# Modelling a composite wall - Stud wall

Hi everyone,
I want to model a stud wall (steel framing) that has no insulation, it would be just air between the osb panels.
In the energyplus library I only find steel frame with insulation, so I have to create a new construction material, but I read on Big Ladder that I should only change the values to the inside or outside layer.

I tried to create a material that would be this air gap between the panels by using the opaque material component, copying the values from the airwall material and just changing it’s thickness. However I guess I’m getting it wrong as the results for the U and R values are not matching with the values I’ve calculated before.

So I wonder how could I model this steel framing wall correctly.
Best regards,
Juliana

@chris any idea?

Regards,
Juliana

Hi @Juliana ,

I’m not sure of the exact question you are asking but there are at least a couple of valid ways to model an air cavity in an opaque wall assembly in E+. If you don’t care about accounting for the heat capacity of the studs, the heat capacity of the air in the cavity is pretty negligible compared to other opaque layers and so you can just model this as a NoMass material with a fixed R-Value. Beyond a certain air cavity thickness like 10 cm, you don’t really get any more thermal resistance out of the air gap like you would for other opaque materials because the air convects in the cavity and transfers heat in more or less the same pattern. So R-0.15 W/m2-K is a good estimate for this cavity resistance for anything 10cm and beyond.

If you care about accounting for the thermal mass and heat capacity of the studs, then you can use a standard opaque material like you have there. You just want to make sure that you set the conductivity and thickness TOGETHER so that, on the whole, they give you the same R-value as you would get with the NoMass approach. Remember:

``````R = thickness / conductivity
``````

So you can use this to adjust the conductivity and make sure that you get an R-value around 0.15 W/m2-K

I’ll try to model it not accounting for the thermal mass and heat capacity of the studs but I was curious about how I should deal with a material that is not homogeneous just like the example before (stud walls).

Thank you @chris!