R - Values for Single Wythe

Concrete Masonry Walls

TEK 6-2A (Energy & IAQ 1996)

INTRODUCTION

Concrete Masonry walls are often constructed of hollow units with cores filled with loose fill material and/or grout. This construction method provides the minimum wall thickness, while allowing insulation and reinforcement to be included to increase thermal and structural performance, respectively.

Determining the thermal insulation values of these walls, however, can be time consuming, especially when the wall is composed of several materials. This TEK facilitates the determination of thermal resistance (R) and thermal transmittance (U) of these single wythe concrete masonry walls.

R-VALUE TABLES

Tables of calculated R-values for hollow block of 6, 8, 10 and 12 in. (152, 203, 254, and 305 mm) thickness, for concrete densities of 85 to 135 lb/ft³ (1362 to 2163 kg/m³) are included. In addition, Table 1 shows the approximate percentage of grouted and ungrouted wall area for different vertical and horizontal grout spacings, which can be used to determine R-values of partially grouted walls. Thermal properties used in compiling the tables are listed in Table 6.

In addition to the core insulations listed in Tables 2 through 5, polystyrene inserts are available which fit in the cores of concrete masonry units. Inserts are available in many shapes and sizes to provide a range of insulating values and accommodate various construction conditions. Specially designed concrete masonry units may incorporate reduced-height webs to accommodate inserts. Such webs also reduce thermal bridging through masonry, since the reduced web area provides a smaller cross-sectional area for heat flow through the wall. To further reduce thermal

bridging, some manufacturers have developed units with two cross webs rather than three. In addition, some inserts have building code approval to be left in the grouted cores, thus improving the thermal performance of fully or partially grouted masonry walls.

The ASHRAE series-parallel method (also called isothermal planes) (ref.1) was used to calculate the base case values (i.e., the row Exposed block, both sides) in Tables 2 through 5. This method accounts for the thermal bridging through the webs of concrete masonry units. R-values of the various finish systems are added to these base values. To determine R-values for walls with 2 in. (51 mm) of rigid insulation (expanded polystyrene, extruded polystyrene, or polyisocyanurate) rather than the 1 in. (25mm) shown in the tables, simply add the appropriate insulation thermal resistivity value from Table 6 to the R-values in Tables 2 through 5.

R-values of concrete masonry walls are correlated to concrete density, since thermal conductivity of concrete increases with increasing density. Tables 2 through 5 list a range of R-values for each density, as well as a single value, which represents a calculated middle-of-the-range. The U-factor is determined by simply inverting the R-value (i.e., U = 1/R).

A range of thermal values is appropriate for concrete products because the thermal conductivity of concrete cannot always be accurately estimated from density alone. The thermal conductivity of concrete varies with aggregate types(s) used in the concrete mix, the mix design, moisture content, etc.

These published values reflect a compendium of historical data on thermal conductivity of concrete (refs. 1,3). Locally available products and local conditions may result in thermal values which fall outside of this range. The middle-of-the-range values are presented for use in cases where more accurate values are not available from local manufacturers.

The values for insulated and grouted cores in Tables 1 through 5 are based on the assumption that all masonry cores are either insulated or grouted. That is, for walls which are either not grouted or are fully grouted, the values in Tables 2 though 5 can be used directly.

a Notes: (hr^.ft^2.oF/Btu) (0.176) = m^2.K/W. Mortar joints are in. (10 mm) thick, with face shell mortar bedding assumed. Unit dimensions based on Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90 (ref. 2). Surface air films are included.

c Installed over wood furring. Includes ½ in. (13 mm) gypsum board and nonreflective air space.

d Installed over wood furring, Includes ½ in. (13 mm) gypsum board and reflective air space.

b Values apply when all masonry cores are filled completely. Grout density is 140 pcf (2243 kg/m³). Lightweight grouts, which will provide higher R-values, are also available in some areas.

c Installed over wood furring. Includes ½ in. (13 mm) gypsum board and nonreflective air space.

b Values apply when all masonry cores are filled completely. Grout density is 140 pcf (2243 kg/m³). Lightweight grouts, which will provide higher R-values, are also available in some areas.

c Installed over wood furring. Includes ½ in. (13 mm) gypsum board and nonreflective air space.

d Installed over wood furring, Includes ½ in. (13 mm) gypsum board and reflective air space.

a Notes: (hr^.ft^2.oF/Btu) (0.176) = m^2.K/W. Mortar joints are in. (10 mm) thick, with face shell mortar bedding assumed. Unit dimensions based on Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90 (ref. 2). Surface air films are included.

b Values apply when all masonry cores are filled completely. Grout density is 140 pcf (2243 kg/m³). Lightweight grouts, which will provide higher R-values, are also available in some areas.

c Installed over wood furring. Includes ½ in. (13 mm) gypsum board and nonreflective air space.

d Installed over wood furring, Includes ½ in. (13 mm) gypsum board and reflective air space.

R-VALUES FOR PARTIALLY GROUTED MASONRY

For partially grouted walls, the values in Tables 2 through 5 must be modified. The first step is to determine how much of the wall area is grouted, from Table 1. The U-factor of the wall is calculated from the area-weighted average of the U-factor of the grouted area and the U-factor of the ungrouted area as follows:

U = (a_gr x U_gr ) + (a_ungr x U_ungr ) and R = 1/U

where:

a_gr = fractional grouted area of wall

a_ungr = fractional ungrouted area of wall

R = total thermal resistance of wall, hrft^2.oF/Btu (m^2.K/W)

U = total thermal conductance of wall, Btu/hr^.ft^2.oF (W/m^2.K)

U_gr = conductance of fully grouted wall, Btu/hr^.ft^2.oF (W/m^2.K)

U_ungr = conductance of ungrouted wall, Btu/hr^.ft^2.oF (W/m^2.K)

For example, consider an 8 in. (203mm) wall composed of hollow 105 lb/ft³ (1682kg/m³) concrete masonry, and grouted at 48 in. (1219 mm) o.c. both vertically and horizontally. The ungrouted cores contain perlite loose fill insulation.

From Table 1, 31% of the wall is grouted and 69% contains insulation. From Table 3, the R-value for a solidly grouted concrete masonry wall is 1.7 hr^.ft^2.oF/Btu (0.30 m^2.K/W). The corresponding U-factor is 1/1.7 or 0.588 Btu/hr^.ft^2.oF (3.3 W/m^2.K). Again from Table 3, a wall containing perlite loose fill insulation has an R-value of 5.2, with a corresponding U-factor of 0.192. The U-factor and R-value of the wall are calculated as follows:

U = a_gr x U_gr + a_ungr x U_ungr

= (0.31 x 0.588) + (0.69 x 0.192)

= 0.315 Btu/hr^.ft^2.oF (1.79 W/ m^2.K)

R = 1/U = 1/0.315 = 3.2 hr^.ft^2.oF/Btu (0.56 m^2.K/W)

REFERENCES

1. ASHRAE Fundamentals Handbook, Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1993.

• Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90-95. American Society for Testing and Materials, 1995.

• Valore, Rudolph C. The Thermophysical Properties of Masonry and Its Constituents, Part I and II. Washington, DC: International Masonry Institute, 1988.

National Concrete Masonry Association

2302 Horse Pen Road, Herndon, VA 22071-3499

Reprinted from the TEK Manual for Concrete Masonry

Design and Construction. Copyright NCMA.

Sound Transmission Class Rating

For Concrete Masonry Walls

TEK 13-1 (Sound 1998)

INTRODUCTION

Unwanted noise is a major distraction, both in the home as well as the work environment. With each technological advance, more and more different types of noise generating machinery and appliances are finding their way into our everyday lives. This unwanted noise is not only disconcerting, but in many cases may also produce permanent injury to hearing when exposure is for prolonged periods of time, depending upon the intensity of the sound.

Two important elements of sound are frequency and pressure. Frequency is a measure of the number of vibrations or cycles per second (cps). One cycle per second is defined as a hertz (HZ). The measurement of pressure is in decibels (dB). For each 20 decibel increase in sound there is a corresponding tenfold increase in pressure. The human ear has the unique ability to reduce its sensitivity as the pressure increases. Therefore, while a 10 decibel increase in sound results in a threefold increase in pressure, the loudness sensation to the ear is only doubled.

The following provides an indication of the decibel as a measure of sound intensity.

 

Decibels	Sound
140 130 120 110 100 90 80 70 60 50 40 30 20 10 3	Jet plane takes off Threshold of discomfort Riveting Thunder - sonic boom Hard rock band Power lawnmower Pneumatic jackhammer Noisy office Average radio Normal conversation Quiet street Quiet conversation Whisper at 4 ft. Normal breathing Threshold of audibility

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Sounds are generated by vibrating objects. The vibrations are transmitted by contact with air, or other mediums, and are carried forward in waves. The speed at which sound travels through a medium depends upon both the density and the elasticity of the medium.

All solid materials have a natural frequency of vibrations. If the frequency of a solid is at or near the frequency of the sound which strikes it, the solid will vibrate in sympathy with the sound, and the sound will be re-generated on the opposite side. The is true for all solids, including walls and partitions. The effect is especially noticeable when the wall or partition is light or thin. Conversely, the vibration is effectively stopped if the partition is of heavy, rigid material. Then the natural cycle of vibration will be relatively slow, and only sounds of low frequency will cause sympathetic vibration.

The human ear can perceive sounds as low as 16 cycles per second to as high as 20,000 cycles per second. However, it is most sensitive to sounds between 500 and 5,000 cycles. For human voices speaking in conversational tones, a frequency of approximately 500 cycles per second is assumed.

Because of its mass and rigidity, concrete masonry is especially effective in reducing the transmission of unwanted sound. This is just one of its many attributes which makes it a most desirable building material.

SOUND TRANSMISSION CLASS

Sound transmission class (STC) is a single-figure rating derived in a prescribed manner from sound transmission loss values. The rating provides an estimate of the performance of the partition in certain common sound insulation problems.

The STC of a wall is determined by comparing its transmission loss curve with a set of standard curves, or contours. The standard curve is superimposed over a plot of the sound transmission loss curve (Figure 1) and shifted upward or downward relative to the test curve until some of the measured TL values of the test specimen fall below those of the STC contour and the following conditions are fulfilled:

1. The sum of the deficiencies (deviations below the contour) shall not be greater than 32 dB, and

2. The maximum deficiency at a single test point shall not exceed 8 dB.

When the contour is adjusted to the highest value that meets the above criteria, the sound transmission class is taken to be the transmission loss value, measured in decibels (dB), corresponding to the intersection of the standard contour and the 500 cycle per second (cps) frequency line.

DESIGN AND CONSTRUCTION

In addition to STC values for walls, there are other factors that have an effect on the acoustical environment of a building. Outside noise levels need to be considered. Low background noise levels, such as exist in rural areas, may indicate the need for partition walls to have a higher STC requirement. Another item that is important to noise insulation is wall layout. In apartment construction, an in-line arrangement, where each apartment will have only one common wall, is preferred. "Mirror-plan" arrangement will generally result in quieter areas, such as bedrooms being located adjacent to each other, and noisy areas, such as kitchens abutting each other. Door and window arrangement may also have an effect on the acoustical environment. Locating apartment doors so that they are not directly opposite each other will diffuse a portion of noise that would otherwise be able to transmit directly across a hall. Windows in exterior walls should be located as far away from common walls as possible in order to

help diffuse noise that may travel from one window to another.

Openings placed in common walls, such as electrical fixtures, plumbing pipes, ducts, and medicine cabinets, are a potential source for noise leaks. Figures 3 and 4 of the TEK illustrate possible methods to control noise penetration through proper detailing and construction.

BUILDING CODE REQUIREMENTS

The model building codes all have similar requirements regulating sound transmission control for partitions that separate adjacent units in multi-family dwellings and similar partitions that separate adjacent units in multi-family dwellings and similar partitions that separate dwelling units from public areas, service areas, or commercial facilities. In the BOCA Code, and the appendix of the Standard Building Code, the requirement is that all partitions serving the above purpose shall have a sound transmission class of not less than 45 Db for air-borne noise when tested in accordance with ASTM E90. The Appendix of the Uniform Building Code establishes a lower limit of 50dB for the same applications.

5.1.1 Hollow units shall be placed so that the face shells of bed joints are fully mortared and head joints are mortared a minimum distance from each face equal to the face shell thickness of the unit.