John,
Like any tool foil insulation it needs to be used in the proper
context. What are you trying to achieve?
We use a product call Low-E with great success is many of our radiant
projects. We use it as a thermal break between the slab and the ground.
The aluminum is sandwiched between two sheets of polyethylene foam so
you get a thermal break, vapor barrier, easy to install, and something
that holds up to 93 PSI worth of punding at a job site.
We also use it for radiant under floor applications to reflect the heat
back up a joist bay.
You can see more info here on how we use it for mostly heating
applications:
http://www.houseneeds.com/shop/misc/insulation/insulationindex.asp
John, this is *exactly* the kind of 'double-talk' you want to look out for.
Stuff like, "Mass insulations (foam boards/extruded polystyrene) even though
they may have high R-values(depending on thickness) will eventually
stabilize at that constant ground temperature of 55 degrees or lower
depending on climate. Therefore the mass insulation provides a constant
drain on the radiant system and the slab. "
Frankly, if they understood heat transfer, they would know that the slab
will *always* reach an equilibrium temperature between the heat flowing into
it from above, and the heat flowing out of it to the ground below. Even
with their product. The best you can hope for is to raise that equilibrium
temperature to a comfortable level by reducing the heat flow to the ground
below. But they're trying to make it look like their product isn't subject
to the laws of physics and it somehow 'breaks' the heat transfer path
preventing heat loss from the slab to the ground underneath.
Aluminum sandwiched between two layers of foam can have some benefit by
minimizing the radiant transfer across foam cells on one side to those on
the other. But this same reduction in heat transfer can be achieved with
simply thicker foam board. But their product does look like it is easy to
install.
But what is the actual heat transfer coefficient for a layer of this product
versus a 1/2" or 1" of foam board? I suspect the *real* difference isn't in
the heat transfer performance, but in the ease of installation, toughness
and price. It would seem if their product were remarkably better, they
would say so with numbers from an independent laboratory, not some paragraph
of mumbo-jumbo about 'thermal breaks' and 'seeking the cold ground below
it'.
Foil insulation spanning under joists of a radiant floor system is the sort
of niche that radiant insulation works very well. It reduces the downward
radiant heat transfer to an unheated basement or crawl space. Because of
the orientation, there is already very little convective heat losses in such
a situation, and no conductive losses. So now that we're down to just
radiant losses anyway, this becomes the 'best bang for the buck'.
daestrom
