Ref ISSN 0701-5216 Sex BUILDING PRACTICE NOTE THE SOUNDPROOF BASEMENT A.C,C. Warnock .AN*LYZED Division of Building Research, National Research Council of Canada O t t a w a , Dkcember 4981 In single-family dwellfngs in Canada, the handyman/homeawner a f t e n converts p a r t of a bare, unfinished hsemnt into a warm, comfortable family worn. Frequently, h l s nuin objective is to build a r o o m where n o i s y activity can be i s d a t e d from the rest of the home. This Note pravldes guidance for those baneowners who want to soundproof room in their basemnts* S e v e r a l books dealing w i t h additions and alterations to homes are available, This Mote is meant to be a supplemnt t a these book.6 and discusses only those factors that affect noise transmission. GENEMIL BACKGROUND It is not necessary t o be an acoustical expert to construct an a c w s tically well-isolated room. It helps, however, to understand the basic p r i n c i p l e s of acoustics in order to d e a l correctly w i t h situations not s p e c i f i c a l l y discussed here. i To begin with, one mst distinguish between materials used ro reduce the transmission of souad between room and those used ta absorb sound i n s i d e a room. To reduce sound transmfssioa between roow, the walls, floor, c e i l i n g , and doors must a l l be made f r o m y e r s of s o l i d materials such as gypsum board, wood or concrete. In theory, f o r a single layer of s o l i d material, the greater t h e weight per unit area, the less the sound transmission. The co-ri practice fa North Amrica is to uae l i g h t w e i g h t w a l k constructed from gypsum board. The sound transmission - through these lightweight walls is greatly reduced by building the walls as mechanically independent double structures wlch an intervening a i r apace. Lightweight prefinfshed fibreboard panels, often used as w a l l f i n i s h e s , are not heavy enough t o reduce sound transmission adequately on their own and are best appl5ed over gypsum board. ' To counteract the build-up of sound due to multiple reflcctiorts to o b j e c t i o n a b l e l e v e l s inside a room, one uses cound-absorbing materials such as acoustic t i l e s , carpets, &rapes, soft furnishings, and ather soft, porous materials. An example af a hfghly e f f e c t i v e souridabsorbing material is exposed glass-fibre thermal Insulation. I n contrast, s aon-porous mterial such a s polystyrene f a a m , although it perfornrs well as a thermal insulator, does not absorb mch sound. Applying sound-absorbing materials such as acoustical tile, cork or carpet to the surface of a wall, floor, or door does not subsean~iallg decrease sound transmissiod from one room to another and reduces sound reflections within the roam only. In mny ways containing sound i s like containing water. Unless t h e room to be isolated is almost at r t i g h t (watertight) t h e sound (water) w i l l "leak" out into adjoifiing spaces. One nust therefore take great care ta caulk and seal thoroughly all cracks and fissures with non-hardening acoustical sealant. In practice, I t i s u n d e s i r a b l e t o achieve complete airtightness since some ventilation is required, hut it is very important to eliminate all unwanted leaks. In the following pages the performance of p a r t i t i o n s as noise barriers i s rated in terns a£ "sound transmission class" (STC). The higher the STC number, the less sound is transmitted through the wall. Plosr people, for example, are s a t i s f i e d if t h e STC for walls between their dwellings is greater than 5 0 , TYPES OF WALL AND FLOOR CONSTRUCTION The sketch in Figure 1Ca) shows the recommended method of f i n i s h i n g a basement c e i l i n g . The r e s i l i e n t metal channels s h w n in Figures 1 and 3 make the two layers of the floor or wall mechanically independent, thereby reducing the transmission of sound between the t w o layers. A further reduction in sound transmission can be obtained by increasing the mass of the floor. This can be done by adding layers of gypsam board or plywood t o the t o p of the floor or t o the underside between the j o f n t s , whichever is more canventent. This is particularly impartant where the f l o o r is composed of tongued and grooved boards s i n c e it has the added benefit of sealing any cracks. Figures 2, 3 and 4 show some recommended w a l l types which use readily available materials. The position of the glass f i b r e within these structures is not important. The thickness needs t o be only a b u t 75 ucm ( 3 in.). All a£ the constructions presented in this Note use two s o l i d layers of material not rigidly linked (as they would be i f nailed to wood studs connecting the two layers, for example) with the cavity between the layers filled with sound-absorbing material. Although this type of construction i s not always superior t o others, it does have the advantage of u s i n g lightweight materials and simple construction techniques. PROV IDIMG SERVICES Containing eound becomes more difficult when one m s t s u p p l y e l e c t r i c a l . pdwer and l i g h t , .air f a r heating and ventilating, and doors to provide access t o a room. A l l these additions cause pcnet.ration of t h e walls or c e i l i n g and therefore p o t e n t i a l sound leaks. However, it is possib Le to provide these services to a room wi thdut seriously i n c r e a s i n g sound transmission, ELECTRICAL OUTLETS. When installing power outlets in a sound-isolating wall, theh o l e s around the boxes should be plugged as mch as p o s s i b l e using caulking or plaster. The power o u t l e t s ofi each s i d e of the wall s h o u l d be o f f s e t by about 0.5 m (19 in.) so that they are not back t o back. If surface-mounted ceiling light fixtures are to be installed, t o caulk a l l t h e holes and thus avoid leaks. Often, however, lack of headroom in a basement requires recessed then i t i s relatively simple l i g h t i n g fixtures. In t h i s case, a box can be constructed around t h e fixture so that an unbroken ceiling layer is preserved (Figure 5). An easier approach is to apply the sheets of s o l i d material d i r e c t l y to the rear surfaces of the fixture; once again, all residual gaps should be caulked, The fixtures used must, of course, be rated for t h i s kfnd of use so that no p o t e n t i a l £Ire hazard is created. Sound leaks can be avofded by mounting the l i g h t s on a wall surface that is n o t an important sound barrierA I R DUCTS In a house with blown-air central heating, the a i r ducts usually present a major problem in reducing sousd transmission. Sound travels very easily into and along air ducts and, unless care is taken, this can ruin an otherwise acceptable acoustical construction. To begin w i t h , all air-duct surfaces should be enclosed by the c e i l i n g or w a l l s or s p e c i a l l y encased. This prevents sound entering or escaping from the duct through the mtal walls and travelling along it from ode room t o another. Sound can s t i l l enter the ducts through the air outlets and travel along them. Using acoustical duct l i n e r will reduce t h i s transmission of sound. Figure 6 shows how glass-fibre duct liner is I n s t a l l e d on the fnside of a duct with a rectangular cross-section. This g l a s s - f i b x e material is t y p i c a l l y about 25 mrn thick and in the form of f l e x i b l e or semi-rigid boards that can be cut t o size, then glued inside the duct. The cut edges are usually s e a l e d with rubber cement. Round, lined d u c t s are also available. Any duct with an outlet in the room to be soundproofed must be treated in t h i s way, It m y also be necessary t o e x t e n d t h e treatment i n t o the main plenum. There is na simple rule to determine how much treatment is necessary, as this depends strongly on the duet dimensions and the number of bends and junctions. However, as a rule of thumb, a m i n i m m length of 3 m of duct liner should be inserted in each d u c t entering the room. Adding sound-absorbing duct liner will, of course, make the duct passages smaller and impede a i r f l o w to so= extent; consequently, the duct may need t o be replaced by a larger one* DOORS L a s t l y , sound isolation may be decreased by the doors leading into the room. The typical door used in hous.es is a l i g h t hollow-core structure, w h i c h does little to prevent the passage of sound, especially when there are large gaps around the edges of the door. The STC is usually in the range of 10 to 15. Commercially-produced acoustical doors are a v a i l a b l e but t h e average homeowner may consider these to be coo expensive, An a l t e r n a t i v e is to use a less c o s t l y , solid-core wood door and t a ensure that the frame is well sealed by gaskets around all the edges. Even then, t h i s type of door has an STC of only about 28. Its performance will not equal that a£ the other structures suggested in t h i s Note unless there is sobuffer space, such as a hallway with another s o l i d door in it between rhe soundproofed room and the other areas. If there is no buffer space, the door alone w i l l control the l e v e l of sound entering or escaping £ram t h e basement. Clearly, it is important to consider t h e layout of the house before any work i s done; other doors i n other room map have to be replaced. If the bornowner considers the effort worthwhile, then t w o independently hung, well-sealed s o l i d doors in the same £ram can be used. Although t h i s could be inconvenient, it may be necessary in some critical cases. AL'PLIAWCE NOISE The noise from electrical appliances such as washers, dryers and furnaces can be reduced by following the p r i n c i p l e s already outlined. The walls of the room where the appliance is located should be constructed as suggested in Figures 2, 3 and 4. To reduce t h e Level of sound in the room, sound-absorbing materials can be added t o the c e i l i n g and p o s s i b l y a l s o to the w a l l s , Ductwork passing through the room should be enclosed, and ducts entering the room should be acoustically lined as shown in Figure 6 . GENERAL CONSIDERATIONS It is important to realize that a well-soundproofed room is o f t e a alnmst airtight. This might create d i f f i c u l t f e s when no forced-air heating or ventilation is used, In some cases it might be necessary to p r o v i d e extra ventilation. Lack of ventilation could also be a problem in a tightly sealed furnace rbom. A supply of air is necessary for combustion, and once again ic may be necessary to provide ventilation, If ducting is used to furnish the required air, then the p o s s i b f l i t y of sound transmfssion v t a the ducts should be considered, Before beginning alterations, one must make sure that the existing waf 1s and floors are properly prepared. Any holes in t h e f l o o r structure should be f113.ed with caulking, or with solid material i f the h o l e is big. h e &thad for dealing with tongued and groaved floorboards has been mentioned earlfer. The importance of goad sealing and caulking techniques cannot be overemphasized. Figure 7 shows some locations where caulking should be added to ensure effective soundpranfing, It is most impartant thar the final layer of gypsum board be well sealed around all the edges. The competent handyman dl1 plan carefully, taktng acoustics and o t h e r f a c t o r s i n t o account, before starting the a c t u a l work; it is always =re difficult, irritating, and expensive to correct errors after the work has been completed. GLASS FIBRE BATT INSULATION ABOUT 75 mm THICK RESILIENT 1 OR 2 LAYERS ENLARGED CHANNELS IN OF GYPSUM BOARD FIG. 1 (b) FIGURE l ( a ) RECOMMENDED F I N I S H F O R A B A S E M E N T CEl L I N G STC = , 5 0 ATTACH GYPSUM BOARD WITH SCREWS FIGURE l ( b ) RE51 L l E N T C H A N N E L : USU.ALLY A P P L I E D 4.0 crn ( 1 4 i n . ) O N C E N T R E S A N D AT R I G H T A N G L E S T O S T U D S OR J O I - S T 5 a) ONE MWR EACH SIDE 4 GUTS BATT INSULATION IN CAVITY /FIBRE STC = 46 TWO LAYERS + ONE LAYER + GFASS FIBRE BATT INSULATION 1N CAVITY 5TC SOUND T R A N S M I S S I ~ N CLASS F - 50 ~ S RT A G G E R E D W O O D STUD W A L L S W l f H D I F F E R E N T SURFACE A N D C A V I T Y TREATMENTS WALLBOARD a) ONE LAYFR EACH SIDE + GLASS F1W BAYT INSULATION IN CAVITY STC = 4d GLASS FIRR BATrS RE51 LlENT CHAlJNELS b) TWO LAYERS -, ONE LAYER + GLASS FIBRE BATT INSULATION 1N CAVlTY STC = 50 FIGULE 3 S O U N D TRANSMISSIQN CLASS FOR W A L L S W I T H W O O D STUDS A N D R E S I L I E N T M E T A L FURRING S T R I P S METAL STUD a) ONE LAYER EACH SIDE GYPSUM WALLBOARD ' FIBRE BATT INSULATION STC = + GLASS IN CAVITY 46 GLASS FIBRE BATTS b) TWO LAYERS + ONE LAYER + GLASS 'FIBRE BATT INSULATION IN CAVITY STC = 51) FIGURE 4 S O U N D T R A N S M I S S I O N CLASS F O R 9 2 m m M E T A L STUD W A L L S (3-5/8 in.) GLASS FIBRE BATT INSULATION CHANNELS FIGURE 5 A M E T H O D OF D E A L I N G W I T H R E C E S S E D L I G H T I N G F I X T U R E USING A B O X INTERNAL L I N I N G - R E I D GLASS FlBRE I NSULATI ON FIGURE 6 I N T E R N A L DUCT L I N E R TO REDUCE SOUND A L O N G DUCTS TRANSMISSION HEADER PLATE GLASS FIBRE BATT I N S U L A T I O N CHANNEL GYPSUM BOARD GYPSUM BOARD (a) CAULKING AT EDGE OF C E I L I N G GYPSUM BOARD SOLE PLATE FLOOR @) CAULKING AT BASE OF WALL FIGURE 7 E X A M P L E S OF C A U L K I N G
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