the relative performance of these products and conventional
fibrous insulations.
Existing frame construction can be insulated pneumatically
using suitable loose-fill insulating material. When loose-fill
materials are used in wall retrofit applications, extra care
must be taken during the installation to eliminate the exis-
tence of voids within the wall cavity. All cavities should be
checked prior to installation for obstructions, such as fire
stop headers and wiring, that would prevent the cavity from
being completely filled. Care must also be taken to install
the material at the manufacturer’s recommended density to
ensure that the desired thermal performance is obtained. Ac-
cessible space can be insulated by manual placement of batt,
blanket, or loose-fill material.
In addition to being properly insulated, the exterior envelope
of all buildings should be constructed to minimize air flow
into or through the building envelope. Air flow can degrade
the thermal performance of insulation and cause excessive
moisture accumulation in the building envelope.
More information on insulation and air flow retarders can be
found in the ASHRAE Handbook of Fundamentals, chapters
22 to 24 (ASHRAE 2005).
Moisture Control
Moisture sources for buildings can be broadly classified as
follows: (1) surface runoff of precipitation from land areas,
(2) ground water or wet soil, (3) precipitation or irrigation
water that falls on the building, (4) indoor humidity,
(5) outdoor humidity, (6) moisture from use of wet building
materials or construction under wet conditions, and (7) er-
rors, accidents, and maintenance problems associated with
indoor plumbing. At a given instant of time, the categories
are distinct from each other. Water can change phase and
can be transported over space by various mechanisms. Water
may therefore be expected to move between categories over
time, blurring the distinctions between categories. Christian
(1994) provides quantitative estimates of potential moisture
loads from various sources.
Moisture accumulation within a building or within parts of
a building can affect human comfort and health, influence
building durability, and necessitate maintenance and repair
activities (or can require that these activities be undertaken
more frequently). Moisture accumulation in the building’s
thermal envelope is also likely to influence the building’s
energy performance. Some problems associated with mois-
ture accumulation are easily observed. Examples include:
(a) mold and mildew, (b) decay of wood-based materials,
(c) corrosion of metals, (d) damage caused by expansion of
materials from moisture (such as buckling of wood floors),
and (e) decline in visual appearance (such as paint peeling,
distortion of wood-based siding, or efflorescence on mason-
ry surfaces). Some problems associated with moisture ac-
cumulation may not be readily apparent but are nonetheless
real; an example is reduced performance of insulated assem-
blies (resulting in increased energy consumption). Detailed
discussions on the effects and the control of moisture in
buildings can be found in an ASTM standard (ASTM 2008),
the ASHRAE Handbook of Fundamentals, chapters 23 and
24 (ASHRAE 2005), and Lstiburek and Carmody (1999).
Mold, Mildew, Dust Mites, and Human Health
Mold and mildew in buildings are offensive, and the spores
can cause respiratory problems and allergic reactions in
humans. Mold and mildew will grow on most surfaces if
the relative humidity at the surface is above a critical value
and the surface temperatures are conducive to growth. The
longer the surface remains above this critical relative hu-
midity level, the more likely mold will appear; the higher
the humidity or temperature, the shorter the time needed
for germination. The surface relative humidity is a complex
function of material moisture content, material properties,
local temperature, and humidity conditions. In addition,
mold growth depends on the type of surface. Mildew and
mold can usually be avoided by limiting surface relative
humidity conditions >80% to short periods. Only for nonpo-
rous surfaces that are regularly cleaned should this criterion
be relaxed. Most molds grow at temperatures approximately
above 4 °C (40 °F). Moisture accumulation at temperatures
below 4 °C (40 °F) may not cause mold and mildew if the
material is allowed to dry out below the critical moisture
content before the temperature increases above 4 °C (40 °F).
Dust mites can trigger allergies and are an important cause
of asthma. They thrive at high relative humidity levels
(>70%) at room temperature, but will not survive at sus-
tained relative humidity levels less than 50%. However,
these relative humidity levels relate to local conditions in
the typical places that mites tend to inhabit (for example,
mattresses, carpets, soft furniture).Figure 17–10. The Alton Sylor Memorial Bridge is a
glulam deck arch bridge crossing Joncy Gorge in An-
gelica, New York. The center three-hinged arch spans
52 m (171 ft) and is the longest clear span in the Unit-
ed States. (Photo courtesy of Laminated Concepts,
Inc., Big Flats, New York.)
General Technical Report FPL–GTR– 190