ASHRAE Journal - April 2021 - 48
BUILDING SCIENCES
COLUMN
Joseph W. Lstiburek
Dirty Harry Does Insulation*
A Good Insulation
Always Knows Its
Limitations
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE
After all these decades, thermal performance should be pretty non-controversial.
How complicated can it be to insulate a building and make it airtight? The physics is
pretty straightforward. But then you get sales people and marketing people involved,
and the physics gets twisted and people get annoyed.† For the record I like all insulations-yup, all of them. I think they are all good, but they all have limitations.
What is nice about where we are right now, after all
these decades, is that we have performance requirements specified in the Model Codes for thermal resistance for roofs, walls and foundations, and we have
airtightness performance requirements specified in the
Model Codes for air barrier materials, air barrier assemblies and air barrier enclosures.‡ We also have airtightness performance requirements for duct distribution
systems in the Model Codes. Buildings are required to
be airtight and are tested for compliance.§ Ducts are
required to be airtight and are air tested for compliance.# In terms of insulations and insulation systems,
install them correctly, and they provide the thermal
resistance expected. Let me repeat that. Install them
correctly, and they work.
Here is something that should be obvious. It is a very,
very bad idea to try to trade off thermal resistance for
increased airtightness because the Model Code minimum airtightness is already so low. Let me repeat. It
does not matter because we are already so " tight. " We
build tight-and sometimes ventilate right ( " Unintended
Consequences Suck, " ASHRAE Journal, June 2013 and
" Deal with Manure & Then Don't Suck, " ASHRAE Journal,
July 2013).
* " Dirty " Harry Callahan. " A good man always know his limitations. " " Magnum Force, " 1973, Clint Eastwood as Harry Callahan.
†In
the 1990s I developed the technology behind unvented and conditioned roof assemblies using spray polyurethane foam (SPF) with
help from the U.S. Department of Energy's " Building America Program " and got it accepted by the Model Codes and managed to irritate the manufacturers of fiberglass, cellulose and mineral wool insulation. Then in the mid-2000s I developed the technology behind
unvented and conditioned roof assemblies using fiberglass, cellulose and mineral wool with help from the U.S. Department of Energy's
" Building America Program " and got it accepted by the Model Codes and managed to irritate the manufacturers of spray polyurethane
foam (SPF) insulation.
48
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ASHRAE Journal - April 2021
Table of Contents for the Digital Edition of ASHRAE Journal - April 2021
Contents
ASHRAE Journal - April 2021 - Intro
ASHRAE Journal - April 2021 - Cover1
ASHRAE Journal - April 2021 - Cover2
ASHRAE Journal - April 2021 - 1
ASHRAE Journal - April 2021 - Contents
ASHRAE Journal - April 2021 - 3
ASHRAE Journal - April 2021 - 4
ASHRAE Journal - April 2021 - 5
ASHRAE Journal - April 2021 - 6
ASHRAE Journal - April 2021 - 7
ASHRAE Journal - April 2021 - 8
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ASHRAE Journal - April 2021 - Cover3
ASHRAE Journal - April 2021 - Cover4
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