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Why houses need mechanical ventilation systems
Haysom, J. C.; Reardon, J. T.
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=13076694-f9ff-4700-905e-f979e1ddae1b https://publications-cnrc.canada.ca/fra/voir/objet/?id=13076694-f9ff-4700-905e-f979e1ddae1bb y J.C. Ha y som a nd J.T. Re a rd on
This Update is the first of tw o that discuss mechanical ventilation systems in
houses. It explains w hy houses need to be mechanically ventilated, and
examines the main characteristics of an ideal system from the standpoint of
design and installation.
C 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 . 1 4
T he N e e d for M e c ha nic a l Ve nt ila t ion
History of Ventilation in Houses
Hou ses n eed to h ave an in d oor/ ou td oor exch an ge of air to rep len ish oxygen u sed by th e occu p an ts an d to rem ove p ollu tan ts gen erated by breath in g, h ou seh old activities an d em ission s from bu ild in g m aterials an d fu rn ish in gs. For m an y years, h ou ses w ere con stru cted w ith ou t m ech an ical ven tilation system s an d relied on air leakage th rou gh th e bu ild in g en velop e to p rovid e th is in d oor/ ou t-door air exchange during the winter months.
In th e p ast, th is n atu ral form of ven tilation w orked fairly w ell. Hou ses bu ilt before th e 1960s ten d ed to be qu ite leaky an d p ressu re d ifferen ces betw een th e in sid e an d ou tsid e, cau sed by w in d or tem p eratu re d ifferen ce, w ere su fficien t to p rovid e a sign ifican t am ou n t of air exch an ge m ost of th e tim e. How ever, a leaky bu ild in g en velop e d oes n ot alw ays gu aran tee ad equ ate air exch an ge. Th e m ovem en t of air requ ires both a p ath w ay (e.g., a leak) an d a p ressu re d ifferen ce, an d even a leaky h ou se w ill exp erien ce p eriod s w h en th ere is n o
in d oor/ ou td oor air exch an ge. Th ese p eriod s are m ost likely to occu r d u rin g th e sp rin g or fall, w h en w in d s are ligh t an d th ere is little or n o in d oor/ ou td oor tem p eratu re d ifferen ce th at can create a stack effect. Th e leakier th e h ou se, h ow ever, th e less frequ en t th e p eriod s of in ad equ ate air exch an ge.
Sin ce m ost fu el-fired system s con su m e air from th e h ou se, an d th is air m u st th en be rep laced by leakage from ou td oors, th e op eration of fu el-fired system s p rom otes som e in d oor/ ou td oor air exch an ge. Th e ch im n eys associated w ith th ese system s also p rovid e a m ajor leakage p oin t, th u s p rom otin g air exch an ge even w h en th e h eatin g system is n ot op eratin g. As w ell, a ch im n ey ten d s to raise th e level of th e n eu tral p ressu re p lan e, th u s red u cin g th e ou tw ard p ressu re d ifferen ce across th e bu ild in g en velop e an d , w ith it, th e
Why Houses Need
Mechanical Ventilation
Systems
Canada Mortgage and Housing Corporation, “Complying with Residential Ventilation Requirements in the 1995 National Building Code” (1996), cover photo
2
p oten tial for in terstitial con d en sation (i.e., con d en sation th at occu rs w ith in th e bu ild in g en velop e) cau sed by air leakin g ou t of th e h ou se.
In h ou ses bu ilt p rior to th e 1960s, th e am ou n t of air exch an ge p rovid ed by leakage w as gen erally regard ed as su fficien t. Bu t in th e ‘60s, a n u m ber of factors ch an ged th is p ictu re, in clu d in g th e in creased u se of electric h eatin g in h ou ses. Un like fu el-fired system s, electric h eatin g system s d o n ot requ ire th e rep lacem en t of air, n or d o th ey requ ire ch im n eys. Con sequ en tly, electrically h eated h ou ses h ave a greater ten d en cy to exp erien ce h igh h u m id ity levels, in terior su rface m ou ld s an d in terstitial con d en sation .
In th e early 1970s, in resp on se to th ese p roblem s associated w ith electrically h eated h ou ses, Can ad a Mortgage an d Hou sin g Corp oration (CMHC) took th e step of requ irin g all NHA-fin an ced electri-cally h eated h ou ses to in corp orate exh au st fan s, a requ irem en t th at w as even tu ally in corp orated in to th e Nation al Bu ild in g Cod e. By th e m id -70s, th ese p roblem s h ad becam e so ap p aren t th at CMHC con tem -p lated n ot allow in g electric h eatin g in h ou ses fin an ced u n d er its Nation al Hou sin g Act m ortgage in su ran ce p rogram .
In ad d ition to th e in crease in th e u se of electric h eatin g, th e 1960s brou gh t th e con -stru ction of h ou ses th at w ere m u ch m ore airtigh t as a resu lt of n ew p rod u cts an d p ractices, w h ich in clu d ed th e su bstitu tion of p an el sh eath in gs, su ch as p lyw ood an d w aferboard , for board sh eath in g; th e rep lace-m en t of p ap er-backed in su lation batts by friction -fit batts an d p olyeth ylen e film ; improved caulking materials; tighter windows and doors; and more efficient heating systems. Wh en th e en ergy crisis d evelop ed in th e early 1970s, considerable emphasis was placed on red u cin g air leakage in ord er to con serve en ergy. Th e u se of electric h eatin g system s w as en cou raged an d h igh er efficien cy fu r-n aces w ere d evelop ed fu rth er red u cir-n g air-change rates in buildings. This trend towards greater airtigh tn ess an d h igh er efficien cy fu rn aces gave rise to con cern s th at th e exch an ge of air in h ou ses by n atu ral m ean s m igh t be in su fficien t in som e in stan ces to p rovid e ad equ ate air qu ality th u s in creasin g th e risk of h ealth p roblem s am on g th e occu -pants. Condensation problems resulting from h igh er h u m id ity levels w ere also a con cern .
How Much Indoor/Outdoor Air Exchange Is Necessary?
Th e air-ch an ge n eed s of h ou ses are n ot u n iform . Not on ly d o th ey vary from h ou se to h ou se accord in g to th e n u m ber of occu p an ts, an d th e p resen ce an d stren gth of variou s p ollu tan t sou rces, bu t, for an y given h ou se, th ey also vary w ith tim e as occu p an ts com e an d go, an d p ollu tan t sou rces w ax an d w an e. Neverth eless, ASHRAE Stan d ard 62, Can ad ian Stan d ard s Association Stan d ard CAN/ CSA-F326 an d th e Nation al Bu ild in g Cod e of Can ad a (NBC) h ave all establish ed levels of air ch an ge th at can be exp ected to m eet th e p eak or n ear-p eak n eed s of a m ajority of n orm al h ou seh old s. (Th e latter tw o are based to som e exten t on ASHRAE Stan d ard 62.)
All th ree ap p roach es su ggest an air ch an ge rate of abou t 0.3 air ch an ges p er h ou r (ach ). Th is is th e level of air ch an ge u sed in tern ation ally as th e n orm in term s of an alyzin g th e su ccess of variou s ven tila-tion sch em es. Again , it is recogn ized th at few, if an y, h ou ses requ ire con stan t air ch an ge at th e rate of 0.3 ach . How ever, if a h ou se is so tigh t th at leakage fails to p rovid e th is level of air ch an ge for sign ifi-can t p eriod s of tim e, it is likely th at m an y su ch p eriod s of sh ortfall w ill coin cid e w ith p eriod s w h en th is level of air ch an ge is requ ired . Wh en th is h ap p en s, p oor in d oor air qu ality, h igh h u m id ity, su rface m ou ld s an d in terstitial con d en sation can resu lt.
How Airtight Are Recently Built Houses?
In 1989, a stu d y to d eterm in e th e airtigh t-n ess of recet-n tly cot-n stru cted h ou ses it-n variou s region s of Can ad a w as con d u cted . Airtigh tn ess w as m easu red by carryin g ou t fan -d ep ressu rization tests on n early 200 h ou ses th rou gh ou t th e cou n try. Th e test resu lts w ere an alyzed to estim ate th e in d oor/ ou td oor air ch an ge rate th at cou ld be attribu ted solely to th e air leakage likely to be exp erien ced by each h ou se over a typ ical h eatin g season . Th e resu lts of th e stu d y allow ed th e research ers to m ake th e follow in g p red iction s:
• More th an 70% of th e su rveyed h ou ses w ou ld h ave an average air-leakage rate of less th an 0.3 ach over th e en tire h eatin g season .
• th ere is su fficien t air exch an ge d u e to w in d or stack effect to m eet th e h ou se-h old ’s n eed s.
Provide needed amount of air exchange
Th e system w ou ld be able to d eliver en ou gh ou td oor air to m eet th e p robable m axim u m n eed s of th e h ou seh old . It w ou ld also be cap able of m od u latin g d eliv-ery so th at it d id n ot d eliver m ore ou td oor air th an requ ired at tim es of red u ced n eed . A system th at d oes n ot h ave th is cap ability is likely to p rovid e too m u ch ou td oor air m ost of th e tim e it is in op eration , resu ltin g in excess en ergy costs an d low h u m id ity. As w ell, a system th at is u n resp on sive can an n oy th e occu p an ts, p ossibly to th e p oin t th at th ey sim p ly tu rn it off altogeth er.
Distribute outdoor air w here needed
It is n ot en ou gh th at th e m ech an ical ven tilation system ch an ge th e air in th e h ou se as a w h ole to m eet th e stan d ard of 0.3 ach . Th e system m u st also be able to d eliver th e ou td oor air to th ose p arts of th e h ou se w h ere th e occu p an ts are likely to sp en d m ost of th eir tim e — th e livin g room , th e kitch en an d th e bed room s.
Be quiet
Th e system w ou ld be qu iet en ou gh so th at th e occu p an ts w ou ld n ot be tem p ted to tu rn it off to red u ce n oise.
Not interfere w ith other systems
Th ere is sign ifican t p oten tial for m ech an ical ven tilation system s to in terfere w ith th e op eration of oth er system s, su ch as certain typ es of fu el-fired h eatin g system s. Un d er th ese circu m stan ces, if th e ven tilation system creates a h igh n egative p ressu re in th e h ou se, th e p rod u cts of com bu stion (w h ich can be h arm fu l to th e occu p an ts) can sp ill in to th e h ou se rath er th an flow in g u p th e ch im n ey to th e ou td oors.
Not interfere w ith the building envelope
Th e system w ou ld n ot create sign ifican t p ositive p ressu re in th e h ou se sin ce th is cou ld d rive h u m id air from th e h ou se th rou gh th e bu ild in g en velop e, resu ltin g in in terstitial con d en sation .
De m a nd-Cont rolle d Ve nt ila t ion Th e first tw o ch aracteristics of th e id eal m ech an ical ven tilation system d escribed above are related to th e issu e of con trol. A system th at em bod ies th ese ch aracteristics is kn ow n as a “d em an d -con trolled ven tila-tion system .” Su ch a system w ou ld id eally • Alm ost 90% of th e su rveyed h ou ses
w ou ld h ave at least on e m on th d u rin g th e h eatin g season w h en th e average air-leakage rate w as less th an 0.3 ach . • Virtu ally all of th e su rveyed h ou ses (99% )
would have at least one 24-hour period over th e h eatin g season in w h ich th e average air-leakage rate w as less th an 0.3 ach .
Th ese resu lts seem to in d icate th at a m ajority of h ou ses bein g bu ilt in Can ad a u sin g n orm al con stru ction p ractices are close en ou gh to bein g airtigh t th at air leak-age th rou gh th e en velop e can n ot be relied on to p rovid e th e rate of air ch an ge d eem ed n ecessary to m ain tain ad equ ate in d oor air qu ality in a typ ical h ou seh old . Wh ile th e rate of air ch an ge th rou gh th e bu ild in g en velop e m ay be ad equ ate m ost of th e tim e, it m ay n ot be all of th e tim e. Th erefore, to en su re th at a satisfactory rate of air ch an ge is attain able at all tim es th rou gh ou t th e h eatin g season , th ese h ou ses m u st be p ro-vid ed w ith m ech an ical ven tilation system s. Cha ra c t e rist ic s of a n I de a l
M e c ha nic a l Ve nt ila t ion Syst e m Cu rren tly available tech n ology is n ot able to p rovid e an id eal m ech an ical ven tilation system for h ou ses. Bu t before lookin g at th e m eth od s of m ech an ically ven tilatin g h ou ses that are available today, it is helpful to identify th e ch aracteristics of an id eal system :
Operate w hen needed
Th e system w ou ld op erate w h en ever ad d i-tion al in d oor/ ou td oor air exch an ge is n eed ed an d w ou ld d o so w ith ou t th e n eed for occu p an t in terven tion .
Operate only w hen needed
Th is is im p ortan t sin ce a m ech an ical ven ti-lation system h as costs associated w ith it — the cost of the electricity to run it and the cost of h eatin g th e ou td oor air th at th e system brin gs in . (Th e latter can be red u ced by incorporating heat-recovery capabilities in the system , bu t can n ot be elim in ated altogeth er.) Th erefore, th e system sh ou ld n ot op erate d u rin g th ose p eriod s w h en n o in d oor/ ou t-d oor air exch an ge is requ iret-d . Th e len gth , tim in g an d frequ en cy of su ch p eriod s vary from h ou seh old to h ou seh old . Air
exch an ge is n ot requ ired w h en : • th ere are n o occu p an ts in th e h ou se • th ere are n o activities or p rocesses
u n d erw ay th at gen erate p ollu tan ts
be con trolled by an array of sen sors — on e for h u m id ity an d on e for every p ossible p ollu tan t th at th e ven tilation system w ou ld h ave to resp on d to, in clu d in g carbon m on oxid e, carbon d ioxid e, form ald eh yd e, volatile organ ic com p ou n d s, etc. Th e system w ou ld brin g in ou td oor air an d / or extract in d oor air u n til all of th ese sen sors d eterm in ed th at sp ecific p ollu tan ts w ere at, or below, p red eterm in ed safe levels.
Wh en ever a sen sor d etected a p ollu tan t above its safe level, th e ven tilation system w ou ld op erate.
A less-th an -id eal d em an d -con trolled ven tilation system w ou ld h ave at least on e sen sor. For exam p le, m an y ven tilation sys-tem s are con trolled by d eh u m id istats: th e system op erates u n til th e d eh u m id istat h as d eterm in ed th at th e h u m id ity in th e h ou se is at a safe level. Excess h u m id ity is on e of th e m ain reason s th at ven tilation is
requ ired , bu t n ot th e on ly on e. Th e am ou n t of ven tilation requ ired to con trol h u m id ity m ay n ot be su fficien t to con trol oth er p ollu tan ts sin ce th is d ep en d s on th e activities of th e occu p an ts, on th e relative stren gth s of oth er p ollu tan ts an d on th e level of h u m id ity.
Carbon d ioxid e (CO2) sen sors are som e-tim es u sed to con trol ven tilation system s in large bu ild in gs, an d th is tech n ology is ju st n ow becom in g available for resid en tial u se. In creasin g CO2 con cen tration is u su -ally a good in d icator of d ecreasin g air qu al-ity bu t it m ay n ot be ad equ ate in cases w h ere th ere are u n u su al p ollu tan ts, su ch as th ose gen erated by certain h obbies.
Th e id eal system requ ires th e fu ll array of sen sors m en tion ed above. How ever, at p resen t th is id eal is n ot attain able becau se: • th ere is in su fficien t kn ow led ge an d
in form ation to d eterm in e
- w h ich p ollu tan ts sh ou ld be m on itored , an d
- w h at th e accep table levels for a p artic-u lar p ollartic-u tan t are.
• p ractical, reliable an d econ om ical d etec-tors for all p ollu tan ts of con cern are n ot available.
Wh ile research an d d evelop m en t is u n d erw ay in m an y cou n tries to try to ad d ress th ese issu es, breakth rou gh s are n ot exp ected in th e n ear fu tu re.
For a d iscu ssion of cu rren t ap p roach es to m ech an ical ven tilation system s for h ou ses, p lease see Con stru ction Tech n ology Up d ate No. 15. Re fe re nc e s
1. ASHRAE 62-1989, Ven tilation for
Accep table In d oor Air Qu ality. Am erican Society of Heatin g, Refrigeratin g an d Air-Con d ition in g En gin eers, Atlan ta, GA. 2. Stan d ard CAN/ CSA-F326-M91,
Resid en tial Mech an ical Ven tilation System s. Can ad ian Stan d ard s Association , Etobicoke, ON.
3. Nation al Bu ild in g Cod e of Can ad a, 1995. Can ad ian Com m ission on Bu ild in g an d Fire Cod es, Nation al Research Cou n cil of Can ad a, Ottaw a.
4. 1989 Su rvey of Airtigh tn ess of New, Merch an t Bu ild er Hou ses. Haysom , J.C., Reard on , J.T., an d R. Mon sou r. In d oor Air ’90: Th e Fifth In tern ation al
Con feren ce on In d oor Air Qu ality an d Clim ate, v. 4, Toron to, 1990.
5. Resid en tial Air System Design . Heatin g Refrigeratin g an d Air-Con d ition in g In stitu te of Can ad a (HRAI), Islin gton , ON, 1986.
6. Com p lyin g w ith Resid en tial Ven tilation Requ irem en ts in th e 1995 Nation al Bu ild in g Cod e. Can ad a Mortgage an d Hou sin g Corp oration , Ottaw a, 1996. 7. Airtigh tn ess an d En ergy Efficien cy of
New Con ven tion al an d R-2000 Hou sin g in Can ad a, 1997. Can ad a Cen tre for Min eral an d En ergy Tech n ology, Natu ral Resou rces Can ad a, Ottaw a.
Mr. John Ha y somis a sen ior tech n ical ad visor with th e Cod es an d Evalu ation Program of th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .
Dr. J.T. Re a rd onis a research officer with 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 .
“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
© 1998
Nation al Research Cou n cil of Can ad a May 1998
