I had a similar dilemna and didn't want to bulk out the post too much with thick insulation (PIR or anything similar). In the end I went and dug into the account and went for aerogel sheets. Stuck that onto the steel post on all four sides, covered with a VCL and then covered that with some timber. Better u-values than PIR, slimmer view and will protect from condensation and reduce thermal bridging.   Aerogel:   use a mask to cut the stuff!   wrap onto p