private void ReadMaterialThermalProps(Document document, Material material)
{
   ElementId thermalAssetId = material.ThermalAssetId;
   if (thermalAssetId != ElementId.InvalidElementId)
   {
      PropertySetElement pse = document.GetElement(thermalAssetId) as PropertySetElement;
      if (pse != null)
      {
         ThermalAsset asset = pse.GetThermalAsset();

         // Check the thermal material type and only read if solid
         if (asset.ThermalMaterialType == ThermalMaterialType.Solid)
         {

            // Get the properties which are supported in solid type
            bool isTransmitsLight = asset.TransmitsLight;
            double permeability = asset.Permeability;
            double porosity = asset.Porosity;
            double reflectivity = asset.Reflectivity;
            double resistivity = asset.ElectricalResistivity;
            StructuralBehavior behavior = asset.Behavior;

            // Get the other properties.
            double heatOfVaporization = asset.SpecificHeatOfVaporization;
            double emissivity = asset.Emissivity;
            double conductivity = asset.ThermalConductivity;
            double density = asset.Density;
         }
      }
   }
}

Private Sub ReadMaterialThermalProps(document As Document, material As Material)
    Dim thermalAssetId As ElementId = material.ThermalAssetId
    If thermalAssetId <> ElementId.InvalidElementId Then
        Dim pse As PropertySetElement = TryCast(document.GetElement(thermalAssetId), PropertySetElement)
        If pse IsNot Nothing Then
            Dim asset As ThermalAsset = pse.GetThermalAsset()

            ' Check the thermal material type and only read if solid
            If asset.ThermalMaterialType = ThermalMaterialType.Solid Then

                ' Get the properties which are supported in solid type
                Dim isTransmitsLight As Boolean = asset.TransmitsLight
                Dim permeability As Double = asset.Permeability
                Dim porosity As Double = asset.Porosity
                Dim reflectivity As Double = asset.Reflectivity
                Dim resistivity As Double = asset.ElectricalResistivity
                Dim behavior As StructuralBehavior = asset.Behavior

                ' Get the other properties.
                Dim heatOfVaporization As Double = asset.SpecificHeatOfVaporization
                Dim emissivity As Double = asset.Emissivity
                Dim conductivity As Double = asset.ThermalConductivity
                Dim density As Double = asset.Density
            End If
        End If
    End If
End Sub