Download (Revised) Text for Sections 5.7 to 5.9 5.7 WHERE AND HOW MUCH

(Revised) Text for Sections 5.7 to 5.9
5.7
WHERE AND HOW MUCH TO INSULATE
The discussion of the thermal properties presented so far leads to two questions:
• Where to insulate a building
• How much insulation (R-value or U-value) to use.
Question 1—where to insulate—is easily answered. Obviously, the insulation is required only
in those components of the building through which heat flow would occur. These include all
components of the building envelope: exterior walls, roof, floor above an unconditioned
basement or crawl space, a concrete slab supported on ground, the walls of a conditioned
basement, and so on, as shown in Figure 5.20. Interior walls and floors need no insulation unless
they are exposed to an unconditioned space.
Question 2—the amount of insulation required—is a function of two factors:
•
Economics and sustainability concerns
•
Climate of the location.
Economics and sustainability: From a purely economic standpoint, the amount of insulation
required in the envelope is a function of the price of energy and the cost of providing and
installing the insulation. If the energy prices are low, the cost of heating and/or cooling a
building is low. In that case, a lower level of insulation is economical. On the other hand, if
energy prices are high, a greater amount of insulation is justified. This is precisely why the
levels of insulation in buildings increased after the abrupt rise in world energy prices in 1973.
Another equally important role of insulation is to improve the sustainability of buildings because
increased levels of insulation generally conserves energy in heating and cooling of buildings,
thereby reducing air pollution, global warming and the buildings’ carbon footprint (see Chapter
10).
Climate of the location: Climate affects the insulation requirement in the following way. In a
severe climate, a greater amount of insulation is needed than in a moderate climate. In a
temperate climate, little or no insulation may be required. From an economic standpoint again,
there generally is an optimum level of envelope insulation for a particular climate. This is
defined as that level of insulation at which the amount saved in energy consumption over the
entire life of the building exceeds the cost of providing and installing the insulation.
For determining the insulation requirements of a building envelope, two climate parameters of
the location are used: (a) exterior air temperature and (b) the exterior humidity level. Based on
the exterior air temperature, the United States has been divided into eight temperature zones,
ranging from Zone 1 to Zone 8. Zone 1 represents a location that is warm throughout the year
(and hence cooling dominated) and Zone 8 refers to a location that is subarctic (hence heating
dominated). Zones 2 through 7 fall within the two extremes represented by Zones 1 and 8.
The energy use in a building is not only dependent on the exterior air temperature of a location
but also on exterior humidity level. To account for exterior humidity, the United States has been
divided into three humidity divisions: Division A (Moist), Division B (Dry) and Division C
(Marine). Temperature zones and humidity divisions have been used by the United States
Department of Energy (DOE) to develop a comprehensive climate zone map of the United
States, Figure 5.21. The DOE’s (hygro-thermal) climate zone map is referenced by the codes to
prescribe the thermal properties of a building’s envelope and other energy efficiency
requirements.
ENERGY CONSERVATION CODE
An important document regulating energy efficiency in buildings in the United States is the
International Energy Conservation Code (IECC). This comprehensive document deals with all
aspects of energy use in buildings, including the amount of insulation in the opaque (unglazed)
portion of building envelope, thermal properties of fenestration (windows and doors), air
infiltration, lighting levels, efficiency of HVAC and other energy-consuming equipment, and so
on.
The discussion presented here focuses on residential occupancies (up to 3 stories in height).
(Interestingly, more energy is used in residential buildings than in commercial and industrial
buildings combined.) Additionally, our discussion in this chapter is limited to the amount of
insulation in opaque (nonglazed) portions of building envelope, a critical factor affecting energy
use in residential buildings. IECC’s requirements for other parts of the building envelope (doors,
windows, glazing, and so on) are covered in Chapters 28, 29 and 30.
Insert NOTE (existing on page 116 of the Text) “Energy Use in the U.S.” here.
The corresponding role of the envelope in commercial (or industrial) buildings is relatively
less critical because their energy use tends to be intermittent and is impacted more by lighting,
occupant load, manufacturing processes, and so on. Therefore, the IECC provisions for envelope
insulation in non-residential buildings are less stringent than those for residential buildings.
However, because of the architectural complexity of commercial and industrial buildings, energy
performance modelling software is generally required in the design of most such buildings.
INSULATION REQUIREMENTS FOR RESIDENTIAL BUILDINGS
The code requirements for minimum R-values of opaque (non-glazed) portions of walls, roofs
and floors of residential buildings (constructed of wood frame), as a function of climate zones,
are given in Figure 5.22. Note that the R-values given refer only to wall insulation. They do not
include the R-value contribution from other components of the wall such as gypsum board,
interior and exterior surface resistances, etc.
Note also that the code does not require a higher R-value of the opaque portions of walls if the
fenestration area (window and door areas) in the walls is large. That is, the prescribed R-value
of the opaque portions of walls is independent of the wall fenestration, as long as the fenestration
meets the maximum U-value (minimum R-value) and other required thermal properties.
Similarly, the required R-value of a residential roof-ceiling assembly is also independent of the
area of skylights.
In other words, the code provisions for residential buildings do not consider the effect of
thermal bridging caused by fenestration, which is generally of much lower R-value than the
opaque parts of walls or roofs. This apparent anomaly was introduced to simplify the code’s
enforcement. (Simpler though less stringent code provisions lead to better enforcement, yielding
greater energy efficiency.) Examples 6 and 7 illustrate the use of Figure 5.22.
Example 6
(Minimum Required R-Value of Insulation in Walls)
Determine the minimum required R-value of insulation in cavities between the studs of a woodframed single-family dwelling in Chicago, Illinois. The area of the opaque portions of the walls
is 1,500 ft2. The total glazed area is 300 ft2.
Solution
Chicago is in Climate Zone 5. The minimum required R-value of insulation in wall cavities for
Climate Zone 5 is R-20, Figure 5.22. The relative areas of glazing and opaque wall are
unimportant, as previously explained. However, the code’s provisions for maximum U-value of
glazing (and other thermal properties of fenestration) must be met. (We will see in Chapter 29
that the maximum permissible U-value of glazing for Climate Zone 5 is 0.35.)
Example 7
(Minimum Required R-Value of Insulation in Roof-Ceiling Assembly)
Determine the required minimum attic (roof-ceiling assembly) insulation for a wood-framed
single-family dwelling in Chicago, Illinois.
Solution
From Figure 5.22, the minimum R-value of attic insulation for Climate Zone 5 is R-38.
Insulation in Floors Over an Unconditioned Basement or Crawl Space
A floor over an unconditioned basement or unconditioned crawl space should be insulated. For a
wood joist floor, the insulation is placed between joist cavities in full contact with the underside
of subfloor, Figure 5.23. Figure 5.24 gives the minimum R-value of such floor insulation.
Insert Figure 5.23 (which is existing as Figure 5.20 on p. 119 of Text) here.
Insulation in Walls of a Conditioned Basement or Crawl Space
The walls of a conditioned basement must be insulated throughout the basement wall’s height,
but the insulation need not extend more than 10 ft below ground. Depending on construction
type, the insulation may be placed within wall’s framing cavities, toward the interior or the
exterior of a concrete or concrete masonry wall.
The walls of a conditioned (unventilated) crawl space should be insulated similarly, and
extend from the floor overhead to the crawl space ground and then at least an additional 2 ft
vertically into the ground or horizontally by the same amount. The minimum R-values of
basement and crawl space wall insulation are given in Figure 5.25. With basement or crawl
space wall insulation provided, there is obviously no need to insulate the floor above.
INSULATION UNDER A CONCRETE SLAB-ON-GROUND
If the floor is a ground-supported concrete slab (a slab-on-ground, also called a slab-on-grade),
it needs to be insulated only along its exposed perimeter. There is no need to place insulation
under the entire slab because the heat loss from a slab-on-ground occurs only from the exposed
perimeter of the slab. The portion of the slab away from the exposed perimeter does not
contribute much to heat loss, as explained shortly.
From Section 5.3, the thermal resistance of an element is directly proportional to its thickness
L. In fact, the quantity L in Figure 5.6 refers to the length of the heat-flow path through the
element. The longer the path, the greater its resistance to heat flow.
A simplified visualization of heat loss from a slab-on-ground shows that there are three
possible paths along which heat may flow from the slab:
• Vertical path—through the thickness of the slab and into the ground,
• Horizontal path through the slab
• Through the ground paths (shown by curved lines in Figure 5.26)
Insert Figure 5.26 (which is existing Figure 5.21 on page 120 of
Text) here
The vertical path represents an extremely long path (as long as the diameter of the earth) and,
therefore, an extremely large resistance to heat flow, implying that almost no heat will flow in
that direction. Heat losses through the other two paths are functions of their respective lengths.
Because the region of the slab away from an exposed edge has long path lengths, it does not
contribute much to heat loss from the slab. Only the region that is close to the exposed edges
affects heat loss. Therefore, a slab-on-ground can be adequately insulated around its exposed
perimeter. Very little is gained by insulating under the entire slab.
Insert NOTE (existing on p. 120 of Text “Frost-Protected Shallow Foundations” here.
Insert (new) NOTE “Properties of Below-Ground Insulation” here.
The standard practice is to insulate under the slab with either horizontal or vertical insulation,
or a combination of both (vertical insulation is generally preferred). Two commonly used details
for insulating a slab-on-ground are shown in Figure 5.27. In both details, the location of the
insulation is meant to insulate the horizontal and curved heat flow paths. The minimum R-value
of the insulation is a function of the climate zone of the location, as given in Figure 5.27. Note
that insulation under a slab-on-ground is not required in climate zones 1 to 3.
Insert Figure 5.27 (which is existing Figure 5.22 on page 121 of Text) here.
Delete existing table and insert new table in this figure entitled “Revised Table for Fig.
5.27”.
5.8 THERMAL CAPACITY
The concepts of heat flow and the related thermal properties of materials discussed in earlier
sections apply to steady-state heat transfer. A steady state is defined as the state in which the
temperatures at all points within an assembly remain constant over time. The opposite of a steady
state is an unsteady state, more commonly referred to as a dynamic state.
With particular reference to the building envelope, a steady state implies that the temperatures
on both sides of the envelope are constant over time. In practice, a perfect steady state does not
occur because although the inside air temperature may be kept constant by heating or cooling,
the outside envelope temperature varies over time.
The outside temperature of the envelope is a function of the outside air temperature and the
intensity of solar radiation. On days when the sun’s rays are absent—that is, when the sky is
cloudy—the diurnal variation in the outside temperature is small. Thus, an approximation of a
steady state is obtained when: (1) the indoor temperature of the building is kept constant and (2)
the outdoor temperature registers little daily variation.
A dynamic state is more common than a steady state. In the dynamic state, the temperature at
a point within a component varies with time due to the variation of outside air and surface
temperatures. This occurs in climates where solar radiation intensities are high, resulting in large
daily and seasonal variations in the outside air temperature.
The distinction between steady and dynamic states has an important bearing on a building
envelope’s thermal properties. The property of the envelope that governs heat transfer under a
steady state is its U-value (or R-value). In other words, under a steady state, it is the insulation in
the envelope that governs the heat flow. Under a dynamic state, both the U-value (insulation) and
the envelope’s ability to store heat come into play, Figure 5.28.
Insert Figure 5.28 (which is existing as Figure 5.23 on p. 121 of Text) here.
THERMAL CAPACITY (THERMAL MASS) OF A COMPONENT
The ability of a component to store heat is referred to as its thermal capacity (TC), which is
defined as the amount of heat needed to raise the temperature of 1 sq. ft of the component by
1F. The greater the thermal capacity of the component, the greater the amount of heat it will
absorb for a given rise in its temperature.
Thermal mass and thermal inertia are other terms that are used synonymously for thermal
capacity. Dense materials, such as concrete, brick, stone, and adobe, have a higher thermal mass
than lightweight materials, such as wood, plastics, and insulating materials.
THERMAL CAPACITY VERSUS INSULATION
Because both thermal capacity and insulation affect heat transfer through the envelope, the
question arises as to the relative importance of thermal capacity and insulation (U-value) of the
envelope. It has been shown that the amount of energy consumed in heating or cooling buildings
is almost independent of the thermal capacity of the envelope, provided the heat flow through the
envelope remains unidirectional—that is, if heat flows from the inside to the outside or from the
outside to the inside.
Under unidirectional heat flow, only the U-value of the envelope (i.e., the insulation)
determines the energy consumption of a building. The envelope’s thermal capacity plays an
insignificant role under unidirectional heat-flow conditions, even when unidirectional heat flow
is dynamic. On the other hand, if heat-flow direction changes over time (from the outside to the
inside for some time and from the inside to the outside at other times), both the U-value and
thermal capacity of the envelope determine energy consumption. The statement of Figure 5.28
can, therefore, be modified as shown in Figure 5.29.
Insert Figure 5.29 (which is existing as Figure 5.24 on p. 122 of Text) here.
An obvious example of unidirectional heat flow is a cold storage room, where the heat flow is
generally from the outside to the inside. Thus, in cold storage buildings, the envelope’s
insulation determines energy consumption. In extremely cold climates, the heat-flow remains
unidirectional (heat flows from the inside to the outside) for most of the year. Here also, the
insulation of the envelope is significant. Thermal capacity plays an insignificant role in such
climates.
In some climates, where the diurnal temperature swing is extreme (say, 90F day-time
maximum and 50F night-time minimum), the heat-flow direction changes in a 24-h period.
Thermal capacity plays a significant role in such climates. A high thermal capacity will stabilize
indoor temperature within the comfort range, and insulation may not be required at all in a welldesigned building.
ENERGY CODE AND THERMAL CAPACITY
The codes takes into account the beneficial role of thermal capacity by requiring low R-values of
walls with high thermal capacity, called mass walls. For example, the minimum R-value
required of mass walls in climate zones 1 to 3 (warm climates where solar radiation produces
dynamic exterior thermal conditions) is much smaller than lightweight (wood and light-gauge
steel frame) walls.
In Climate Zones 4 to 6, the difference between the minimum required R-values for mass
walls and light-weight walls decreases progressively. For Climate Zones 7 and 8, the difference
between the two wall types is insignificant. In other words, mass walls in Climate Zones 7 and 8
do not contribute to thermal efficiency of the envelope. A mass wall is defined by the Code as a
wall made of concrete blocks, brick, stone, adobe, heavy timber logs, etc.
Insert (NEW) NOTE “Alternatives to Prescriptive Envelope Properties” here.
5.9 THE MOST EFFECTIVE FACE OF THE ENVELOPE FOR
INSULATION
The relative locations of insulation and thermal capacity in the envelope are also important.
Placing the insulation on the outside of the assembly is more effective in reducing energy
consumption than if the insulation is placed in the middle of the envelope section or toward the
interior of the building because it eliminates or reduces thermal bridging.
The placement of high-thermal-capacity (heavyweight) materials on the interior side of the
envelope helps to moderate interior temperature fluctuations. This is valid in buildings that are
continuously heated or cooled. For a building that is heated or cooled intermittently, such as a
church hall, the placement of high-thermal-capacity material on the interior of the building
envelope implies that it will take longer to achieve comfort conditions.
PRACTICE QUIZ
Each question has only one correct answer. Select the choice that best answers the question.
28. The U.S. Department of Energy’s Climate Zones are based on
a. the exterior air temperature.
b. the exterior humidity.
c. precipitation.
d. (a) and (b).
e. (b) and (c).
29. The coastal region of the Western United States lies in
a. Climate Zone 1(A).
b. Climate Zone 2(A).
c.
Climate Zone 3(A).
d. Climate Zone 4(A).
e.
none of the above.
30. The U-value and R-value of an assembly are related to each other by which relationship?
a. U = R
b. U + R = 1
c. (U)(R) = 1.0
d. U2 + R2 = 1.0
e. None of the above
31. A building assembly with a higher U-value is thermally more insulating than one with a
lower U-value.
a. True
b. False
32. Which of the following metal stud wall assemblies gives a higher overall R-value? In both
assemblies the total amount of insulation is R-15.
a. Assembly A has an exterior insulating sheathing of R-5 and within-stud insulation of R10.
b. Assembly B has an exterior insulating sheathing of R-3 and within-stud insulation of R12.
33. In a cold climate, a concrete slab-on-ground must be insulated under its entire area.
a. True
b. False
34. The walls of a heated basement must be insulated over the entire depth of the wall.
a. True
b. False
35. In which of the following buildings will heat flow occur unidirectionally?
a. Continuously heated or cooled buildings
b. Intermittently heated buildings
c. Intermittently cooled buildings
d. All the above
e. None of the above
36. For energy conservation, materials with a high thermal capacity are effective in climates in
which the heat flow through the building envelope is
a. bidirectional within a 24-hour period during a large part of the year.
b. unidirectional within a 24-hour period during a large part of the year.
c. absent during a large part of the year.
37. Insulation is most effective if it is placed
a. on the outside face of the assembly.
b. on the inside face of the assembly.
c. in the center of the assembly.
Answers: 28-d, 29-e, 30-c, 31-b, 32-a, 33-b, 34-b, 35-a, 36-a, 37-a.