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Material data
Most modern simulation tools incorporate 2 'structures' for entering construction data
The material data base contains basic thermal data on on the material. Most tools, whatever the complexity of the algorithms (calculation method) will need 3 parameters for each material:
It's important to take care with units, that the model expectations and your recorded data are compatable. It is possible that you will encounter just 2 parameters volumetric heat capacity and thermal conductivity, rather than the three above The units should give you an idea of what the parameters represent: Specific Heat Capacity is usually quoted in J/kg/oC. Volumetric Heat Capacity is usually quoted in J/m3/oC. Specific Heat Capacity (shc) represents the heat required to heat 1kg of the material by 1 degree centigrade (more generally unit mass (kg,lb etc) by unit temperature (oF,oC,1K). Similarly Volumetric Heat Capacity (vhc) represents the heat required to heat unit volume of the material by 1 degree of temperature. vhc actually has more meaning for buildings as we largely think in terms of material dimensions rather than their masses/weights. A construction database contains details of the layers of materials which make up a given construction. It will need to access the material database and assign the following to each layer of the construction:
Both material and construction library/database may organise materials into different categories, in particular materials which transmit solar radiation and those which do not. A further catagory may be air cavities. This is because heat will be transferred by both conduction and convection. Air (or other 'fluids') cannot be treated as simply another material with the given 3 properties (in particular thermal conductivity). Heat transfer in an air gap depends upon the properties of the surfaces which make up the enclosure and whether or not it is ventilated, more than its thickness. This doesn't prevent air gaps and cavities being defined in this way. This is generally not a problem as long as you are aware of it and how to deal with it. The two most common ways of representing air gaps are via:
Air resistance is probably the safer way to represent the air gap: if you enter a thickness it will probably not effect the resistance. This resistance is largely dependent upon
If a thermal conductivity is entered, the material may have a default thickness and situation. If these adequately represent your situation then this should be OK. If the thickness, in particular, is wrong, you will have to find a more appropriate value. Air resistances and thermal conductivities may be found in standard texts eg CIBSE Guide A, ASHRAE Fundamentals. However, if the software calculates U value and you know the U value of the glazing/cavity construction then you can find an appropriate resistance/conductivity by trial and error. |
Material data |