ASHRAE Journal - June 2022 - 50

BUILDING SCIENCES
COLUMN
Joseph W. Lstiburek
Piltdown Man Does
Thermal Resistance*
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE
I, as an engineer, am born with a genetic defect, as all engineers are-the " efficiency
gene. " We can't help ourselves. The more efficient a thing is, the better it is-in our
engineering brains. I love thermal resistance. I love insulation. I love all insulations-fiberglass,
cellulose, mineral wool, expanded polystyrene, extruded polystyrene,
polyisocyanurate, low density spray foam, high density spray foam, phenolic
insulation.†
Every now and then something gets to me. Actually,
a couple of things. Here goes. We are going through a
bunch of significant fundamental changes in the way
we build buildings. We are supposedly getting serious
about energy efficiency. I go visit the local construction
supply outlet and make the mistake of reading what is
being sold. Ouch.
For the record, radiant barriers do not have R-value.
There is no such thing as R-11 radiant barrier paint.
There is no such thing as R-4 3/8 in. (9.5 mm) thick
bubble wrap reflective insulation under a concrete
slab. There is no such thing as R-4 ¼ in. (6.4 mm) thick
*Only folks from my generation recall one of the greatest scientific
frauds of the last century. I leave it to you youngsters to look it up.
†I think I got them all. Oops, forgot about Gloria Vanderbilt ground
up blue jeans-the ones with borates.
§Did you know that if you use two layers of ¼ in. (6.4 mm) thick
bubble wrap with two air spaces of minimum thickness 4.75 in.
(121 mm) in floor joists you get R-21? Yup, right from the packing
slip from the roll you buy at the lumber yard under the " Read
This Before You Buy-What You Should Know About R-Values "
information.
#Didn't we go through the " mass-and-glass thing " in Santa Fe,
N.M., in the '70s? And who can forget the Trombe Wall?
50
ASHRAE JOURNAL ashrae.o rg
J U N E 2022
bubble wrap reflective insulation on a duct. There is no
such thing as R-50 1/16 in. (0.06 mm) reflective plaster.
There is more.§
And there is still more. Thermal mass is not very
good thermal resistance unless you have reliable diurnal
temperature swings and effective coupling to the
enclosure.#
Let me remind you youngsters: " light and
tight " beat " mass and glass. " " Mass and glass " referred
to masonry and stone exterior walls that were externally
insulated. The mass was the masonry or stone. The glass
was hugely thermally inefficient south-facing almost
entirely glass window walls. The idea was that solar
radiation collection was maximized with lots of southfacing
glass, and this solar energy was stored in the interior
masonry and stone.
Note that when I was a youngster I did mass and glass.
Yup, I got sucked in. Now I am older and wiser and
embarrassed. See Photo 1 and Photo 2-two " mass and
glass " houses I built in 1980. I even did the R-15 " reflective
insulation roll blinds " on the inside of the windows
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.

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ASHRAE Journal - June 2022

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