system
Given the following post, respond as if you're commenting on Reddit:
human
Does aluminum-coated roofing insulation work better than matte insulation?

I have recently been exposed to a lot of information about building materials. One debate which came up more than once deals with roofing insulation. Some of the water vapor barriers used under drywall in Poland have a reflective aluminum coating. The marketing materials as well as some people in the construction industry say that this causes it to insulate better. 

Barring the added thickness of the aluminum, would a reflective coating make the material a better insulator? Please keep in mind that drywall gets attached directly to this, so there is no radiation heat that I can think of which could be reflected. Can someone set this debate straight?
gpt
As you imply, a reflective foil coating will only make a difference with regards to radiant heat.  As such, foil coated insulation can make a minor difference with regards to insulation placed inside attic spaces -- the type you place on the floor of an attic, or on the floor and walls of attic crawlspaces.  
  
The problem is, using such a coating is mostly counter productive.  In order to have any effect, the foil must be on the hot side of the insulation.  It will do nothing at all if it is placed against a wall or ceiling.  But, the proper place for a vapor barrier is between the living space and the insulation.  Placing a barrier on the hot side of the insulation will trap moisture, which will degrade the insulation over time.  The minor gain in thermal efficiency is quickly outweighed by the harm it causes to the insulation.