Hamed H. Saber

3D heat and air transport model for predicting the thermal resistances of insulated wall assemblies

A wall energy rating (WER) system has been proposed to account for simultaneous thermal conduction and air leakage heat losses through a full-scale insulated wall system. Determining WER requires performing two standard tests on a full-scale wall specimen: a thermal resistance test and an air leakage test. A 3D model representation of the wall specimen is developed to combine the results of these tests to obtain an accurate prediction of the wall thermal resistance (apparent R-value) under the influence of air leakage. Two types of wall configurations were tested and simulated. The first one was a standard 2” × 6” wood stud frame construction, made of spruce, spaced at 16” (406 mm) o/c in 2.4 m × 2.4 m full-scale wall specimens. The second wall configuration was similar to the first one except that it included through-wall penetrations. The cavities of the two types of wall configurations were filled with different types of insulation, namely glass fibre batts and two different types of open cell spray polyurethane foams (light density, 6.8 and 12 kg/m3 nominal), a total of six walls. The present 3D model was used to predict the R-values of different types of wall assemblies (with and without air leakage). This model is a new hygrothermal tool that was recently developed and benchmarked against hygIRC-2D that was previously developed at the National Research Council of Canada, Institute for Research in Construction. The 3D version of this model was benchmarked by comparing its predictions of R-values for different types of wall assemblies against the measured R-values in the guarded hot box at no air leakage. Results showed that the present model predicted R-values of six walls to within ±5%. The 3D model was then used to investigate the effect of air leakage rate on the apparent R-values for these same walls. The results showed that the apparent R-values decreases linearly with air leakage rate less than ∼0.1 L/(m2 · s). At air leakage rate greater than ∼0.1 L/(m2 · s), the apparent R-values decrease asymptotically.

Thermal response of basement wall systems with low-emissivity material and furred airspace

In basement wall systems, airspaces can contribute in obtaining a higher thermal resistance, if a low-emissivity material such as reflective foil is installed within a furred-airspace. In this study, numerical simulations were conducted using the hygrothermal model ‘hygIRC-C’ that was developed at the National Research Council of Canada’s Institute for Research in Construction to investigate the steady-state and transient thermal performance of basement wall systems. This model solves simultaneously the energy equation in the various material layers, surface-to-surface radiation equation in the furred-airspace assembly, Navier–Stokes equation for the airspace, and Darcy and Brinkman equations for the porous material layers. The wall systems used in the simulations incorporate a low-emissivity material (foil with emissivity = 0.04) bonded to a moulded/expanded polystyrene foam that is installed in a furred-airspace assembly. The furring is installed horizontally and covered with a gypsum board. The structural element of the wall (external layer) is a poured-in-place concrete. Walls with and without furred-airspace assembly were considered in this study. Also, consideration was given to investigate the effect of the above-grade and below-grade portions of the wall on the thermal performance when these walls are subjected to two different Canadian climates. Results showed that at steady-state condition, the effective thermal resistance (R-value) of the wall with a furred-airspace assembly depends on the soil, outdoor, and indoor temperatures. Additionally, these wall configurations resulted in an energy savings of ~17% compared to walls without furred-airspace assembly when these walls are subjected to two different climate conditions.

Numerical modeling and experimental investigations of thermal performance of reflective insulations

Reflective insulations are being used in attics, flat roof, and wall systems. Numerical modeling and experimental investigations were conducted to assess the thermal performance of assemblies with reflective insulations. In this article, the present model was used to verify the use of the ASTM C-518 test method for measuring the effective thermal resistances (R-values) of sample stacks comprising reflective insulations. Two tests were conducted on sample stacks using heat flow meter apparatus. The sample stack consists of two expanded polystyrene layers and a reflective insulation installed in between. The model predictions agreed with the measured heat fluxes within ±1%. The article also discusses the combined effect of heat transfer by convection and radiation in the airspace facing the reflective insulation, showing that the derived R-value from the test data resulted in underestimation of the effective R-value of the sample stack.

Thermal performance of wall assemblies with low emissivity

In wall systems, airspaces can increase thermal resistance if a reflective material such as foil with low emissivity is installed in a furred-airspace assembly. In this article, the present model, hygIRC-C, was used to investigate the steady-state thermal resistance of wall assemblies that incorporate foil adhered to expanded polystyrene foam in a furred assembly. To investigate the effect of the furring orientation, the furring was installed horizontally and vertically and compared to walls with no furring. For wall with vertical furring, the three-dimensional version of the present model was used to capture the three-dimensional effect of the thermal bridges. Because the foil emissivity can be affected by dust accumulation and/or water vapor condensation on the foil surface, consideration was given to investigate the effect of both varying foil emissivity and outdoor temperature on the thermal performance of the various wall specimens. The results showed that the thermal resistance (R-value) of the reference wall (no furring) is greater than the wall specimens with furring. Also, the results showed that the contribution of the furred-airspace assembly to the R-value of wall specimen with vertical furring is higher than that for wall specimen with horizontal furring.

Energy retrofit using vacuum insulation panels: An alternative solution for enhancing the thermal performance of wood-frame walls

Field monitorings of thermal performance of residential 2 x 6 wood-frame wall systems that had been retrofitted using vacuum insulation panels (VIPs) and extruded polystyrene foam (XPS) panels were undertaken in May 2011 – May 2012 at the Field Exposure of Walls Facility (FEWF) of NRC-Construction. The main objective of this research was to measure the steady-state and transient thermal performance of three wall assemblies (4 ft x 6 ft), two of which incorporated VIPs within an XPS Tongue and Groove (T&G) configuration and VIPs within an XPS Clip-On (C-O) configuration, and a third assembly incorporating only XPS. The three wall assemblies were installed in the FEWF for 1-year cycle of exposure to outdoor natural weather conditions. The hygIRC-C model was used in this study. The results of the model calculations were in good agreement with the experimental data. Given that the VIPs could be punctured during the installation process or could fail during normal operating conditions, additional model calculations were used to predict the thermal resistance in cases where one or more VIPs failed. The model was also used to predict the yearly cumulative heat losses across these wall systems. It is important to point out that the aging effect and the effect of the thermal bridging due to envelope (i.e. skin) of the VIPs are not accounted for in this study. However, sensitivity analysis of the thickness and thermal conductivity of the VIP envelope was conducted to investigate the effect of these parameters on the effective thermal resistance of VIP.

Practical correlation for thermal resistance of horizontal enclosed airspaces with downward heat flow for building applications

The 2009 ASHRAE Handbook of Fundamentals (Chapter 26) provides a table that contains the thermal resistances (R-values) of enclosed airspaces for different values of airspace thickness, effective emittance, mean airspace temperature, and temperature differences across the airspace. This table is extensively used by modelers, architects, and building designers in the design for thermal resistance of building enclosures. The effect of the airspace aspect ratio (length/thickness) on the R-value is not accounted for in the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) table. However, in previous studies, it was shown that the aspect ratio of the airspace can affect its R-value. In this article, the previous studies by the author who focused on determining the R-value for vertical enclosed airspaces and horizontal enclosed airspace under upward heat flow condition are extended to investigate the effect of the aspect ratio on the R-value of horizontal enclosed airspaces under a downward heat flow condition for different airspace thicknesses and having a wide range of values for effective emittance, mean temperature, and temperature differences across the airspaces. The R-values predicted from numerical simulation are compared with those provided in the ASHRAE table. Considerations were also given to investigate the potential increase in the R-values of enclosed airspaces when a thin sheet is placed horizontally in the middle of the airspace and whose surfaces have different values of emissivity. Thereafter, practical correlation was developed for determining the R-values of horizontal enclosed airspaces for future use by modelers, architects, and building designers. The simplicity of this correlation derived for horizontal airspaces under downward heat flow condition together with those that were previously developed for vertical airspaces and horizontal airspaces under upward heat flow condition suggests that these correlations could be included in the ASHRAE Handbook of Fundamentals.

Optical model for tubular hollow light guides (1415-RP)

Tubular hollow light guides are found in many lighting and daylighting systems to transport collected light into deep spaces of building interiors. Linear straight guides are popularly used due to their high optical efficiency, but non-linear guides with bent sections are sometimes required to fulfill some installation restrictions. The optical performance of such bent guides is, however, unknown. This article presents the development, validation, and application of an optical model to compute the transmittance of light guides with and without bends. The model is based on the ray-tracing technique and can handle segmented guides with connection elbows. Measurement of the light transmittance of a light guide with two connection elbows is conducted using an outdoor large integrating box to benchmark the model. The model predictions are in good agreement with the measurement and public data for vertical light guides without bends. The model predictions for bent light guides installed in a northern mid-latitude location show that orienting the middle pipe section of the guide toward the northern direction results in better control of sunlight and solar heat gains than other orientations.

Optical model for prismatic glazing (1415-RP)

Prismatic glazing is found in many building applications, such as complex fenestration systems to control solar heat gains and glare and re-direct sunlight to building interior spaces and daylighting (and lighting) systems to enhance their optical and lighting performance. However, modeling and simulation of such prismatic glazing has been a very difficult task due to its versatile and complex geometrics. This article presents the development and validation of a simplified model to compute the optical characteristics and dominant directions of the transmitted and reflected beam rays of sawtooth-like prismatic glazing. The model was based on tracing the average ray and was extensively validated using third-party data derived from ray tracing computer simulations and measurement using integrating spheres and goniophotometers. The models predictions for the transmittance and reflectance of single and double prismatic panes compared well overall within the accuracy of the third-party data over all incidence angles.

Tubular daylighting devices. Part I: Development of an optical model (1415-RP)

Tubular daylighting devices are systems that collect and channel daylight from building roofs into deep interior spaces. To meet high standards of building energy efficiency and glare-free indoor environments, tubular daylighting device technologies have been rapidly and continuously evolving over the past two decades. However, this pace has been counteracted by a lack of reliable computer design tools. This article presents the development of analytical models to compute the optical characteristics (transmittance, reflectance, and layer absorptances) of various types of complex tubular daylighting devices New metrics for the optical and lighting performance are developed. The optical models are based on the ray-tracing technique, and account for the spectral (monochromatic) or broad-band optical properties of tubular daylighting device glazing panes. Experimental validation of these models is presented in an accompanying paper (Laouadi et al., 2013c).

Tubular daylighting devices. Part II: Validation of the optical model (1415-RP)

This article presents the development of a methodology to measure the visible transmittance of complex configurations of tubular daylighting devices under direct sunlight, and conducts a comparison study between the measurements and computer simulations using the new optical model developed in the first part of the study. A large integrating box was built and calibrated, and the procedure was benchmarked by comparing the measurement of a transparent glazing sample with the manufacturer data. Two commercially available tubular daylighting devices with prismatic and frosted elements built into the glazing and a custom-made tubular daylighting device with a complex pipe having roof and ceiling elbows were selected for the comparison study. The model predictions were overall in good agreement with the measurement for the tested tubular daylighting device configurations, and the sources of discrepancies were clearly identified.