When code (e.g. ASHRAE 90.1) says the minimum insulation value for a wall is R-13 in a stud framed wall, R-13 is not the input you would use in an energy modeling program. This is primarily due to the thermal bridging of the studs, which occur every 16 to 24 inches.
In today's post, I will show how Revit automatically accounts for this.
The effective R-Value of an insulated wall accounts for many things, such as how it's installed, age, dryness, and -- the topic of the day -- when installed in a stud wall. Each stud represents a thermal bridge, where there is effectively no insulation in the wall. When you consider studs are every 16-24" O.C., the effective R-value of the wall is significantly affected, so the R-value must be derated.
Revit has a list of wall types we can select from, which in turn applies thermal properties to everything that is translated into an exterior wall, regardless of the wall type/settings. These walls have real-world materials with derated r-values. I wrote about this 'list of walls' previously: Revit and Insight - Wall Properties for Early Energy Analysis.
Note the numbered items in the image above:
- ASHRAE 90.1 Code Minimums (R-13 + R-3.8 c.i. Mtl Studs)
- Insight override for EAM exterior walls (R-13 + R-3.8 Mtl Studs)
- Detailed definition of wall assembly
- Resultant derated R-value for selected analytical surface (EAM)
EAM = Energy Analysis Model (created in Revit)
Notice there is a wall construction option (#2 & 3) that has the code minimum R-values in the name... this is not a coincidence. They are one and the same thing i.e. the item in the list highlighted contains ‘contructions|layers|material’ elements (in gbXML) that amount to a total r-value of 9.
When you look inside that element (see gbXML image below) you will see the ‘minwool batt R13 incl. mtl framing factor’ shows an r-value of 4.7, way lower than 13. As such, it has been derated but that calculation isn’t shown, just an overall r-value for that layer.
Note, gbXML always exports in metric so some conversion is required...
Thus, the Revit + Insight workflow for early energy modeling is accurate in this example. The correct r-value will be fed into DOE2.2 to calculate the pEUI.
Watch for another post on how to use product literature, open cell insulation in this case, to create a custom thermal asset. This asset can be associated with a material layer within a wall/roof/floor type in Revit. Then, with the correct Energy Settings, this same data can be used in the energy model. Thus, you don't just have to use the canned options mentioned above.
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