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Effect of electrical outlet boxes on sound isolation of gypsum board
walls
Nightingale, T. R. T.; Quirt, J. D.; Sultan, M. A.
https://publications-cnrc.canada.ca/fra/droits
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NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=42394591-4da6-4457-9bca-aa8c4f0c9117 https://publications-cnrc.canada.ca/fra/voir/objet/?id=42394591-4da6-4457-9bca-aa8c4f0c9117by T.R.T. Nightingale, J.D. Quirt and M.A. Sultan
This Update discusses the effect of electrical outlet boxes on the sound
isolation of gypsum board w alls and provides recommendations w ith
respect to box type and location, and methods for improving existing boxes.
The information is based on the results of an industry-supported research
project conducted at the NRC’s Institute for Research in Construction.
1C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 2 7
Th e Nation al Bu ild in g Cod e (NBC) requ ires p artition w alls th at sep arate u n its in m u ltifam ily d w ellin gs to be both fire an d sou n d -rated . Th e Can ad ian Electrical Cod e requ ires th ese an d oth er w alls in th e d w ellin g to h ave a certain n u m ber of electrical ou tlet boxes p er u n it len gth of w all. How ever, p en etration s associated w ith electrical ou t-let boxes can d ecrease th e sou n d isolation or acou stical p rivacy offered by th e w all. Un til IRC research ers con d u cted a system -atic stu d y, th e effect of ou tlet box typ e an d location , an d sou n d -absorbin g m aterial (su ch as bu ild in g in su lation m ad e from glass fibre, m in eral fibre or cellu lose fibre) in th e cavity on th e sou n d isolation of gyp su m board w alls w as n ot kn ow n . Th e stu d y also qu an tified th e effectiven ess of several p ractical retrofit m eth od s.
Te st M e t hod a nd Spe c im e ns
Th e stu d y exam in ed w alls con stru cted w ith sin gle, staggered an d d ou ble w ood stu d s.
Th is Up d ate focu ses on d ou ble-stu d w alls becau se th is typ e of w all a) is m ost likely to be u sed to ach ieve m axim u m sou n d isolation betw een d w ellin g u n its an d b) allow ed m ore con figu ration s of th e sou n d -absorbin g m aterial to be tested .
Basic Wall Configurations Tested
Th e th ree typ es of d ou ble-stu d w alls tested w ere as follow s:
1. Walls w ith n o sou n d -absorbin g m aterial in th e cavity (see Figu re 1, No In su lation ) 2. Walls w h ere th e sou n d -absorbin g m aterial
is p u sh ed asid e so it d oes n ot cover th e box, th u s on ly p artially blockin g th e p ath betw een th e ou tlet boxes (see Figu re 1, Disp laced In su lation );
3. Walls w ith th e sou n d -absorbin g m aterial in th e cavity com p letely coverin g th e back of th e ou tlet box (Figu re 1, Fitted In su lation ), so sou n d m u st travel th rou gh th e sou n d -absorbin g m aterial to get to th e oth er box.
Th e electrical ou tlet boxes, each w ith a d u p lex ou tlet, p lastic facep late an d all w irin g n ecessary to sim u late typ ical field in stallation , w ere p osition ed in th ree d iffer-en t w ays:
1. Back-to-back, w ith zero h orizon tal offset (Figu re 1, Back-to-Back Con d ition ) 2. With in th e sam e stu d cavity, w ith a h
ori-zon tal offset of 350 m m (Figu re 1, Sam e Stu d Cavity Con d ition )
Effect of Electrical Outlet
Boxes on Sound Isolation
of Gypsum Board Walls
Th e fin d in gs an d recom m en d ation s p ertain in g to staggered an d sin glestu d assem blies w ere in fact sim ilar to th ose for d ou bleglestu d assem -blies.2 Th e effect of electrical ou tlet boxes in steel-stu d w alls w ith
sim ilar fram in g (i.e., sin gle stu d , staggered stu d an d d ou ble stu d ) is exp ected to be com p arable to th at for w ood -stu d assem blies. In som e of th e test w alls, th e sp ace betw een stu d s w as filled w ith glass-fibre insulation while in others it was left empty. It is expected that walls containing fibrous sound-absorbing material other than glass-fibre insu-lation, with similar thickness, will exhibit comparable performance.
2
3. In ad jacen t stu d cavities, w ith a h orizon tal offset of 400 m m (Figu re 1, Ad jacen t Stu d Cavity Con d ition ).
Effe c t of Diffe re nt Type s of Out le t Box
Th ere are tw o basic typ es of electrical ou tlet box: stan d ard m etal boxes an d p lastic vap ou r-barrier boxes. Th e tests con d u cted on th e assem -blies w ith n o sou n d -absorbin g material in the cavity (Figure 1, No In su lation ) d em on strated th at w h en p lastic vap ou r-barrier ou tlet boxes w ere u sed , th e effect of p en etra-tion s on th e sou n d isolaetra-tion of th e w all assem bly w as n egligible (see Table 1).
Th e tw o typ es of box p er-form d ifferen tly (see Table 1) becau se of th e d ifferen ce in th e n u m ber of p en etration s in each . Stan d ard m etal boxes h ave m an y h oles to allow a) electrical wires to enter the box an d b) th e box to be fasten ed to th e stu d . In con trast, p lastic vap ou r-barrier boxes h ave closed -cell foam gaskets, w h ich form an air-tigh t seal w h ere th e w ires en ter th e box, as w ell as tabs on th e ou tsid e of th e box, w h ich are u sed to fasten th e box to th e stu d so th at th ere are n o h oles in th e box. Plastic vap ou r-barrier boxes also typ ically h ave a backer p late at th e m ou th to seal th e box to th e gyp su m board . Th ese airtigh tn ess fea-tu res im p ed e th e tran sm ission of sou n d as w ell as th e flow of air, an d can com p en sate for p roblem s cau sed by box p roxim ity an d / or p oor in su lation p lacem en t.
Becau se th ere w as virtu ally n o ch an ge in th e sou n d isolation of th e w all assem blies with plastic vapour-barrier boxes, irrespective of w h ere th e boxes w ere located , th e effect of ad d in g sou n d -absorbin g m aterial to th e cavity w as in vestigated for m etal boxes on ly.
Effect of Box Placement and Sound-Absorbing M a t e ria l in t he Ca vit y
Figu re 1 sh ow s th e ch an ge in sou n d isolation relative to th e referen ce case (th e w all w ith n o p en etration s) for m etal ou tlet boxes located in variou s p osition s. Th e effect of electrical boxes on th e sou n d isolation of a gyp su m board w all assem bly can be large — in th e w orst cases, th e STC d ecreased by 6 p oin ts.
Ra t ing t he Sound I sola t ion
Sou n d isolation is th e ability of a p artition to stop th e tran sm ission of airborn e n oise, an d h en ce its ability to p rovid e acou stical p rivacy. In th is Up d ate, each w all assem bly is assign ed a sin gle n u m ber ratin g, STC (sou n d tran sm ission class). STC is a stan d ard ized ratin g sch em e (ASTM E413) th at allow s assem blies to be ran ked accord in g to th eir ability to stop airborn e n oise w ith a sp ectru m sim ilar to th at of sp eech or th e n oise from office equ ip m en t. Th e h igh er th e STC ratin g th e greater th e sou n d isolation or acou stical p rivacy. Wh en com p arin g resu lts in th is Up d ate, a ch an ge of on e STC p oin t sh ou ld n ot be con sid ered sign ifican t; a ch an ge of tw o or m ore p oin ts sh ou ld be con sid ered sign ifican t. Figure 1.Sketches of the double-stud constructions used in the study indicating the locations of the electrical outlet boxes, and the presence and method of installation of the sound-absorbing material in the cavity. The sound isolation is expressed as STC, or a change in STC (in parentheses), relative to the reference wall, which did not have any penetrations. All walls are constructed with 2 rows of 38-mm x 89-mm wood studs set 25 mm apart and 2 layers of 12.7-mm Type X gypsum board on each side.
3 Table 1.Measured sound isolation expressed as an STC rating for metal outlet boxes and plastic vapour-barrier outlet boxes in the double-stud wall without sound-absorbing material in the cavity (Figure 1, No Insulation)
Th e ch an ge in sou n d red u ction d ep en d s on several factors:
• th e sep aration (h orizon tal offset) of th e electrical boxes
• th e con stru ction of th e w all assem bly • th e location of th e sou n d -absorbin g
m aterial in th e cavity.
Th ese factors can n ot be fu lly sep arated , bu t gen eral tren d s can be id en tified :
• Th e greatest red u ction in STC occu rred w h en th ere w as a sh ort u n im p ed ed p ath betw een boxes; th at is, w h en th e sou n d d id n ot h ave to travel th rou gh th e sou n d -absorbin g m aterial or th rou gh th e n arrow gap betw een op p osite stu d s in to th e ad jacen t stu d cavity. Th is is con firm ed by th e STC ratin gs (see Figu re 1) for: - boxes in th e back-to-back p osition w ith
no sound-absorbing material in the cavity; - boxes in th e back-to-back p osition w ith
d isp laced sou n d -absorbin g m aterial in th e cavity;
- boxes located within the same stud cavity w ith a h orizon tal offset of 350 m m or less w ith n o sou n d -absorbin g m aterial in th e cavity.
• Wh en th e sou n d h ad to travel th rou gh th e sou n d -absorbin g m aterial, th e effect of th e boxes w as greatly red u ced w h en com p ared to th e case in w h ich it d id n ot h ave to p ass th rou gh th e m aterial.
• Very little red u ction in STC w as n oted for w alls w ith th e electrical boxes located in an ad jacen t stu d cavity an d at least 400 m m ap art. Th e p resen ce of sou n d -absorbin g m aterial fu rth er en su res th at th e electrical boxes h ave a n egligible effect on STC.
• Th e tren d of in creased sou n d isolation w ith in creased sep aration breaks d ow n on ly w h en com p arin g th e con d ition s w ith both boxes w ith in th e sam e stu d cavity an d n o sou n d -absorbin g m aterial. Th is lack of im p rovem en t in sou n d isolation w ith in creased sep aration of boxes is likely d u e to stan d in g-w ave p attern s in th e cavity, w h ich are m ost p ron ou n ced w h en th ere is n o sou n d -absorbin g m aterial p resen t.
Th e figu re sh ow s th at for boxes located in back-to-back p osition s, in stallin g a layer of sou n d -absorbin g m aterial betw een th em greatly im p roves th e STC. Wh ere p ossible, th e sou n d -absorbin g m aterial arou n d th e boxes sh ou ld n ot be d isp laced becau se its effectiven ess w ill be red u ced .
Possible Re m e dia l M e a sure s: Box Tre a t m e nt s
Wh en an existin g w all h as d ecreased sou n d isolation becau se of im p rop erly located electrical boxes, th ere are several p ossible
rem ed ial m easu res, all of w h ich in volve m od ifica-tion s in an d arou n d th e
ou tlet box. Measu res in vestigated in th e stu d y are as follow s (see Table 2 w h ere th ey are ran ked in ord er of in creasin g effectiven ess):
Caulking.A bead of cau lk can be u sed to fill th e gap betw een th e electrical box an d th e gyp su m board . STC Rating
Electrical Box Location
(see Figu re 1)
Box Type Reference Wall Boxes located back-to-back Boxes separated by at least one stud
(No boxes) No h orizon tal offset 400-m m h orizon tal offset
Plastic Vap ou r-Barrier 55 55 55
Metal 55 51 53
Table 2.Improvement in sound isolation expressed as the mean change in STC for various treatments applied to two wall types (double stud and single stud) when the outlet boxes were located back-to-back.
Mean Ease
Treatments Improvement and
in STC Efficiency
Worst Non e 0
Cau lkin g (gap ) +1 Sim p le
Draft stop p er (u n d er facep late) +3 Sim p le Cau lkin g (gap ) an d box in sert +3 Difficu lt
Mastic-typ e m aterial (lin in g) +5 Mod erate
Th is sim u lates th e flan ge an d seal th at form th e airtigh t seal to th e gyp su m board in a p lastic vap ou r-barrier box.
Draft stopper.A d raft stop p er is a closed cell foam gasket, com m on ly sold in h ard -w are stores, -w h ich is p laced bet-w een th e gyp su m board an d th e facep late coverin g th e electrical ou tlet. Draft stop p ers are easy to in stall, bu t th e im p rovem en t in sou n d isolation w as fou n d to be variable becau se an ad equ ate seal can n ot be form ed if th e gyp su m board op en in g h as been p oorly cu t or th e box skew ed so it p rotru d es beyon d th e face of th e gyp su m board .
Electrical box inserts.Th ese in serts, m ad e from th in ru bber or p lastic, red u ce airflow if th ey are tigh tly fitted in sid e th e box. Th e d egree of airtigh tn ess an d resu ltan t im p rovem en t to th e sou n d isolation of th e w all is largely d eterm in ed by a) h ow w ell th e h oles or slits for th e electrical w ires are sealed w ith cau lkin g an d b) h ow w ell th e in sert is sealed to th e gyp su m board . (If p oorly cu t op en in gs in th e gyp su m board d o n ot allow th e in sert to be p rop erly sealed to th e gyp su m board , a bead of cau lk sh ou ld be ap p lied .)
Mastic-type materials.Lin in g th e in terior of an electrical box w ith a m astic m aterial th at is im p erviou s to air w ill in crease th e
sou n d isolation of th e w all, esp ecially if th e m aterial can be sealed to th e gyp su m board . Wh ile a w id e ran ge of m aterials, e.g., cau lkin g, can p rovid e th e n ecessary acou stical p rop erties, it is p ossible th at th ey m ay h ave oth er p rop erties, su ch as electrical con d u ctivity, th at m ake th em u n su itable for th is ap p lication . Before in stallation , th e su itability of th e m aterial for p lacem en t in electrical boxes sh ou ld be con firm ed by th e local electrical au th ority.
Conc lusions
Poorly p laced electrical ou tlet boxes can sign ifican tly d ecrease th e sou n d isolation of gyp su m board w alls — a d ecrease of u p to 6 STC p oin ts w as observed . Th is can be m in im ized by:
• en su rin g th at (u n treated ) m etal boxes are offset by 400 m m or m ore in ad jacen t stu d cavities rath er th an bein g p laced w ith in th e sam e stu d cavity, or • u sin g p lastic vap ou r-barrier boxes, or • u sin g retrofit tech n iqu es th at em u late th e
attribu tes of p lastic vap ou r-barrier boxes. Th e p resen ce of sou n d -absorbin g m aterial in th e cavity h elp s to fu rth er m in im ize th e effect of p oor p lacem en t, esp ecially w h en in stalled so th at th e m aterial blocks th e lin e of sigh t betw een th e boxes (see Figu re 1, Fitted In su lation ).
Re fe re nc e s
1. Th e p roject w as su p p orted by Can ad a Mortgage an d Hou sin g Corp oration (CMHC).
2. T.R.T. Nigh tin gale, “Effect of electrical ou tlet boxes on sou n d in su lation of a d ou ble leaf w all,” IRC-IR-772, Nation al Research Cou n cil, October 1998.
Dr. T.R.T. Nightinga leis a research officer in th e In d oor En viron m en t Program of th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .
Dr. J.D. Q uirtis a sen ior research officer an d grou p lead er of th e acou stics activity in th e sam e p rogram .
Dr. M.A . S ulta nis a sen ior research officer with IRC’s Fire Risk Man agem en t Program .
Ele c t ric a l Out le t s a nd Fire Re sist a nc e
Th is Up d ate sh ow s th at w h ile th e p resen ce of electrical ou tlet boxes n eed n ot sign ifican tly affect th e sou n d red u ction of gyp su m board w alls, it m ay ad versely affect th e fire resistan ce of th ese w alls. Th ree n on stan d ard tests w ere p erform ed at IRC on n on -load -bearin g, sm all-scale gyp su m board w all assem blies w ith w ood stu d s 400 m m o.c., to d eterm in e th e fire-resistan ce ratin g of th e w all w h en a sin gle electrical box w as in stalled . Th e resu lts of th ese tests w ere as follow s:
Referen ce w all assem bly (n o ou tlet box) 142 m in
Wall assem bly w ith m etal box 121 m in
Wall assem bly w ith p lastic vap ou r-barrier box 130 m in Th ese resu lts are n ot con clu sive an d can n ot be ap p lied d irectly to fu ll-scale assem blies or to cases w h ere th ere are boxes on both faces of th e w all, bu t th ey su ggest th at th e p resen ce of electrical ou tlet boxes m ay be of con cern w ith resp ect to th e fire-resis-tan ce ratin g of w alls. In th e absen ce of an y d ata for fu ll-scale w all assem blies, on e sh ou ld avoid p lacin g electrical ou tlet boxes in fire-rated assem blies, w h ere p ossible. How ever, if ou tlet boxes m u st be p laced in th e w all, u se th e sm allest n u m ber p er-m itted by ap p licable cod es, sp aced as far ap art as p ossible. Fu rth er w ork is n eed ed to assess th e effect of n u m ber, typ e an d location of electrical ou tlet boxes on fire resistan ce in fu ll-scale w all assem blies.
“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.
For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6
Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca
© 1999
Nation al Research Cou n cil of Can ad a May 1999
