Other Basement Insulation Question

I'm glad this topic came up, but I have a different question.
I have a set of stairs that lead from my main level up to a master suite in an old, converted attic space. The stairs lie directly above the stairs that lead from the main level down into the basement.
Naturally, the stairs going up to the suite stay rather cold in the winter. I'm wanting to insulate them from the underneath, but I don't have any room (a thick layer of rolled fiberglass insulation would greatly reduce the already tight space below).
What would you recommend as a foam sheet to insulate that area? I'm thinking of the sheeting that is applied to the exterior of new construction before the siding is added, but I don't know what rating to use.
Thanks for your advice! Squanklin
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On 12/22/2004 8:51 AM US(ET), Squanklin took fingers to keys, and typed the following:

They make foam sheets for interior use that come in various thicknesses. Just decide how much height you want to lose and go with that thickness. http://www.owenscorning.com/around/insulation/products/foamular.asp
--
Bill

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The higher the rating the better. EPS and Styrofoam are about R5 per inch. Some more expensive foams have higher ratings. Put as much as you can.
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