ASHRAE Journal - June 2022 - 51

COLUMN BUILDING SCIENCES
PHOTO 1 Mass and Glass. A concrete block wall insulated on the exterior with
6 in. (152 mm) of expanded polystyrene. Note the " high heal " roof trusses so
ceiling insulation does not get " squeezed " at the perimeter. Yup, I did a few
things correctly.
PHOTO 2 Exposed and Coupled Interior Thermal Mass. Looks pretty good. Very
expensive. Only the main floor thermal mass was coupled and distributed to the
inside and solar gain. But, all the " light and tight " super insulated frame buildings
I did later worked better and were much less expensive.
(Photo 3). " Light and tight " meant extremely well insulated
wood frame walls in a building that was signifi -
cantly airtight. How many folks remember the Illinois
Lo-Cal House and the Leger House.1 (Yes, we were here
before: " Building America, " ASHRAE Journal, December
2008.)
And still more. Radiant barriers in attics only work
well in poorly insulated attics. Read on. Yup, still more.
Refl ective insulation never worked on the inside of block
walls in Florida.II
So what is radiant barrier paint? There is no such
thing. A radiant barrier is a low-emittance ( " low E " )
surface facing an air space or vacuum. The American
Society of Testing and Materials (ASTM) actually
defi nes these things. ASTM defi nes radiant barriers as
a material or layer that has a refl ective/low-emittance
of 0.10 or less.2. There are interior radiation control
coatings (IRCC), but no radiant barrier paints. The
applicable ASTM standard for measuring emittance
is C1371-15, Standard Test Method for Determination of
Emittance of Materials Near Room Temperature Using Portable
Emissometers.3
Recall the Stefan-Boltzmann Law and the fourth power
of the thermodynamic temperature thing. If you want
a radiant barrier to do anything, it should be on the
high temperature side of things-and it needs to face
an " open " air space. Wait for it. If there is no air space
and the radiant barrier is touching something, we call
it conduction, not radiation anymore-no more radiant
barrier. Radiant barriers do not have R-values-I think I
already mentioned that-they only deal with radiation.
R-values deal with conduction (and sometimes convection-they
should always deal with convection-but that
is for a different story and different time).
So radiant barriers don't make sense on the inside of a
wall or on the inside of a roof or ceiling under insulation
because those locations are not on the high temperature
side of things or the real cold side of things. They certainly
don't make any sense under a fl oor slab or in crawl
space fl oor framing. The ground isn't that hot or that
cold-the fl oor isn't that hot or that cold-who knew?
So what is refl ective bubble wrap? It is a roll product
with sealed air bubbles between two layers of plastic
or aluminum foil (Photo 4). It is refl ective insulation.
Refl ective insulation is different from radiant barriers.
Refl ective insulation relies on enclosed air spaces to
have any benefi t. It is typically between ¼ in. and 3/8 in.
thick (6.4 mm to 9.5 mm)-with claims that it has a thermal
resistance of anywhere between R-4 and R-6. Think
of the possibilities. You could take four layers of the R-6
bubble wrap-with the right air gaps-and wrap a R-13
cavity-insulated wood frame wall and add an additional
R-24. Awesome-an R-40ish wall. And the cost is way
less than adding 5 in. (127 mm) of extruded polystyrene
or 3.5 in. (89 mm) of polyisocyanurate. What a bunch of
losers we old engineers are.
Didn't Lt. Commander Montgomery Scott ( " Scotty " )
invent " transparent aluminum " in Star Trek IV?
Actually, we do have a version of transparent aluminum
today-the " low E " coatings on insulated glass assemblies.
But work with me on this. We could replace the
aluminum foil layers in the 3/8 in. (9.5 mm) bubble
IISpring Breakers take note of why you are uncomfortable in your hotel roomsÑand by the way enjoy the mold smell.
J U N E 2 0 2 2 ashrae .o rg ASHRAE JOURNAL
51

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