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Window condensation in historic buildings that have been adapted for
new uses
Brown, W. C.
https://publications-cnrc.canada.ca/fra/droits
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https://nrc-publications.canada.ca/eng/view/object/?id=86662de6-d20b-4932-8f89-fada238a447f https://publications-cnrc.canada.ca/fra/voir/objet/?id=86662de6-d20b-4932-8f89-fada238a447fb y W.C. Brow n
When a heritage building is adapted for a new use, changes to the indoor
environment can lead to condensation problems. This Update describes
an evaluation of selected w indow s undertaken by IRC researchers at
Ottaw a’s Laurier House (now being used as a museum) to determine their
effectiveness in controlling condensation.
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 . 5
Th e evalu ation of a h eritage or ‘h istoric’ bu ild in g to d eterm in e h ow it w ill p erform u n d er n ew con d ition s in volves th e follow in g step s: id en tifyin g th e sp ecific u ses to w h ich th e bu ild in g w ill be p u t; establish in g th e tem p eratu re an d relative h u m id ity n eed ed to su p p ort th is u se; d eterm in in g th e clim atic con d ition s in w h ich th e bu ild in g h as to op erate (i.e., th e w in ter d esign tem p eratu re, w h ich is read ily available for an y location in Can ad a) an d d eterm in in g th e actu al p erfor-m an ce of th e bu ild in g en velop e th rou gh a p rogram of m on itorin g. Su ch a p rogram m ay d em on strate th at th e level of p erform an ce th e bu ild in g p rovid es is ad equ ate for th e p rop osed u se u n d er th e p rop osed con d ition s an d th at n o m od ification s are requ ired .
Th e objectives of th e IRC p roject w ere: 1) to exam in e th e bu ild in g, review th e
cu rren t situ ation an d d eterm in e w h ich bu ild in g en velop e elem en ts requ ired fu rth er in vestigation ; an d
2) to recom m en d op eratin g/ refu rbish in g strategies, if n ecessary, to im p rove th e m oistu re p erform an ce of th e variou s bu ild in g en velop e elem en ts.
A N e w U se for La urie r H ouse Lau rier Hou se, a d esign ated Nation al Historic Site, w as bu ilt in 1878. Th e form er resid en ce of Can ad ian Prim e Min isters Sir Wilfrid Lau rier an d Macken zie Kin g is n ow bein g u sed as a m u seu m .
For th e form er in h abitan ts of Lau rier Hou se, in d oor con d ition s in w in ter p robably in clu d ed a tem p eratu re of 18°C an d a relative h u m id ity (RH) of betw een 20% an d 30% . How ever, th e exten sive collection of h istorical artifacts n ow h ou sed th ere requ ires con stan t in d oor con d ition s in w in ter of 21°C an d 35% RH.
Th e u se of Lau rier Hou se as a m u seu m requ ires th at th e bu ild in g en velop e be able to con trol con d en sation u n d er th e p rop osed op eratin g con d ition s. Th is con trol is a fu n ction of th e d esign an d con stru ction of th e elem en ts th at com p rise th e bu ild in g en velop e, an d of th e in d oor an d ou td oor en viron m en ts. (Th e w in ter d esign tem p era-tu re for Ottaw a is -25°C.)
Window Condensation in
Historic Buildings that Have
Been Adapted for New Uses
2
Th e h igh h u m id ity levels requ ired by th e n ew u se com bin ed w ith Ottaw a’s low w in ter d esign tem p eratu re su ggested th at a large qu an tity of con d en sation m igh t form on th e w in d ow s an d in th e w alls. Wh ile m ain te-n ate-n ce record s available for Lau rier Hou se d id n ot p rovid e ad equ ate in form ation abou t past condensation performance, water staining on the sills of many of the windows indicated th at con d en sation h ad occu rred .
Becau se of th is evid en ce of p reviou s con d en sation p roblem s, com bin ed w ith th e fact th at w in d ow s are p articu larly su scep tible to con d en sation (becau se th ey u su ally h ave th e low est th erm al resistan ce am on g th e variou s elem en ts th at m ake u p th e bu ild in g en velop e), th e evalu ation of th e w in d ow s at Lau rier Hou se w as con d u cted first.
It sh ou ld be n oted th at th e p resen ce of con d en sation on th e w in d ow s d oes n ot n ecessarily m ean th at con d en sation is occu rrin g in th e w alls. It is also th e case th at strategies for d ealin g w ith con d en sation on w in d ow s can d iffer from th ose requ ired to ad d ress con d en sation in w alls.
In ord er to d eterm in e w h eth er effective con d en sation con trol w as bein g ach ieved by th e w in d ow s as-fou n d , an d to p red ict th eir con d en sation p erform an ce u n d er th e p rop osed op eratin g con d ition s, selected w in d ow s, in th eir origin al (‘h istoric’) con d ition , w ere in stru m en ted w ith th erm o-cou p les an d m on itored by IRC research ers. Te m pe ra t ure M onit oring of
‘H ist oric ’ Window s
Lau rier Hou se h as tw o typ es of w in d ow s: • w ood -fram e, sin gle-h u n g w in d ow s • m etal-fram e, lead ed -glass casem en t
w in d ow s.
All w in d ow s are sin gle-glazed an d all storm w in d ow s are w ood -fram e. For th e sin gle-h u n g w in d ow s, th e storm w in d ow s are m ou n ted on th e exterior, as u su al; for th e casem en t w in d ow s, th ey are m ou n ted on th e in terior (becau se th e casem en t w in d ow s op en ou tw ard ).
To evalu ate con d en sation resistan ce, th e research ers selected tw o w in d ow s of each typ e (see Figu re 2). For each typ e, on e of th e w in d ow s w as less airtigh t th an th e oth er, Princ iple s of Conde nsa t ion Cont rol
For con d en sation to occu r, a su rface h as to be at or below th e d ew p oin t tem p eratu re of th e ad jacen t air. Th is is th e tem p eratu re at w h ich th e vap ou r in an air-vap ou r m ixtu re begin s to con d en se. Con d en sation form s on th e glazin g, sash , or fram e elem en ts of a w in d ow m ore read ily th an elsew h ere in th e bu ild in g en velop e becau se th e w in d ow is typ ically cold er.
On e m easu re of th e con d en sation p oten tial of th e in d oor en viron m en t is its d ew p oin t tem p eratu re, w h ich is related to its m oistu re con ten t. For air at a tem p eratu re of 21°C an d an RH of 35% , for exam p le, th e d ew p oin t tem p eratu re is abou t 5°C.
Th e tem p eratu re in d ex (Ix), w h ich relates th e in terior su rface tem p eratu re of a w in d ow or w all to th e exterior
tem p eratu re an d th e th erm al ch aracteristics of th e w in d ow or w all, is a m easu re of con d en sation resistan ce. Th e in d ex is d efin ed as
w h ere
Tx is th e su rface tem p eratu re;
Te is th e w eath er-sid e (exterior) air tem p eratu re; an d
Ti is th e room -sid e (in terior) air tem p eratu re.
(Note: Th e tem p eratu re in d ex can also be exp ressed as a p ercen tage.)
Wh at th e form u la for tem p eratu re in d ex actu ally states is th at th e m ore th e room-side surface temperature of the window can be like the indoor temperature, the greater the condensation resistance. This relationship therefore links the con-densation resistance of glazing to its thermal performance and thereby to its design.
In creased con d en sation resistan ce is ach ieved by im p rovin g th e th erm al p erform an ce of th e w in d ow.
Th e tem p eratu re in d ex w ill n orm ally be con stan t over th e ran ge of con d ition s to which buildings are subjected and it can be treated as a property of the assembly. A high temperature index is required for applications with a high indoor relative humidity, or a cold outdoor design temperature, or both, in order to minimize condensation on the interior surface.
Wh en th e tem p eratu re in d ex for a p articu lar w in d ow or w all in com bin ation w ith th e w in ter d esign tem p eratu re of th e location of th e bu ild in g elem en t is kn ow n , th e in terior relative h u m id ity su stain able by th at bu ild in g elem en t w ith ou t con d en sation occu rrin g can easily be d eterm in ed . Usin g th e sam e ap p roach , th e tem p eratu re in d ex (Ix) th at a w in d ow requ ires to su stain a p articu lar RH can be d eterm in ed . For exam p le,
w h ere an RH of 30% an d an in d oor tem p eratu re of 21°C is requ ired , w in d ow s w ith a tem p eratu re in d ex of 0.60 are n eed ed to p reven t con d en sation .
I
x=
( T
x- T
e)
( T
i- T
e)
Glass Exterior Te Interior Ti Txtem p eratu re in d ices. Th e con d en sation p oten tial of a w in d ow can be evalu ated in con d ition s oth er th an th ose th at actu ally p rod u ce con d en sation — th at is, th e evalu a-tion d oes n ot h ave to be carried ou t u n d er con d en sation con d ition s.
Th e tem p eratu re an d relative h u m id ity of th e in terior air, exterior air, an d th e air from a d iffu ser located in th e floor w ere also m easu red as w as th e relative h u m id ity in th e sp ace betw een th e p rim e an d storm w in d ow s of on e of th e sin gle-h u n g w in d ow s.
Wh ile th e w in d ow s w ere bein g tested , th e in terior tem p eratu re w as m ain tain ed betw een 22°C an d 23°C at an RH betw een 20% an d 30% .
To obtain rep resen tative d ata, th e room en viron m en ts w ere kep t in th eir n orm al op eratin g state for th e d u ration of th e m on itorin g so th at th e th erm al en viron m en t of th e w in d ow s w ou ld n ot ch an ge. For example, blinds for the single-hung windows, n orm ally h alfw ay closed , w ere left in th is p osition w h ile th e w in d ow s w ere m on itored . No blin d s w ere m ou n ted in fron t of th e m etal casem en t w in d ow s.
Results of Evaluating the Window s For both typ es of w in d ow s, th e m easu red tem p eratu re in d ices sh ow ed th at con d en sa-tion w ou ld occu r at exterior tem p eratu res of -10°C an d low er, if th e in terior con d ition s w ere m ain tain ed at 21°C w ith an RH of 35% . Given th at th e w in ter d esign tem p eratu re for Ottaw a is -25°C, it is clear th at sign ifican t con d en sation w ill occu r on th e w in d ow s u n less m easu res are taken to p reven t it.
Single-hung wood windows. Condensation w ill occu r on th e (exterior) storm w in d ow s, u n less th e (in terior) sin gle-h u n g w in d ow s are m ad e airtigh t to p reven t air leakage th rou gh h oles an d air-p erm eable m aterials. No con d en sation , h ow ever, w ill be fou n d on th e w ood fram es of th e p rim e w in d ow s.
Metal casement w indow s. Th e tem p eratu re in d ices on th e m etal casem en t w in d ow s in d icate th at con d en sation w ill occu r u n less th e storm w in d ow s, fitted on th e room sid e, are m ad e airtigh t.
De a ling w it h Ex c e ssive
Condensation on ‘Historic’ Window s Th ree op tion s are available to ad d ress w in d ow con d en sation at Lau rier Hou se: 1. Accep t th at con d en sation w ill occu r an d
be p rep ared to
a) rem ove it (by m op p in g, etc.) d aily, an d b) refin ish th e w in d ow su rfaces an n u ally. i.e., on e w as left u n altered , w h ile th e oth er
w as sealed at th e in n er su rface. All windows w ere m on itored for a m in im u m of on e w eek d u rin g late Febru ary/ early March u sin g th erm ocou p les in stalled at variou s location s on th e w in d ow s to record th e tem p eratu res.
Th e collected d ata w ere u sed to calcu late th e tem p eratu re in d ex (Ix) at th ese variou s
location s, an d th e con d en sation resistan ce (con d en sation p oten tial) of a p articu lar w in d ow w as th en d ed u ced from th e 3 STORM WINDOW SINGLE-HUNG WINDOW a bc c b a c b a a a a b c a = 6 mm b = 12 mm c = 12 mm C e n t r e of glass C e n t r e of glass C e n t r e of glass C e n t r e of glass a a 1092 1346 762
Figure 2. Windows showing locations of the thermocouples. Thermocouples were located at the corners of the windows (where the temperature tends to be the coldest and therefore where condensation is most likely to occur) as well as at the centre (where condensation is least likely to occur). Not only does this arrangement allow for measuring the temperature index at a particular point, but also for determining the area of the window that would likely experience condensation. For the single-hung windows, thermocouples were located around the frame in order to assess whether the construction of the frame (i.e., the presence of counter weights contained in the frame) had an adverse effect on the condensation potential of the frame. As well, thermocouples were placed on the storm window to help assess how it contributes to the total performance of the window.
SOUTH NORTH
LEADED-GLASS CASEMENT WINDOW
419 419 a a a = 6 mm b = 12 mm c = 12 mm C e n t r e of glass C e n t r eof glass a c b a c b a c b a a a 1168 INSIDE ST ORM WINDO W 1264 OUTSIDE CASEMENT WINDO W b c
2. Ch an ge th e in terior en viron m en t ad jacen t to th e w in d ow so th at th e w in d ow su rface is at or above th e d ew p oin t tem p eratu re th u s p reven tin g con d en sation from occu rrin g on it.
3. Im p rove th e th erm al p erform an ce of th e w in d ow s.
Th e first op tion requ ires th e least in ter-ven tion w ith regard to th e ‘h istoric’ fabric of th e bu ild in g, bu t requ ires a p erm an en t com -m it-m en t on th e p art of th e ow n er to en su re th at m oistu re accu m u lation is tem p orary an d th at fin ish es are m ain tain ed . Th e secon d op tion requ ires sign ifican t in terven tion in th at a sou rce of h eat or d ry air, or both , is requ ired . Th ese m easu res sin gly or in com -bin ation w ou ld en su re th at th e d ew p oin t of th e air ad jacen t to th e w in d ow is less th an th e tem p eratu re of th e w in d ow. Th e th ird op tion p robably p rovid es th e best lon g-term con trol of con d en sation on th e w in d ow s an d w arran ts a closer look.
In th e case of an h istoric bu ild in g, th e ch oice of strategy d ep en d s on th e d egree to w h ich h istorical au th en ticity an d / or th e origin al aesth etic m u st be m ain tain ed an d on th e m ain ten an ce an d op eratin g bu d gets available. So w h ile it ap p ears th at Op tion 3 is th e best on e, th ere m ay be factors w h ich ren d er it u n accep table or n ot viable. I m proving t he T he rm a l
Pe rform a nc e of ‘H ist oric ’ Window s Recom m en d ed im p rovem en ts d ep en d on th e typ e of w in d ow, an d on th e con d ition s in w h ich th ey h ave to fu n ction .
Single-hung w ood w indow s. Th e th erm al p erform an ce of th e glazin g of th e sin gle-h u n g w in d ow s can be im p roved by im p lem en tin g on e of tw o m easu res:
• rep lacin g th e sin gle-glazed p an e in th e prime window with low-emissivity (low-e), sealed d ou ble-glazed u n its; or
• ad d in g a th ird layer (th e storm w in d ow p rovid es th e secon d layer) of glazin g, eith er sin gle or d ou ble, to th e in terior of th e w in d ow.
Eith er m easu re w ill p rovid e su fficien t im p rovem en t in th erm al p erform an ce to red u ce con d en sation on th e glazin g to an accep table level. Note: Th e exterior storm w in d ow s w ou ld be left u n ch an ged in both cases.
In the case of Laurier House, the sash of the w ood w in d ow s h ap p en s to be th ick en ou gh to accom m od ate a sealed , d ou ble-glazed u n it w ith 3-m m -th ick glass an d a 13-m m -d eep air space. Such a replacement u n it w ith an alu m in u m sp acer and a low-e coating h as a th erm al resistan ce of ap p roxi-m ately RSI-0.35 in th e cen tre of th e glass an d RSI-0.20 at th e ed ge. Warm-edge tech-nology (WET) sp acers w ou ld fu rth er im p rove th e th erm al p erform an ce at th e ed ge of th e sealed u n it, esp ecially at th e corn ers.
Ad d in g a th ird layer of glazin g to th e in terior of th e w in d ow w ou ld form an ad d ition al air sp ace, h elp in g to in crease th e RSI valu e of th e sin gle-h u n g w in d ow. Th at, in tu rn , w ou ld in crease th e tem p eratu re of its room -sid e su rface, th ereby p reven tin g con d en sation .
Metal casement w indow s. Th e th erm al p erform an ce of th e m etal casem en t w in d ow s can be im p roved by rep lacin g th e sin gle-glazed storm (i.e., in terior) w in d ow s w ith low -e, sealed , d ou ble-glazed u n its. Th ese u n its w ou ld be th e sam e as th ose su ggested as replacements for the single-hung windows.
Th ese in n er storm w in d ow s w ou ld n eed to be in stalled in an airtigh t m an n er so th at th ere is n o m oistu re from leakin g air th at cou ld con d en se on th e casem en t w in d ow s. (Note: In an y w in d ow system , it is alw ays th e in n erm ost w in d ow th at m u st be airtigh t.) Conc lusions
By establish in g th e con d en sation ch aracter-istics of th e w in d ow s, ow n ers of h istoric bu ild in gs con sid erin g th eir reu se can d eter-m in e w h eth er eter-m easu res h ave to be taken to p reven t con d en sation from occu rrin g on th e w in d ow s or w h eth er th ey are able to p rovid e accep table p erform an ce u n d er th e p rop osed u se an d op eratin g con d ition s in th eir as-fou n d con d ition .
W.C. Brow nis a sen ior research officer in th e Bu ild in g En velop e Laboratory 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
© 1997
Nation al Research Cou n cil of Can ad a May 1997
