Insulation For Plastic Piping: How Much is Needed?

Christopher P. Crall

September 1, 2012

Introduction

Plastic piping for
domestic hot and cold service water systems, and for HVAC systems in buildings,
has been in use for many years and has become the dominant piping material for
residential construction. One source1 estimates that plastic pipe
systems are currently used in 75% of the potable piping systems in new
residential construction, and this number is predicted to rise to 80% by 2015.
Plastic piping is also used routinely in commercial and industrial
applications.

Compared to metallic piping systems, plastic piping materials
have a significantly lower thermal conductivity, which translates to lower heat
transfer between the fluid and the ambient air. For some piping applications,
this can be advantageous. For example, city water lines entering a building
will often sweat due to the relatively cold temperature of the water entering
the building. Depending on the ambient conditions, plastic piping may minimize
or eliminate the surface condensation and associated dripping from cold water
pipes. However, when insulation is required by energy codes, the impact of the pipe wall material on the overall heat
transfer is generally small. For this reason, energy codes do not differentiate
insulation requirements based on pipe wall material.

How much insulation is needed on
plastic pipe? As is often the case, the answer depends primarily on the design
objectives. There are a number of reasons to insulate piping. The Mechanical
Insulation Design Guide lists seven design objectives: Condensation Control,
Energy Conservation, Fire Safety, Freeze Protection, Personnel Protection,
Process Control, and Noise Control.2

Often, designers are faced with multiple design objectives
(e.g., energy conservation and fire safety). The amount of insulation required
depends on the design objectives and the specifics of the application. In some
cases (e.g., condensation control or freeze protection) plastic piping may need
no insulation. In other situations, additional insulation may be required,
relative to metallic piping. Requirements need to be determined on a
case-by-case basis by analyzing the expected operating conditions.
Significantly, when the objective is energy conservation (i.e., compliance with
energy codes and standards), plastic piping generally requires the same amount
of insulation as metallic pipe.

Plastic Piping Materials

A number of different
plastic materials are used in piping systems, including:

  • ABS
    (Acrylonitrile Butadiene Styrene)

  • CPVC
    (Chlorinated Polyvinyl Chloride)

  • PB
    (Polybutylene)

  • PE
    (Polyethylene)

  • PEX
    (Cross-linked Polyethylene)

  • PP
    (Polypropylene)

  • PVC
    (Polyvinyl Chloride)

  • PVDF
    (Polyvinylidene Fluoride)

These plastics have various properties that make them more or
less appropriate for various applications. A key property for hot systems is
maintaining strength at high temperatures. Since all plastics lose strength as
temperature increases, the use of plastic piping is limited to operating
temperatures less than 220°F. For domestic hot and cold water piping systems,
CPVC and PEX are the most common materials. For chilled water distribution
piping, many different materials may be used. 

When it comes to limiting the transfer of heat, the key
factors are thermal conductivity and the wall thicknesses of pipe products. As
expected, the thermal conductivity of plastic pipe materials varies. Table 1 is
extracted from various sources and shows the range of conductivity values
reported in the literature. Values range from a low of 0.8 Btu?in./(h?ft.2?°F)
for PVDF to a high of 3.2 Btu?in./(h?ft.2?°F) for PEX. For
comparison purposes, the conductivity of copper is approximately 2,720
Btu?in./(h?ft.2?°F) at a temperature of 75°F; while steel has a
conductivity of approximately 314 Btu?in./(h?ft.2?°F).

Plastic piping is manufactured to a number of different size
standards. CPVC is available in either nominal pipe sizes (NPS) from ¼” to 12″
or in copper tube sizes (CTS) from ¼” to 2″. NPS sizes are available in either
Schedule 40 or Schedule 80 wall thicknesses. CTS sizes for wall thickness have
a standard dimension ratio (SDR) of 11 (i.e., the outside diameter is 11 times
the wall thickness).3

PEX is available in CTS sizes from ¼” to 3″ with SDRs of
approximately 9. Dimensions used in this study were taken from the National
Association of Home Builders (NAHB) Research Design Guide “Residential PEX
Water Supply Plumbing Systems.4

Heat Transfer Calculations

The data from Table 1 demonstrates that
the thermal conductivity of metallic piping is 30 to 3,000 times higher than
typical plastic piping materials. However, the impact on heat transfer to or
from the fluid will depend not only on the relative thermal resistances of the
pipe wall, but also on the other thermal resistances in the system. For bare
piping, the air surface coefficient normally represents the largest thermal
resistance in the system. The wind speeds at the surface, along with the
thermal emittance of the surface material, are dominant. As insulation is added
to the system, the resistance of the insulation layer begins to dominate and
other resistances become less important. Figure 1 compares the heat loss from a
horizontal 2″ tube containing water at 140°F in still air at 75°F. For the bare
case, the heat loss from the CPVC tubing is significantly less than the copper
tubing. At insulation thicknesses above ½”, the difference in the heat loss
becomes small. Flexible elastomeric insulation was assumed for this example.

The relative magnitude of these effects
will vary according to the situation, but they can be estimated using
well-established calculation procedures. The procedures for these calculations
are outlined in ASTM Standard C 6805 and in many heat transfer
textbooks.
 

A few example applications were
selected to help illustrate the relationships.

All of these examples compare thin-walled (Type M) copper tubing to
standard-size CPVC and PEX tubing. These materials were chosen because,
together, they represent the largest share of products in the marketplace and
because they effectively span the range of thermal conductivities for piping.
Table 2 shows the conductivities and surface emittance used in this analysis.

Example 1 assumes a 2″ CTS domestic hot
water (DHW) line located within a commercial building. The operating
temperature of this line is 140°F and the ambient conditions are assumed to be
75°F with 0 mph wind speed. For calculation purposes, the insulation material
is flexible elastomeric insulation (ASTM C 534 Grade 1). The 2012 International
Energy Conservation Code (2012 IECC) Energy Code requirement for this
application calls for 1″ of insulation. Calculated heat losses per foot of piping
run are summarized in Table 3.

Example 2 involves a 1″ CTS heating hot
water (HHW) line in a commercial building. The line operates at a temperature
of 180°F and runs through a return-air plenum with an air temperature of 75°F
and an air velocity of 3 mph. For this example, we will use fiberglass
insulation (ASTM C 547 Type I). The 2012 IECC insulation requirement for this
application is 1 ½.” Results of the calculations are shown in Table 4.
 

Example 3 is a 2″ CTS chilled water
supply (CWS) line operating in a mechanical room of a commercial building. The
operating temperature is 40°F; the ambient temperature is 80°F; and the wind
speed is 1 mph. The insulation material is flexible elastomeric insulation
(ASTM C 534 Grade 1). The 2012 IECC insulation thickness requirement for this
application is 1″. Results from this example are shown in Table 5.

Results for all three of these examples are similar and
reveal the following important points:

  • As expected, the heat loss or
    gain depends on both the thickness of the insulation as well as the choice of
    the pipe material. However, the effect of insulation thickness is considerably
    more significant than the choice of pipe material. In Example 1, adding 3/8 ”
    of insulation to the bare copper line reduces the heat loss by 61%; while
    changing the “bare pipe” material from copper to CPVC reduces the heat loss by
    21%.

  • For bare piping, the effect of
    base pipe material on heat flow is significant. The largest effect is for the
    CPVC cases (as CPVC has the lower thermal conductivity). Compared to the copper
    case, the CPVC cases show reductions of heat flow of 21%, 34%, and 27% for the
    three examples, respectively. Reductions for the PEX case have a smaller effect
    and average an 8% reduction of heat flow. For the still air case, the lower
    emittance of the copper surface (Ɛ=0.6) contributes some thermal resistance relative to the
    plastic cases (
    Ɛ=0.9).

  • The impact of the base material
    decreases as the amount of insulation increases. In Example 1 with 1 inch of
    insulation, the heat loss for the CPVC material is 7% less than the comparable
    copper case. At 2″ of insulation, the difference is below 5%. Considering all
    three examples, the impact at 2″ of insulation averages 4.4%

  • Based on these examples, trading
    insulation thickness for lower conductivity pipe material would not work. In
    Example 1, at the code-required insulation thickness of 1″, the heat loss for
    the copper pipe system is 12.2 Btuh/ft. The alternate design of CPVC with ¾” of
    insulation (the next smaller increment for this insulation material) yields a
    higher heat loss of 12.9 Btuh/ft. Examinations of the other cases yield a
    similar conclusion: Plastic pipe lowers the heat flow, but not enough to
    justify removing an increment of insulation.

Energy Code Requirements for Piping

All
of the current model energy codes contain insulation requirements for domestic
hot water and HVAC piping. Although the details vary somewhat, the requirements
are generally given as a minimum insulation thickness without regard to pipe
material. As an example, requirements for service water heating from the 2012
IECC are given in Section C 404.5 and read as follows:

C404.5 Pipe insulation. For automatic-circulating hot
water and heat-traced systems, piping shall be insulated with not less than 1
inch (25 mm) of insulation having a

conductivity not exceeding 0.27 Btu ?inch/(h?ft2?°F).
The first 8 feet (2438 mm) of piping in non-hotwater-supply temperature
maintenance systems served by equipment without integral heat traps shall be
insulated with 0.5 inch (12.7 mm) of material having a conductivity not
exceeding 0.27 Btu?in./(h?ft2?°F).

The only qualifier here is that the insulation has a
conductivity not exceeding 0.27 Btu?in./(h?ft.2?°F). Insulation thickness requirements are the same
whether the base material is copper, schedule 40 steel, schedule 80 stainless
steel, CPVC, or PEX. While the choice of base material will impact the heat
loss or gain of insulation systems, the effect is relatively small for
insulated piping. 

The 2012 IECC requirements for piping for HVAC systems in
commercial buildings are summarized in Table 6. The thickness requirements here
are differentiated by operating temperature and by nominal pipe or tube size.
As before, the thickness requirements are not differentiated by pipe
base material or wall thickness. 

Thickness requirements are again
independent of the insulation material, as long as the conductivity of the
material falls within the specified range. If the conductivity of the
insulation layer is outside the specified range, the required insulation
thickness must be adjusted based on the equation in Table 6’s footnote b. Note
that since the emittance of the outer surface is not addressed in Table 6, the
thickness requirements are independent of the outer jacket material as well.

The code requirements for piping do not address some other
system variables known to impact thermal performance. For example, thickness
requirements are independent of location within the building. While it can
certainly be argued that hydronic piping to a reheat coil routed through a
return air plenum, where moving air is increasing heat loss, should have more
insulation than a similar line run through a closed cavity in still air, energy
codes do not require different insulation thicknesses. 

When considering these energy code requirements, they may
appear to be overly simplistic. However, one of the goals of code writing organizations
is to state the requirements as simply as possible, while still meeting the
intent of the code. Buildings are complicated, with literally thousands of code
requirements subject to verification. A good code requirement must be simple
and easily verifiable.

While the 2012 IECC minimum thickness
requirements for pipe insulation are not dependent on pipe material, it is
recognized that code officials may be receptive to alternatives based on a
technical analysis demonstrating that the thermal performance of an alternative
design is as good as or better than a baseline case meeting the code. For
example, the ASHRAE 90.1-2010 Standard (which formed the basis for the 2012
IECC requirements) has a footnote to the requirement table:

The table is based on steel pipe. Non-metallic pipes
Schedule 80 thickness or less shall use the table values. For other
non-metallic pipes having thermal resistance greater than that of steel pipe,
reduced insulation thicknesses are permitted if documentation is provided
showing that the pipe with the proposed insulation has no more heat transfer
per foot than a steel pipe with insulation shown in the table.

This specifically provides flexibility to designers to use
thick-walled plastic piping with reduced levels of insulation, provided the
alternative design has no more heat transfer than the baseline design.

A number of “Green Codes” or “Stretch
Codes” have been developed with the intent of going beyond the minimum
requirements in base codes. These model codes are available for jurisdictions
or owners who desire improved building performance. Examples include the
International Green Construction Code (IgCC), the International Association of
Plumbing and Mechanical Officials (IAPMO) “Green Plumbing and Mechanical Code
Supplement,” and ASHRAE Standard 189.1-2011
“Standard for the Design of
High-Performance Green Buildings.” While none of these model codes specifically
call out exceptions for insulation on plastic piping, alternative designs are
generally allowed if justified by technical analysis. The wording in section
102.1 of the IAPMO Green Supplement is typical:

102.1 General. Nothing in this supplement is intended to
prevent the use of systems, methods, or devices of equivalent or superior
quality, strength, fire resistance, effectiveness, durability, and safety over
those prescribed by this supplement. Technical documentation shall be submitted
to the Authority Having Jurisdiction to demonstrate equivalency. The Authority
Having Jurisdiction shall have the authority to approve or disapprove the
system, method, or device for the intended purpose.


Conclusion

All current building
energy codes and standards require pipe insulation on service hot water and
HVAC piping. Requirements vary, but none of the model codes differentiate pipe
insulation requirements based on pipe material.

For uninsulated or bare pipe, the
higher thermal resistance of the plastic pipe walls can significantly reduce
heat flow (by roughly 30%) compared to copper piping. As insulation levels are
increased, the impact of pipe wall resistance decreases significantly. At the
insulation levels required by current energy codes and standards, the impact of
pipe wall material on overall heat transfer is small.

In some applications (e.g., condensation control or freeze
protection), the lower conductivity of plastic compared to metallic piping
materials could be advantageous and may obviate additional thermal insulation.
For other applications, additional insulation may be required, depending on the
design objectives and the specifics of the situation.

Thermal insulation for mechanical
systems has proven to be a simple and cost-effective technology for reducing
heat losses and gains in building systems. As energy codes and regulations
(both prescriptive and holistic), become more stringent and building owners,
operators, and tenants strive for higher performing and more sustainable
buildings, designers should be focusing on how and where to use more, not less,
insulation. For example, some designers are considering using pipe insulation
to conserve scarce water resources, as well as energy, in domestic hot water
delivery systems.6 Since the expected useful life of buildings can
be 50 years or more, it is significantly easier and more cost-effective to plan
for and install proper mechanical insulation systems at the time of
construction than to retrofit or upgrade the insulation systems later.
Likewise, when facilities are being renovated or repaired, the opportunity to
upgrade mechanical insulation systems should not be overlooked. Efforts to
sacrifice mechanical insulation levels to minimize initial costs are
counterproductive, and building owners would be better served to focus on
examining the long-term performance of building systems.

This article was developed by the National Insulation
Association (NIA) and the North American Insulation Manufacturers Association
(NAIMA).


References:

  1. Barrett, Stephen R.
    “Potable and Process Pipe and Fitting Advances Utilizing Radio
    Frequency Fusion Welding.” IAPMO Emerging Technologies Symposium, May 1, 2012

  2. National Institute of
    Building Sciences, “The Mechanical Insulation Design Guide,” www.wbdg.org/design/midg.php

  3. Plastic Pipe and Fitting
    Association, “Installation Handbook: CPVC Hot & Cold Water Piping,” 2002.

  4. NAHB Research Center,
    “Design Guide: Residential PEX Water Supply Plumbing Systems,” November 2006.

  5. ASTM C680-10, “Standard
    Practice for Estimate of the Heat Gain or Heat Loss and the Surface
    Temperatures of Insulated Flat, Cylindrical, and Spherical Systems by Use of
    Computer Programs.” ASTM International, West Conshohocken, Pennsylvania. 2010.

  6. Klein, G., “Hot Water Distribution Research,” Insulation
    Outlook,
    December 2011.