Steam boiler cladding thickness

[Penrhos1920]

What was the typical thickness of the insulation/cladding on a steam boiler?  I'm building a loco and the only information I have is a poor drawing and the fact that the boiler was 4'7" diameter.  Am I right to think that the insulation would be 2-3" thick?  So about 5' diameter over the cladding.

[Wickham Green too]

In the right sort of ball park is the SECR D1 :  4'7&7/8'' Dia Outs boiler & 5'0&1/8'' Dia Outs Clothing ............ so that's 4&1/4'' difference or a 2&1/8'' layer .............................................................................................. in other word YES !