Attic Design Research Articles

NUMERICAL SIMULATION OF NATURAL CONVECTION FOR RAISED-CEILING ROOFTOP HEATED FROM BELOW

Attic design has a significant impact on buildings’ energy performance thus making investigation of its thermal performance characteristics imperative. Raised-ceiling is among the common complex attic configuration found in pitched-roof buildings. In this study, natural convection inside a raised-ceiling rooftop heated isothermally from the base wall (ceiling) is numerically investigated at a certain pitch angle range while keeping the enclosure base-length unchanged. The results show that the roof pitch and the kinks on the ceiling have a strong influence on the heat and fluid flow patterns. At low roof pitch, high heat transfer rate between the cold and hot walls leads to a multi-cellular flow structure. The number, size and strength of the counter-rotating cells change with the pitch angle. The kinks caused air velocity to double for every 15° increase in the roof pitch. The rate of convective heat transfer is enhanced with decreasing value of the aspect ratio.

Moisture-safe attic design in extremely cold climate: Hygrothermal simulations

Attic ventilation is a commonly used method for the removal of moisture build-up in attics through air leakage from indoors, which is generally applied to cold climates. Un-ventilated attic is a fully sealed construction, which can prevent wind-driven rain penetration or snow accumulation. However, it does not provide an effective path for moisture removal, which may cause moisture-related issues. Northern Canada suffers from extremely cold conditions and very fine snow particles caused by extremely low temperature have high chances of penetrating into attics through vents or un-intentional openings during periods of high winds. To provide adequate attic design recommendations for this climate, hygrothermal models of ventilated and un-ventilated attics were firstly validated by comparing with field measurements and then used for a parametric study. The attic ventilation rate, ceiling air leakage rate and un-intentional air infiltration rate are set as variables for the parametric study to evaluate their effect on the hygrothermal performance of attics. Mold growth index (MGI) on sheathing is used as the performance indicator. For the ventilated attic, there is no mold growth risk, while for the un-ventilated attic, the MGI increases with the increase of ceiling air leakage rates and the decrease of un-intentional air infiltration rates. North-facing roof sheathing has higher MGI compared to south-facing sheathing. The parametric study results show that attic ventilation is required for the extremely cold climate and an un-ventilated attic has greater risks of moisture problems unless the ceiling air leakage from indoors is well controlled.

Moisture-Safe Cold Attics in Humid Climates of Europe and North America

During the last few decades, unheated attic spaces (so-called “cold attics”) have been one of the most moisture-problematic spaces in the building practices of Europe and North America. Problems with such spaces are usually associated with condensation, frost formation, and mould growth on the lower surfaces of the roof deck. To solve these problems, a number of qualitative studies were performed. Although these studies revealed many important pieces of information, some of them are seemingly not in accordance with one another. Therefore, there has not yet been presented cold attic design which is suitable (especially moisture-safe) for the whole target region. The aim of this paper is to determine whether such a design can be determined or state the reasons why it is not possible. Based on a review of published studies, 38 cold attic designs were selected for detailed analysis. Their comparative parameters were established, quantified, and compared. The moisture-safeness of each design was evaluated using an original approach, and the designs were sorted into groups based on their similarities. Despite many uncertainties, it was found that the criteria for moisture-safe cold attic design in the target region can likely be stated.

A COMPARISON OF SEALED AND VENTILATED ATTIC SPACES: A CASE STUDY OF RESIDENTIAL ATTIC DESIGN

Two attics were constructed as part of a building renovation project at the University of West Florida. The first attic is described as a traditional ventilated attic, with openings in the soffits and a small dormer vent on the roof. The second attic is described as a sealed attic (with no ventilation), and open-cell spray foam insulation installed on the underside of the roof deck. The study was undertaken to demonstrate hypothesized performance differences between attic types. Thermal and relative humidity sensors were installed to measure the condition of the air in the two attics spaces, and measurements were taken at 15 minute intervals. Measurements of relative humidity were later calculated as dew point and specific humidity. Similar studies are often conducted by comparison of attics in separate buildings under different use conditions. This project offers a unique opportunity to explore data collected from a single structure, and provides support for existing research on attic design in southern regions. The resultant data show significant differences in attic temperatures, with the sealed attic exhibiting a much more thermally stable pattern. There were also significant differences in attic dew points and specific humidity, although these differences appear to be much less pronounced. Data were analyzed using independent t-tests to establish significant differences between means. Overall, the sealed attic performed better than the ventilated attic, although dew point and specific humidity remain concerns.

OPTIMAL DESIGN OF HOUSING ATTICS WITH INTEGRATED SOLAR COLLECTORS

In order to reduce the increasing energy consumption for the domestic demands of existing single-family housing and take advantage of frequent building enlargements, this paper presents a methodology and supporting software tool for determining the optimal design configuration of an attic with integrated solar collectors. The analysis procedure is based on parametric modeling, energy simulation and the use of evolutionary algorithms for finding optimal designs. It has been implemented as a Web-platform for public use that provides users with a proposal of an attic shape with maximum solar energy collection, maximum living space and minimum construction envelope for each house according its size and orientation. The attic integrates PV, thermal and hybrid solar panels on one side of the roof. This paper describes the methodology and software design, assessment of the Web-platform usage and case-studies to verify its behavior. In a matter of minutes, the Web-platform enables users to select a specific attic design for each house that has integrated solar collectors that can produce energy to cover almost 100% of domestic energy consumption. The attics designed provide a nearly 30% increase in living space through the extension of one to four rooms, and the construction cost of the envelope is similar to that of a standard housing extension.

Exploring future climate trends on the thermal performance of attics: Part 1 – Standard roofs

Attic spaces experience complex heat and mass transfer in comparison to the rest of the building envelope. The thermal energy evolution of these unique spaces is further complicated due to complex geometries, multi layered material components, their non-linear material properties, and geographical locations. The proper design of attics plays a significant role in the amount of thermal energy transferred across the ceiling to the occupied space and the energy usage of the building envelope. This is especially important in the context of changing climatic conditions which affects the energy demand of residential as well as commercial buildings.This study investigates the impact of current and predicted future climates on different attic forms in multiple locations across the U.S. For this analysis we utilize a recently developed, validated attic energy analysis model, the Fraunhofer attic thermal model. Six attic geometries (constructed according to architectural standards) are considered. These geometries are analyzed under four climatic scenarios in six major U.S. cities. We investigate several energy characteristics of roofs including thermal loads, peak roof deck temperatures, and attic air temperatures. The identification of energy usage and robustness to climatic changes can contribute to improved attic construction design standards. We envision this work informing architects and engineers in designing robust attic spaces that create more sustainable buildings for the future.

Longitudinal and Transverse Rolls in a Triangular Cavity

Natural convection in a triangular cavity is present in many domestic and industrial systems. Increasing investigators have been devoted to the study of natural convection and heat transfer in the triangular cavity due to its application in the attic design. In this study, the three dimensional (3D) numerical simulation of natural convection in the triangular cavity with the top cooling and the bottom heating has been carried out. The onset and development of natural convection flows in the cavity are investigated for various Rayleigh numbers and aspect ratios. The results show that the 3D structures of convection rolls in the cavity such as transversal rolls and longitudinal rolls, similar to the Rayleigh-Bénard-Poiseuille flow, are clear. The scenario of the roll formation is described.

Numerical simulation of buoyancy-driven turbulent ventilation in attic space under winter conditions

Attic design and construction have significant impacts on residential buildings’ energy performance. In order to understand how passive ventilation rates affect ridge-vent attic's performance, a two-dimensional steady-state finite volume model is employed to simulate the buoyancy-driven turbulent ventilation and heat transfer in a triangular attic space of a gable-roof residential building under winter conditions. The modeled attic has a pitch of 5/12 and a passive ventilation system, consisting of continuous ridge and soffit vents. The v2f model is used to analyze the turbulent air flow and natural convection heat transfer inside the attic. The effects of ambient air temperature, vent size, and ceiling insulation on heating load and ice dam formation are investigated. The thermal performance of the vented attic is compared with a sealed attic as well. The simulation results reveal that symmetrical air flow patterns exist in a vented attic, in contrast to the asymmetrical air flow patterns found in a sealed attic. In addition, it is suggested that increasing vent size results in higher ventilation air flow rate but barely affects the attic heating load, and that both sufficient ventilation and insulation are needed to ensure the proper functions of the attic and its energy efficiency.