Does anyone use builders insulation wrap under their sleeping mats?

I think Tyvek is the brand name you're talking about. I personally have never used it but a lot of guys on the Boundary Waters forum I visit swear by it.

Mik, I use DuPont Tyvek, white on both sides. I have used an aluminized mylar sheet in the past, as a way to insulate from cold ground but then had to deal with condensation as the mylar didn't breath like the Tyvek does. What is the brand name of your product?

Many of the non-Tyvek building wraps (Barricade is one) are woven plastic, and much less water resistant than Tyvek. Also, all of them (including Tyvek building wrap) are quite a bit heavier than polycryo (that's GG's name for it–many of us buy it in the form of window insulation kits). If you want an aluminized groundsheet, a cut down emergency blanket would be a lightweight choice.

@ Steve:
Curious about the condensation. Was it on top of the sheet or underneath? If underneath (as I'd expect, since that's what I experience with polycryo), I'd consider that an advantage of a non-breathable sheet, in that it's keeping all that moisture out of the sleep system.

Steven, not sure of the product name but the builders basically wrap the entire house walls with this insulation paper as part of the Australian energy ratings for houses. Was thinking that instead of taking the groundsheet that I ordered with my TT Rainbow (haven't got it yet it's in the post) that is cut to the entire tent's footprint I will just cut this insulation wrap paper big enough for the sleeping mat. I think it is a heavier item per square meter but being cut to only my mat's size I should be saving weight.

I think this product is designed to reflect thermal activity and that is the main issue for me. I feel the cold easily and have had many a sleepless night due to my body heat leeching into the ground :(

sounds like you are talking about Tyvek.. which is what TT will be sending with your tent.

Tyvek is not popular at all here in Australia (I have to drive across town to get mine…)
The stuff Mik is talking about is very different from HomeWrap although designed to do the same thing.
It is blue on the outside and has a silver foil on the inside (the against the wall side)
Could be worth a try.
I have considered several times getting some from the leftovers from a building site but never done so..
Franco
looks like this :

That's it Franco. I am getting the Tyvek with the Rainbow but will make a decision when I have thje 2 products in my hands.

I've never seen that material on a jobsite, but Tennessee has a pretty mild climate so our building codes are a bit more relaxed in some areas. I will say that not all house wraps are equal. Around here most of the wraps used are the plastic weave kind because everyone is using wall material with the vapor barrier built in. I say if you have access to it then try it! It could work out great for you.

The stuff you see at construction sites (I grew up in the trades) is called foil-faced kraft paper, and is often found pre-attached to residentual insulation.

Here it is in rolls:

http://www.alibaba.com/product-gs/558851791/Aluminum_Foil_Kraft_Paper.html

I would not take it hiking as it will absorb water.