Insulation, Radiant and Vapor Barriers - Layering board insulation

I have some board insulation which you can see in these two images.

First image:
http://i423.photobucket.com/albums/pp311/Ramledpb/Home%20images/Oct17_001.jpg

Second image:
http://i423.photobucket.com/albums/pp311/Ramledpb/Home%20images/Oct17_002.jpg


One side has a reflective foil, which I tried to capture in the second photo. Is this a vapor barrier?

Can this be layered on top of itself?

If I put this in combination with fiberglass rolled insulation (I think referred to as 'bat' insulation?) does it matter if the fiberglass has the paper backing? Does it matter which side the reflective side of the board faces? I think you get the idea... :D

I have read about not layering vapor barriers in insulation, but am confused about this foil and the usual advice to layer the board insulation.

Thanks.

Hi Ignis, I'll see if I can sort things out for you. The reflective foil surface is a radiant heat barrier as well as a vapor barrier. First the radiant properties. A highly reflective metal surface will neither absorb radiant heat traveling to it nor radiate heat from it's surface when it gets hot (typically 98% efficient). The be effective, a radiant barrier should face an air gap, usually a minimum of 1/2". Now, there are three modes of heat transfer, conduction, convection, and radiation. By creating layers, one can block the movement of heat in all three modes. An air gap does not allow conduction. Fill that air gap with foam and it blocks convection. Add a foil surface to both sides and the product will reflect heat from one side and not transmit it from the other. It is the combining of these various elements that gives way to the layering concept.

The drawback is the air gap and the potential for moisture to enter and things to grow, so the most common application is simply foil faced rigid foam. If you apply a layer over the studs with the foil facing the unfaced fiberglass insulation, it will essentially be facing an air gap. If there is foil on the sheetrock side, it will be of no additional value if the sheetrock is in contact. Strapping can be added before the sheetrock goes up to create the required air gap.

The guidelines for vapor barriers is: use only one and place it on the warm side of the insulation. In the case of rigid foam with foil on both sides, there is no place for moist air to get between the foil surfaces, so it is considered as one.

There are more details and I'm not the greatest expert on the topic, but I do hope this helps. I will attach a link to give you some reading.

Bud
Energy Savers: Vapor Barriers or Vapor Diffusion Retarders (http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810)

Thanks for the post and the link!