Impact of Stud Spacing on Thermal Value of Walls

[gas163]

When establishing thermal values to components of a wall to generate an overall R value is there a strategy that accounts for stud spacing? The thermal performance of a stud is different from the thermal performance of the insulation in the stud cavity so the thermal performance of a wall with studs 24" O.C. is going to be different than a wall with studs 16" O.C. Should I just create insulation types with varying thermal values accounting for different stud spacing? Or maybe there is something I'm missing?

[dkoch]

When establishing thermal values to components of a wall to generate an overall R value is there a strategy that accounts for stud spacing? The thermal performance of a stud is different from the thermal performance of the insulation in the stud cavity so the thermal performance of a wall with studs 24" O.C. is going to be different than a wall with studs 16" O.C. Should I just create insulation types with varying thermal values accounting for different stud spacing? Or maybe there is something I'm missing?

We have gotten away from having insulation in stud cavities for this reason, and because insulation on the outside side of the stud cavity also moves the dew point away from the studs. But if you are going to have an insulated stud cavity and you are assigning thermal values to your Wall layers for analytical purposes, then yes, you would want to assign a value to the stud/insulation layer that accurately reflects the "real-world" performance, which would require different values for the "same" insulation installed with different stud spacings.

[Steve_Stafford]

When you define a wall type you set the thickness of each layer in the wall. There is no value for stud spacing. When you assign a material to a wall's layer you are probably starting with the stock library of materials and assets that Autodesk provides. For example, the stock interior wall types use a material called Metal Stud Layer. There isn't a material for wood studs unless you consider the "Softwood, Lumber, Structure, Wood Joist/Rafter Layer" or "Softwood, Lumber, Structure, Wood Joist/Rafter Layer, Batt Insulation" eligible for that role.

When you examine the thermal asset settings for the material "Softwood, Lumber, Structure, Wood Joist/Rafter Layer" you'll find that Autodesk has provided values for Basic Thermal: Thermal Conductivity, Specific Heat, Density, Emissivity, Gas Viscosity, and Compressibility.

When you look at the thermal asset for the material "Softwood, Lumber, Structure, Wood Joist/Rafter Layer, Batt Insulation" you'll find a slightly different collection, these additional properties: Behavior, Permeability, Porosity, Reflectivity, and Electrical Resistivity. The Gas Viscosity and Compressibility settings that are part of the other material are missing from this material's list of properties.

I'm not qualified to challenge the values they've provided us for these materials and assets. If you search the web for resources on thermal properties and construction materials you'll find that some agree with a number of the values that are assigned to them. You'll most likely find some that don't. The way materials are defined doesn't appear to take the spacing of materials into account, at least not in an obvious way. If they did perhaps the material name would declare it, Metal Stud Layer 16 OC? If you have experience with this then perhaps you can tell whether the values are valid for 16" O.C. studs instead of 24" O.C.?

Shorter answer, you need to create additional materials that use settings that you believe to be accurate for a wall that is built with different stud spacing. Keep in mind that Revit is adding up the data for each layer in the wall to arrive at a total "R value" and Thermal Mass value.

[Simon.Whitbread]

So, what we're saying here is, that for the HVAC Engineer / Designer who is into building analysis for heating and cooling load calculations, they are still better off using the material overrides on a linked architectural model rather than trying to rely on the data architects actually publish.
And I suppose this all comes down to communication once again. If the MEP Consultant knows what the actual total R value is of any given set of materials (from ASHRAE / CIBSE / "whateveryourlocal" standards are), then for everyone on the design team - share the knowledge.

[Mike L Sealander]

There is a free software program from the Department of Energy called Therm that takes about an hour to learn. It is a two-dimensional thermal analysis program. You can use it to determine the U-value of composite wall sections, such as wood or metal studs at different spacings, with different insulation parameters. It's really quite neat. The values you get from it can be used in Revit materials.