试验研究

在麦秸砖墙传热机理研究的基础上,对不同密度麦秸砖墙导热系数进行试验研究,结果表明,麦秸砖墙导热系数小,是一种可持续发展、低能耗、经济的绿色墙体绝热材料.同时,试验结果还表明麦秸砖墙导热系数随密度增大呈先减小后增大的变化规律.另外,根据麦秸砖的制作、砌筑和试验结果,确定了秸秆砖墙导热系数较小的合理密度范围.     

Assessing the environmental performance of stabilised rammed earth walls using a climatic simulation chamber

The SHU climatic simulation chamber is a novel piece of apparatus that allows testing of full-sized walls with realistic inner and outer wall climatic conditions. Four SRE test walls were successfully constructed and tested over four separate regimes to measure physical properties such as pressure-driven moisture ingress, rate of moisture penetration, and internal/interstitial condensation. The walls far exceeded a series of cyclic pressure-driven rainfall penetration tests based on BS 4315-2. After 5 days of exposure to static pressure-driven moisture ingress there was no evidence of moisture penetration or erosion. The embedded sensor array detected no significant increase in the relative humidity or liquid moisture content inside the test walls, throughout a range of temperature differentials with high levels of humidity, indicating a negligible risk of internal or interstitial condensation.     

地下洞室多孔墙体热湿传递的数值模拟

提出了一种以温度与相对湿度为驱动势的多孔墙体热湿耦合传递的数学模型,该模型同时考虑了水蒸气与毛细孔内液态水的传递。采用控制容积法将理论方程组离散并编制了计算程序,对深埋地下洞室的墙体进行了热湿传递的数值模拟,得到了墙体温度、相对湿度、热流率、湿流率的变化规律。结果表明,墙体传热过程趋于稳定的时间远小于传湿过程。     

多层墙体热湿耦合传递模型及验证

从非平衡热力学角度论证了多层墙体热湿耦合过程采用水蒸气分压力和温度作为驱动势的合理性。由于水蒸气分压力是含湿量和温度的函数,利用全微分思想,建立了多层墙体热湿耦合传递模型,该方法可避免Budaiwi方法在热湿耦合模型建立过程中采用的空气含湿量与相对湿度间的近似表达式,而且简化了方程系数,便于方程的求解。通过对多层墙体求解结果的对比,验证了该模型的有效性。     

Assessment of temperature gradient effects on moisture transfer through thermogradient coefficient

The objective of this paper is to describe moisture transfer through porous material due to temperature gradient. For that purpose, an experimental device was set up to assess moisture flux under isothermal and non-isothermal conditions. This involves placing samples between two compartments with controlled air conditions and monitoring relative humidity and temperature profiles inside the samples over the time. To interpret these results, we proposed to express the mass flux in terms of two driving potentials: water vapor content gradient and temperature gradient. Accordingly, thermogradient coefficient was calculated and discussed. It represents the difference between the moisture fluxes under isothermal and non-isothermal conditions. The impact of temperature gradient on the moisture buffer value (MBV) was also considered through a numerical experiment taking into account thermogradient coefficient. Results show that temperature gradient implies a relative variation of the MBV for about 14%. Thus, it would be better to consider non-isothermal conditions for its assessment.     

湿传递对建筑墙体传热性能的影响

建筑物的耗能与建筑围护结构的传热传湿密切相关,了解建筑墙体内部的热湿传递对建筑节能有重要影响。以相对湿度和温度梯度为驱动势建立墙体一维非稳态热、湿和空气耦合传递模型(HAM模型),并利用有限元法进行了数值求解,重点关注了湿传递对传热的影响。数值结果表明:考虑传湿时墙体内部温度波动小,墙体进行热湿传递会产生湿积累,降低墙体使用年限;考虑传湿时通过墙体总传热量比不考虑传湿时多7.5%;考虑传湿时内壁面最大平均数比不考虑传湿时大0.78。     

关于气候分区问题的探讨

针对《全国民用建筑工程设计技术措施》暖通空调.动力篇(2009年版)中以"最湿月平均含湿量"作为气候分区判断依据一问题,探讨了含湿量(绝对湿度)和相对湿度概念的不同,指出含湿量是空气加湿或减湿的程度,而相对湿度表示的是空气接近饱和的程度,并进一步分析了湿球温度是空气接近饱和程度的一种度量,是一个独立的空气状态参数。最后笔者通过一些气候分区的举例,进一步说明了湿球温度这一参数在气候分区中的重要性。     

外保温节能墙体保温层重量湿度含量的研究

在能源日益紧张的今天，外保温墙体是我国严寒地区实现建筑节能第二阶段目标的重要措施。按照《热工规范》对苯板外保温墙体在不同室内温度及空气相对湿度下保温层的重量湿度含量进行了计算分析，并总结和提出了一些有效的措施，控制保温层的凝结界面的中重量湿度含量在规范允许的范围之内。     

外墙外保温系统对室内温度和湿度影响

通过连续监测实验房的室内墙体表面和空气的温度和湿度 ,观察外墙外保温系统对室内居住环境的影响。观察结果显示 ,外墙外保温系统能够减小室内墙体表面温度受室外气温变化的影响 ,避免冬季的冷墙效应 ,使室内的温度和相对湿度保持较为恒定 ,改善了室内居住环境。     

新建节能建筑墙体材料层及排布对干燥过程的影响

建筑围护结构内的热湿耦合传递是一个非常复杂的过程,其研究是降低建筑能耗、评估和预防湿害、提高室内热舒适性、室内卫生及优化围护结构性能的基础。新建节能建筑墙体具有初始含湿量大的特点,若墙体湿积累过大,则容易出现墙体表面剥蚀、渗漏、发霉甚至结构出现损坏的现象。墙体干燥时,传热传质过程同时发生且相互耦合。目前相关热物性仿真软件、理论研究和设计规范主要建立在热传递的基础上,忽略了湿传递的影响,对新建建筑墙体干燥不适用。WUFI~? Pro热湿仿真软件充分考虑了材料本身含湿量、风驱雨、太阳辐射、长波辐射、毛细传输和夏季结露等典型气候的影响,实现了对自然气候条件下建筑构件非稳态热湿性能的真实计算。节能墙体多在外墙添加内外保温层来增加围护结构的传热热阻,且在保温层内外两侧分别添加隔汽层和空气层的措施来防止保温层受潮,最终提高围护结构的保温性能。为墙体美观,多在围护结构的内外两侧分别黏贴墙纸和釉面砖。采用WUFI~? Pro对北京地区2种典型的建筑墙体进行热湿耦合传递模拟,分析新建建筑墙体在不同保温层材料和位置时的干燥过程,以及保温层两侧的隔汽层和空气层、墙体两侧的墙纸和釉面砖对墙体干燥过程的影响。模拟用室外条件为北京典型气象年小时室外气象参数,室内条件设定室内冬季供暖温度T_1=20℃,夏季室内温度设计值T_2=25℃,全年平均相对湿度为50%。模拟外围护结构属于西向,墙体温湿度初始条件为:相对湿度为100%,温度为15℃。模拟结果表明:内保温层的设置非常不利于围护结构的干燥,容易在内保温层和砌块之形成湿积累,降低围护结构的耐久性;EPS、PU和XPS都能降低围护结构含湿量,但EPS更有利于墙体干燥;隔汽层和空气层的添加可一定程度上阻止保温层受潮,避免造成湿积累,进而提高围护结构的保温性能;釉面砖和墙纸的黏贴将严重延缓围护结构的干燥过程,降低围护结构的保温性能,缩减建筑构件的使用寿命。     

Tauwasserausfall in Eissporthallen

Condensation in Ice Sports Arenas. Because of damage to the wooden structure or mould on the suspended ceiling, examinations were made in three ice sports arenas in Northern Germany. Measurements of temperature and relative humidity inside and outside these arenas gave interesting results: During relatively warm winter periods the measured surface temperature at the bottom of the roof above the ice rink was lower than the air temperature in the arena, resulting in a significantly higher relative humidity at the roof than outside the building. Thus, condensed water was dripping from the roof construction to the ice. Depending on the construction, at the end of the winter the moisture content of the wooden structure was very high, partially resulting in wood destroying fungi, or the steel parts were corroding, respectively. In this paper the climatic conditions in ice sports arenas are simulated including conductivity, convection and radiation in a simplified model. The results were discussed for different parameters. Finally two different solutions are shown to lower the moisture content in the timber construction or the steel corrosion, respectively: – An insulated suspended ceiling below the roof construction may protect timber or steel from the low ice temperature with the resulting high humidity. – An air‐conditioning system may dry the air in the arena. By that means the relative humidity can descend as low as necessary for a durable construction.     

炭化工艺参数对竹炭调湿性能的影响

以平衡吸放湿量、吸放湿效率和调湿稳定性来评价竹炭调湿性能,研究了炭化温度、升温速率以及保温时间这3个炭化工艺参数与竹炭调湿性能之间的关系.结果表明:较低的炭化温度和较长的保温时间有利于竹炭平衡吸放湿量的提升,在较低的升温速率下竹炭的平衡吸放湿量较高;炭化工艺参数对竹炭的吸放湿效率影响不大;在升温速率低、保温时间短的情况下竹炭的调湿稳定性更好.     

秸秆复合墙体研究进展及发展前景

对秸秆这种天然可再生资源用于建筑领域具有的特点和优势进行了分析;对植物纤维水泥板、插芯式秸秆水泥砌块、秸秆草砖墙体、秸秆人造板、秸秆纸面草板等秸秆基复合墙体的主要类型的研究应用现状进行了总结,并对其优缺点进行了比较。对秸秆资源应用于建筑领域的发展方向进行了阐述,认为节能性好、可工业化生产、便于装配式建筑应用的秸秆纸面草板具有良好的发展前景,特别是应用于农村建筑节能具有广阔空间。     

Modeling the effect of hygroscopic curtains on relative humidity for spaces air conditioned by DX split air conditioning system

The use of hygroscopic materials for moisture buffering is a passive way to moderate the variation of indoor humidity. Through absorption and desorption, surface materials in the indoor environment, such as curtains, carpets and wall paper, are able to dampen the moisture variations. The moisture buffering capacity of these materials may be used to improve the relative humidity of the indoor environment at reduced energy costs.The objectives of this paper are threefold. The first objective is to derive a theoretical model for the transient moisture transfer between a curtain system and the indoor air for the case where the curtain is placed in front of a wall. The second objective is to conduct experiments inside environmental chambers to validate the theoretical model and to test the ability of curtains to moderate indoor humidity. It is shown that the experimental results for the curtain moisture uptake and the relative humidity inside the chamber compared well with the model simulation results. The third and final objective is to test and evaluate the model under “real environment conditions” for a case study of a hygroscopic cotton curtain, placed in a “typical” office space in the city of Beirut with an area of 25 m² that uses direct expansion (DX) air conditioning system. It is found that hygroscopic curtains maintain humidity of less than 65% during part load operation compared to the upper limit of 70% relative humidity when no curtain is used. On the other hand, it is found that the energy use, as determined by the daily electrical power consumption of the DX system, is almost the same for the two cases, (with and without a curtain), where approximately 20 kWh of energy input is required 13 kWh of sensible energy and 7 kWh of latent energy.     

Experimental data set for validation of heat,air and moisture transport models of building envelopes

This paper reports experimental studies on heat, air and moisture (HAM) transfer through a full scale light weight building envelope wall under real atmospheric boundary conditions. The main objective of the article is to generate informative data so that it can be used for numerical validation of HAM models. The considered wall is a multilayered structure built up from outside to inside of external board, vented cavity, fibreboard sheathing, mineral wool between wooden studs and interior finishing. The global wall has a surface area of (1.80 × 2.68) m²; and is subdivided into three vertical parts. The parts differ from each other by the applied interior finishing. Between the different layers of each part and on the surfaces of the wall humidity, temperature and heat flux sensors are placed in a 3D matrix. At the outer surface of the wall, the applied sheathing is a bituminous wood board. In the board nine removable specimens are included. By regularly weighing the fibreboard samples, their moisture content could be quantified. Using data collected over a total time span of about two years, insight about the hygrothermal behaviour of the different envelope parts is obtained and at the same time a well-documented data set is generated that can be used for hygrothermal envelope model validation purposes.