Overview of Thermal Performance of Air Cavities and Reflective Insulations

Saber, Hamed H.

doi:10.1007/978-3-030-98693-3_3

Cited by 2 publications

(4 citation statements)

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“…This section presents thermal performance data for three cases: (a) vertical airspaces (θ = 90°) with heat-flow horizontal to represent building components such as walls, windows and curtain walls with Reflective Insulations (RIs), (b) horizontal airspaces (θ = 0°) with heat-flow up to represent building components such as flat roofs or skylights with RIs during the cold season, and (c) horizontal airspaces (θ = 0°) with heat-flow down to represent building components such as flat roofs or flat skylights with RIs during the hot season. Results for enclosed airspaces with the full range of inclination angles (0°–90°) subjected to heat-flow up and down have been previously published (Saber, 2022).…”

Section: Resultsmentioning

confidence: 99%

“…In November of 2020, the full capabilities and features of this tool were presented to the Reflective Insulation Manufacturers Association International (RIMA-I (RIMA International, 2020)). For enclosed airspaces, subjected to various heat-flow directions, the ranges of the airspace parameters that are covered by this tool include: (a) airspace effective emittance, e eff from 0.0 to 0.82, (b) airspace length, H from 51 mm to 2438 mm, (c) airspace thickness, d from 13 to 143 mm, airspace average temperature, T avg from 50°C to 250°C, temperature difference across the airspace, DT, from 0°C to 30°C, and airspace inclination angle, u, from 0°(horizontal) to 90°(vertical) (Saber et al, 2022). Briefly, the model solves simultaneously the 2D and 3D moisture transport equation, energy equation, surface-to-surface radiation equation (e.g.…”

Section: Numerical Modelmentioning

confidence: 99%

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Numerical modeling for evaluation of the thermal resistance of reflective airspaces with and without defects

Saber

Yarbrough²

2023

Journal of Building Physics

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The thermal resistances (R-values) of airspaces depends on the emittance of all surfaces around an airspace, dimensions, heat-flow direction, and the temperatures of the bounding surfaces. Assessing the energy performance of building envelope components and fenestration systems requires accurate results for the R-values of any enclosed spaces. The evaluation of reflective insulation R-values has evolved to include use of computational fluid dynamics and surface-to-surface radiation to quantify convective and radiative contributions to the heat transfer across airspaces of all types. This paper compares an advanced and validated model for calculating enclosed airspace R-values with the widely-used ISO 6946 and airspace R-values in the ASHRAE Handbook-Fundamentals. The impact of construction and installation defects on the thermal performance of reflective insulation has not been previously evaluated. In this research, an advanced model was used to evaluate a construction defect and dimensional aspect ratios that one-dimensional methods do not address. Imperfect installation and defects that result in air movement into or through a reflective insulation assembly reduces the thermal resistance of the assembly. Additionally, the amount of thermal resistance reduction depends on the amount and temperature of invasive air or the size of internal defects that allows natural convection inside the reflective airspace. In this study, these performance issues are evaluated quantitatively using computer simulation techniques. The differences in results obtained using methods that are currently being used to evaluate the R-value and the advantages of the advanced method for evaluating the reflective insulation performance for different applications are discussed. For the case considered in this study, the results showed that the failure to achieve parallel surfaces results in less than a 5% decrease in thermal resistance. Also, the results showed that internal air gaps between airspaces result in negligible loss in R-value unless air gaps that allow circulation between airspaces are created.

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Section: Resultsmentioning

confidence: 99%

Section: Numerical Modelmentioning

confidence: 99%

Numerical modeling for evaluation of the thermal resistance of reflective airspaces with and without defects

Saber

Yarbrough²

2023

Journal of Building Physics

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“…As provided in [40] for various airspace parameters including thicknesses (D), aspect ratios (A R ), average temperatures (T avg ), temperature differences (∆T), and effective emittances (E), the present model was used to develop practical correlations for determining the R-values as functions of these parameters for vertical enclosed airspaces (θ = 90 • ) with horizontal heat flow, horizontal enclosed airspaces (θ = 0 • ) with heat flow up and down, 45 • sloped airspaces with heat flow up and down, and 30 • sloped airspaces with heat flow up and down. Additionally, the model has been used to develop and design an optimization tool called the "Reflective Airspace Tool" for use by the technical community to determine thermal resistances for a variety of unventilated/enclosed airspaces with different dimensions, orientations, effective emittances, and operating conditions [41].…”

Section: Performance Assessment Of Reflective Insulations Using the M...mentioning

confidence: 99%

Section: Performance Assessment Of Reflective Insulations Using the M...mentioning

confidence: 99%

Assessing the Effect of Air Intrusion on Reflective Insulations Performance with Horizontal Heat Flow

Saber,

Yarbrough

2023

Buildings

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The determination of thermal resistances (R-values) for enclosed airspaces, including those with one or more low-emittance surfaces, has advanced from one-dimensional heat transfer between large parallel planes to multi-dimensional heat flow in a wide variety of physical configurations. The key elements in this advancement, however, are the evaluation of the heat flow due to conduction-convection and the solution for radiation that includes all surfaces bounding the region of interest. The model used in this study has been validated against several sets of laboratory test data, including the data from the U.S. National Bureau of Standards for the thermal resistance of airspaces, which has been the basis for handbook values for reflective airspaces for five decades. In addition, this model has been previously used to determine the reductions in the R-values of reflective insulations assemblies due to imperfect installations and internal defects in multilayer reflective systems with cross-airflow between the layers. In this study, the model is used to examine the impact on R-value of air intrusion of different air changes per hour (ACH) at various exterior air temperatures into reflective insulation assemblies with a range of effective emittance from 0 to 0.82. Two cases are considered in this study. In the first case, called “infiltration”, exterior air enters the assembly through an opening located in the hot side of the assembly and exits through another opening located in the cold side of the assembly. In the second case, called “wind washing”, the exterior air enters the assembly through an opening located in the hot side of the assembly and exits through another opening located in the same side of the assembly. To quantify the reductions in the R-values due to infiltration and wind washing conditions, a reference case is included in this study for the case in which no air intrusion occurs in the reflective insulation assemblies. Finally, consideration is given to investigating the effect of the aspect ratio on the R-values of reflective insulation assemblies without air intrusion and with air intrusion of different ACH at various exterior air temperatures. The results show that the aspect ratio has a significant impact on the R-value of reflective insulation assemblies with and without air intrusion. Additionally, the results show that the impact of infiltration and wind washing on R-values of reflective insulation assemblies increase as the difference between the exterior air temperature and the undisturbed temperature of the cavity increases.

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Overview of Thermal Performance of Air Cavities and Reflective Insulations

Cited by 2 publications

References 20 publications

Numerical modeling for evaluation of the thermal resistance of reflective airspaces with and without defects

Numerical modeling for evaluation of the thermal resistance of reflective airspaces with and without defects

Assessing the Effect of Air Intrusion on Reflective Insulations Performance with Horizontal Heat Flow

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