ASHRAE Journal - February 2020 - 60

COLUMN BUILDING SCIENCES
Joseph W. Lstiburek

Interior Spray Foam
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE

We looked at exterior spray foam previously ("Exterior Spray Foam," ASHRAE Journal,
November 2010). Exterior spray foam typically is used in commercial applications.
Most residential applications use interior spray foam. We are going to look at interior
spray foam and focus on residential applications.
Houses are environmental separators-they keep the
on its exterior surface. A cladding is installed over this
outside out and the inside in. Houses have walls, roofs
water control layer. An air gap is provided between the
and foundations that each have to be connected to
cladding and the water control layer to provide drainage
each other (Figure 1). To function as
of rainwater that penetrates the cladFIGURE 1 Houses have walls, roofs and foundations
environmental separators, the walls,
ding and thereby controlling hydrothat each have to be connected to each other
providing continuity of the water control layer,
roofs and foundations have to be able
static pressure. The cavity insulation
air control layer, vapor control layer and thermal
to handle water, air, vapor and heat.
can be low-density open-cell or highcontrol layer.
How they handle water, air, vapor and
density closed-cell spray foam.* Both
heat depends on the materials used
foam types work in most climates. As
and the design. Spray foam is unique
long as spray foam is sprayed to the
in that it has the ability to handle all
minimum depth classiﬁed as an air
four. However, most typical residenimpermeable insulation, the foam
tial applications do not require spray
does not need to completely ﬁll the
foam to handle all four-residential
cavity. Of course, the minimum code
water control (rainwater and groundrequirement for thermal resistance
water) is typically handled by exterior
still has to be met.
water control layers-not spray foam.
In the International Energy
We are going to look at spray foam in residential walls,
Conservation Code (IECC) Climate Zones 6 and higher,
roofs and foundations installed to interior of the extehigh-density closed-cell spray foam provides additional
rior water control layer.
condensation control and will qualify as a Class II vapor
retarder at a thickness greater than 1.5 inches (38 mm).
Walls
Low-density open-cell spray foam can be utilized with
The most common residential wall is a wood frame
an interior vapor retarder to control condensation. The
wall with wood-based sheathing. The wood-based
typical interior vapor retarder in such an application
sheathing typically has a water control layer installed
is a vapor retarder paint coating applied to the interior
*High-density closed-cell spray polyurethane foam is typically 2 lb/ft3 (32 kg/m3) whereas low-density open-cell spray polyurethane foam
is 0.5 lb/ft3 (8 kg/m3). The 2 lb/ft3 high-density foam is also referred to as "closed-cell" spray-applied foam and the 0.5 lb/ft3 low-density
foam is also referred to as "open-cell." The "cell" designation relates to vapor control. The 2 lb/ft3 high-density foam is around 3 perms per
inch of thickness and the 0.5 lb/ft3 low density foam is around 50 perms per inch of thickness. The difference is a big deal. There is more.
The 2 lb/ft3 high-density foam has a thermal resistance-R-value-of 6.5 per inch, the 0.5 lb/ft3 low-density foam is around 3.5 per inch.

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ASHRAE Journal - February 2020

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