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Materials with a low thermal emittance surface have been used for many years to create reflective insulations that reduce the rate of heat flow across building envelopes. Reflective insulation technology is now being combined with other energy conserving technologies to optimize overall thermal performance. The basis for the performance of reflective insulations and radiant barriers will be discussed along with the combination of these materials with cellular plastic or mineral fiber insulations to form hybrid insulation assemblies. Calculations of thermal resistance for enclosed reflective air spaces and current field data from Southeast Asia will be presented. These data show that reductions in heat transfer across the building enclosure can be effectively reduced by the use of enclosed reflective air spaces and attic radiant barriers. Reflective technology increases the overall thermal resistance of the building enclosure when used to insulate poured concrete structures.

Enclosed reflective air spaces have significant thermal resistance making them useful as thermal insulation for a building enclosure. The term reflective air space means that there is at least one surface with low emittance (emissivity), ε, perpendicular to the direction of heat flow.^{ }The statement “enclosed space” means that air is not moving in or out of the space. The low-emittance surface is commonly provided by polished aluminum foil with total hemispherical emittance in the range 0.03 to 0.05 or metallized polymer film with a coating for protection against oxidation with emittance in the range 0.04 to 0.06. The radiative heat flux, q_{rad}, across an air space (net heat transfer by radiation from a warm surface at absolute temperature T_{1} to a cool surface at absolute temperature T_{2}) can be calculated for large parallel surface using Equation (1), the Stefan-Boltzmann Law [

The radiative heat flux, q_{rad}, is readily calculated from the above equations for large parallel surfaces and used to estimate the radiative flux for enclosed regions in the building enclosure. _{rad} is directly proportional to the reduction in E.

The total heat flow across an air space includes conduction and convection in addition to radiation. For small air spaces, convective transport is often negligible and the sum of radiative and conductive heat transport can be readily calculated [

The RSI (m^{2}∙K/W) for a large planar enclosed air space depends on several variables.

The total steady-state heat flux, q_{total} for an enclosed region can be obtained from Equation (3) where the dimensionless Nusselt Number, Nu (=h∙L/λ), is the ratio of the heat flux for conduction plus convection to the heat flux for conduction where the characteristic length, L, is the distance across the enclosed region in the direction of heat flow. Equation (4) is of practical important since it shows how Nu can be obtained from measurements of q_{total}. The total heat flux across a building element can be determined from a hot-box measurement [

Correlations for Nu in terms of the dimensionless Grashoff Number, Gr, which is ΔT∙L^{3}∙g_{c}/T∙γ^{2} where T is the

Emittances | |||
---|---|---|---|

Description | Surface 1 | Surface 2 | E |

Masonry-masonry | 0.90 | 0.90 | 0.818 |

Masonry-foil | 0.90 | 0.03 | 0.030 |

Foil-foil | 0.03 | 0.03 | 0.015 |

Factor | Importance | Comment |
---|---|---|

Heat flow direction | Major | Convection |

Emittance | Major | Radiation |

Depth | Medium | Conduction, convection |

Temperature difference | Major | All mechanisms |

Average temperature | Minor | Conduction |

Aspect ratio (depth/width) | Major | Radiation, convection |

absolute temperature and γ is the kinematic viscosity of air at the average temperature of the space. The function f in Equation (4) which depends on the heat flow direction has been determined from a large number of hot box tests for heat flow: up, 45˚ up, horizontal, 45˚ down, and down [_{total}. Enclosed reflective air spaces in series can also be calculated be an iterative procedure in which the total temperature difference is partitioned into {ΔT_{i}} where the subscript indicates the i^{th} air space.

The calculation of the set {RSI_{i}} can be accomplished using the Method of Successive Approximations with a starting set {ΔT_{i}} taken to be equal to f_{i}∙ΔT where f_{i} is the fraction of the total space occupied by region i. A correction based on radiation intersecting supporting materials between the warm and cool surfaces can be made using an analysis from Glicksman [

A ventilated space in a building with a low-emittance surface facing the space is called a radiant barrier system (RBS) [

The thermal performance of RBS is generally determined from computer simulations of the heat flow and air movement in the ventilated space [

Up | Horizontal | With 2D | Down | With 2D | |
---|---|---|---|---|---|

One air space | 0.36 | 0.60 | 0.55 | 0.77 | 0.64 |

Two air spaces | 0.77 | 0.87 | 0.83 | 0.87 | 0.79 |

Three air spaces | 0.91 | 0.91 | 0.88 | 0.90 | 0.85 |

% Reduction | |||
---|---|---|---|

Summer | Winter | Reference | |

RB (ε = 0.03) below roof deck | 33 | 8 | 17 |

RB (ε = 0.03) on rafters below roof deck | 50 | 9 | 17 |

Coating (ε = 0.23) on surfaces below roof deck | 19 | 5 | 17 |

Field studies | 23 - 45 | 9 - 18 | 16 |

The instrumentation in the test huts allows a record of temperatures and heat flux, q, across the ceiling in each unit. These data are used in Equation (6) to calculated U-value based on the area of the ceiling C. The corresponding overall thermal resistant can be obtained from the reciprocal of U.

If the average ΔT is calculated for the region between the ceiling and the radiant barrier material (the attic space), then the result is the U and RSI for the attic space.

An attic radiant barrier reduces the temperature on the inside of the unconditioned test huts. As an example, when the outside air temperature was 37.1˚C, the maximum interior temperature without a radiant barrier was observed to be 38.8˚C. The interior hut temperatures were reduced to 33.9˚C or 35.7˚C depending on the type of radiant barrier that was installed. The lower interior temperature was observed with a radiant barrier that has a small intrinsic (material) RSI-value. _{ave}. These data show the increase in RSI that result in the reduction in air conditioning that would be needed to maintain a constant temperature space below the ceiling; an energy conserving feature. The vertical axis is the measured

R-value at the time indicated by the horizontal axis. The blue data are for a test hut with no radiant barrier. The red and green data are for huts with a radiant barrier installed between the roof deck and the ceiling. The negative heat flux data occurs at night when the hut is cooling and heat is being transferred to the outside. These data

are used in Equation (6) along with the temperature difference to calculated RSI at a specific time.

The average RSI, RSI_{ave}, is obtained over a time period (t_{1}, t_{2}) using Equation (7) for n data points taken at even time steps, Δt.

Enclosed reflective air spaces (Reflective Insulation Systems) ae commonly used to add thermal resistance to the low thermal resistance of masonry walls (blocks or poured concrete). This is accomplished by attaching spacers

to the interior surface of the wall to provide space for the reflective insulation. The reflective insulation is installed between the spacers.

The thermal resistance attributed to enclosed reflective air spaces is readily calculated from published correlations representing a large number of hot-box tests. Multiple enclosed reflective air spaces significantly increase the thermal resistance that can be created in a given region in the building envelope. Low-emittance material (radiant barriers) installed between the roof and ceiling in a structure with a low emittance surface facing the attic space significantly reduces the transport of heat across the air space and the ceiling. Insulations can be combined to create hybrid insulation systems with high thermal resistance and demonstrated energy conservation potential.

The authors extend thanks to William A. Lippy, President, Fi-Foil Company USA and San Miguel Yamamura Woven Products Malaysia and the Universiti Kebangsaan Malaysia for use of graphics and test data.

David W. Yarbrough,Khar San Teh,Lim Chin Haw,Elias Salleh,Sohif Mat,M. Yusof Sulaiman, (2016) Hybrid and Reflective Insulation Assemblies for Buildings. Journal of Power and Energy Engineering,04,23-31. doi: 10.4236/jpee.2016.47004