Expert: Ted Barnhill (Principal, Design45 LLC) - 10/21/2007

Question
Ted I'm looking to add a wlakin closet to my master on a pre-existing slab
outside.  there's a shed roof and the slab is at the same level as the interior
floor.  I'm looking for ideas about how to insulate the floor of the closet
(assuming I need to- I live in a very cold climate in Idaho) and how to tie it in
with the master

Answer
First, you need to insure that your slab has footings to frost.  Otherwise, you may have soil heaving and a host of resulting problems with your addition.  If your slab does not, you could look into frost-protected shallow foundations (link: http://oikos.com/esb/43/foundations.html).  It might, but might not, be possible to retrofit insulation for your slab using this theory.

If you do have frost footings, I recommend adding rigid insulation on the exterior of your foundation wall, as deep as you can conveniently dig.  This will prevent some heat loss through the floor/foundation.  

For comfort, you will probably want to carpet the closet, or add radiant floor heat, or both.  If you have hot water heat in your home, you may be able to add hydronic radiant heat (e.g. WIRSBO) at a reasonable cost.  You can use  a range of flooring types, including wood, tile, and carpet, over this type of floor heat.  Otherwise, electric under-tile heat adds comfort but isn't a source of primary heat in the room; you would need to extend your home's heating system into the new space, or add a space heater (typically gas or electric).

Good luck with your addition.

P.S. Sorry for the delayed response, I was out of the office last week.