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You don't need paperwork to back up use of timber windows. The u-value of timber is known, as is that of various glazing and coatings.
Sometimes you might need the unit filled with argon instead of air, other times not.
U value of timber is known and variuos glazing etc.... think you'll find you will need documentary evidence to conform to building regs .... ( as you would with Pvc too , that also has ' known ' U values...)
As for Argon Gas ...since the invention of Window energy ratings....ALL sealed units in new frames in a domestic property MUST have Argon and Low e glass at the very least