Material Thermophysical Properties

General tab on Materials Dialog.

Materials are used to define the properties of construction layers.  There are 2 types of material:

1. Detailed properties including the thermophysical properties, surface properties and visual appearance for the material.
2. Simple resistive material with no thermal mass.  This option will typically be used to model air gaps.

Extremely thin and/or highly conductive layers - technical note on applicability

 

When a Material is used for the Construction of a building surface, care should be taken to not attempt to model assemblies that were not included in the intended scope of applicability for the underlying heat transfer models. The building surface models are for normal applications to building energy efficiency where the main focus is on assemblies with some thermal resistance. Extremely thin and/or highly conductive material layers should be neglected from the Construction rather than included because they will not contribute to the assembly’s overall thermal resistance or heat capacity. For some cases, thin and/or highly conductive materials are a serious problem for the heat transfer modelling and the values for thickness, conductivity, density and specific heat are checked for appropriateness. This check calculates the Material’s thermal diffusivity from the inputs for conductivity, density, and specific heat and compares it to a maximum threshold of 1.E-5 (m2/s). If the diffusivity is above this threshold, then the program checks if the layer is sufficiently thick and may issue a warning if it is too thin and highly conductive.

Material Thickness

Force thickness

If the material should only be used in layers of a particular thickness then check this box and enter the material thickness that applies below. When materials having this option set are selected on the Constructions dialog the layer thickness will be fixed with the material thickness described below.

This option may be useful for materials such as bricks, tiles, insulation and PCMs which are usually supplied with a particular thickness.

Material thickness

The fixed layer thickness of the material (in m or in) when selected.

This should be the dimension of the layer in the direction perpendicular to the main path of heat conduction. Modelling layers thinner (less) than 0.003 m is not normally recommended; rather, add those properties to one of the adjacent layers.

Detailed properties

Thermal bulk properties

Thermal conductivity

This field is used to enter the thermal conductivity of the material layer. Thermal conductivity must be greater than zero.

This field is used to enter the thermal conductivity of the material layer (in W/m-K or Btu-in/h-ft2-F). Thermal conductivity must be greater than zero. Modelling layers with conductivity higher than 5.0 W/(m-K) is not normally recommended.

Specific heat

This field represents the specific heat of the material layer (in J/kg-K or Btu/lb-F).

Note: EnergyPlus uses scientific SI units but data reported in textbooks and references will often use kJ/kg-K or J/g-K instead. Take care!

Only values of specific heat of 100 or larger are allowed. Typical ranges are from 800 to 2000 J/kg-K.

Density

This field is used to enter the density of the material layer in (kg/m3 or lb/ft3). Density must be a

positive quantity.

Note: Some textbooks and references may use units of g/m3. Take care!

Simple resistance

Thermal resistance

This field is used to enter the thermal resistance (R-value) of the material layer (in m2-K/W or ft2-F-hr/Btu).

Note: Materials defined using an R-value do not contribute to construction (and hence building) thermal mass in the calculations and so should be used with care, especially if you plan to use the model for dynamic thermal simulations. You are especially advised not to use R-value materials on the innermost layer of constructions and you should not generally use them for representing materials that have mass. They are best used for modelling air gaps and insulation layers.

Vapour Resistance

You can enter vapour diffusion properties here if the material is to be used within a construction which is have a condensation analysis carried out. Otherwise this data does not have to be entered for general simulations.

Note: The moisture diffusion data provided in the literature tends to be very approximate, much more so than for thermal properties. This is due to large variations in these properties that have been measured in lab tests.

Vapour resistance definition

There are 3 ways to define the vapour diffusion properties of the material:

• 1-Factor, in which case the Vapour factor is entered below.
• 2-Resistivity, in which case the Vapour resistivity is entered below.
• 3-Non-permeable, defines the material is not permeable to moisture vapour and no further data is required. Use this option for vapour barriers and plastic materials which do not transmit water vapour.

Vapour factor

Enter the vapour factor for the material. There are no units.

Vapour resistivity

Enter the vapour resistivity (in MNs/g).