ASHRAE Journal - October 2024 - 15

COLUMN RESIDENTIAL APPLICATIONS
engineers and students that can accelerate workforce
training on the topic of building science and moisture
durability by providing a more intuitive way to learn
building science.
The Tool
BSA is a free, web-based expert system tool that
provides guidance on the moisture durability and
energy effi ciency of new and retrofi t wall assembly
designs. BSA leverages industry knowledge, fi eld studies
and literature, along with a database of thousands of
pre-simulated hygrothermal models to provide rapid
feedback on moisture durability of a proposed design.
The tool also provides guidance on wall assembly
design, as well as access to a library of building science
knowledge.
The tool is comprised of three parts: 1) the user
interface, 2) the knowledge base that contains the
rules that determine the wall performance and 3)
the inference engine that takes the user inputs and
summarizes the performance of the wall based on the
rules in the knowledge base.3
The tool guides users through selecting the various
components of a wall system for new construction
and retrofi t wall assemblies. When examining new
construction, the BSA will assess the proposed design.
For retrofi ts, it provides recommendations on how to
address the retrofi t in a manner that will not create a
durability problem.
The user interface is simple and intuitive. Moisture
durability is provided in fi ve easy steps: 1) defi ne
climate, 2) specify existing or proposed wall details, 3)
specify retrofi t type (as needed), 4) select a proposed
retrofi t assembly and 5) review the outputs. It is
imperative that the wall details are known; guessing if a
vapor retarder is resident in a wall assembly could lead
to erroneous recommendations due to the signifi cant
role that a vapor retarder can play in moisture control.
The tool allows four different types of construction
practices to be evaluated. The options that are available
are new construction, exterior retrofi t, interior retrofi t
and a gut retrofi t. The new construction mode allows
the user to select all the components of the wall system
from a series of drop-down menus. The exterior retrofi ts
involve improvements to the building enclosure from
the exterior. In some cases, the cladding is removed to
facilitate the addition of continuous exterior insulation.
With the cladding removed, a water control layer or
water resistive barrier could be incorporated to improve
durability by improving the rejection of water leaks
through the cladding. All wall system components
outboard of the wall structure and the cavity can be
modifi ed with this option.
Interior retrofi ts involve improvements from the
interior of the structure while preserving the exterior
cladding and sheathing. Depending on the approach,
the interior part of the enclosure can be removed or
preserved. All wall system components inboard of the
wall structure and the cavity can be modifi ed with this
option. The gut retrofi t removes both the interior and
exterior sheathings, exposing the structural elements
from both sides. All wall system components except the
wall structure can be modifi ed with this option. Pros
and cons of each retrofi t selection are also offered.
When evaluating a new wall assembly, the tool
presents the moisture and thermal performance of
the wall with specifi c guidance about improving the
performance of the wall. For the moisture durability,
gas-gauge type dials are used with a needle that displays
a " fail, " " risky " or " pass " rating. Gauges provide both
" Mold Risk " and " Moisture Sheathing Content " ratings.
The moisture durability is based on the mold index
calculated in accordance with ANSI/ASHRAE Standard
160-2016, Criteria for Moisture-Control Design Analysis
in Buildings.4 To determine if the walls are moisture
durable, the database of pre-simulated hygrothermal
models are evaluated for mold index. The mold index
uses a subjective scale ranging from 0 to 6. A 0 rating
would indicate that there is no mold present, while
a 6 rating indicates that the surface is covered with
visible mold. According to Standard 160-2016: " In order
to minimize problems associated with mold growth
on the surfaces of components of building envelope
assemblies, the mold index shall not exceed a value of
three (3.0). " We modifi ed this requirement by yielding
scores of " fail, " " risky " or " pass " where " fail " has a mold
index more than 3.0, " risky " and " pass " have mold index
ranges of 2.0 - 3.0 and less than 2.0, respectively.
The sheathing moisture content analysis evaluates
the moisture content levels that are calculated in the
wall exterior sheathing when that sheathing is woodbased
or organic and whose performance may be
compromised if the sheathing is maintained at high
levels of relative humidity for extended periods of time.
O CTO B E R 2 0 2 4 ashrae.org ASHRAE JOURNAL
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http://www.ashrae.org
ASHRAE Journal - October 2024

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