COLUMN BUILDING SCIENCES
FIGURE 4 Spray polyurethane foam can also be applied to the interior of rigid foam
exterior sheathings such as foil faced isocyanurates and extruded polystyrene
(XPS). In these assemblies the water control layer is typically the taped joints of
the exterior rigid insulation. The cavity insulation can be low density open cell or
high density closed cell spray foam.
Cladding
FIGURE 5 Unvented conditioned attics can be constructed by installing low-density
open-cell or high-density closed-cell spray foam directly to the underside of the
roof deck.
Shingles
Roofing Paper
Roof Sheathing
Air Impermeable Insulation
(aka Spray Polyurethane Foam)
Air Gap
Rigid Foam Exterior Sheathing
("Taped Sheathing Joints")
Spray Polyurethane Foam (SPF)
Gypsum Board
Acrylic Latex Paint
Notes: High-density closed-cell foam can be used in all IECC Climate Zones. Where lowdensity open-cell foam can be used the thickness or thermal resistance of the rigid foam
exterior sheathing is specified by the International Residential Code (IRC) based on Climate
Zone and the thickness of the wall framing.
Note that we were here previously as well ("Hybrid Attics
and Hybrid Walls," ASHRAE Journal, October 2017).
Spray polyurethane foam can also be applied to the
interior of rigid foam exterior sheathings such as foil
faced isocyanurates and extruded polystyrene (XPS). In
these assemblies the water control layer is typically the
taped joints of the exterior rigid insulation. The cavity
insulation can be low-density open-cell or high-density
closed cell spray foam (Figure 4). Where low-density
open-cell spray foam is used, the thickness or thermal
resistance of the rigid foam exterior sheathing is specified by the International Residential Code (IRC) based
on climate zone and thickness of the wall framing
(Table 1).
Roofs
The most common residential roof is an attic that is
wood framed, typically using trusses. It can be either
an unvented conditioned attic or a vented unconditioned attic. Unvented conditioned attics are common in warm climates where basement construction
is not common. The absence of basements typically
62
ASHRAE JOURNAL
ashrae.org
FEBRUARY 2020
results in mechanical systems and ductwork being
located in attic spaces. Locating mechanical systems
and ductwork in vented unconditioned attic spaces is
a large thermal penalty, and in hot humid and mixed
humid climates results in significant condensation
issues. Vented unconditioned attics are common in
cold climates where basement construction is typical.
Mechanical systems and ductwork in cold climates
are typically located in basements and interior floor
framing rather than in attics, avoiding associated large
thermal penalties.
Unvented conditioned attics can be constructed by
installing low-density open-cell or high-density closedcell spray foam directly to the underside of the roof
deck (Figure 5). Both foam types work in most climates.
In IECC Climate Zones 5 and higher only high-density
closed-cell spray foam should be used. The reason is that
it is difficult and impractical to install an effective vapor
control layer to the surface of low density open cell spray
foam. Vapor retarder paint coatings have had some limited success. Note the words "limited success."
Hybrid attics can be constructed in IECC Climate
Zones 5 and higher using a combination of high-density
closed-cell spray foam and low-density open-cell spray
foam (Figure 6). The thickness or thermal resistance of
the high-density closed-cell spray foam is specified by
the International Residential Code (IRC) based on climate zone (Table 2). A further variation is where the low
density open cell spray foam is replaced with fiberglass
or cellulose insulation (Figure 7). Again, the thickness or
thermal resistance of the high-density closed-cell spray
foam is specified by the International Residential Code
(IRC) based on climate zone (Table 2).