ASHRAE Journal - April 2020 - 22

TECHNICAL FEATURE

FIGURE 6 Heating degree days (HDD) environment for the aboveground part of the
house versus the insulated basement (Seattle).

FIGURE 7 Heat loss due to envelope leakage for a basement in the Seattle area,
C=0.3 (insulation coefficient).

700
900
600

700

Heat Loss to Wall (Btu/h)

800
House

600
HDD

500
400

Basement

300
200

500

250

400
200
300
150
200
100

100
0

2
300 Leakage Area (in. )

100
1

2

3

4

5

6 7
Month

8

9

10 11

12

0

50
0

1

2

3

4

5

6
7
Month

8

9

10

11

12

the foundation at the footing are also sepaFIGURE 8 Main elements of depressurization system (a) wall only and (b) wall and floor.
rately exhausted by this system.
A.
B.
The sealed envelope system is conJoists
tinuously depressurized by at least 2 Pa
Blower
Top Edge Seal
Drywall
(0.04 lb/ft2) year-round, typically by a
Plywood or
flow in the 10 cfm to 30 cfm (4.7 L/s to 14
Fiberboard Floor
L/s) range, with air taken from the living
Foil
space and exhausted outdoors. In setting
1 in. × 4 in.
Sleepers
a minimum year-round exhaust rate, take
Drain
into account that winter stack pressures to
Concrete
Slab
be offset typically are about 2 Pa (0.04 lb/ft2)
Drained and
Drained and
higher than those in summer. Exhausting
Depressurized Wall
Depressurized Wall
And
Sub-Slab
And Subfloor
this air provides continuous energy-efficient
low-level house ventilation and envelope air
contaminant and moisture exhaust. It is energy efficient
The system is robust, surviving even sewer backups
with the exhaust air drying the envelope insulation, warm- that raise water levels above the floor. Some of these
ing the envelope, while house radiant heat loss warms the systems are three decades old and have maintained
replacement air entering via aboveground walls leaks. At
their same tightness. The system helps prevent living
the same time, the exhaust air with its soil gases, such as
space interior condensation and high humidities by
radon, microbial, concrete and envelope sealant matepreventing the humid air in the envelope from enterrial off-gases, cannot enter the living space; otherwise,
ing the basement living space by removing the soil
these can only be addressed by considerably less efficient
moisture wicking through the concrete footings and
whole-house ventilation dilution and filtration.
slabs. This moisture can be as much as 2 gallons (7.6
The occupants will know if the system is working prop- L) per day in summer-moisture that would otherwise
erly by observing the depressurization instrumentation
be trapped by standard basement finishing and sumshown in Figure 9 and, if necessary, they can adjust the
mer stack pressures. Note that house living space air
blower exhaust rate to ensure system depressurization
humidity above 65% can cause microbial growth inside
relative to the living space at all times. Some wall and
the living space. So, while this system helps reduce
floor areas of the basement are not covered by this syshouse humidity, it does not replace air conditioners
tem (e.g., the electrical circuit board and the furnace
or dehumidifiers. These devices are required to mainand hot water tank).
tain living space humidity below 65% (target 50% in an
22

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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
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ASHRAE Journal - April 2020 - Cover3
ASHRAE Journal - April 2020 - Cover4
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