Roof Ventilation Research Articles

Hygrothermal Characteristics of Cold Roof Cavities in New Zealand

The New Zealand Building Code contains minimum durability requirements for components. For roof structures the requirement is 50 years if the component is structural or 15 years if it is not. Metal roof claddings are very common in New Zealand, and roof spaces are typically not deliberately ventilated. Recently, a number of roofs are failing to meet their durability requirement, and the lack of deliberate ventilation is a contributory factor in some cases. In this paper, we consider roof failures and analyse them using the hygrothermal simulation software WUFI® 2D (version 4.1). Using the National Research Council of Canada’s Guideline on Design for Durability of Building Envelopes, we evaluate to what extent the guideline can be used for such more complex models. Experimental data from a residential dwelling where excessive roof moisture issues were discovered shortly after occupancy are presented. A novel remedial solution using daytime-only ventilation to the roof cavity was trialled, and the data were used to benchmark a two-dimensional numerical simulation of the roof space using WUFI® 2D. A larger hygrothermal data set for 71 dwellings is presented together with relevant climatic conditions. The study works towards evidence-based building code changes for roof ventilation and is an example of using the guideline document for more complicated building envelope assemblies.

Adaptasi Kinerja Bangunan Rumah Tinggal dengan Ventilasi Atap Responsif

Thermal comfort is a condition where occupants can feel comfortable doing activities in the building. One of the ways to get comfort is done by passive design, in which the building is designed to adapt weather change outside the building. Because the weather outside is always changing, the planned building design must be responsive to these weather changes. This study uses a responsive roof ventilation model, that the movement of the roof vent covering grille, which serves as an attempt to regulate the exit of wind from the inlet. This study method begins with a literature review on the thermal comfort of residential building in the humid tropics, as a study of the application of passive design principles as building adaptation to weather changes. Furthermore, an analysis of the ventilation area is made based on the need for thermal comfort in the building. Finally, the planning of building performance systems automatically using the Arduino temperature and humidity sensors. The result is the concept of a framework as applying automation to responsive roof ventilation. This preliminary study is still basic and it is hoped that can be continued in research with the application and experimentation of the roofs in small building

Growth responses of light demanding and shade tolerant peat swamp forest saplings to elevated CO2

Aim: To determine the growth responses of two peat swamp forest species, Shorea platycarpa, a shade-tolerant slow-growing species and Macaranga pruinosa, a light-demanding fast-growing species under elevated atmospheric CO2 concentration. Methodology: The saplings of both species were grown in a shade house at ambient (400±50 µmol mol-1) and in an open roof ventilation greenhouse at elevated atmospheric CO2 concentration 800±50 µmol mol-1 for seven months. The temperature in both environments ranged between 25-33°C with 55–60% sunlight transmittance and the saplings were thoroughly watered twice a day. Plants growth measurements were estimated at frequent intervals. Saplings biomass characteristics were examined using destructive methods after seven months of treatment and non-destructive method was used for determination of leaf area. Results: Elevated CO2 enhanced all the growth characteristics in M. pruinosa with a significant increase was observed particularly on both height and diameter relative growth rate and biomass characteristics. The height relative growth rate and leaf area were significantly reduced under elevated CO2 in S. platycarpa but did not affect the shoot or root diameter and biomass significantly. A positive correlation (r =0.77, p>0.05) between stem biomass and basal diameter for plants under elevated CO2 was recorded for M. pruinosa, but not in S. platycarpa. Both species showed negative correlation (S. platycarpa; r = -0.53, M. pruinosa; r = -0.46, p>0.05) between stem growth and stem biomass at elevated CO2. Interpretation: These results unveiled profound effects of elevated CO2 on the growth of light-demanding species M. pruinosa, while shade-tolerant species S. platycarpa was not relatively affected by elevated CO2. This underscored the necessity to analyse different species performance to elevated CO2, thereby improving the ability to predict tropical swamp forest ecosystem responses to rising CO2.

A Study of Energy Efficiency in Flats, Case Study: The A-B-C Tower of Tambora Flats in Jakarta, Indonesia

Flats are one of the government's solution to provide decent housing for low-income people. However, as mostly flats overlook energy efficiency aspects, residents must pay high rental and operational costs. Therefore, this paper aims to study the principles of green buildings, especially in the building envelope that can bear the operating costs of flats with the case study on the A-B-C Tower of Tambora Flats. The A-B-C Tower of Tambora Flats is one of the flats building in Jakarta that has rental cost issues. This study employed the EDGE application that provided energy saving values in the application of green building principles, such as building mass and orientation, wall and roof finishing materials, natural shading and ventilation elements. From these values, it was only the application of green building principles that was significant for the Tambora flats. The results of this study revealed that the optimization of natural ventilation for flats was still important and having a significant impact on energy savings

3D numerical simulation of a novel ventilated roof: thermal performance analysis and fluid flow behavior

This paper presents a numerical simulation of a ventilated roof based on a novel plastic support especially designed to reduce the solar heat gains by improving the airflow distribution as compared to a traditional one. In order to evaluate the benefits of the new configuration, a 3 D numerical model was developed allowing for a deep understanding of the airflow distribution inside the ventilated roof channels. The simulations were performed by varying the solar irradiance from 600 to 1000 W m−2 assuming buoyancy-driven airflow. The investigation was conducted by comparing the new proposed ventilated roof assembly to the traditional layouts. In particular, two roof types were considered as benchmarks: an insulated roof and a non-insulated one. When compared to the traditional ones, the results revealed that the innovative ventilated roof led to a great reduction (up to 70%) of the total amount of solar heat gains for all the simulated scenarios.

Performance Improvement Plan of Air Circulation-Type Solar Heat-Storage System Using Ventilated Cavity of Roof

Indoor solar-heating systems that use ventilated roofs have drawn attention in recent years. The effectiveness and efficiency of such air-heating systems vary depending on the design and operation methods. In Japan, by introducing outside air into a ventilated roof cavity and circulating the air indoors, systems that simultaneously obtain ventilation, solar heating, and heat-storage effects have been actively developed. The conventional systems intake a large volume of outside air to increase the solar heat collection effect. However, there is a risk of heat loss and over-drying when a large amount of cold dry air during winter is introduced. In this paper, plans are presented for improving these solar heating and heat-storage effects by preventing over-drying using indoor air circulation via ventilated cavities in the roof and indoor wall. By comparing the results of the proposed system with those of the conventional system via numerical simulation, the heating load is found to be reduced by 50% or more by circulating indoor air to the ventilated roof and storing the heat in the indoor wall. Moreover, an increased relative humidity of approximately 10% was confirmed by reducing the intrusion of the outside air and keeping the moisture indoors.

Model of thermal buoyancy in cavity-ventilated roof constructions

Timber roof constructions are commonly ventilated through an air cavity beneath the roof sheathing in order to remove heat and moisture from the construction. The driving forces for this ventilation are wind pressure and thermal buoyancy. The wind driven ventilation has been studied extensively, while models for predicting buoyant flow are less developed. In the present study, a novel analytical model is presented to predict the air flow caused by thermal buoyancy in a ventilated roof construction. The model provides means to calculate the cavity Rayleigh number for the roof construction, which is then correlated with the air flow rate. The model predictions are compared to the results of an experimental and a numerical study examining the effect of different cavity designs and inclinations on the air flow rate in a ventilated roof subjected to varying heat loads. Over 80 different test set-ups, the analytical model was found to replicate both experimental and numerical results within an acceptable margin. The effect of an increased total roof height, air cavity height and solar heat load for a given construction is an increased air flow rate through the air cavity. On average, the analytical model predicts a 3% higher air flow rate than found in the numerical study, and a 20% lower air flow rate than found in the experimental study, for comparable test set-ups. The model provided can be used to predict the air flow rate in cavities of varying design, and to quantify the impact of suggested roof design changes. The result can be used as a basis for estimating the moisture safety of a roof construction.

Improvements in the energy performance of buildings in summer, through the integration of ventilated envelopes on north-facing facades and roofs. The case of Mendoza, Argentina

The proposal of energy efficiency measures in the residential sector in Argentina requires analyzing the architectonic possibilities of building rehabilitation using technologies that reduce energy consumption, that are feasible to implement locally. In regions with high solar radiation levels, as is the case of the city of Mendoza, heat fluxes transmitted inside can be reduced by the natural ventilation of the layers in the envelope, both on facades and roofs, thus obtaining significant savings in consumption for cooling purposes. This work evaluates the potential for improvement with the integration of ventilated envelopes. The work methodology is structured in two stages: i) survey of residential buildings by morphological typology and analysis of rehabilitation possibilities with ventilated facades, considering the exposed envelope surfaces by orientation; ii) simulation of a case study - previously validated with onsite measurements - using the EnergyPlus software. On integrating ventilated facades and roofs important energy savings of around 32% were achieved, considering the building without users (unoccupied). In the case of units on the top floor, with roofs exposed to the outside, energy savings of 260% were recorded.

Experimental study on the thermal performance of composite phase change ventilated roof

With the improvement of people's living standards, building energy consumption presents a trend of the increasing year by year. The phase-change energy storage technology is applied in the building so that the building envelope has good thermal insulation ability, and thermal storage performance the energy consumption of the building heating and air conditioning system will be greatly reduced. A new ventilated roof composed with multiple phase change material (VR-MPCM) was proposed as the experimental room, and a room of the same size conventional ventilated roof (CVR) was set as the contrast room. The PCM panel of myristic acid and tetradecanol was installed outside the test room, and the PCM panel of lauric acid and tetradecanol was installed inside. Then three kinds of summer experiments were carried out: stage Ⅰ, natural cold storage; stage Ⅱ, the ventilation layer regularly opened cold storage; stage Ⅲ, the ventilation layer regularly turned on and composite cooling. The research results show that compared with the comprehensive outdoor air temperature, stage Ⅱ can reduce the peak temperature in the experimental room by 41.1-44.9%, the minimum indoor air temperature increased by 19.2-27.7%; stage Ⅲ can reduce the peak temperature in the experimental room by 43.2-47.0%, and increase the minimum indoor air temperature by 26.7-43.9%. At the same time, stage Ⅱ reduces the dissatisfaction rate of the indoor thermal environment (DR) by 15.5% and stage Ⅲ by 40.2%. Regional comparison indicated that both stage Ⅱ and stage Ⅲ can effectively reduce the outer surface of the roof and indoor air temperature in severe cold areas, and reduce the fluctuation range of indoor air temperature, which has obvious potential for energy saving. Among them, stage Ⅲ will make the indoor environment more comfortable.

Simulation Study on Dynamic Thermal Performance of a New Ventilated Roof with Form-Stable PCM in Southern China

Latent heat storage in phase change material (PCM) is an efficient technology that can be applied in building envelopes. Installing PCM in building roof has been effective in altering space cooling loads. However, the heat absorbed by the PCM during the daytime will be released at night; the cooling load is shifted to the night. So, this study proposed a new ventilated roof with form-stable PCM (VRFP). The night cool air is used for ventilation during summer to remove the solidification heat of PCM and to store the cooling energy in the roof. Form-stable PCM is placed in the upper layer and ventilation duct is placed in the middle layer. The inner surface temperature of this roof is reduced sharply compared with the conventional PCM roof. The thermal performance of this PCM roof with night ventilation in Wuhan, a city in southern China, was studied by through Computation Fluid Dynamics (CFD) simulation. A three-dimensional dynamic numerical model of this roof was built. The effects of melting temperature range, thickness of Form-stable PCM layer and ventilation strategy on the thermal performance were analyzed. Results show that, in Wuhan city, the optimal melting temperature range is 35~38 °C, the appropriate thickness of PCM layer is 30~40 mm and the optimal ventilation speed is 2.4~2.5 m/s. This structure can effectively prevent the stored heat of PCM transferring from the exterior to the interior during the summer and reduce cooling energy consumption.

Study using a CFD approach of the efficiency of a roof ventilation closure system in a multi-tunnel greenhouse for nighttime microclimate optimization

ABSTRACT The thermal efficiency of naturally ventilated greenhouses is limited due to the permanent exchange of air through the vents, especially during the night hours. The objective of the work consisted in evaluating a system of inflatable air ducts that close the roof vents during the night as a strategy to reduce the energy loss during these hours. For the development of this work, we applied the computational fluid dynamics (CFD) method to a passive multi span greenhouse operating under the dominant nocturnal climatic conditions of the Bogota savannah (Colombia). The results indicated that the use of the ducts system reduces the value of the negative thermal gradient between the interior and exterior of the greenhouse. The CFD model used was validated by comparing experimental data and simulated data and by calculating goodness-of-fit parameters, finding that the numerical model predicts satisfactorily and with an adequate degree of fit the actual thermal behavior of the greenhouse evaluated.

Filling of smoke due to fire in a room with roof ventilation

Displacement ventilation is one of the ventilation techniques employed in maintaining the interior thermal environment of low-energy buildings. Warm air rising from occupants in a room is removed through a vent in the ceiling, while cool fresh air flows in through the floor level openings. The entire flow is driven by buoyancy forces due to temperature difference of air. In the event of fire in such a room, the smoke from the fire rises to the ceiling and begins filling it from the top, while also draining through the ceiling vent. In this article we investigate filling of smoke due to fire and examine how it is affected by heat loss through walls and ceilings, along with other parameters such as the room's geometry, ceiling vent area, and fire's power. We validate our model using experimental results of Harrak et al., 2019, International Journal of Thermal Sciences. We show that heat loss reduces smoke-layer's steady state depth, and the said depth is unique in the limit of large heat losses. For small aspect-ratio rooms, steady state properties of the smoke-layer can be significantly different for the insulated and conducting wall cases, even for small conductivity in the latter case. We derive formulas for bounds on the smoke-layer's steady state depth and density (and hence temperature), as well as obtain a useful estimate for the time taken for the smoke-layer to reach its steady state depth.

Evaluation of airflow pattern and thermal behavior of the arched greenhouses with designed roof ventilation scenarios using CFD simulation.

Natural ventilation is an effective energy-saving strategy conducive to promoting sustainable agricultural production. A comprehensive numerical simulation is performed to predict the airflow pattern and thermal behavior in different arched greenhouses. The defined arc chord angle and position angle are employed to examine the natural ventilation process and corresponding roof vent scenarios. The numerical simulation is compared with the experimental data and good agreements are observed. Various configurations of ventilated structures, wind conditions and ventilation layouts are simulated on a high-resolution polyhedral grid based on a grid sensitivity analysis, which is beneficial to the optimization of greenhouse cooling combined with the water circulation heat collection system. The cooling effect in summer is analyzed by estimating the ventilation flow rate and microclimate inhomogeneity. The results demonstrate that the position angle of 85° of the arched greenhouses is an optimum ventilation direction and its impact on the microclimate is marginally affected by the change of the ventilation structure. The designed ventilation scheme has perfect air exchange capacity and cooling effect because the average air temperature can be reduced by 1.5°C more than the existing greenhouse in 10 minutes of ventilation. Likewise, the results show that the temperature and velocity inhomogeneities are approximately decreased by 33.3% and 11.89%, respectively. The practical value of the research is expected to provide basic quantitative conclusions for evaluating the natural ventilation performance.

Parametric analysis of the thermal behavior of a single-channel solar chimney

This work presents the design, construction, instrumentation and thermal evaluation of a single-channel solar chimney under a warm-humid climate. A solar chimney (0.8 m width and 2 m length) with a capacity for varying the air gap (0–0.28 m), the inclination angle (0 − 90°), and the amount of energy absorbed by the absorber plate. A parametric study was performed with a total of 60 tests under laboratory conditions. A maximum temperature difference of 16.7 °C was obtained for an angle of 15° and a 0.075 m air gap, and when the air gap was varied, the highest air speed was 0.38 m s−1. The results show that a solar chimney can be used as a passive ventilation system, where up to 5 times more air flow can be removed in an enclosure, with a maximum air change rate (ACH) of 27.11 h−1, and a maximum efficiency of 88.9%. The data obtained can be used not only to select the configuration of a chimney but also for other applications where heating or the removal of air is required, such as for drying or in ventilated roofs.

Indirect evaporative cooling by sub-roof forced ventilation to counter extreme heat events

In a pitched ventilated roof, an air stream enters a cavity below the roof surface through openings at the level of the gutters, then it flows up along the roof and is finally discharged through openings along the ridge. The heat brought by the absorbed solar radiation is removed thanks to the flow established by natural ventilation, but only in part because the flow is relatively weak and unstable. As shown in previous work, a stronger and more regular air flow can be ensured by forced ventilation, extracting the air by means of a fan after it has been collected along the ridge by a properly designed manifold. This setup can be further enhanced by humidifying and saturating the air as it enters the cavity, thus significantly lowering the downstream air temperature. An indirect evaporative cooling system is thus obtained, by which an inhabited space under the roof can be cooled through the ceiling while remaining sealed and unaffected by the evaporative process. In this work, the setup is analyzed by a mathematical model and the concept tested by means of a small scale test bed, showing that it can be used to counter extremely hot ambient conditions with a relatively low consumption of liquid water and almost negligible energy need.

Suitability of Wooden Shingles for Ventilated Roofs: An Evaluation of Ventilation Efficiency

Wooden shingles are a traditional roofing material on many culturally important building artifacts. Currently, the roof space of many traditional buildings is used for residential purposes and, consequently, cold roof constructions with ventilation layers are applied. In this study, it is evaluated whether the moisture content of wooden shingles is adversely affected by such constructions, compared with unvented shingle roofs over cold attics and whether a temporary closing of the ventilation gaps at the eaves contributes to a lower wood moisture content. Various sensors were installed in and around a building with wooden shingles on a ventilated roof and temperature, air moisture, and air speed in the ventilation layer were measured throughout a year. The findings show that the air speed in the ventilation layer can be adjusted from 0.06 to 0.25 m/s depending on the layout of the eaves. A hygrothermal model was applied to evaluate the effects of different ventilation operation modes and cardinal orientations of the roof on the moisture content of the wooden shingles. The results show that roof ventilation results in a 1% lower shingle moisture content on average than an unventilated roof over a cold attic. Finally, it is shown that the wood moisture content repeatedly reaches dangerous levels above 25% throughout a year, which is worse on north-facing roofs and, hence, measures to increase the dry-out are relevant.

Spatiotemporal variability of leaf photosynthesis and its linkage with microclimates across an environment-controlled greenhouse

Environmental controls in a greenhouse improve microclimates, thereby enhancing photosynthesis, but they create spatiotemporal non-uniformity of photosynthesis, with implications for unstable crop production. However, there has been no research focussing on the spatiotemporal variability of photosynthesis arising from greenhouse environmental controls. Here we visualise spatiotemporal distributions of leaf photosynthetic rate ( A ) and assess its linkages with microclimates [air temperature ( T a ), water vapour concentration ( W a ), CO 2 concentration ( C a ), and leaf-boundary-layer conductance ( g a )] across a strawberry greenhouse during daytime under roof ventilation and CO 2 enrichment, using physical, physiological, and biochemical models for A and mobile observations of the microclimates. The distributions of A were variable during the daytime and were non-uniform across the greenhouse under the influence of the microclimate distributions arising from the environmental controls. In particular, with the roof ventilation, spatial variations of T a and g a were most associated with non-uniformity in A through the physical process of the energy budget determining the leaf temperature and thus affecting leaf physiological properties (photosynthetic capacities and stomatal conductance). With CO 2 enrichment, in addition to the roof ventilation, spatial variations of C a further increased non-uniformity in A through large variations of Rubisco-limited and RuBP-limited rates in the biochemical process of leaf photosynthesis. Spatial non-uniformity of A arising from the environmental controls ranged from 15% to 69% during the daytime. These findings indicate the importance of considering the spatiotemporal variability of photosynthesis with respect to its physical, physiological, and biochemical processes, in addition to that due to microclimates, for optimising greenhouse environmental controls. • Spatiotemporal variability of leaf photosynthetic rate ( A ) in a daytime greenhouse. • Multipoint model simulation of A was assisted by mobile observation of microclimates. • Greenhouse environmental controls varied microclimates in space and time. • Microclimate distributions enhanced spatiotemporal non-uniformity in A . • Spatial non-uniformity in A varied from 15% to 69% during the daytime.

A review of traditional multistage roofs design and performance in vernacular buildings in Myanmar

Myanmar’s territory mostly experiences tropical monsoon climate, where temperatures are normally not extreme, but humidity can increase discomfort. In response, vernacular architecture strategies have evolved to deal with excess heat and humidity. One of the most prominent of these strategies is the use of high multistage roofs with ventilation. Over the years, many of the traditional buildings were altered but the use of multistage roof design has remained remarkably resilient in Myanmar. Nevertheless, little is known about their contribution to thermal comfort and their vulnerability to overheating risks due to the pervasive threat of the climate crisis.In the work presented here, a thorough review of multistage roof typologies was followed by an investigation of their performance when building parameters including form, ventilation and materials were varied. Twenty-four dynamic simulations were performed using three building typologies and thirty-two computational fluid dynamic simulations were performed using two building typologies. In all cases, indoor volumes were kept the same. The results suggest that with the use of a typical light-weight permeable envelope, the indoor temperatures follow outdoor ambient temperature closely; although a heavier-weight set of materials did not impact significantly on the maximum air temperatures, it has made a different with regard to the lowest temperatures and overall comfort. The variable that impacted the most on the results was roof ventilation mode, with the best results being 3.5 % of a year better than the worst. The multistage roof was found to help reduce heat gains form solar radiation.The findings showed that Myanmar’s vernacular buildings with multistage roofs offer an opportunity to improve indoor comfort in tropical climates and therefore its ability to moderate indoor temperatures through the use of simple building physics and geometry should be honoured.

Experimental and numerical study on the thermal performance of ventilated roof composed with multiple phase change material (VR-MPCM)

In the hot summer, roof is the prime location absorbed the most solar radiation for both residential and commercial buildings. Hence, enhancing the thermal performance of roofs is essential for reducing the heat gain thus building load. In this study, a novel ventilated roof composed with multiple phase change material (VR-MPCM) was proposed and its thermal performance was investigated experimentally and numerically. A contrast experiment was conducted in two full-scale chambers. The research results indicated that VR-MPCM can reduce the indoor peak temperature by 16.9–18.8%, and push back the peak temperature occurrence time by 30–50 min compared with the conventional ventilated roof (CVR). In addition, the cumulative heat flux during 4320 min demonstrated that the energy conservation rate of VR-MPCM can go up to 97.1% compared with the CVR. A verified three-dimensional transient heat transfer model was introduced for further research. Through the simulation research, intermittent opening of vents was found to be the best operating strategy for reducing heat gain. Regional comparison indicated that VR-MPCM in severe cold and cold climate zones had obvious energy saving potential. In conclusion, VR-MPCM has a potential of energy saving by reducing the diurnal heat gain and utilizing the nightly free cooling.

The Energy Conservation With A Ventilated Roof Of Joglo’s House

The ventilation of buildings in hot and dry zones has to function differently from the warm and humid zones. When outside temperatures are rising higher than human body temperatures, cross ventilation through wide openings would not be useful; on the contrary, it would be a burden. To emphasize the sustainability of tropical homes, it is expected to maximize the use of the role of the roof opening in the building envelope in a passive manner. One passive cooling strategy is natural air. Circulation of natural ventilation in traditional Javanese built objects in humid tropical climates is used to achieve thermal comfort. Therefore, the problem that arises in this study is how far the Joglo-based roof element is in natural ventilation to form a comfortable space. This paper contains the elements of a Joglo-based house roof as a natural ventilation element. This study uses a computer simulation based on the program of CFD; the name is CFX-5, which is used to track the pattern and direction of air movement as well as the amount of air velocity and air direction in the Joglo-based roof ventilation object. The final value of these observations is between 0.389 to 0.489 m/s, which provides the majority cooling effect in the room, range between 0.489 O C - 0.589 O C. This was shown in the modified roof conditions, roof ventilation of the Joglo-Mangkurat-based roof is able to increase the speed of the air rate in the room. Keywords: energy conservation, ventilated roof, Joglo’s house.