ASHRAE Journal - April 2020 - 14

TECHNICAL FEATURE

Substituting

ρ= P/RT

(2)

FIGURE 1 (a) Stack pressure at the top of insulated basement stud wall sealed at floor, (b) basement stud wall
open at floor. Room air temperature = 70°F; air pressure = 14.694 psia.

Flow (cfm)

Stack Pressure at Top of Wall (Pa)

Stack Pressure at Top of Wall (Pa)

Gives
A.
B.
Wall Sealed at Floor
Average Temperature
ΔP = P1 (1/T1 - 1/T2) × g × h/R (3)
Wall Open at Floor
3
Behind the Insulation (°F)
6.5
where
0
2.5
5.5
ΔPr = Pressure differential at the
10
4.5
20
2
reference altitude
30
3.5
ρ1 = Air density in the living
1.5
40
2.5
space (slug/ft3; one slug is
50
1
1.5
32.1740 lb)
0.5
0.5
ρ2 = Air density behind the insu6 7
8 9 10
0
Wall Height (ft)
lation (slug/ft3)
6 7
8
9 10
Wall Height (ft)
h = Height above the neutral
plane (ft)
P1 = Living space air pressure (lb/ft2)
FIGURE 2 Conditioned airflow behind basement insulation versus ELA, stack pressure, ΔP. Air pressure = 14.694 psia. Envelope leakage Cd = 0.6.
T1 = Temperature in room (°R)
T2 = Temperature behind the insulation (°R)
1,100
R = Individual gas constant for dry air = 1,716 ft2/(s2·°R)
1,000
4.5
Top of Wall Stack Pressure (Pa)
g = Acceleration due to gravity (32.2 ft/s2)
900
3.5
ΔP = Stack pressure (lb/ft2) or customarily in units of
800
Pa (47.88 Pa/lb·ft2)
700
2.5
600
Using Equation 3, the stack pressure at the top of a
2.0 1.5
500
basement wall with the air barrier sealed at the floor is
1.0
400
shown in Figure 1a for a range of temperatures behind
300
the insulation and wall heights. Leaving the insulation
0.5
200
envelope air barrier open at the floor doubles the stack
100
pressures, which are shown in Figure 1b.
0
The soil in cold climates is colder than the living
0 50 100 150 200 250 300 350 400 450 500
Equivalent Leakage Area (ELA) (in.2)
space year-round (e.g., 46°F to 50°F [7.8°C to 10°C] at
foundation depth and 26°F to 32°F [-33°C to 0°C] at
ELA = Equivalent or effective leakage area, ft2
the ground surface in winter and near average air tem(= in.2/144)
peratures at the ground surface in summer). Hence,
ρ = Mass air density, 0.002379 slug/ft3
temperatures between the insulation and the founda(=0.07659 lb/ft3/g) at sea level and 59°F
tion wall inside the building are always colder than in
ΔPr = Reference pressure difference (lb/ft2) or custhe living space.
tomarily in units of Pa (= 47.88 × lb/ft2)
Airflows Behind the Insulation
Figure 2 shows a range of conditioned air circulation
The airflow rate through the insulation envelope, Q, is
behind the insulation for a range of peak stack pressures
governed by the following equation
(see Figure 1b) under winter conditions and a range of air
barrier total (in + out) leakage areas.
Q = ( ELA) Cd ( 2 ∆p ) / ρ
(4)
In home basement stud wall insulation envelopes that
are sealed at the wall top and bottom, we have measured
where
flows through the envelope of much greater than 100 cfm
Q = Predicted flow rate, ft3/s (= cfm/60)
(47 L/s) under winter stack pressures. Air barrier seals
Cd = Discharge coefficient (varies between 0.6 for
at discontinuities such as at pipes, wall plugs, electrical
small sharp-edged openings and 1 for large
breaker or around furnaces in subfloors also may be leaky.
openings)
In some basement wall insulation, the air barrier has
14

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ASHRAE Journal - April 2020

Table of Contents for the Digital Edition of ASHRAE Journal - April 2020

Contents
ASHRAE Journal - April 2020 - Intro
ASHRAE Journal - April 2020 - Cover1
ASHRAE Journal - April 2020 - Cover2
ASHRAE Journal - April 2020 - 1
ASHRAE Journal - April 2020 - Contents
ASHRAE Journal - April 2020 - 3
ASHRAE Journal - April 2020 - 4
ASHRAE Journal - April 2020 - 5
ASHRAE Journal - April 2020 - 6
ASHRAE Journal - April 2020 - 7
ASHRAE Journal - April 2020 - 8
ASHRAE Journal - April 2020 - 9
ASHRAE Journal - April 2020 - 10
ASHRAE Journal - April 2020 - 11
ASHRAE Journal - April 2020 - 12
ASHRAE Journal - April 2020 - 13
ASHRAE Journal - April 2020 - 14
ASHRAE Journal - April 2020 - 15
ASHRAE Journal - April 2020 - 16
ASHRAE Journal - April 2020 - 17
ASHRAE Journal - April 2020 - 18
ASHRAE Journal - April 2020 - 19
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ASHRAE Journal - April 2020 - 21
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ASHRAE Journal - April 2020 - Cover4
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