ASHRAE Journal - June 2020 - 79

COLUMN BUILDING SCIENCES

FIGURE 6 Controlling Condensation. It is not necessary to install all of the insulation on the top of the roof sheathing to elevate its temperature sufficiently to
control condensation and moisture accumulation. Some insulation can be located
above the top of the roof sheathing and some of the insulation can be located on
the underside of the roof sheathing.

TABLE 1 Insulation for condensation control.a
CLIMATE
ZONE

RIGID BOARD OR
AIR IMPERMEABLE
INSULATION

CODE
REQUIRED
R-VALUE

RATIO OF RIGID BOARD INSULATION
OR AIR IMPERMEABLE R-VALUE TO
TOTAL INSULATION R-VALUE

1,2,3

R-5

R-38

10%

R-10

R-49

20%

4A, 4B

R-15

R-49

30%

R-20

R-49

40%

R-25

R-49

50%

R-30

R-49

60%

R-35

R-49

70%

aAdapted

from Table R 806.5 2015 International Residential Code

Table 6.1

mineral wool and rigid ﬁberglass.
There must be an air control layer ("air barrier")
under the rigid insulation boards. The air control layer
can be a fully adhered membrane on the roof deck, or
the air control layer can be the roof deck itself (airtight
sheathing with taped or sealed seams). The rigid board
insulation should be installed in multiple layers, with
joints offset to limit three-dimensional airﬂow networks
within the multiple layers of this assembly.
It is not necessary to install all of the insulation on
the top of the roof sheathing to elevate its temperature
sufﬁciently to control condensation and moisture accumulation. Some insulation can be located above the top
of the roof sheathing, and some of the insulation can be
located on the underside of the roof sheathing (Figure 6).
Sufﬁcient insulation should be located above the roof
sheathing to keep the moisture content of the sheathing
below 20% by weight for the coldest part of the winter.
Roof sheathing moisture content drops quite quickly in
the spring. The roof sheathing moisture content should
be below 16% for the summer and the fall. These moisture content limits have been shown to also address
mold growth-based on historical experience. How much
insulation should be located above the roof sheathing
depends on the climate zone and the interior moisture
load? The model codes specify the amount of insulation necessary based on climate zone. The model codes
assume an interior moisture load based on historical

experience and test hut experimentation over several
decades.
The model codes assume a residential occupancy-a
moisture load of approximately 35% relative humidity at
68°F (20°C) in winter.
The model codes specify a speciﬁc performance
requirement: the temperature of the roof sheathing
should be maintained above 45°F (7°C). For calculation
purposes, an interior air temperature of 68°F (20°C) is
assumed and the exterior air temperature is assumed to
be the monthly average outside temperature of the three
coldest months. This is an engineering equation providing boundary conditions derived from observed experimentation and ﬁeld experience.
This is an "insulation ratio"-the R-value on the top
of the roof sheathing compared to the R-value on the
underside of the roof sheathing and the ratio changes
based on climate severity. The model codes specify the
ratios based on climate zone (Table 1). Note how the ratio
changes from approximately 10% to 70%, as the assembly
moves from hot climates to cold climates.
The key requirement is that the ratio of the thermal
resistance of the exterior rigid insulation versus the
thermal resistance of the interior ﬁberglass or mineral
wool must comply with the "ratio" in Table 1.
Another way to control condensation or moisture accumulation on the underside of the roof sheathing is the
"ﬂow through" approach.
The "ﬂow through" approach is limited to hot, dry climates such as Las Vegas and Phoenix (Climate Zones 2B
and 3B). The roof cladding needs to be "back-vented,"
and the roof sheathing and rooﬁng paper, rooﬁng felt,
or rooﬁng membrane need to be vapor open. The most
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