I have extra 1" dow foam board left over from my under slab insulation. Should i place this against the outside of the Superior Walls, outside of the plastic drain sheating to gain alittle more R value before i backfill? What would be the downside. The board was free and i am starting to backfill in a couple days. My only concerns are insects and the fact that the airgap behind the plastic drain sheet would negate any gain. Thanks Michael

What is the name of that membrane you have already in place ?

Oh and sorry im a newbie and unsure about the specifics of your question or I would help in a heartbeat.

brawler, There was a report regarding the problem of XPS water absorption. Search the site. Under your slab should not be a problem, but around the perimeter I would be hesitant. You are probably better off adding to the inside of the wall cavities and/or backing the perimeter floor bands of your webbed trusses.

Dow foam board is used for that all the time around where I live. One of the most important reasons to use it is that it protects your waterproof coating during backfill, besides the extra Rs.

Since it was a freebie, why not! Please do install on the outside face for all advantages are outside use. I don't think the air gap should be of any concern. Overall I only see advantages.
Cheers!

Thanks for the replies guys. I think I will use what i have. Soryy for the delay, had to leave town for a couple days. I will let you know how it goes.

newbejohn, typar Delta drain