internal CLS treated or untreated timber?

[Moonshine]

As the title says, locally there is about a £1 difference between treated and untreated C16 CLS (38 x 89mm £1.98/m vs £3.10/m)

 

is it worth paying the extra? and is it necessary.

Edited November 8, 2021 by Moonshine

[Mr Punter]

Treated is not required internally, although you could use it for sole plates in case of minor leaks / floods.