ASHRAE Journal - April 2021 - 49

COLUMN BUILDING SCIENCES

PHOTO 1 Foil-Faced Polyisocyanurate Boards. The rigid insulation boards are

PHOTO 2 Extruded Polystyrene (XPS) Boards. The rigid insulation boards are

installed on the exterior of framing with their joints taped/sealed and act as the
water control layer, air control layer, vapor control layer and thermal control
layer-all four control layers.

Here is another thing that is obvious. Some insulations
can also act as air control layers (aka air barriers), in
addition to acting as thermal control layers. It gets even
more interesting. Some can also act as vapor control layers-or not-and some can also act as water control layers
(aka water resistive barriers [WRBs] or drainage planes).
Some can act as all of the above. Some can't. Let's look at
a bunch of common insulations and see what they can
do-and what they can't do.
Foil-faced polyisocyanurate boards installed on the
exterior of framing with their joints taped/sealed can act
as the water control layer, air control layer, vapor control
layer and thermal control layer-all four control layers (Photo 1). So can extruded polystyrene (XPS) boards
(Photo 2).
Mineral wool board insulation installed on the exterior
of framing can act as the thermal control layer, but it
needs to be coupled with a water control layer, air control layer and vapor control layer (Photo 3). It can do only
one of the control layers ( " Hot and Wet But Dry, " ASHRAE

PHOTO 3 Mineral Wool Board Insulation. The insulation boards installed on the

exterior of framing can act as the thermal control layer, but they need to be
coupled with a water control layer, air control layer and vapor control layer.
Mineral wool board insulation can do only one of the control layers.

Journal, June 2016).
Let's look at structural insulated panels (SIPs). They
can act as the air control layer, vapor control layer and

‡These

requirements came from my work also through the U.S. Department of Energy's " Building America Program. " Air impermeable insulation is 0.02 L/s·m2 at 75 Pa (.004 cfm/ft2 at 0.01 psi)-basically the leakage through gypsum board (aka " drywall " ). Air
barrier enclosures for buildings are 2.00 L/s·m2 at 75 Pa (0.4 cfm/ft2 at 0.01 psi)-see " Understanding Air Barriers, " ASHRAE
Journal, July 2005. Residentially, the Model Codes call out 3 ach at 50 Pa (0.007 psi) for houses constructed in cold and mixed
climates and 5 ach at 50 Pa (0.007 psi) for houses constructed in hot climates. The reason for the difference between the climates is that in cold and mixed climates we have basements-great places for ductwork and mechanical systems. Where we have
slabs with vented attics and vented crawlspaces with vented attics with ductwork and mechanical systems in them, it is not easy
to get to 3 ach at 50 Pa (0.007 psi). Hence, the " push " to unvented and conditioned roof assemblies and unvented and conditioned attics and crawlspaces-easy to get to 3 ach at 50 Pa (0.007 psi). The problem is not leaky ducts, but all those penetrations the ducts and boots go through.

§It is amazing when you think about this: in the 1980s and 1990s, houses were 5 ach at 50 Pa (0.007 psi) to 15 ach at 50 Pa,
and now they are at 3 ach at 50 Pa to 5 ach at 50 Pa.

#It

is even more amazing that in the 1980s and 1990s, ductwork leakage was typically between 10% and 30% at
25 Pa (0.004 psi), and now it is typically between 3% and 5% at 25 Pa.

Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Westford, Mass. Visit www.buildingscience.com.
APRI L 2021

ashrae.org

ASHRAE JOURNAL

http://www.buildingscience.com http://www.ashrae.org

ASHRAE Journal - April 2021

Table of Contents for the Digital Edition of ASHRAE Journal - April 2021