The Use Of Attic Space For Cooling and Dehumidification

Traditionally, attic space in buildings is perceived as a source of nuisance where the moisture condensation occurs in winter encourages mildew growth, and heat build up in the attic space in summer increases the cooling load. However, if the attic is integrated in a holistic design and control strategy, it can function as a solar collector, a heat exchanger, and a desiccant!. This research investigates energy saving by optimizing direct and indirect ventilation through attic to pre-cool buildings and reduces humidity. This strategy was examined in a double story house with attic in a moderatehumid climate. The built up heat in the attic space and outside air ventilation were used to dry up roof construction materials during the day. When outside air cools down during the night but maintains high humidity, the indoor air circulates through the attic space. The attic construction materials absorb moisture from the indoor air. Thus indoor air loses both heat and moisture. EnergyPlus Simulation software was used to simulate this cooling and dehumidification strategy. The simulation results showed significant passive cooling and dehumidification in the building. Key notes: Dehumidification, Nighttime ventilation, Indirect Ventilation, Cooling load Introduction Traditionally, utilizing nighttime ventilation and thermal mass is a major strategy for passive cooling in dry climates. However, in moderate and humid climates, passive cooling through ventilation is more difficult to achieve. In these climates, the outside air is relatively cold during the early evening, but its relative humidity is high. Therefore, it is not suitable for nighttime ventilation. Later at night, outside relative humidity reaches the upper limit of the comfort zone, but the outside air enthalpy remains equal or more than the inside air enthalpy. Thus, introducing outside air to the space may reduce the inside air temperature without reducing the air enthalpy. Instead it could lead to more moisture absorption by inside building materials and furniture [1]. In the daytime, more moisture is produced inside the building due to the required ventilation and other inside latent energy sources. In most cases, dehumidification is needed to extract the excess moisture. Thus, more active cooling is needed to extract the extra moisture, which is admitted to the space during nighttime ventilation [2]. Conventional building materials usually store heat efficiently for short period of time. However, moisture content of building materials and furniture varies widely in their capabilities to store moisture. In addition, to achieve comfort, the air temperature tolerance in a space usually should not exceed 8-11 Fo(5-6 Co), but the comfort level can be achieved with wide relative humidity range of approximately 30-60%. Thus, the "enthalpy storage" capacity of the building materials can significantly contribute to cooling load. In addition, during the day where active cooling is required, if inside relative humidity is lower, thermal comfort can be achieved at higher air temperatures, and less dehumidification is required. During sunny winter days, solar radiation heat gain in the attic space may exceed the conduction and convection heat loss. Thus attic space will act as a passive heating collector. In summer, the attic space can work as a desiccant, here, the desiccant materials are the attic wood construction, and the regeneration energy is the solar energy collected in the roof. Summer Dehumidification Unlike heat transfer in building materials, which is clearly defined, moisture transfer in buildings is rather complex and involved many mechanisms that are still not fully explained [3]. There are at least 9 different mechanisms of water transport in solids which are; molecular vapour diffusion, molecular liquid diffusion, capillary flow, Kundsen diffusion, surface diffusion, Stefan diffusion, evaporation condensation, Poiseuville flow, and movement due to gravity. To get a general idea of moisture transfer in building materials and to explain the concept of using the attic space to dehumidify buildings let us consider the following example; In a typical clear sky summer day in a moderate climate, if the outside air temperature reaches 90Fo(32Co) and relative humidity of 50%, the attic air temperature in a typical wood building can reach 108Fo(42Co) and relative humidity 29%. During the night, if the air inside the attic is mixed with the house living space, we can assume that relative humidity in the attic will equal the living space relative humidity and let us say that it is 50%. Under steady state condition, the moisture isotherm curve shows that moisture content in the particleboard insulation will drop from .08 to .05 lb/lb (kg/kg) [4]. Isetti showed that the particleboard could approach the steady state in 12 hours [1]. This suggests that in a house with a roof area of 861 F2 (80 m2), if the relative humidity fluctuated between 50% and 29%, roof particleboard insulation will have a daily moisture capacity of approximately 353 lb (160 kg) of water. From another hand, the wood attic construction has also high moisture absorption capacities, which can reaches .15 lb/lb (kg/kg) under relative humidity of 50%, and .1lb/lb (kg/kg) under relative humidity of 29% [4]. In addition, wood can reach the balance point in more than 3 months [5]. In this case attic wood construction can contribute to moisture control in the short and the long term. Thus, in a typical wood house with attic space, moisture capacity of the attic construction materials is more that moisture produced in the house due to ventilation and internal latent heat gain (Figure 1)(Figure 2). The moisture balance in buildings can be defined by the following equation [6]; ∑ = × − × − − × × − = × ∂ n i A csur ci cu ci V n G V dt ci