ASHRAE Journal - April 2020 - 13

TECHNICAL FEATURE

air below, sinks to the floor, pulling warm living space
air into the envelope behind the insulation after it, via
leaks. The falling cold air behind the insulation goes to
the floor and then recirculates through leaks back into
the living space. In summer, this circulation around
the insulation does not occur to the same extent, as
now the coldest air behind the insulation is at the base
of the wall, not the top.
This paper provides the science behind these stack
pressures and their effects and analyzes the current situation for batt-insulated basements and their
energy implications. It also provides details for creating
tightly sealed and insulated perimeter walls and low-E
floors that can be depressurized throughout with small
exhaust airflows (10 cfm to 30 cfm [4.7 L/s to 14 L/s]) to
the outdoors; this efficiently eliminates typical basement dampness and mold odors, soil and envelope
moisture, humidity, and air contaminants without relying on less energy-efficient whole-house ventilation
and dehumidification systems. This insulation envelope
sealing and depressurization system has been successfully used in several hundred residential basements
in Canada and the U.S., in some cases for over three
decades.
Foamed-in-place insulation over interior foundation
walls as an alternative solution to depressurized batt or
shimmed board insulation envelopes is not practical for
a number of reasons, including cost. Further, it does not
solve water leakage problems.
This paper describes an integrated method for the
construction and mechanical ventilation of basements
to improve air quality. It eliminates or reduces mold and
similar airborne contaminants, excess humidity, radon
gas and energy loss.
The suggested method, while requiring some extra
time to make the seal tight enough, is in principle simple to implement, using existing construction methods
and materials. Careful attention is given to sealed air
barriers and continuity of the insulated wall (and sometimes floor) cavity with low-level ventilation of this cavity to the exterior.
The suggested method is supported by a demonstration of the results of stack effect within typical existing
basement wall assemblies, which contribute to heat loss,
mold growth and poor air quality. The addition of very
low-level continuous ventilation greatly improves the
passive performance of the physical construction.

This is the first of a two-part series of papers on the
subject of batt and blanket-insulated envelope stack
pressures. Part 2 addresses aircraft.
Most of us are aware of the water leakage, weeping tile
failure, sewer backup, sump failures, mold growth, moisture problems with cracked foundation walls and interior
wood stud walls and subfloors, and radon entry problems
that finished basements can suffer. However, the winter
energy loss due to envelope air circulation is not on the
list of concerns, but should be. This loss is caused by stack
pressures across the insulation vapor barrier and drywall
finish that draw basement air behind the insulation via
leakage pathways at the insulation envelope air barrier at
the top of the wall and push it out into the room after it is
cooled at the bottom of the wall.
The leakage pathways at the top of the wall are unintentional and the result of poor sealing of air barriers to
the underside of the floor above. Leakage pathways at the
bottom are sometimes intentional-intended to ensure
water condensation does not occur on the cold foundation
by allowing house air to travel behind the insulation. The
amount of this insulation bypass is at least 100 cfm (47 L/s)
in a typical 1,000 ft2 (93 m2) finished basement even when
the insulation is kept 1 ft (0.3 m) or more above the floor,
and the vapor barrier is not intentionally poorly sealed.
The insulation bypass could be several times that when
the vapor barrier is intentionally left open at the bottom
to prevent winter condensation collecting behind the
insulation on the aboveground portion of the foundation,
a potential major moisture problem in humidified houses
and in new houses.

Stack Pressures

Stack or buoyancy pressure differentials across perimeter walls and foundations are created by indoor and
outdoor temperature differences. In winter, these
pressures increase with increased height of the cold air
column in the insulation envelope and increased temperature difference. If the bottom of the wall is open to
the room, the stack pressure is based on the full height
of the wall. However, the floor can act as a flow blocker
if the attachment to the wall is sealed. If this is done,
the stack pressure is reduced by half. Air will continue
to enter and exit the envelope through other cracks and
seams throughout the wall.
Stack pressures are predicted by1,2
ΔPr = (ρ2 − ρ1) × g × h
(1)



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 - 11
ASHRAE Journal - April 2020 - 12
ASHRAE Journal - April 2020 - 13
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ASHRAE Journal - April 2020 - Cover3
ASHRAE Journal - April 2020 - Cover4
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