Thermal Insulation

With the many improvements being made in heating and air conditioning equipment for residences, commercial and office buildings, hotels, industrial buildings and the like, it is becoming increasingly important for architects and engineers to have ready access to information on the thermal insulation properties of building materials. Since the mechanical engineer designs the heating and cooling plant on the basis of the total hourly heat transmission through the exterior parts of the building, optimum efficiency in economical structural design requires analysis of relative heat loss through the elements of the structure.

Since thermal insulation varies inversely with unit weight, lightweight concrete has better thermal insulation properties than normal-weight concrete. The thermal conductivity, k, for lightweight concrete is usually between 2.5 and 3.6 Btu per hour per inch thickness per square foot per degree Fahrenheit for oven-dry concretes weighing 80 to 100 Ibs. per cubic foot. The corresponding k value for normalweight concrete is generally between 9.0 and 12.0, depending upon the unit weight of the concrete.

The table below shows R-values for Solid Concrete (Normally Dry)

TABLE 2A. R–values - Solid Concrete (Normally Dry)*

Insulating
Lightweight



Structural
Lightweight



Structural
Normal-
Weight

  20
  30**
  40
  50
  60
  70
  80
  90
 100
 110***
 120
 130
 140
 145****
 150

 1.43 
 1.00 
  .83 
  .67 
  .52 
  .45 
  .37 
  .30 
  .24 
  .19 
  .14 
  .11 
  .083
  .075
  .065

 2.86
 2.00
 1.66
 1.34
  .04
  .90
  .74
  .60
  .48
  .38
  .28
  .22
  .16
  .15
  .13

 5.72
 4.00
 3.32
 2.68
 2.08
 1.80
 1.48
 1.20
  .96
  .76
  .57
  .43
  .33
  .30
  .26

 8.58
 6.00
 4.98
 4.02
 3.12
 2.70
 2.22
 1.80
 1.44
 1.13
  .86
  .65
  .50
  .45
  .39

 11.44
  8.00
  6.64
  5.36
  4.16
  3.60
  2.96
  2.40
  1.92
  1.51
  1.14
   .87
   .67
   .60
   .52

 14.30
 10.00
  8.30
  6.70
  5.20
  4.50
  3.70
  3.00
  2.40
  1.89
  1.43
  1.09
   .83
   .75
   .65

 17.16
 12.00
  9.96
  8.04
  6.24
  5.50
  4.44
  3.60
  2.88
  2.27
  1.71
  1.31
  1.00
   .90
   .78

Conductivity, (k), Btu in./hr. sq. ft. ·F R(l/k)

NOTE: Typical increases in conductivity and unit weight, from the PCA Development Bulletin D114. General Relation of Heat Flow Factors to the Unit Weight of Concrete.

Thermal Insulation of Concrete Masonry

Concrete masonry is energy efficient. A National Bureau of Standards study reveals that masonry out–performs other systems in dynamic heating and cooling conditions more effectively than steel stud. Masonry's mass delivers vital economy by reducing heat loss in cold weather and heat gain in warm weather. Its inherent thermal storage qualities assist in reducing heating and cooling loads, thus providing substantial long-term savings. The ability of concrete masonry backup to conserve energy is further supported by the fact that insulation utilized in conjunction with a masonry system resists moisture more effectively than the fiberglass batt insulation used in a steel stud system.

The tables below show R-values for Hollow Concrete Block Walls

TABLE 1A. Resistance R-values for Hollow Concrete Block Walls*

 60
 80
100
120
140

 3.36
 2.79
 2.33
 1.92
 1.14

 2.07
 1.68
 1.40
 1.17
 0.77

 5.59 
 4.59
 3.72
 2.95
 1.59

 2.25
 1.83
 1.53
 1.29
 0.86

 7.46
 6.06
 4.85
 3.79
 1.98

 2.30
 2.12
 1.75
 1.46
 0.98

 9.35
 7.46
 5.92
 4.59
 2.35

 3.0
 2.40
 1.97
 1.63
 1.08

10.98
 8.70
 6.80
 5.18
 2.59

 3.29
 2.62
 2.14
 1.81
 1.16

*Values do not include effects of surface resistance **Unit weight of concrete in the block

Lightweight Concrete Masonry

Expanded shale, clay and slate lightweight concrete masonry is one of the smartest wall systems available, providing structural integrity, beauty and energy efficiency all in one. Lightweight concrete masonry walls offer a unique energy efficient system by combining high R-values, high thermal mass and low thermal bridging. In addition, lightweight blocks provide superior fire resistance, sound absorption ability, and resistance to vermin, rust and rot. Lightweight blocks also increase mason productivity thus making load-bearing lightweight concrete masonry wall systems cost competitive and unsurpassed in quality and value.

ASHRAE/IES 90.1-1989

The latest version of ASHRAE's (American Society of Heating, Refrigerating and Air-Conditioning Engineers) building energy conservation standard clearly demonstrates that lightweight block walls are indeed energy efficient. This latest version, ASHRAE/IES 90.1-1989, provides state-of-the-art guidance regarding the design of energy efficient buildings. Standard 90.1 looks at the performance of many building characteristics, rather than concentrating on wall R-values as earlier versions did. These building characteristics include the effects of wall thermal mass, thermal bridging and insulation position.

Thermal Mass - Heat Capacity

The effects of wall thermal mass are well known. Massive walls, such as concrete masonry, poured concrete or clay brick have the ability (due to their high heat capacity) to delay and reduce the impact of outdoor temperature changes on conditioned indoor environments. This means less heat gain or loss, depending on the season, that must be offset by energy consuming HVAC equipment. ASHRAE 90.1 quantifies thermal mass effects based on a wall's heat capacity. Table 1 lists block heat capacity and Table 2 lists heat capacity for other common building materials. ASHRAE defines heat capacity as wall weight per square foot times specific heat.

Thermal Bridging

Thermal bridges in buildings take place anywhere an insulating material is interrupted by a highly conductive material; e.g., steel studs interrupting batt insulation continuity or metal fasteners in heavily insulated exterior walls. Simply put, thermal bridges occur where differences in material thermal conductivities allow for lateral heat flow; e.g., heat flowing along the surface of a wall and then flowing through the wall via a steel stud. ASHRAE 90.1 considers many thermal bridges. Table 3 (Table 8C-2 in ASHRAE 90.1) lists the effects of thermal bridging in metal stud walls. Table 1 lists block R-values calculated according to ASHRAE's series parallel method recommended by National Concrete Masonry Association which accounts for thermal bridging in block. Because of its low thermal bridging lightweight block walls with hollow cores (no insulation) have higher R-values than normal weight block walls with core insert insulation. This is clearly shown in the following table of R-Values (information taken from Table #1).