Okna historyczne oraz ich wpływ na bilans energetyczny budynku

Dominika Strzałka-Rogal

doi:10.37190/arc240207

Streszczenie

W związku z tym, że coraz większy nacisk kładzie się na to, aby istniejące budynki były energooszczędne, wymiana okien w obiektach o zabytkowym charakterze stała się tematem ważnym i kontrowersyjnym. W prezentowanej pracy przedstawiono współczesne tendencje o zasięgu europejskim i światowym dotyczące wymiany historycznej stolarki na nową. Na podstawie analizy literatury przedmiotu omówiono zagadnienia konserwacji, naprawy i termomodernizacji starych okien, by spróbować określić, w jaki sposób oraz w jakim stopniu wymiana nie tylko niekorzystnie wpływa na estetykę budynku, ale również na aspekt ekonomiczny i ostateczny bilans energetyczny budynku. Celem artykułu jest ukazanie zabytkowego okna jako detalu architektonicznego, który warto za wszelką cenę zachować podczas przeprowadzanej renowacji budynku, modernizując je w taki sposób, by sprostało współczesnym wymogom energetycznym. Autorka zwraca w pracy uwagę także na kwestię zrównoważonego rozwoju oraz szacunek do otaczającego dany obiekt dziedzictwa architektonicznego.

Podgląd pełnego artykułu jest możliwy wyłącznie na większych ekranach.

2024

2(78)

Dominika Strzałka-Rogal*

Historical windows and their impact on the energy balance

of the building

Introduction

The loss of original, antique woodwork from historic

buildings is one of the main threats to our heritage. Tradi-

tional window frames and glazing are important to historic

areas, and their value goes far beyond aesthetics.

Originally, the windows were openings in the wall cov-

ered with skins or fabrics, which, with the passage of time,

were replaced by wooden shutters, set on hinges. Shutters can

be considered the rst protections from weather conditions in

history. Over time, the function of the window became not

only to allow light to enter the interior, but to protect it from

the elements. Window openings also had a de co rative func-

tion, enriching the architectural form of the building façade.

The article pays special attention to wooden windows, among

which the following types can be distinguished: loom,

frame,

box, composite, single-frame [1, pp. 7–9].

By building tangible connections to the past, windows

contribute to a unique sense of place, strengthening the

connection between communities and their heritage. They

DOI: 10.37190/arc240207

Published in open access. CC BY NC ND license

are a testament to craftsmanship as well as artisanship and

can be important artifacts in their own right, often made

with great care and ingenuity, using materials of higher

quality than those generally available nowadays (Fig. 1).

The distinctive look of historic handmade glass is not eas-

ily imitated by modern glazing. Original historic window

woodwork also represents an environmental value, as the

maintenance and restoration of existing windows reduces

the need for new production and promotes sustainability.

Windows are particularly sensitive building elements

be cause they are relatively easy to replace or change. Such

work often has a profound aesthetic and compositional im-

pact not only on the building itself, but also on the appear-

ance of the street and neighbourhood. The depth and thick-

ness of the frames and sills, the width and visual weight of

the sash elements, the materials, colour and pattern of the

light reecting o the glass – all complement and empha-

size the architectural style, texture and age of the build-

ing. Modern counterparts tend to have wider mullions that

provide less light to the interior (Fig. 2). Much of this char-

acter is lost when windows are replaced with their modern

counterparts, which lack these features. There are simple

ways to repair and upgrade window frames that are fea-

* ORCID: 0000-0001-6437-048X. Faculty of Architecture, Cra -

cow University of Technology, Poland, e-mail: d.strzalka@admin.pk.edu.pl

Abstract

With the increasing emphasis on making existing buildings energy ecient, window replacement in historic buildings has become an important and

controversial topic. In the presented work, contemporary trends of European and global scope regarding the replacement of historic woodwork with

new woodwork are presented. Based on an analysis of the literature on the subject, the issues of maintenance, repair and thermal modernization of old

windows are discussed to try to determine how and to what extent the replacement not only adversely aects the aesthetics of the building, but also the

economic aspect and the nal energy balance of the building. The purpose of the article is to show the historic window as an architectural detail that

is worth preserving at all costs during the renovation of the building, upgrading it to meet modern energy requirements. Attention is also given to the

issue of sustainability and respect for the architectural heritage of the building’s surroundings.

Key words: historic windows, renovation, sustainability, energy balance

66 Dominika Strzałka-Rogal

sible for the building owner or craftsman, and that benet

the building’s energy balance [2].

The topic addressed in the article remains controversial

for many researchers, as arguments about historic values

and energy eciency collide. In the available literature on

the subject one can nd items presenting solid arguments

for both preservation and replacement of historic windows.

In her paper, the author focused on analysing a group of re-

search papers in which historical value is put before energy

eciency, citing, among others, the research collected in

the article Windows in historic buildings: sustainable, re-

pairable Susan D. Turner [2], the methods described in the

guide Traditional windows. Their care, repair and upgrad-

ing by David Pickles et al. [3], research in What replace-

ment windows can’t replace: the real cost of removing his-

toric windows by Walter Sedovic and Jill Gotthelf [4] and

Repair or replace by Craig Sims and Andrew Powter [5].

The purpose of this article is to show the historic window

as an architectural detail that aects the energy balance of

a building – an element that is worth preserving at all costs

during a building renovation.

Window replacement

Although old windows are often seen as the elements

responsible for heat loss in a building, preserving historic

woodwork is crucial to maintaining the integrity and au-

thenticity of buildings.

An economic analysis of the life cycle of a single win-

dow has shown that replacing historic windows to reduce

heating costs is largely a myth and remains the result of

misinformation disseminated by manufacturers

[2]. It

turns out […] that high-quality equivalent replacements

cost up to three times more than restoring a historic win-

dow [4, p. 29].

Windows with vinyl or aluminium cladding do not re-

quire cyclic painting, as is the case with wood, but they

scratch and fade, factory seals fail, and joints can separate

[4]. These forms of deterioration cannot be stopped by main-

tenance. In modern windows, serious irreparable damage to

the sealing joints occurs within 10–25 years [5]. Currently,

most seals are only warranted for 8 to 10 years. Regardless

of its resemblance to the architectural original, a replica will

also never convey the nuances of the original (Fig. 2) [6].

Renovation of historic windows

A successful historic window restoration project re-

quires a clear understanding of the existing condition and

the owner’s expectations for the maintenance and life of

the windows [7]. If possible, a comprehensive study of

Sims and Powter cite a belief dating back to the energy crisis of the

1970s. According to them, this was a view repeated by all Canadian gov-

ernment agencies concerned with helping homeowners save energy [5].

Fig. 1. Cracow, Poselska 7, antique window joinery:

a) interior view, b) view of exterior façade (photo by D. Strzałka-Rogal)

Il. 1. Kraków, ul. Poselska 7, zabytkowa stolarka okienna:

a) widok od wnętrza, b) widok elewacji zewnętrznej (fot. D. Strzałka-Rogal)

a b

Historical windows and their impact on the energy balance of the building 67

existing windows can more accurately tailor a restoration

approach that achieves the desired eect [8]. Based on

this experience, it is estimated that at least 95% of original

windows can be restored – there are countless examples of

windows that are 250 years old with a remaining useful life

of 100 years with proper restoration and maintenance [6].

Assessing the signicance of the window or windows

and their contribution to the overall character of the build-

ing is the rst step in deciding on the appropriate course of

action [7]. The preservation of a historic original façade is

an irreplaceable heritage resource that should be left and

epaired whenever possible [8]. The signicance of a histo -

ric building, both as a whole and in terms of its component

parts, can be assessed by considering its heritage value [9].

Historic window systems are typically built with great at-

tention to detail (such as weather protection) and with good

quality materials (such as heavy-duty wood) [2].

Prob-

lems associated with the passage of time – peeling paint,

cracked glass and missing putty – can look unsightly, but

they are easily repaired (Fig. 3) [10].

Some materials (such as PVC) used in replacement win-

dows are inherently unsuitable for maintenance. Once the

material degrades, it will need to be replaced. One of the

biggest advantages of historic windows is the quality of the

wood from which they are constructed [11]. Historic win-

dows use wood that has a denser, more natural grain struc-

ture than what is available today. In addition, in the past,

more attention was paid to milling methods, such as quarter

Fig. 2. Cracow, Poselska 7, antique window joinery:

a) interior view, b) construction of a casement window (photo by D. Strzałka-Rogal)

Il. 2. Kraków, ul. Poselska 7, zabytkowa stolarka okienna:

a) widok od wnętrza, b) konstrukcja okna skrzynkowego (fot. D. Strzałka-Rogal)

a b

sawing or radius sawing. The resulting window is charac-

terized by greater stability than its modern counterpart.

In many cases, traditional windows are slated for repla-

cement due to their poor thermal properties. Research shows

that thermal performance can also be improved through

relatively inexpensive methods such as the use of blinds,

shades and curtains. Further increases in performance are

possible with the introduction of methods such as second-

ary glazing, which allows the historic character of the win-

dow to be preserved [12].

The results of a study cited by Søren Vadstrup [11],

prove that existing old/original wood single-glazed win-

dows with mastic sealed frames can be energy retrotted

with interior double-glazed windows or frames coupled

with energy-ecient glazing, saving 30–50% more heat

than equivalent new wood, plastic, aluminium or wood-al-

uminium double-glazed windows or the latest low-energy

thermopanes

[11]. Even if an energy window has very low

heat loss through the centre of the glass (centre U-factor),

the energy loss calculated for the entire window (including

edge losses, for example) will be higher than for a similar

old putty window with energy glass inside. If all energy-

ecient glazing is used in the interior glazing, the dier-

A material composed of two plates of at window glass, glued to-

gether exactly at the edges, leaving a gap of about 6 mm, which is provid-

ed by strips of glass inserted between the plates; slit glass, hermetic glass.

68 Dominika Strzałka-Rogal

ence is even greater in favour of the old windows [11]

This solution is also the best in terms of trac noise attenu-

ation, the risk of external condensation on the glazing, and

the several-hundred-year lifespan of the entire window. In

addition, in terms of environmental performance and CO

balance, the energy-ecient old window also achieves

the best values. In a similar study, it was also documented

that ongoing maintenance for 30 years is also signicantly

cheaper for old energy-ecient wood windows if they are

maintained according to regulations with linseed oil paint,

compared to similar new thermal windows made of wood,

plastic, aluminium or wood-aluminium.

In December 1999. The Raadvad Centre was awarded a re-

search project under the Danish Energy Agency’s Project Window (J.nr.

75661/99-0006) on improving the energy performance of existing win-

dows in older buildings. The Raadvad Centre, together with various oth-

er collaborators, also conducted work on life cycles, life cycle analyses,

sound and light quality measurements for old and new windows. The

result of the research clearly indicates that new windows with insulat-

ing glass or energy glazing are not the right choice in terms of energy

or heating management in older homes, compared to renovation of old

windows complemented by replacement windows with energy glazing

or energy-saving glass. This means that in buildings older than 60–70

years, old wooden windows should be preserved and repaired, and en-

ergy-ecient glazing or energy-ecient glazing should be installed in-

side them as coupled frames or front windows. This improves heating

eciency and overall economy, while preserving the architecture of the

building [11].

Energy balance

When considering the impact of older windows on the

energy balance, it is important to note that individual fac-

tors such as climate, building design, insulation levels and

other energy-saving measures also play a signicant role.

Typically, the most eective ways to improve the energy

performance of a historic building are to control all sources

of air leakage and have an ecient heat source (furnace,

boiler or other) [13].

The age of a building’s windows can aect the energy

balance, primarily in terms of energy eciency [14]. Over

time, gaps or cracks can develop in the windows, allowing

air to enter or leave the building (Fig. 4). This air inltra-

tion can lead to drafts, loss of conditioned air and increased

energy consumption as heating or cooling systems com-

pensate for temperature changes.

Replacing historic windows with modern, airtight dou-

ble-glazed units is one of the most expensive, short-lived,

least eective, yet most popular ways to reduce energy

consumption in historic buildings [15]. Sometimes man-

ufacturers of new windows commonly boast of low U-

values. However, U-values are often mistakenly reported

as the

value for the entire window, when in fact it is the

value for the centre of the glass (the place with the best

U-value), not the sash or the average for the entire window.

To be sure the data is properly presented, ask for U-val-

ues published by the National Fenestration Rating Coun-

cil (NFRC), which assesses the performance of the entire

window. When U-values are realistically presented for the

entire window assembly, they are often much worse (i.e.,

higher) due to inltration around the frame and unevenness

of the opening. To compare windows in a scientic way,

you must rst calculate and measure the entire window,

not just the glass. Second, you need to compare windows

with the same dimensions and with exactly the same frame

and glazing bars. It is also important to pay attention to

comparative analyses: some manufac turers of replacement

windows compare their window units to an “equivalent”

single-pane aluminium window [3].

Heat loss/heat gain

Heat losses are often discussed, while heat gains re-

main marginalized. In summer, heat gains can signicantly

increase the energy costs associated with cooling a build-

ing. Long wavelengths of the daylight spectrum that enter

through the glass must be able to exit, otherwise they turn

into heat, which must then be absorbed by the building’s

cooling system. Glass with a low heat transfer coecient

(“low-e” or “soft low-e”) performs this task best, improving

thermal performance by reecting infrared (long-wave) ra-

diation reachingthe window.

Inltration of outside air – not heat lost through the glaz-

ing – is the main culprit aecting the heat balance. It can

account for up to 50% of a building’s total heat loss [3].

When retrot windows are installed on or within an existing

window frame, the argument for preservation already ex-

ists: restoring the integrity of the t between the frame and

the building wall should be the priority of a preservation

Fig. 3. Cracow, Poselska 7, a deadbolt after renovation

(photo by D. Strzałka-Rogal)

Il. 3. Kraków, ul. Poselska 7, zasuwka po renowacji

(fot. D. Strzałka-Rogal)

Historical windows and their impact on the energy balance of the building 69

approach. Sash pockets, pulleys and rails are areas prone

to air inltration in double-hung windows. Replacement

window manufacturers themselves acknowledge that even

in replacement, double-hung windows present the greatest

challenge in terms of reducing heat loss, as inltration oc-

curs most often at the sash-sash and sash-frame interface,

which is highly dependent on the quality of installation [4].

Insulated glass

Listed windows almost always contain insulating glass

units. The eectiveness of double glazing depends largely

on the depth of the air space between the inner and outer

panes, as well as the type, type and amount of desiccant and

gaskets used around the perimeter of the glass. Although

Insulated Glass Unit (IGU) manufacturing techniques con -

tinue to improve, replacement in the event of failure is di-

cult and time-consuming, and the extra weight and thick-

ness of IGUs prevent their use as retrot elements in historic

wood or metal sashes [4]

Laminated glass as an alternative

Laminated glass remains an often overlooked alternative

to insulating glass units, perhaps due to the industry’s empha-

sis on advertising it as “safety” glass. While l aminated glass

cannot compete with high-tech, complex insulating glass

units, it does oer better U-values

than monolithic glass,

without having to signicantly alter the mullions of the his-

toric sash in which it is installed. It should be remem

bered,

The heat transfer coecient U indicates how much energy (ex-

pressed in watts) penetrates 1 m

of a partition (wall, roof, window, door)

when the temperature dierence on either side of it is 1 K. The unit of

heat transfer coecient is W/(m

-K). Typical dierences in U-values

can be: for single glazing: 4.8–5.8 W/m

K, for double 1.2–3.7 W/m

(depends on the type), and for triple <1 W/m

however, that the U-factor is not the only criterion for de-

termining the relative thermal eciency of a window. Light

and solar transmittance also aect the window’s properties,

and the benets can be greater if laminated glass is chosen.

It is worth noting that for historic buildings, the heat transfer

coecient U

may not be the same as for newly built build-

ings or other non-historic buildings undergoing thermal up-

grading

[16]. On the one hand, energy audits, improvement

of the emissivity of buildings, environmental certicates are

imposed on investors, and on the other hand, the circles lob-

bying these trends turn a blind eye to such aspects as the

carbon footprint of manufactured plastic windows.

Studies show that the best solutions in terms of energy,

and thus heating economics, are achieved by keeping the

existing wooden windows in older houses and retrotting

them with interior double-glazed windows with energy-

ecient glass or glazing [4].

Sustainability

When properly repaired and upgraded, old windows

can usually be brought to a condition comparable to new

windows. This approach is “sustainable” on many levels.

The Deutsches Institut für Normung standard species that the

external wall U-value should be 0.20 W/(m

-K), for historic buildings

insulated from the outside it allows U = 0.45 W/(m

-K), and for build-

ings insulated from the inside, especially for frame buildings, it allows

U = 0.65 W/(m

-K). It is not possible to treat historic buildings the same

way as buildings currently under construction and strive at all costs to

achieve a U-value for exterior walls at the statutory level – currently

0.20 W/(m

-K).

According to Article 3, paragraph 4, rst indent of the Usta wa

z dnia 29 sierpnia 2014 r. o charakterystyce energetycznej bu dyn ków

[Law on Energy Performance of Buildings], the obligations to prepare

an energy performance certicate do not apply to buildings subject to

protection under the provisions on the protection and care of historical

monuments [16].

Fig. 4. Cracow, Poselska 7, historic window joinery:

a) photo taken with a thermovision camera showing heat loss within the window, b) existing condition of the window (photo by D. Strzałka-Rogal)

Il. 4. Kraków, ul. Poselska 7, zabytkowa stolarka okienna:

a) zdjęcie wykonane kamerą termowizyjną ukazujące utraty ciepła w obrębie okna, b) obecny stan okna (fot. D. Strzałka-Rogal)

a b

70 Dominika Strzałka-Rogal

building renovation carried out. At the same time, improv-

ing the thermal and functional parameters of old windows,

while respecting the historic character (and often even sav-

ing money) is possible [8]. When examining old original

wooden windows (pre-1950) that are being replaced with

new double-glazed windows, it can be seen that many of

them are in excellent condition and that they can be easily

repaired and energy upgraded at a much lower cost than re-

placing them with equivalent new double-glazed windows

[2]. In a situation where old original wooden windows in

historic buildings can achieve better energy performance,

have less maintenance, and have a lifespan 10–20 times

longer than equivalent new double-glazed windows, there

is no energy or economic argument, let alone an environ-

mental one, in favour of replacing old windows with new

double-glazed windows [5].

In fact, replacement windows can have unrealistic pay-

back periods, and many are dicult to maintain and pre-

serve. In many cases, given the life-cycle costs and mainte-

nance of the historic fabric, repairing and restoring historic

windows proves to be not only environmentally and aes-

thetically sound, but also economical [20]. Alternative ma-

terials, such as aluminium or vinyl and berglass, which

have been used to replace historic original windows, result

in a short lifespan. The decision to replace or refurbish win-

dows also requires thinking in terms of sustainability. The

author points out the trade-o between material gains and

the long-term impact of the action taken, not only on the

building in question, but also on the architectural, environ-

mental and economic context, in order to nd appropriate

energy eciency solutions.

Retrotting is generally less costly than collective repla-

cement and preserves the greatest amount of historic ma-

terial. It also allows preservation of the invested energy

put into material processing, labour, transportation. Em-

bedded energy is lost when windows go to the landll.

Scrapping must also be added to the costs incurred [3].

In addition to the loss of architectural value as a result

of replacing carpentry elements, a lot of technical damage

to the building is observed. Old windows are often con-

structed of old lumber with good structural details, mak-

ing them more durable and easier to maintain than new

replacement windows [17]. With proper maintenance,

they can last for decades, and repair costs over the life of

a building are less than the cost of periodic replacement

[18]. Repair costs typically

also represent an investment

in labour that benets the local economy and does not in-

volve gathering resources or shipping products from a dis-

tant production site [5]. Sustainable construction is about

using fewer non-renewable resources. The goal of preser-

vation and renovation by eye will not just be to reduce the

monthly heating bill, but the overall and long-term envi-

ronmental impact of the actions taken [19]. Wasting the

energy used to extract raw materials, manufacture, trans-

port, install and maintain an existing window worsens the

overall balance of energy.

Conclusions

Based on the analysis of research results presented in

selected literature, it can be concluded that the preserva-

tion of historic window frames aects the success of the

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Historical windows and their impact on the energy balance of the building 71

Streszczenie

Okna historyczne oraz ich wpływ na bilans energetyczny budynku

W związku z tym, że coraz większy nacisk kładzie się na to, aby istniejące budynki były energooszczędne, wymiana okien w obiektach o zabytko-

wym charakterze stała się tematem ważnym i kontrowersyjnym. W prezentowanej pracy przedstawiono współczesne tendencje o zasięgu europejskim

i światowym dotyczące wymiany historycznej stolarki na nową. Na podstawie analizy literatury przedmiotu omówiono zagadnienia konserwacji,

naprawy i termomodernizacji starych okien, by spróbować określić, w jaki sposób oraz w jakim stopniu wymiana nie tylko niekorzystnie wpływa na

estetykę budynku, ale również na aspekt ekonomiczny i ostateczny bilans energetyczny budynku. Celem artykułu jest ukazanie zabytkowego okna

jako detalu architektonicznego, który warto za wszelką cenę zachować podczas przeprowadzanej renowacji budynku, modernizując je w taki sposób,

by sprostało współczesnym wymogom energetycznym. Autorka zwraca w pracy uwagę także na kwestię zrównoważonego rozwoju oraz szacunek do

otaczającego dany obiekt dziedzictwa architektonicznego.

Słowa kluczowe: okna historyczne, renowacja, zrównoważony rozwój, bilans energetyczny

[17] Lynch M., Caring for your old windows, “Historic New England

Magazine”, Fall 2000, 1.

[18] Window rehabilitation guide for historic buildings, C. Fisher,

D. Sla ton, R. Shier (eds.), Historic Preservation Education Foun-

dation, Washington, D.C., 1997.

[19] Mattinson B., DePaola R., Arasteh D., What should I do about my

windows?, “Home Energy” 2002, Vol. 19, No. 4, 24–31.

[20] Sidler S., What to do with old wood windows, “Fine Homebuilding”

2021, Iss. 296, 78–84, https://www.nehomebuilding.com/2020/11/

09/what-to-do-with-old-windows [accessed: 15.05.2023].