I am building a new 2-story timber frame home in upstate New York (zone 5). The foot print is 24 x 40. It will have a below grade finished basement with 9′ foundation walls with a walkout at one end of the building (approximately 16′ wide). The slab will be about 7 feet below grade. 

The foundation walls will be insulated from the interior and the original plan was to insulate underneath the slab with 2″ of Foamular 250 XPS which would butt up to the interior wall insulation. There would be a 15 mil. vapor barrier over the 2″ of XPS that would carry up the foundation walls. 

I am reviewing my plans and I just realized my architect didn’t include the slab insulation detail in the plan. While I would be comfortable laying 2″ of XPS under a slab, the problem is there are two interior footings (each 42″ x 42″ by 21″ thick). Each footing will  support a 2-3 1/2 steel pipe column which, in turn, will carry the load from a 8″x8″ x 24′ vertical post which runs from the first floor all the way to the ridge beam. The ridge beam runs 40′ (the length of the house) and is supported by the two posts in the middle and the two exterior walls. So, the long and the short of it is that the two interior footings are going to be carrying a lot of weight and I am trying to figure out what is the best way to insulate these footings. 

I am reaching out to my architect for guidance, but I wanted to see if anyone could provide some advice. I have seen this post which seems to be similar to my situation:

https://www.greenbuildingadvisor.com/question/rigid-insulation-between-interior-bearing-wall-footings-and-slab

What make senses here? Insulate above the interior footing with some beefier XPS (i.e., Foamular 400) given the amount of weight that will be transferred? 

Does it even make sense to insulate my basement slab? Some of articles on GBA seem to indicate that it isn’t necessary. Curious to see what people’s thoughts are? 

Thanks so much!