I have two exterior cinder block walls in an apartment that are being redone
on the inside. The furring strips are in place and the walls will be
getting sheetrocked. I have 1-inch styrofoam insulation that will be going
in the spaces between the furring strips. The styrofoam insulation has foil
on one side.
Which side should I keep the foil on? -- the inside facing the room, or the
outside facing the cinder block wall?
I'm thinking the inside since that's the "warm" side, but I don't know.
An excerpt from
"All radiant barriers have at least one reflective (or low emissivity)
surface, usually a sheet or coating of aluminum. Some radiant barriers
have a reflective surface on both sides. Both types work about equally
well, but if a one-sided radiant barrier is used, the reflective
surface must face the open air space. For example, if a one-sided
radiant barrier is laid on top of the insulation with the reflective
side facing down and touching the insulation, the radiant barrier will
lose most of its effectiveness in reducing heating and cooling loads."
The foil radiant barrier requires an air space - it's not supposed to
touch anything. If it is touching the back of the wall covering it
loses some of its effectiveness.
The foil also acts as a vapor barrier, which may or may not be
necessary depending on what type of rigid foam insulation it's on.
Thanks. I checked out the link and read the article, and I appreciate you
providing the key excerpt. I had tried doing some Internet searches before
posting my question but didn't seem to get to any sites that had the
information I wanted.
The bottom line is that the styrofoam is going up with the foil facing the
inside of the house and with the insulation slightly back so it creates a
space between the foil covering and the inside of the sheetrock.
White foam is polystyrene. The typical expanded polystyrene stuff
(Styrofoam being the biggie in brand names) is made up from little
beads of polystyrene bonded together and used in everything from
packaging to coffee cups. It has one of the lower insulation values
of the rigid foam insulations - something around R3 or R4 per inch.
More info at:
It definitely looks like white "coffee-cup-like" polystyrene, with foil on
one side. I'll get the exact brand name etc. and post it.
Here's what it says on the 4x8 sheets of insulation:
Cellofoam North America, Inc.
Here's their website:
That site crashes my IE. Polystyrene is white and only maybe R5 new
4.6 settled. Polyisocyanurate is white, [not as bright] and is R7.2
new and settles to R 6.8 or so, it is superior in R value for spaces
with little depth. 1" or even R 10 is not much on an exterior wall,
unless area is moderate temperature.
I bet both sides have foil but one side is painted as my Celotex board
was, try alcohol or laquer thinner or some paint remover to see. One
inch is only R 7.2". 2" is still substandard in most zones for R
value, I used 2-4.
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