Unable to change R value in the 'Increase R value' measure for exterior walls and roof below a certain limit. Why?

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Can you provide more detail. When you say it "refuses to accept any value" how are you running the measure? Assuming you are using a GUI like the OpenStudio Application or PAT I assume it will accept any value, but is just not altering the model in how you expected. What do the log messages from the measure say? If you dig into the modeler description you will see that it works by inferring the insulation layer by finding the construction layer with the most thermal resistance, and then increases (or optionally) decreases the thickness. to get the desired value. It doesn't alter additional layers, so if had a construction with multiple insulation layers this would only impact one of the two layers, as a result there is a limit to how much you can reduct the R value. To reduce even the one layer you have to change the default value of "false" for Allow reduction to "true".