COLUMN BUILDING SCIENCES
FIGURE 7 A further variation of hybrid attics is where the low-density open-cell
spray foam is replaced with fiberglass or cellulose insulation.
FIGURE 6 Hybrid attics can be constructed using a combination of high-density
closed-cell spray foam and low-density open-cell spray foam.
Shingles
Roofing Paper
Shingles
Roofing Paper
Roof Sheathing
Roof Sheathing
Air Impermeable Insulation
(aka Spray Polyurethane Foam)
Air Impermeable Insulation
("Closed-Cell" Polyurethane Foam)
Air Permeable Insulation (Fiberglass Batts,
Netted Blown Cellulose, Netted Blown
Fiberglass, Spray Applied Fiberglass)
Air Permeable Insulation
("Open-Cell" Spray Polyurethane Foam)
TABLE 2 Insulation for condensation control.1
CLIMATE
ZONE
AIR IMPERMEABLE HIGHDENSITY CLOSED-CELL
SPRAY FOAM INSULATION
CODE REQUIRED RATIO OF AIR IMPERMEABLE
R-VALUE HIGH-DENSITY CLOSED-CELL
SPRAY FOAM INSULATION R-VALUE
TO TOTAL INSULATION R-VALUE
1, 2, 3
R-5
R-38
10%
4C
R-10
R-49
20%
4A, 4B
R-15
R-49
30%
5
R-20
R-49
40%
6
R-25
R-49
50%
7
R-30
R-49
60%
R-35
R-49
70%
8
1Adapted
from Table R 806.5 2018 International Residential Code.
Vented unconditioned attics are typically constructed
using spray foam in conjunction with fiberglass or cellulose insulation. The key to the performance of vented
unconditioned attics is continuity of the air control
layer located at the ceiling plane. It is difficult to provide
airtightness at the ceiling plane using typical sealants.
Spray foam is used to seal the ceiling plane in an airtight
manner (Figure 8). It is critical to provide air sealing at
the perimeter to control wind washing of air permeable
insulation. The spray foam creates an airtight "bathtub"
that is then filled with air permeable insulation such as
fiberglass or cellulose. Typically, high-density closed-cell
spray foam is used for this application.
In low slope roof construction similar approaches are
used to those used with unvented conditioned attics.
Most low slope roof construction using spray foam use a
hybrid approach where a combination of high-density
closed-cell spray foam is used with fiberglass or cellulose insulation (Figure 9). Again, the thickness or thermal
FIGURE 8 Vented unconditioned attics are typically constructed using spray foam
in conjunction with fiberglass or cellulose insulation. The key to the performance
of vented unconditioned attics is continuity of the air control layer located at the
ceiling plane.
Cellulose or Spray Fiberglass
HD Spray Foam
Gypsum Board
2×6 Frame Wall
Cellulose or Spray Fiberglass
OSB/Plywood
Sheathing
Drainage Plane
HD Spray Foam
FIGURE 9 Most low slope roof construction using spray foam use a hybrid approach
where a combination of high-density closed-cell spray foam is used with fiberglass
or cellulose insulation.
Roofing Membrane
Roof Deck
Spray Polyurethane Foam
OSB Sheathing
Rim Closure Material
OSB Sheathing
Water Control Layer
Drainage Mat
Stucco Over
Paper-Backed Lath
Cavity Insulation
Gypsum Board With Semi-Permeable
(Latex) Paint
Cavity Insulation
resistance of the high-density closed-cell spray foam is
specified by the International Residential Code (IRC)
based on climate zone (Table 2).
FEBRUARY 2020
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