What type of barrier should be used in between the steel frame and treated timber?

For H1 treated timber normal building paper will act as a separator and will suffice if the area is closed. If the situation is sheltered or exposed with H3 timber then a more substantial separator should be used, such as window tape. This separation is more important if the site is ranked severe or very severe marine. If the frame is not clad, then an additional paint coat is necessary. 

NASH provides guidance on barriers to use in all applications of steel framing and there is a clear and reliable New Zealand standard on which this information is based.