I'm going to put a plastic vapor barrior on the inside of the house I'm
building. My question is this: Most vapor barriors are clear or black, but
there is some plastic out there that is white on one side. Would using a
white plastic facing the interior, reflect radiant heat back into the house?
Years ago, lot of insulation had foil facing but I can't find any.
You might want to read the literature on BuildingScience.com regarding
vapor barriers in the wall before you jump into this. In many climate
zones, wall vapor barriers are no longer recommended because they were
found to do more harm than good - trapping moisture in the walls and
rotting them out.
As for the radiant barrier ability, there are two issues:
1: in order for a radiant barrier to reflect heat, you need an air gap
of about an inch or more between the surface of the radiant barrier
and the wall. If anything comes in contact with the shiny face, it
loses its radiant reflectivity and becomes a simple heat conductor.
2: Effective radiant barriers are typically aluminized foil which have
very low emissivity / high reflectivity across many wavelengths.
Normal white materials, vs. black have essentially the same
emissivity. White reflects well in the visible spectrum but not in the
long wavelength that is low temperature heat. The reason white-roofs
work is that they're reflecting direct sunlight, which has most of its
energy closer to visible wavelengths.
See this graph:
For reference, room temperature objects emit infrared at a wavelength
far off the right end of the graph
So, to answer your question, no, white plastic will do you no good in
your wall and in many circumstances, the installation of a plastic
vapor barrier may lead to your wall rotting out.
Thanks Astro - It's been a decade since I've built and it seems there is a
lot of new knowledge out there that supersedes old code like vapor barriers.
The previous thought was that interior room air is moist and should be kept
out of the wall.
Now - there is an aluminum reflective insulation with two sides of aluminum
with polyethylene bubbles sandwiched in the middle. In their advertising,
they suggest it can be used under a heated slab as it will reflect heat back
up through the slab. But - if you need an air gap for this to happen, it
would seem their advertising is false.
In order for a radiant barrier to work it needs an air space.
Radiant barrier insulation is often over rated and or misleading. It
is generally recommended to add Extruded polystyrene under slabs,
remember to address the perimeter.
Building Science Consultant
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