ASHRAE Journal - April 2021 - 53

COLUMN BUILDING SCIENCES

say ( " Don't be Dense With Insulation, "
ASHRAE Journal, August 2010). Same
with netted blown ﬁberglass.
What else do we want to look at?
Lot's of stuff, but one more biggie-ﬁre. Lots of insulations burn,
and lots do not. Those that do
not can be made to work ( " Rain
Screens, Claddings, and Continuous
Insulation: Great Fire of London, "
ASHRAE Journal, August 2017). What
about embodied energy, carbon
footprint, global warming potential?
Ah, save that for another day.II
Now to sales people and marketing people. I got this argument years
ago-and thought we crushed it-but
alas, no. Here goes the argument:
" If I make my house airtight, I don't
need as much insulation. " Or " my
insulation is so good it makes the
house airtight, so I don't need as
much insulation. "
This argument typically comes from
people whose insulation is more
expensive than their competitors-so
to compete on a cost basis, they want
to reduce the amount of insulation.
With today's levels of airtightness and
duct tightness, you can't reduce thermal insulation and expect the house
to perform the same.
The ﬁrst time I dealt with this
was in Florida when I was using
low-density spray polyurethane
foam to create conditioned attics.
Yes, the entire house became more
airtight. Yeah! The roof deck/insulation layer became the air barrier
and was connected to the top of the
walls. But you could also get to the
house being more airtight by taping/sealing the joints of the plywood
or OSB roof sheathing and installing sealed wood blocking between
the rafter tails or the roof trusses.
You did not need spray foam to get

PHOTO 10 Fiberglass and Mineral Wool Batts.

PHOTO 11 Faced Insulation Batts. Airflow and

When they are installed in wall cavities, roof cavities, on the ceilings of attics or on the underside
of roof/attic sheathing, they can't act as the
water control layer, the air control layer or the
vapor control layer. They can act as the thermal
control layer if airflow through them is limited or
controlled.

convection can be effectively controlled by facers
adhered to batt insulation. Note that faced insulation batts should have the facers installed to the
face of the stud framing, not inset stapled. Inset
stapling creates voids and air channels that can
lead to convective loops and a loss of thermal
resistance.

FIGURE 2 Cavity Insulation Airflow Pathways. There

PHOTO 12 Netted and Damp Spray Cellulose. They

are 12 possible airflow pathways that can reduce
the thermal resistance of cavity insulations. Cavity
insulations can act as effective thermal control
layers. The Thermal Metric Project1 showed that
all cavity insulations functioned if they were
combined with air control layers and if convection
was controlled. Let me be specific and obvious.
If batts are installed and fitted " tightly " without
voids, they work.

can only act as the thermal control layer. They
can't act as the air control layer or the vapor control layer. Same with netted blown fiberglass.

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ASHRAE Journal - April 2021

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