Revisiting Design Objectives

February 1, 2013

The Mechanical Insulation Design Guide (MIDG) contains
extensive information on mechanical insulation materials and their properties,
as well as design information. Based on excerpts from the MIDG, here is an
overview of design objectives and considerations that should be taken into
account for each insulation system.

Most
engineers, architects, and end users are familiar with the use of insulation to
reduce heating and cooling loads, and to control noise in building envelopes.
Insulations used for pipes, ducts, tanks, and equipment are not as familiar;
and the installed cost of these materials is usually a small part of the total
cost of a project. As a result, mechanical insulation is often overlooked,
undervalued, or improperly specified and maintained in commercial and
industrial construction projects.

Mechanical insulation is used primarily
to limit heat gain or loss from surfaces operating at temperatures above or
below ambient temperature. It may be used to satisfy one or more of the
following design objectives (answering the question: why insulate?).

  • Condensation
    control: minimizing condensation and the potential for mold growth by keeping
    surface temperature above the dew point of the surrounding air

  • Energy
    conservation: minimizing unwanted heat loss/gain from systems. There are three primary
    reasons to conserve energy

  • Energy: minimize
    the use of scarce natural resources

  • Economics:
    minimize return on investment (ROI) and minimize life-cycle cost

  • Environment:
    minimize emissions associated with projects’ energy usage

  • Fire safety: protecting critical building elements and slowing the
    spread of fire in buildings

  • Freeze
    protection: minimizing energy required for heat tracing systems and/or
    extending the time to freezing in the event of system failure

  • Personnel
    protection: controlling surface temperatures to avoid contact burns (hot or
    cold)

  • Process control: minimizing temperature change in processes where close
    control is needed

  • Noise control:
    reducing/controlling noise in mechanical systems

In
addition to these objectives, a number of design considerations may require
attention when designing a mechanical insulation system.

  • Abuse resistance

  • Corrosion under
    insulation

  • Indoor air
    quality

  • Maintainability

  • Regulatory
    considerations

  • Service and
    location

  • Service life

Designing insulation systems can be complicated, as they are
intended to satisfy a number of design objectives and in some projects, must
satisfy multiple design objectives simultaneously. For example, the objective
may be to provide the economic thickness of insulation and avoid surface
condensation on a chilled water line. The chilled water line may pass through
various spaces within the project. Those various spaces may have differing
temperature and humidity conditions, so it is likely that different insulation
materials, thicknesses, and coverings may be required for a single line. Since
projects may involve many lines operating at various service temperatures in
various environmental conditions, it is clear that a systematic approach is
required for all but the simplest projects.

Design Objectives Condensation Control

For
below-ambient systems, condensation control is often the overriding design
objective. The design problem is best addressed as two separate issues 1)
avoiding surface condensation on the outer surface of the insulation system,
and 2) minimizing or managing water vapor intrusion.

Avoiding surface condensation is desirable for a number of
reasons (1) it prevents dripping that can wet surfaces below, (2) it minimizes
mold growth, and (3) it avoids staining and possible damage to exterior
jacketing.

The design goal is to keep the surface
temperature above the dew-point temperature of the surrounding air. Calculating
surface temperature is relatively simple, but selection of the appropriate
design conditions is often confusing. The appropriate design condition is
normally the “worst-case” condition expected for the application. For
condensation control, however, a design that satisfies the worst-case condition
is sometimes impossible.

For outdoor applications (or for
unconditioned spaces vented to outdoor air), there are always some hours per
year where the ambient air is saturated or nearly saturated. For these times,
no amount of insulation will prevent surface condensation. Therefore, for
outdoor applications and mechanical rooms vented to outdoor conditions, it is
suggested to design for a relative humidity (RH) of
90%. Appropriate water-resistant,
vapor-retarder jacketing or mastics then must be specified to protect the
system from the inevitable surface condensation. Additional design dew-point
data can be found in Chapter 28 of the 2005 ASHRAE Handbook of Fundamentals
(www.ashrae.org).

For indoor designs in conditioned
spaces, care must still be exercised when selecting the design conditions.
Often, the HVAC system will be sized to provide indoor conditions of 75
°F/50% RH on a design summer day.
However, those  conditions would not represent the worst-case indoor conditions
with respect to the insulation design. Part load conditions could result in
higher humidity levels, or night and/or weekend shutdown could result in more
severe conditions.

Water Vapor

Note
that the discussion has focused so far on designing to avoid condensation on
the exposed surface. Another important design consideration is minimizing or
managing water vapor intrusion. Water vapor intrusion is extremely important
for piping and equipment that is operating at below ambient temperatures.
Water-related problems include thermal performance loss, health and safety
issues, structural degradation, and aesthetic issues. Water entry into an
insulation system may be through diffusion of water vapor, air leakage carrying
water vapor, and leakage of surface water.

During those periods when the operating temperature is below
the dew point of the surrounding ambient air, there will be a difference in
water vapor pressure across the insulation system. This vapor-pressure
difference serves as the driving force for diffusion of water vapor from the
ambient toward the cold surface. Piping and equipment typically create an
absolute barrier to the passage of water vapor, so any vapor-pressure
difference imposed across the insulation system results in the potential for
condensation either within the insulation or at the cold surface. While these
pressure differences seem small, the impact over many operating hours can be
significant.

A number of fundamental design principles are applied in
managing water vapor intrusion. One method is to reduce the driving force by
reducing the moisture content of the surrounding air. While the insulation
designer typically will not have control of the location of the piping,
ductwork, or equipment to be insulated, there are opportunities for the
mechanical engineer to influence the ambient conditions. Certainly, locating
cold piping, ductwork, and equipment in unconditioned portions of buildings
should be minimized. Consideration should be given to conditioning mechanical
rooms, if feasible.

Another fundamental design principle involves the
moisture-storage design. In many systems, some condensation can be tolerated,
with the amount depending on the water-holding capacity or water tolerance of a
particular system. The moisture storage principle permits accumulation of water
in the insulation system but at a rate designed to prevent harmful effects.
This concept is applicable when 1) unidirectional vapor flow occurs, but
accumulations during severe conditions can be adequately expelled during less
severe conditions; or 2) reverse-flow conditions regularly occur on a seasonal
or diurnal cycle. Design solutions using this principle include 1) periodically
flushing the cold side with low dew point air (this procedure requires a supply
of conditioned air and a means for distribution), and 2) use of an insulation
system supplemented by selected vapor retarders and absorbent materials such
that an accumulation of condensation is of little importance. Such a design
must ensure  adequate expulsion of the accumulation.

Another common method is the application of moisture-blocking
design. The moisture-blocking principle is applied in a design wherein the
passage of water vapor is eliminated or minimized to an insignificant level.
The design must incorporate: 1) a vapor retarder with suitably low permeance;
2) a joint and seam sealing system that maintains vapor retarding system
integrity; and 3) accommodation for future damage repair, joint and seam resealing,
and reclosing after maintenance.

A vapor retarder is a material or system that will adequately
reduce the transmission of water vapor through the insulation system. The vapor
retarder system is seldom intended to resist the entry of surface water,  or to
prevent air leakage, but can occasionally be considered the second line of
defense for these moisture sources.

Energy Conservation – Financial Considerations

Energy
is in short supply, and our dependency on foreign energy sources is at best
volatile. Throughout the developed world, the primary source of energy is the
burning of fossil fuels. Energy supply and demand plays an increasingly vital
role in national security and the economy.

Minimum insulation levels for ductwork and piping often are
dictated by these energy standards, which are often adopted by model building
code organizations and/or various jurisdictions as building energy codes.
Energy codes address energy conservation in buildings, but typically do not
address additional design objectives (such as condensation control, personnel
protection, or noise control) that may be important on a specific project. The
adoption of energy codes varies by state and locality, and the level of
enforcement varies. For the current status of statewide energy codes, see www.bcap-energy.org
or www.energycodes.gov.

As an aid to understanding the relationships between energy,
economics, and emissions for insulated systems, simple spreadsheet calculators
have been developed for equipment, vertical flat surfaces, and horizontal pipe
applications.

Calculated results are given over a range of insulation types
and thicknesses, and include 1) surface temperature, 2) heat flow, 3) annual
cost of fuel, 4) payback period, 5) annualized rate of return, and 6) annual CO2
emissions.

The pipe spreadsheet estimates the heat flows through
horizontal piping. Information concerning a hypothetical insulation system
(e.g., the length of run, pipe size, operating temperature, ambient temperature
and wind speed, insulation material, and surface emittance of a proposed
insulation system) may be input by the user. Calculated results are given over
a range of insulation types and thicknesses, and include 1) surface
temperature, 2) heat flow, 3) annual cost of fuel, 4) payback period, 5) annualized
rate of return, and 6) annual CO2 emissions.

Economic Considerations – ROI

Insulation
systems are frequently designed with the objective of minimizing costs.
Properly designed systems can reduce heat loss (or gains) from (or to)
mechanical systems by 90-98%. When energy must be purchased to offset these
heat flows, insulation systems can quickly pay for themselves in reduced energy
costs.

Insulation projects (like many energy conservation projects)
generally involve making an initial investment that will result in future cost
savings. A number of approaches can be used to measure the financial
desirability of an insulation project. All require estimates of the initial
investment (the installed cost of the insulation system) and the resulting
future savings. Some of these financial measures are simple, like ROI and
Simple Payback Period. Others are more complicated and take into account the
time value of money, inflation, and taxes.

In today’s economic environment,
companies are requiring that investments be subject to review at various levels
in the approval process. The Mechanical Insulation Financial Calculator was
developed to provide a simple example of specific financial measures related to
investments in mechanical insulation. It can be used for an overall mechanical
insulation project or a simple, small mechanical insulation investment?such as
insulating a valve or replacing a section of insulation. Tax implications of
the investment have not been considered in the Financial Calculator. Users
should consult a financial advisor for specific and/or tailored financial
calculations
.

Economic Thickness Considerations

The
concept of economic thickness of insulation considers the initial installed
cost of the insulation system plus the ongoing value of energy savings over the
expected service lifetime. Economic thickness is defined as the thickness that
minimizes the total life-cycle cost (see Figure 1). The installed costs (labor
and material) of the insulation increase with thickness.

Insulation often is applied in multiple layers (1) because
materials are not manufactured in single layers of sufficient thickness, (2) to
accommodate expansion and contraction of insulation and system components, and
(3) to minimize thermal short circuits at joints. Figure 1 shows initial
installed costs for a multilayer application. The curve is discontinuous and
increases with the number of layers because labor and material costs increase
more rapidly as thickness increases. Figure 1 also shows the present value of
the “lost energy” cost over the expected life of the project, which decreases
as the insulation thickness is increased. The total cost curve, which is the
sum of the installed insulation cost and the present value of the lost energy
cost curves, shows a minimum value at point A. This point on the total cost
curve corresponds to the economic insulation thickness, which, in this example,
is in the double-layer range.

Initially, as insulation is applied, the total life-cycle
cost decreases rapidly because the value of incremental energy savings is
greater than the incremental cost of insulation. Additional insulation reduces
total cost up to a thickness where the change in total cost is equal to zero.
At this point, no further reduction can be obtained; beyond it, incremental
insulation costs exceed the additional energy savings derived by adding another
increment of insulation.

An economic thickness analysis should
consider the time value of money, which can be based on a desired rate of
return for the insulation investment. Energy costs are volatile, and a fuel
cost inflation factor is sometimes included to account for the possibility that
fuel costs may increase more quickly than general inflation. Insulation system
maintenance costs should also be included, along with any identifiable cost
savings associated with the ability to specify lower capacity equipment.

Any economic analysis must, by nature,
deal with quantifiable benefits and costs. Some benefits difficult to quantify
(e.g., thermal comfort of occupants, increased life of mechanical equipment,
reduced dependency on energy imports, and/or reduced emissions). Designers
therefore should consider the economic thickness of insulation to be a minimum.

Economic considerations also come into focus during the
owner/contractor “value engineering” (VE) process. Efforts by the contractor or
owner/developer to reduce the initial cost of a building or structure by
applying the principles of value engineering to the building design and
construction must be carefully weighed by the architect/engineer-of-record
against the best long-term interests of the owner/end-user. This is
particularly true with regard to mechanical insulation. In too many instances,
the term “value engineering” is used as a euphemism for cost reduction. Design
choices of insulation materials, thicknesses, and jacketing systems made by the
design professional are arbitrarily considered cost-prohibitive for a project
and are eliminated during the VE process. In some cases, the costs associated
with correcting problems that result from VE decisions can be significant, and
the work highly disruptive. The importance of documenting the criteria used in
the design, and highlighting these criteria during the VE process, is
emphasized.

Environmental Considerations – Sustainability

Along
with the benefits associated with conserving energy resources, insulation
contributes to the reduction in emissions associated with using those energy
resources. Insulation products are among the few building materials considered
to be “carbon negative,” in that reductions in carbon emissions over the life
of the materials far exceed the carbon emissions associated with manufacture,
transport, and installation of the insulation products.1 In
addition, well-insulated mechanical systems often can result in smaller 
equipment, which also contributes to sustainability.

Fire Safety

Mechanical
insulation materials often are used as a component in systems or assemblies
designed to protect buildings and equipment from the effects or spread of fire
(i.e., fire-resistance assemblies). Specific designs are tested and assigned
hourly ratings based on performance in full-scale fire tests. Note that
insulation materials alone are not assigned hourly fire-resistance ratings;
ratings are assigned to a system or assembly that may include specific
insulation products, along with other elements such as framing members,
fasteners, wallboard, etc.

Materials used to insulate mechanical equipment generally
must meet the requirements of local codes adopted by governmental entities
having jurisdiction over the project, so local code authorities should be
consulted for specific requirements. In the United States, most local codes
incorporate or are patterned after model codes developed and maintained by
organizations such as the National Fire Protection Association (NFPA) and the
more recent International Code Council (International Codes).

Fire-resistance ratings often are developed using ASTM E119.
This test exposes assemblies (walls, partitions, floor or roof assemblies, and
through-penetration fire stops) to a standard fire exposure controlled to
achieve specified temperatures throughout a specified time period. The
time-temperature curve is intended to be representative of building fires where
the primary fuel is solid, and specifies a temperature of 1,000°F at 5 min,
1,700°F at 1 h, and 2,300°F at 8 h.

Most codes related to insulation product fire safety refer to
the surface burning characteristics as determined by the Steiner tunnel test
(ASTM E84, NFPA 255, UL 723, or CAN/ULC S-102). These similar test methods
evaluate the flame spread and smoke developed from samples mounted in a 25 ft
long tunnel and subsequently exposed to a controlled flame. Results are given
in terms of flame-spread and smoke-developed indices.

For pipe and duct insulation products,
samples are prepared and mounted in the tunnel per ASTM E2231, which directs
that “the material, system, composite, or assembly tested shall be
representative of the completed insulation system used in actual field
installations, in terms of the components, including their respective
thicknesses.” Samples are constructed to mimic the products, as closely as
possible, the products as they will be used?including any facings and
adhesives, as appropriate.

Duct insulation generally requires a
flame-spread index of not more than 25 and a smoke-developed index of not more
than 50, when tested in accordance with ASTM E84. Codes often require
factory-made duct insulations (e.g., insulated flexible ducts, rigid fibrous
glass ducts) to be listed and labeled per UL 181, Factory-Made Air Ducts and
Air Connectors. This standard specifies a number of other fire tests (e.g.,
flame penetration and low-energy ignition) as part of the listing requirements.

Some building codes require that duct
insulations meet the fire-hazard requirements of NFPA 90A or 90B to restrict
spread of smoke, heat, and fire through duct systems; and to minimize ignition
sources.

For pipe insulation, the requirement is
generally a maximum flame-spread index of 25 and a maximum smoke-developed
index of 450. For insulation materials exposed within supply and return ducts
and plenums, the typical requirement is that the materials be noncombustible or
have a maximum flame-spread index of 25 and maximum smoke-developed index of
50.

Freeze Prevention

It
is important to recognize that insulation
retards heat flow; it does not stop it completely. If the surrounding air
temperature remains low enough for an extended period, insulation cannot
prevent freezing of still water or water flowing at a rate insufficient for the
available heat content to offset heat loss. Clean water in pipes usually
supercools several degrees below freezing before any ice is formed, then, upon
nucleation, ice forms in the water and the temperature rises to freezing. Ice
can be formed from water only by the release of the latent heat of fusion (144
Btu/lb) through the pipe insulation. Well-insulated pipes may greatly retard
this release of latent heat. Gordon’s research in 1996 showed that water pipes
burst not because of ice crystal growth in the pipe, but because of elevated
fluid pressure within a confined pipe section occluded by a growing ice
blockage.2 Insulation can prolong the time required for freezing, or
prevent freezing if flow is maintained at a sufficient rate.

Personnel Protection – Safety

In
many applications, insulation is provided to protect personnel from burns. In
addition, there are safety and comfort concerns related to personnel working in
high-temperature, high-radiant exposure locations.

The potential for burns to human skin is a complex function
of surface temperature, surface material, and time of contact. See ASTM
Standard Guide ASTM C1055 – 03(2009) for a discussion of these factors. ASTM
Standard Practice C1057 – 03(2010) provides a mathematical model for
“Determination of Skin Contact Temperature from Heated Surfaces….” Standard
industry practice is to specify a maximum temperature of 140°F for surfaces
that may be contacted by personnel. This temperature would cause no more than
first-degree burns for all surfaces for contact times up to about 5 sec. Note
that for non-industrial settings (e.g., retail stores) it may be prudent to
design for longer contact times (i.e., lower surface temperatures).

When evaluating an insulation system for the potential for
burn hazards, select the appropriate worst-case ambient condition. For indoor
applications, maximum air temperatures depend on the facility and location, and
are typically lower than for outdoor conditions. For outdoor installations,
base calculations on summer design ambient temperatures with no wind (i.e., the
worst case). Surface temperatures increase because of solar loading, but are
usually neglected because of variability in orientation, solar intensity, and
many other complicating factors. Engineering judgment must be used in selecting
ambient and operating temperatures, and wind conditions, for these
calculations.

The choice of jacketing
also strongly affects a surface’s relative safety. Higher-emittance jacketing
materials (e.g., plastic, painted metals) can be selected to minimize the
surface temperature. Jacketing material also affects the relative safety at a
given surface temperature. For example, at 175°F, a stainless steel jacket
blisters skin more severely than a nonmetallic jacket at equal contact time.

Process Control

Insulation systems often are designed to minimize
variation of temperatures in processes. A common example is the use of
insulation on tanks or vessels used as chemical reactors, where the yield of
the reaction is dependent on the temperature. Insulating the reactor helps to
isolate the reaction from variations in ambient temperature and minimizes the
capacity of process heating and/or cooling equipment.

Another example is the use of insulation
to minimize temperature change (drop or rise) of a fluid from one location to
another. Consider the case of a hot fluid flowing down a pipe or duct. Note
that, for a cold fluid flowing down a pipe, the temperature drop will have a
negative sign, which indicates a temperature rise in the fluid stream.

Noise Control

Noise Radiating from Pipes

Noise
from piping can be reduced by adding an absorptive insulation and jacketing
material. The expected level of noise reduction in the field can be estimated
from knowing knowing the sound insertion loss of insulation and jacketing
material combinations. A range of jacket weights and insulation thicknesses can
be used to reduce noise. Jackets used to reduce noise are typically referred to
as being mass-filled. Some products for outdoor applications use mass-filled
vinyl (MFV) in combination with aluminum.

Pipe insertion loss is a measurement (in dB) of the reduction
in sound pressure level from a pipe as a result of application of insulation
and jacketing. Measured at different frequencies, the noise level from the
jacketed pipe is subtracted from that of the bare pipe. The larger the
insertion loss number, the larger the amount of noise reduction.

ASTM Standard E1222 describes how to determine insertion loss
of pipe jacketing systems.

It is important that sound sources be well identified in
industrial settings, otherwise it is possible to treat specfic noise sources
effectively and have no significant influence on ambient sound measurement
after treatment. All sources of noise above desired levels should receive
acoustical treatment, beginning with the largest source.

Noise from Ducts

HVAC
ducts act as conduits for mechanical equipment noise and carry office noise
between occupied spaces. Additionally, some ducts can create their own noise
through duct wall vibrations or expansion and contraction. Lined sheet metal
ducts and fibrous glass rigid ducts can greatly reduce transmission of HVAC
noise through the duct system. The insulation also reduces cross-talk from one
room to another through the ducts.

Attenuation loss is noise absorbed within the duct. In
uninsulated ducts, it is a function of duct geometry and dimensions, as well as
noise frequency. Internal insulation liners generally are available for most duct
geometries. Chapter 47 in the 2007 ASHRAE Handbook?HVAC Applications provides
attenuation losses for square, rectangular, and round ducts lined with fibrous
glass, and gives guidance on the use of insulation in plenums to absorb duct
system noise. Internal linings can be very effective in fittings such as
elbows, which can have two to eight times more attenuation than an unlined
elbow of the same size. For alternative lining materials, consult individual
manufacturers.

Breakout noise is from vibration of the
duct wall caused by air pressure fluctuations in the duct. Absorptive
insulation can be used in combination with mass-loaded jacketings or mastics on
the duct exterior to reduce breakout noise. This technique is only minimally
effective on rectangular ducts, which require the insulation and mass composite
to be physically separated from the duct wall to be effective. For round ducts,
as with pipes, absorptive insulation and mass composite can be effective even
when directly applied to the duct surface.

For More Information

Additional information on these topics and more can be found
at www.wbdg.org/midg or www.insulation.org.

References

1.
Gordon, J., 1996. An investigation into freezing and bursting water pipes in
residential construction. Research Report 90-1. University of Illinois Building
Research Council.

2.
Russell, A., 2007, The Basics of Life Cycle Assessment and Insulation, ASTM C
16 Forum. April 2007.