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Insulation of Building Against Highway Noise

Chapter 2 Noise Reduction Design Procedures

2.1 INTRODUCTION

When noise enters a building from the outside, it undergoes a reduction in level we have defined as Noise Reduction (NR). The amount of NR obtained depends on the type of construction used for the walls of the building, the sizes and types of windows and doors, the presence of noise leaks such as ventilation openings, and the amount of acoustically-absorptive material inside the building. Noise Reduction can be measured directly (this is discussed in Chapter 3), or it may be calculated from the following relation:

where:

EWNR = Exterior Wall Noise Rating.

10 log (S/A) = Room Absorption Factor.

1. S is the area of exterior wall through which the noise is transmitted, and
2. A is the total room absorption.

The Exterior Wall Noise Rating (EWNR) is a scheme for evaluating the Transmission Loss (TL) of a building element based on a typical highway noise spectrum.^* In a typical building, many walls have windows and doors so that the EWNR must be known for these elements. Furthermore, the composite EWNR must be determined for the wall system with windows and doors installed. This chapter contains EWNR values for commonly-used wall constructions, window and door units, and miscellaneous elements such as through-the-wall air conditioners. In addition, procedures are included for combining the effects of two or more elements to determine the composite EWNR. Finally, a step-by-step procedure is given to evaluate the Noise Reduction value in the above relation. Material contained in this chapter can also be used to select building modifications to increase Noise Reduction to the desired amount. The steps of the overall manual procedure that will be carried out in this chapter are shown in Figure 5, where L_o is the given highway noise level and L_c is the design noise level.

^*See Appendix A for details on the development of the Exterior Wall Noise Rating concept.

2.2 EWNR VALUES FOR WALLS

Table 1 is a matrix presentation of EWNR values of 91 commonly-used basic exterior wall constructions, each made up of an exterior and an interior surface with no absorptive material in the stud space. The matrix may be entered either on the horizontal listing of interior wall surfaces or on the vertical listing of exterior wall surfaces.

Typical constructions found in existing buildings are often a variation of one of the basic constructions listed in Table 1. For example, an existing building may have fiberglass insulation in the stud spaces, or the stud spacing may be 24 inches rather than 16 inches. For this reason, it is convenient to consider standard constructions as having a basic EWNR value with some additional value added to account for construction variations. This approach may also be used to determine the NR benefits that may be expected by modifying an existing construction.

Modifications to basic constructions usually involve changes to the weight per unit area of wall panels, addition of absorption to the stud space, or changes to the limpness of the wall panels. These three modification categories act independently and when modifications are made to different categories, the full benefit of each modification will be realized. Adjustments to be added to the EWNR for basic constructions are given in Table 2 for the three categories. Category 3 contains several different modifications that increase the limpness of a construction. If two or more Category 3 modifications are used, count the value of the largest adjustment plus one-half of the value of the next largest adjustment.

Modification Category 1: Mass Increases EWNR, dB Double Mass One Side 3 Double Mass Both Side 4

Modification Category 2: Stud Space Absorption EWNR, dB Absorption in Stud Space² 4

Modification Category 3: Limpness Increases EWNR, dB Fiberboard Under Both Panels 8 Resilient Mounting of One or Both Panels 8 Staggered Studs 6 24-inch Stud Spacing 2 Metal Channel Studs 5

1. To obtain the Total EWNR adjustment for multiple modifications: add the adjustments for each of the three categories. If more than one Category 3 modification is used, count the value of the largest adjustment plus one-half of the value of the next largest.

2. If fiberboard is used for a Category 3 modification, count Category 2 stud space absorption as only 2 dB.

1. Example 1:
2. 1. An existing building located near a highway has a solid exterior wall facing the highway. The wall is constructed of standard wood studs 16 inches on center with an interior surface of ½ inch thick plaster and an exterior surface of ½ inch thick wood siding. What is the EWNR of the wall? What would the EWNR be if the exterior siding were removed, fibrous absorption installed in the stud spaces, and the wood siding reinstalled?
3. Solution:
4. 1. From Table 1, a standard 16-inch on-center stud wall is located in Area (above the solid line). At the intersection of Exterior Surface Row D and Interior Surface Column 4, an EWNR value of 31 dB for the wall is found. From Table 2, the addition of stud space absorption is determined to provide an adjustment of +4 dB. The modified wall would therefore have an EWNR of 31 + 4 = 35 dB. These calculations should be carried out on Worksheet No. 1, a portion of which is reproduced as follows. (A complete Worksheet No. 1 is included at the end of the manual.)

2.3 EWNR VALUES FOR ROOF-CEILING CONSTRUCTIONS

If the room being evaluated is just below the roof, the EWNR of the Roof-Ceiling Construction must be determined. Two general types of Roof-Ceilings in common use are the Single Joist and Attic Space Type, as shown in Figure 6.

Basic EWNR values for Typical Roof-Ceiling Constructions are given in Table 3 for Single Joist as well as Attic Space Constructions. These basic values must be adjusted to account for attic venting, acoustical absorption in the attic or ceiling joist space, and self-shielding of the roof by the building.

If the roof system is not vented and is lined with acoustically absorptive material (such as fiberglass or mineral wool), add an adjustment from Table 4 to the basic EWNR value.

If the roof system is a vented attic space system, use Table 5 to adjust the EWNR values. Find the row corresponding to the basic EWNR value from Table 3 and read the new EWNR value in the appropriate column to account for the absorptive material. Single joist constructions are not normally vented, so Table 5 does not apply to these systems.

Finally, the Roof-Ceiling EWNR values must be adjusted to account for building self-shielding which is a function of the slope of the roof line. The noise level due to an exterior source will not be as great just outside the center of the building roof as it will be just outside the building walls. This self-shielding effect of the building may be accounted for by adding the adjustment factors given in Table 6 to the EWNR values determined in Tables 3, 4, and 5.

1. Example 2:
2. 1. An existing building has a sloped roof consisting of composition shingles. The attic space is vented in accordance with the Uniform Building Code and there is no acoustical absorption in the attic space. If the ceiling construction is ½ inch gypsumboard, what is the EWNR of the Roof-Ceiling System? What would the EWNR be if the attic space were treated with acoustic absorption?
3. Solution:
4. 1. The basic EWNR for this Roof-Ceiling System is found at the intersection of Column 1 and Row G of Table 3-40 dB. Since the attic space is vented, we must refer to Table 5 which indicates that a gypsumboard construction with a basic EWNR of 40 to 42 dB will have an EWNR of 25 dB with venting. Since the roof line is sloped, we add a final adjustment of 3 dB from Table 6 to account for self-shielding. The resulting EWNR is 28 dB.M

      If absorption were added to the vented attic space, the use of Table 5 indicates an EWNR of 32 dB. With the adjustment for self-shielding, the resulting EWNR is 35 dB. The absorption has provided a beneficial increase in the EWNR of 7 dB.

      These calculations should be carried out on Worksheet No. 1,a portion of which is reproduced as follows. (A complete Worksheet No. 1 is included at the end of the manual.)

2.4 EWNR VALUES FOR WINDOWS AND DOORS

If the wall being evaluated has one or more windows or doors/ the EWNR values of these elements must be determined. Table 7 lists EWNR values for common window assemblies and Table 8 lists EWNR values for commonly-used doors.

Note: The addition of a storm window to an existing single glazed or jalousie window will increase the EWNR by 5 dB.

^*If the window is fully open (such as a fully open jalousie or crank type window) its EWNR value is 4 dB. If the window is not completely open (usually the case for sliding windows) use 4 dB for the open area, the given value for the closed or unopenable area, and combine the two values using the procedures of Section 2.6.

Note: The addition of a weather-stripped single glazed storm door to an existing door will increase the EWNR by 5 dB.

2.5 EWNR VALUES FOR THROUGH-THE-WALL AIR CONDITIONERS

If a through-the-wall-type air conditioning unit is present in an existing wall being evaluated, or if such a unit is being considered, the EWNR values of the air conditioners must be determined. Table 9 lists EWNR values for through-the-wall air conditioners for vents open and vents closed. Treat these units as windows in the remaining analysis in this manual.

2.6 COMPOSITE EWNR VALUES FOR TWO OR MORE ELEMENTS

In most typical constructions of interest, there will be one or more windows, doors, and through-the-wall air conditioners existing as part of the walls being evaluated. Two components - such as a single door in a wall - may be combined and treated as one single component with an EWNR value that is less than the EWNR of the higher-rated element. The amount by which the EWNR of the higher rated element (usually the wall) must be reduced to account for the presence of the lower-rated element (usually the window or door) is a function of the relative areas of both components and the magnitude of the difference between their individual EWNR values.

For more than two components, the following procedure may be repeated as many times as necessary taking the elements two at a time in any order.

PROCEDURE To Combine the EWNR Values of Two or More Elements

1. Determine the surface area of each of two elements to be combined. Note that if two elements have the same EWNR values, combine them directly by considering them as one element with the same EWNR and a combined surface area equal to the total area of both elements.
2. Determine the percentage area of each element to the total area of both elements.
3. Record the EWNR of each element. (This should be the adjusted EWNR where applicable.)
4. Subtract the EWNR of the lower-rated element from the higher-rated element.
5. Enter Figure 7 on the horizontal axis at the value determined in Step 4. (Difference between EWNR values of two elements.)
6. Draw a vertical line up to the curve corresponding to the percentage of the total area occupied by the lower-rated EWNR element. (Interpolate between percentages if necessary.)
7. Draw a horizontal line from this point to the left to intersect the vertical axis to find the difference between the higher-rated element and the composite of both elements.
8. Subtract this value (difference between higher-rated element and composite of both elements) from the higher-rated element to obtain the composite EWNR for both elements in combination.
9. Repeat the procedure as necessary to determine the composite rating of three or more wall components.

1. Example 3:
2. 1. It is proposed that the solid wall of Example 1 (without absorption) be modified to include a 1-¾" (4.4 cm) hollow core door (fully weather-stripped). If the wall is 8' 0" x 30' 6" (2.4 x 9.3m) and the door is 3' 6" x 7' 0" (1.1 x 2.1 m), what will the composite EWNR be for the wall with door installed?
3. Solution:
4. 1. Following the numbered steps of the described Procedure:

1. The door area is 24.5 square feet (2.3m²). The wall area (with door cut out) is 219.5 square feet (20.4m²). The total area is 244 square feet (22.7m²).
2. The door is 10 percent and the wall 90 percent of the total area.
3. The EWNR of the door is 17 dB (from Table 8). The EWNR of the wall is 31 dB (from Example 1).
4. The difference between the EWNR values is 14 dB.
5. Enter Figure 7 at 14 dB on the horizontal axis.
6. Draw a line up to the curve marked 10 percent.
7. Draw a line horizontally to the left to intersect the left axis at 5 dB (round off to the nearest integral value).
8. The composite EWNR of the wall with door installed is 31-5 = 26dB.

These calculations should be carried out on Worksheet No. 2, a portion of which is reproduced as follows. (A complete Worksheet No. 2 is included at the end of the manual.)

2.7 INTERIOR ABSORPTION ADJUSTMENT

When exterior noise is transmitted to the interior of a structure, it undergoes absorption due to materials located in the room such as draperies, carpeting, and upholstered furniture which usually provide most of the absorption in typical buildings.

Increasing the interior room absorption in an attempt to reduce noise produced by an exterior source is not usually an effective technique, since a great deal of absorption must be added to an existing interior space to obtain only a small noise reduction benefit.

The presence of normal amounts of interior room absorption must be accounted for, however, when determining the noise reduction of an existing structure. The value of the adjustment for room absorption will depend on the total amount of absorption usually referred to by the term A and the total surface of the exterior wall, S (this is the area of the exterior wall that covers the room being evaluated).It is not necessary to actually determine what S and A are, since the combined room absorption adjustment - 10 log (S/A) - is normally a constant value for various types of rooms in typical structures. (See Appendix C for a detailed discussion of this.) Adjustment factors for room absorption are listed in Table 10 for three types of rooms with either one exterior wall or two exterior walls (corner rooms).

2.8 NOISE REDUCTION PROCEDURE

Once the appropriate EWNR values are determined for the structure, the Noise Reduction (NR) can be calculated from the following relationship

where 10 log (S/A) is the room absorption correction factor given in Table 10. Use the following procedure to determine the NR for an existing structure or for one that is being modified.

1. For each interior room to be evaluated, identify all elements that will transmit noise from outside the structure. These elements will include exterior walls, roof-ceilings, windows, doors, and air conditioners.
2. Determine the area of each element. For roof-ceiling constructions, use the ceiling area of the room being evaluated.
3. Determine the EWNR of all wall elements. Find the basic EWNR in Table 1. Add appropriate adjustments from Table 2.
4. Determine the EWNR of the roof-ceiling if the room being evaluated adjoins the roof. Find the basic EWNR from Table 3. For nonvented ceiling/joist spaces, add an appropriate adjustment for absorption from Table 4. For vented spaces (attics only), use Table 3 and Table 5 to determine the EWNR. Add the appropriate adjustment from Table 6 to account for self-shielding of the structure.
5. Determine the EWNR for all windows, doors, and through-the-wall air conditioners from Tables 7, 8, and 9.
6. Determine the composite EWNR of each wall element that contains a window, door, or air conditioner, using the procedure of Section 2.6. If a wall contains more than one window or door, repeat the procedure as necessary to account for all windows and doors.
7. Combine the composite EWNR of all wall elements if the room has more than one exterior wall. Use the procedure of Section 2.6 as required to determine the EWNR for all composite wall elements.
8. Combine the EWNR for all composite wall elements with the adjusted EWNR of the roof-ceiling construction, if appropriate, to obtain the total EWNR for the structure.
9. Determine the room absorption correction from Table 10 for the type of room being evaluated.
10. Subtract the quantity (10 log S/A + 6) from the total structure EWNR to determine the structure's NR.

1. Example 4:
2. 1. A single-story residence, located near a highway that is being widened, is being evaluated to determine if acoustical modifications will be required. The exterior walls are constructed of aluminum siding on ½" (1.3cm) wood nailed to 2"x4"(5.1 x 10.2cm) studs with fiberglass thermal insulation in the stud spaces, and an interior finish of 5/8" (1.6cm) gypsum board. The house has a vented attic space with fiberglass thermal insulation installed. The sloped roof is wood shingle, and the interior ceilings are ½" (1.3cm) gypsumboard. The bedroom has one exterior wall 8' 0" x 15' 6" (2.4 x 4.7m) containing one 1/8" (0.3cm) window 316" x 3' 6" (1.1 x 1.1 m). The bedroom ceiling is 15' 6" x 12' 0" (4.7 x 3.7m). Calculate the NR of the structure for the bedroom .
3. Solution:
4. 1. Following the numbered steps in the procedure of Section 2.8:

These calculations should be carried out on Worksheet No. 1 and Worksheet No. 2, portions of which are reproduced as follows: (Complete Worksheets are included at the end of the manual.)

Steps 1 to 3 Portion of Worksheet No. 1

Step 4 Portion of Worksheet No. 1

Steps 5 and 6 Portion of Worksheet No. 2

2.9 MODIFICATIONS TO INCREASE NOISE REDUCTION

When interior noise (from nearby highways) exceeds the Design Noise Level, modifications to increase the building's noise reduction are required. A preliminary selection of modifications can be made by considering the three basic paths by which noise enters a structure. The actual increases in noise reduction which would result from these modifications can then be determined using methods presented earlier in this chapter. If insufficient noise reduction is obtained, the cycle of selection and evaluation should be continued until interior noise no longer exceeds the Design Noise Level.

The three major paths by which noise can be transmitted to the interior of a structure are illustrated conceptually in Figure 8. These paths are:

• Air Infiltration (gaps and cracks)
• Small Wall Elements (Windows and Doors)
• Main Wall Elements

Air infiltration paths are the small gaps and cracks that normally exist around doors and windows. Naturally, the more such leakage paths there are, the lower the noise reduction of the building will be - even if the EWNR of the wall panels themselves is relatively high. This explains why it is often not sufficient simply to calculate the interior noise level using known properties of wall materials, because this method cannot easily take leakage into account. The usual calculated value of interior noise provides the minimum value that can be obtained in the absence of leakage paths. Thus, the principal difference between the calculated and appropriately measured values of interior noise may be attributed to noise leakage. The improvement in NR that can be obtained merely by treating the leakage paths without modifying the windows, doors, or other building elements is shown, in Table 11, to be as high as 4 dB. Thus, the usual first step in increasing the NR of buildings is to

Figure 8. Conceptual Illustration of the Three Major Types of Paths By Which Noise is Transmitted to Building Interiors.

seal all infiltration cracks using weather-stripping, nonhardening caulking, and door threshold seals. If the sealing of cracks and leaks will not add sufficiently to the NR of the building, then modifications of the building elements will be required.

Since small wall elements such as windows and doors usually have EWNR values less than that of the surrounding wall, it is usually beneficial to consider them in the second stage of modifications. This second stage should upgrade small wall elements to an EWNR which approaches that of the surrounding wall. This is usually done by replacement with improved elements and, as shown in Table 11, can result in NR increases of up to 10 dB. One basic small element modification is the installation of storm doors and windows. These can provide substantial increases in door and window EWNR, but these elements must remain in place if the benefits are to be realized year-round.

The final alternative, if the previous two approaches do not provide adequate noise reduction, is modification of the main wall elements, the basic^* - wall and roof construction. This major noise attenuating technique can provide substantial increases in NR but may not be justifiable on a cost/benefit basis. Two of the simpler construction modifications are addition of absorbing material to the stud space, and resilient mounting of the interior wall panels. The NR benefits to be expected from these and more extensive modifications can be obtained from tables presented earlier in this chapter.

Table 11 may be used to select the modification type based on the required increase in NR. .Relative modification costs are also included to assist in this selection. Procedures for estimating quantitative changes in heating and ventilation requirements due to the modifications are presented in Chapter 4, and procedures for estimating the actual costs of various modification alternatives are given in Chapter 5.