I want to hear your thoughts on radiant barriers.

[Erin]

OK, so we talked to a P2000 http://www.p2insulation.com/flashfiles/index.html sales rep last week.  P2000 is touted as an insulative foam board with a radiant barrier on either side of it.

Ie, it's plain old, styrofoam with Mylar adhered to both sides. The demonstration made me think this might be exactly what it promises to be.  I also happen to know the post & beam company in my hometown uses P2000 in their houses (2" in the ceiling, 1.5" in walls) with great success for our hot summers & cold winters. 
The price, on the other hand, was nearly double the cost of a 4x8 sheet of EPS and two sheets of Mylar!   
So of course I'm now looking for a cheaper option. 
If anyone gets Mother Earth News, there's a diagram of a timber (log) framed house in this month's with the type of "panel" application we're planning.  (inside, moving out:  an interior shiplap or panelling, pink foam insulation, plywood skin)  vapor barrier is dependent upon how we do the radiant...

I know a radiant barrier is supposed to have dead space to work appropriately, but I also know that the dead space doesn't need to be that big. Seems the dept. of Energy says as little as 1" will work....

So what do you think?  I realize the attic application is most common, but it seems a radiant barrier on the interior and exterior of the wall insulation would also work to reflect radiant heat back where you want it.  (Not to mention be pretty hard to have dust build up on a vertical surface...)

[Woodswalker]

I used insulation sheets with a radiant barrier in the roof of my cabin.  Check my thread for pics and description of the application.  Installed 1 x 2's along each side of each rafter up against the roof sheathing to provide 1.5" of vent space above the foil from top of walls to ridge vent.  That also provides enough open space above the barrier to make it effective.  Think I read once that 3/4" or more is needed.  Used the cabin during hot (upper 90's), sunny weather last summer, and was pleased with interior temp not going above 82F.  Have shade for the windows during most of the day.  Radiant barrier insulation should work well in walls too, since in that vertical application the reflective surface will not accumulate dust that reduces its effectiveness.

Steve

[Erin]

Thanks for replying, Steve.  I'm off to track down your thread.   

Question for you:  Did you use double-sided foam or single?  I see you put a (the?) foil side facing down, which is why i'm wondering...