PRESERV ATION 

BRIEFS 

The Repair and 

Thermal Upgrading of 

Historic Steel Windows 

Sharon C. Park, AlA 

U.S. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

The Secretary of the Interior's "Standards for Rehabilitation" require that where historic windows are individually significant features, or where 

they contribute to the character of significant facades, their distinguishing visual qualities must not be destroyed. Further, the rehabilitation 

guidelines recommend against changing the historic appearance of windows through the use of inappropriate designs, materials, finishes, or colors 

which radically change the sash, depth of reveal, and muntin configuration; the reflectivity and color of the glazing; or the appearance of the 

frame. 

Windows are among the most vulnerable features of 

historic buildings undergoing rehabilitation. This is 

especially the case with rolled steel windows, which are 

often mistakenly not deemed worthy of preservation in the 

conversion of old buildings to new uses. The ease with 

which they can be replaced and the mistaken assumption 

that they cannot be made energy efficient except at great 

expense are factors that typically lead to the decision to 

remove them. In many cases, however, repair and retrofit 

of the historic windows are more economical than wholesale replacement, and all too often, replacement units are 

unlike the originals in design and appearance. If the windows are important in establishing the historic character of 

the building (see fig. 1), insensitively designed replacement 

windows may diminish-or destroy-the building's historic 

character. 

This Brief identifies various types of historic steel 

windows that dominated the metal window market from 

1890-1950. It then gives criteria for evaluating deterioration 

and for determining appropriate treatment, ranging from 

routine maintenance and weatherization to extensive 

repairs, so that replacement may be avoided where possible. I This information applies to do-it-yourself jobs and to 

large rehabilitations where the volume of work warrants the 

removal of all window units for complete overhaul by professional contractors. 

This Brief is not intended to promote the repair of ferrous metal windows in every case, but rather to insure 

that preservation is always the first consideration in a 

rehabilitation project. Some windows are not important 

elements in defining a building's historic character; others 

are highly significant, but so deteriorated that repair is infeasible. In such cases, the Brief offers guidance in 

evaluating appropriate replacement windows. 

Fig. 1 Often highly distinctive in design and craftsmanship, rolled steel 

windows play an important role in defining the architectural character of 

many later nineteenth and early twentieth century buildings. Art Deco, 

Art Moderne, the International Style, and Post World War II Modernism depended on the slim profiles and streamlined appearance of metal 

windows for much of their impact. Photo: William G. Johnson. 

'The technical information given in this brief is intended for most ferrous (or 

magnetic) metals, particularly rolled steel. While stainless steel is a ferrous metal, 

the cleaning and repair techniques outlined here must not be used on it as the finish 

will be damaged. For information on cleaning stainless steel and non-ferrous 

metals, such as bronze, Monel, or aluminum, refer to Metals in America's Historic 

Buildings (see bibliography). 
HISTORICAL DEVELOPMENT 

Although metal windows were available as early as 1860 

from catalogues published by architectural supply firms, 

they did not become popular until after 1890. Two factors 

combined to account for the shift from wooden to metal 

windows about that time. Technology borrowed from the 

rolling industry permitted the mass production of rolled 

steel windows. This technology made metal windows cost 

competitive with conventional wooden windows. In addition, a series of devastating urban fires in Boston, 

Baltimore, Philadelphia, and San Francisco led to the 

enactment of strict fire codes for industrial and multistory commercial and office buildings. 

As in the process of making rails for railroads, rolled 

steel windows were made by passing hot bars of steel 

through progressively smaller, shaped rollers until the appropriate angled configuration was achieved (see fig. 2). 

The rolled steel sections, generally 118" thick and 1" -

1 112" wide, were used for all the components of the windows: sash, frame,and subframe (see fig. 3). With the addition of wire glass, a fire-resistant window resulted. 

These rolled steel windows are almost exclusively found in 

masonry or concrete buildings. 

A byproduct of the fire-resistant window was the 

strong metal frame that permitted the installation of 

larger windows and windows in series. The ability to have 

expansive amounts of glass and increased ventilation 

dramatically changed the designs of late 19th and early 

20th century industrial and commercial buildings. 

The newly available, reasonably priced steel windows 

soon became popular for more than just their fireresistant qualities. They were standardized, extremely 

durable, and easily transported. These qualities led to the 

use of steel windows in every type of construction, from 

simple industrial and institutional buildings to luxury 

commercial and apartment buildings. Casement, doublehung, pivot, projecting, austral, and continuous windows 

differed in operating and ventilating capacities. Figure 4 

outlines the kinds and properties of metal windows 

available then and now. In addition, the thin profiles of 

metal windows contributed to the streamlined appearance 

of the Art Deco, Art Moderne, and International Styles, 

among others. 

The extensive use of rolled steel metal windows continued until after World War II when cheaper, noncorroding aluminum windows became increasingly 

popular. While aluminum windows dominate the market 

today, steel windows are still fabricated. Should replacement of original windows become necessary, replacement 

windows may be available from the manufacturers of 

some of the earliest steel windows. Before an informed 

decision can be made whether to repair or replace metal 

windows, however, the significance of the windows must 

be determined and their physical condition assessed. 

Cover illustration: from Hope's Metal Windows and Casements: 

1818-1926, currently Hope's Architectural Products, Inc. Used with permission. 

2 

ROLLING SECTION FROM BAR 

1ST PASS 2ND PASS 3RD PASS 4TH PASS 

5TH PASS 6TH PASS 7TH PASS 8TH PASS 

FULL SIZE 

Fig. 2. The process of rolling a steel bar into an angled section is illustrated above. The shape and size of the rolled section will vary slightly depending on the overall strength needed for the window opening and 

the location of the section in the assembly: subframe, frame, or sash. 

The 1/ 8 " thickness of the metal section is generally standard. Drawing: 

A Metal Window Dictionary. Used with permission. 

?A?H· 

Fig. 3 A typical section through the top and bottom of a metal window 

shows the three component parts of the window assembly: subframe, 

frame, and sash. Drawings: Catalogue No. 15, January 1931; International Casement Co, Inc., presently Hope's Architectural Products, Inc., 

Jamestown, NY. Used with permission. 
EVALUATION 

Historic and Architectural Considerations 

An assessment of the significance of the windows should 

begin with a consideration of their function in relation to 

the building's historic use and its historic character. Windows that help define the building's historic character 

should be preserved even if the building is being converted 

to a new use. For example, projecting steel windows used 

to introduce light and an effect of spaciousness to a 

warehouse or industrial plant can be retained in the conversion of such a building to offices or residences. 

Other elements in assessing the relative importance of 

the historic windows include the design of the windows 

and their relationship to the scale, proportion, detailing 

and architectural style of the building. While it may be 

easy to determine the aesthetic value of highly ornamented 

windows, or to recognize the importance of streamlined 

windows as an element of a style, less elaborate windows 

can also provide strong visual interest by their small panes 

or projecting planes when open, particularly in simple, 

unadorned industrial buildings (see fig. 5). 

One test of the importance of windows to a building is 

to ask if the overall appearance of the building would be 

changed noticeably if the windows were to be removed or 

radically altered. If so, the windows are important in 

defining the building's historic character, and should be 

repaired if their physical condition permits. 

Physical Evaluation 

Steel window repair should begin with a careful evaluation 

of the physical condition of each unit. Either drawings or 

photographs, liberally annotated, may be used to record 

the location of each window, the type of operability, the 

condition of all three parts-sash, frame and subframe-and the repairs essential to its continued use. 

Specifically, the evaluation should include: presence and 

degree of corrosion; condition of paint; deterioration of 

the metal sections, including bowing, misalignment of the 

sash, or bent sections; condition of the glass and glazing 

compound; presence and condition of all hardware, 

screws, bolts, and hinges; and condition of the masonry 

or concrete surrounds, including need for caulking or 

resetting of improperly sloped sills. 

Corrosion, principally rusting in the case of steel windows, is the controlling factor in window repair; 

therefore, the evaluator should first test for its presence. 

Corrosion can be light, medium, or heavy, depending on 

how much the rust has penetrated the metal sections. If 

the rusting is merely a surface accumulation or flaking, 

then the corrosion is light. If the rusting has penetrated 

the metal (indicated by a bubbling texture), but has not 

caused any structural damage, then the corrosion is 

medium. If the rust has penetrated deep into the metal, 

the corrosion is heavy. Heavy corrosion generally results 

in some form of structural damage,through delamination, 

to the metal section, which must then be patched or spliced. A sharp probe or tool, such as an ice pick, can be used to determine the extent of corrosion in the metal. If 

the probe can penetrate the surface of the metal and brittle strands can be dug out, then a high degree of corrosive 

deterioration is present. 

In addition to corrosion, the condition of the paint, the 

presence of bowing or misalignment of metal sections, the 

amount of glass needing replacement, and the condition 

of the masonry or concrete surrounds must be assessed in 

the evaluation process. These are key factors in determining whether or not the windows can be repaired in place. 

The more complete the inventory of existing conditions, 

the easier it will be to determine whether repair is feasible 

or whether replacement is warranted. 

Rehabilitation Work Plan 

Following inspection and analysis, a plan for the 

rehabilitation can be formulated. The actions necessary to 

return windows to an efficient and effective working condition will fall into one or more of the following 

categories: routine maintenance, repair, and weatherization. The routine maintenance and weatherization 

measures described here are generally within the range of 

do-it-yourselfers. Other repairs, both moderate and major, require a professional contractor. Major repairs normally require the removal of the window units to a 

workshop, but even in the case of moderate repairs, the 

number of windows involved might warrant the removal 

of all the deteriorated units to a workshop in order to 

realize a more economical repair price. Replacement of 

windows should be considered only as a last resort. 

Since moisture is the primary cause of corrosion in steel 

windows, it is essential that excess moisture be eliminated 

and that the building be made as weathertight as possible 

before any other work is undertaken. Moisture can accumulate from cracks in the masonry, from spalling mortar, from leaking gutters, from air conditioning condensation runoff, and from poorly ventilated interior spaces. 

Finally, before beginning any work, it is important to 

be aware of health and safety risks involved. Steel windows have historically been coated with lead paint. The 

removal of such paint by abrasive methods will produce 

toxic dust. Therefore, safety goggles, a toxic dust 

respirator, and protective clothing should be worn. 

Similar protective measures should be taken when acid 

compounds are used. Local codes may govern the 

methods of removing lead paints and proper disposal of 

toxic residue. 

ROUTINE MAINTENANCE 

A preliminary step in the routine maintenance of steel 

windows is to remove surface dirt and grease in order to 

ascertain the degree of deterioration, if any. Such minor 

cleaning can be accomplished using a brush or vacuum 

followed by wiping with a cloth dampened with mineral 

spirits or denatured alcohol. 

3 
Double-hung industrial windows 

duplicated the look of traditional wooden 

windows. Metal double-hung windows were 

early examples of a building product adapted to meet stringent new fire code requirements for manufacturing and high-rise 

buildings in urban areas. Soon supplanted 

in industrial buildings by less expensive 

pivot windows, double-hung metal windows regained popularity in the 1940s for 

use in speculative suburban housing. 

Pivot windows were an early type of industrial window that combined inexpensive first cost and low maintenance. Pivot 

windows became standard for warehouses 

and power plants where the lack of screens 

was not a problem. The window shown 

here is a horizontal pivot. Windows that 

turned about a vertical axis were also 

manufactured (often of iron). Such vertical pivots are rare today. 

Projecting windows, sometimes called 

awning or hopper windows, were perfected 

in the 1920s for industrial and institutional 

buildings. They were often used in "combination" windows, in which upper panels 

opened out and lower panels opened in. 

Since each movable panel projected to 

one side of the frame only, unlike pivot 

windows, for example, screens could be 

introduced. 

Austral windows were also a product of 

the 1920s. They combined the appearance 

of the double-hung window with the increased ventilation and ease of operation 

of the projected window. (When fully 

opened, they provided 70070 ventilation as 

compared to 50% ventilation for doublehung windows.) Austral windows were 

often used in schools, libraries and other 

public buildings. 

Casement windows adapted the English 

tradition of using wrought iron casements 

with leaded carnes for residential use. 

Rolled steel casements (either single, as 

shown, or paired) were popular in the 

1920s for cottage style residences and 

Gothic style campus architecture. More 

streamlined casements were popular in the 

1930s for institutional and small industrial 

buildings. 

Continuous windows were almost exclusively used for industrial buildings requiring high overhead lighting. Long 

runs of clerestory windows. operated by mechanical 

tension rod gears were typical. Long banks 

of continuous windows were possible 

because the frames for such 

windows were often 

structural elements 

of the building. 

Fig. 4 Typical rolled steel windows available jrom 1890 to the present. The various operating and ventilating capacities in combination 

with the aesthetics oj the window style were important considerations in the selection oj one window type over another. Drawings: 

Sharon C. Park; AlA. 

If it is determined that the windows are in basically 

sound condition, the following steps can be taken: 1) 

removal of light rust, flaking and excessive paint; 2) priming of exposed metal with a rust-inhibiting primer; 3) 

replacement of cracked or broken glass and glazing compound; 4) replacement of missing screws or fasteners; 5) 

cleaning and lubrication of hinges; 6) repainting of all 

steel sections with two coats of finish paint compatible 

with the primer; and 7) caulking the masonry surrounds 

with a high quality elastomeric caulk. 

Recommended methods for removing light rust include 

manual and mechanical abrasion or the application of 

chemicals. Burning off rust with an oxy-acetylene or propane torch, or an inert gas welding gun, should never be 

attempted because the heat can distort the metal. In addition, such intense heat (often as high as 3800 0 F) 

vaporizes the lead in old paint, resulting in highly toxic 

fumes. Furthermore, such heat will likely result in broken 

glass. Rust can best be removed using a wire brush, an 

aluminum oxide sandpaper, or a variety of power tools 

4 

Fig. 5 Windows ojten provide a strong visual element to relatively simple or unadorned industrial or commercial buildings. This 

design element should be taken into consideration when evaluating the significance oj the windows. Photo: Michael Auer. 
adapted for abrasive cleaning such as an electric drill with 

a wire brush or a rotary whip attachment. Adjacent sills 

and window jambs may need protective shielding. 

Rust can also be removed from ferrous metals by using 

a number of commercially prepared anti-corrosive acid 

compounds. Effective on light and medium corrosion, 

these compounds can be purchased either as liquids or 

gels. Several bases are available, including phosphoric 

acid, ammonium citrate, oxalic acid and hydrochloric 

acid. ¥ydrochloric acid is generally not recommended; it 

can leave chloride deposits, which cause future corrosion. 

Phosphoric acid-based compounds do not leave such 

deposits, and are therefore safer for steel windows. 

However, any chemical residue should be wiped off with 

damp cloths, then dried immediately. Industrial blowdryers work well for thorough drying. The use of running 

water to remove chemical residue is never recommended 

because the water may spread the chemicals to adjacent 

surfaces, and drying of thes~ surfaces may be more difficult. Acid cleaning compounds will stain masonry; 

therefore plastic sheets should be taped to the edge of the 

metal sections to protect the masonry surrounds. The 

same measure should be followed to protect the glazing 

from etching because of acid cop.tact. 

Measures that remove rust will ordinarily remove flaking paint as well. Remaining loose or flaking paint can be 

removed with a chemical paint remover or with a 

pneumatic needle scaler or gun, which comes with a series 

of chisel blades and has proven effective in removing flaking paint from metal windows. Well-bonded paint may 

serve to protect the metal further from corrosion, and 

need not be removed unless paint build-up prevents the 

window from closing tightly. The edges should be feathered by sanding to give a good surface for repainting. 

Next, any bare metal should be wiped with a cleaning 

solvent such as denatured alcohol, and dried immediately 

in preparation for the application of an anti-corrosive 

primer. Since corrosion can recur very soon after metal 

has been exposed to the air, the metal should be primed 

immediately after cleaning. Spot priming may be required 

periodically as other repairs are undertaken. Anticorrosive primers generally consist of oil-alkyd based 

paints rich in zinc or zinc chromate.

2 Red lead is no 

longer available because of its toxicity. All metal primers, 

however, are toxic to some degree and should be handled 

carefully. Two coats of primer are recommended. Manufacturer's recommendations should be followed concerning application of primers. 

REPAIR 

Repair in Place 

The maintenance procedures described above will be insufficient when corrosion is extensive, or when metal window sections are misaligned. Medium to heavy corrosion 

that has not done any structural damage to the metal sections can be removed either by using the chemical cleaning 

process described under "Routine Maintenance" or by 

sandblasting. Since sandblasting can damage the masonry 

surrounds and crack or cloud the glass, metal or plywood 

shields should be used to protect these materials. The 

sandblasting pressure should be low, 80-100 pounds per 

square inch, and the grit size should be in the range of 

#10-#45. Glass peening beads (glass pellets) have also been 

successfully used in cleaning steel sections. While sandblasting equipment comes with various nozzle sizes, 

pencil-point blasters are most useful because they give the 

operator more effective control over the direction of the 

spray. The small aperture of the pencil-point blaster is 

also useful in removing dried putty from the metal sections that hold the glass. As with any cleaning technique, 

once the bare metal is exposed to air, it should be primed 

as soon as possible. This includes the inside rabbeted section of sash where glazing putty has been removed. To reduce the dust, some local codes allow only wet blasting. 

In this case, the metal must be dried immediately, generally with a blow-drier (a step that the owner should consider 

when calculating the time and expense involved). Either 

form of sandblasting metal covered with lead paints produces toxic dust. Proper precautionary measures should 

be taken against toxic dust and silica particles. 

Bent or bowed metal sections may be the result of 

damage to the window through an impact or corrosive expansion. If the distortion is not too great, it is possible to 

realign the metal sections without removing the window to 

a metal fabricator's shop. The glazing is generally removed and pressure is applied to the bent or bowed section. 

In the case of a muntin, a protective 2 x 4 wooden bracing can be placed behind the bent portion and a wire 

cable with a winch can apply progressively more pressure 

over several days until the section is realigned. The 2 x 4 

bracing is necessary to distribute the pressure evenly over 

the damaged section. Sometimes· a section, such as the 

bottom of the frame, will bow out as a result of pressure 

exerted by corrosion and it is often necessary to cut the 

metal section to relieve this pressure prior to pressing the 

section back into shape and making a welded repair. 

Once the metal sections have been cleaned of all corrosion and straightened, small holes and uneven areas 

resulting from rusting should be filled with a patching 

material and sanded smooth to eliminate pockets where 

water can accumulate. A patching material of steel fibers 

and an epoxy binder may be the easiest to apply. This 

steel-based epoxy is available for industrial steel repair; it 

can also be found in auto body patching compounds or in 

plumber's epoxy. As with any product, it is important to 

follow the manufacturer's instructions for proper use and 

best results. The traditional patching technique-melting 

steel welding rods to fill holes in the metal sections-may 

be difficult to apply in some situations; moreover, the 

window glass must be removed during the repair process, 

or it will crack from the expansion of the heated metal 

sections. After these repairs, glass replacement, hinge 

lubrication, painting, and other cosmetic repairs can be 

undertaken as necessary. 

'Refer to Table IV, Types of Paint Used for Painting Metal in Metals in America's 

Historic Buildings, p, 139. (See bibliography). 

5 
To complete the checklist for routine maintenance, 

cracked glass, deteriorated glazing compound, missing 

screws, and broken fasteners will have to be replaced; 

hinges cleaned and lubricated; the metal windows painted, 

and the masonry surrounds caulked. If the glazing must 

be replaced, all clips, glazing beads, and other fasteners 

that hold the glass to the sash should be retained, if possible, although replacements for these parts are still being 

fabricated. When bedding glass, use only glazing compound formulated for metal windows. To clean the hinges 

(generally brass or bronze), a cleaning solvent and fine 

bronze wool should be used. The hinges should then be 

lubricated with a non-greasy lubricant specially formulated for metals and with an anti-corrosive agent. 

These lubricants are available in a spray form and should 

be used periodically on frequently opened windows. 

Final painting of the windows with a paint compatible 

with the anti-corrosive primer should proceed on a dry 

day. (Paint and primer from the same manufacturer 

should be used.) Two coats of finish paint are recommended if the sections have been cleaned to bare metal. 

The paint should overlap the glass slightly to insure 

weathertightness at that connection. Once the paint dries 

thoroughly, a flexible exterior caulk can be applied to 

eliminate air and moisture infiltration where the window 

and the surrounding masonry meet. 

Caulking is generally undertaken after the windows 

have received at least one coat of finish paint. The 

perimeter of the masonry surround should be caulked 

with a flexible elastomeric compound that will adhere well 

to both metal and masonry. The caulking used should be 

a type intended for exterior application, have a high 

tolerance for material movement, be resistant to 

ultraviolet light, and have a minimum durability of 10 

years. Three effective compounds (taking price and other 

factors into consideration) are polyurethane, vinyl acrylic, 

and butyl rubber. In selecting a caulking material for a 

window retrofit, it is important to remember that the 

caulking compound may be covering other materials in a 

substrate. In this case, some compounds, such as silicone, 

may not adhere well. Almost all modern caulking compounds can be painted after curing completely. Many 

come in a range of colors, which eliminates the need to 

paint. If colored caulking is used, the windows should 

have been given two coats of finish paint prior to caulking. 

Repair in Workshop 

Damage to windows may be so severe that the window 

sash and sometimes the frame must be removed for cleaning and extensive rust removal, straightening of bent sections, welding or splicing in of new sections, and reglazing. These major and expensive repairs are reserved for 

highly significant windows that cannot be replaced; the 

procedures involved should be carried out only by skilled 

workmen. (see fig. 6a-6f.) 

6 

As part of the orderly removal of windows, each window should be numbered and the parts labelled. The 

operable metal sash should be dismantled by removing the 

hinges; the fixed sash and, if necessary, the frame can 

then be unbolted or unscrewed. (The subframe is usually 

left in place. Built into the masonry surrounds, it can only 

be cut out with a torch.) Hardware and hinges should be 

labelled and stored together. 

The two major choices for removing flaking paint and 

corrosion from severely deteriorated windows are dipping 

in a chemical bath or sandblasting. Both treatments require removal of the glass. If the windows are to be dipped, a phosphoric acid solution is preferred, as mentioned 

earlier. While the dip tank method is good for fairly evenly distributed rust, deep set rust may remain after dipping. 

For that reason, sandblasting is more effective for heavy 

and uneven corrosion. Both methods leave the metal sections clean of residual paint. As already noted, after 

cleaning has exposed the metal to the air, it should be 

primed immediately after drying with an anti-corrosive 

primer to prevent rust from recurring. 

Sections that are seriously bent or bowed must be 

straightened with heat and applied pressure in a 

workshop. Structurally weakened sections must be cut 

out, generally with an oxy-acetylene torch, and replaced 

with sections welded in place and the welds ground 

smooth. Finding replacement metal sections, however, 

may be difficult. While most rolling mills are producing 

modern sections suitable for total replacement, it may be 

difficult to find an exact profile match for a splicing 

repair. The best source of rolled metal sections is from 

salvaged windows, preferably from the same building. If 

no salvaged windows are available, two options remain. 

Either an ornamental metal fabricator can weld flat plates 

into a built-up section, or a steel plant can mill bar steel 

into the desired profile. 

While the sash and frame are removed for repair, the 

subframe and masonry surrounds should be inspected. 

This is also the time to reset sills or to remove corrosion 

from the subframe, taking care to protect the masonry 

surrounds from damage. 

Missing or broken hardware and hinges should be 

replaced on all windows that will be operable. Salvaged 

windows, again, are the best source of replacement parts. 

If matching parts cannot be found, it may be possible to 

adapt ready-made items. Such a substitution may require 

filling existing holes with steel epoxy or with plug welds 

and tapping in new screw holes. However, if the hardware 

is a highly significant element of the historic window, it 

may be worth having reproductions made. 

Following are illustrations of the repair and thermal 

upgrading of the rolled steel windows in a National 

Historic Landmark (fig. 6). Many of the techniques 

described above were used during this extensive rehabilitation. The complete range of repair techniques is then summarized in the chart titled Steps jor Cleaning and Repairing Historic Steel Windows (see fig. 7). 
• 

Fig. 6 a. View of the wing of the State Capitol where the 

rolled steel casement windows are being removed for repair. 

Fig. 6 c. View of the rusted frame which was unscrewed from the 

sub frame and removed from the window opening and taken to a 

workshop for sandblasting. In some cases, severely deteriorated sections of the frame were replaced with new sections of milled bar 

steel. 

Fig. 6 e. View looking down towards the sill. The cleaned frame 

was reset in the window opening. The frame was screwed to the 

refurbished subframe at the jamb and the head only. The screw 

holes at the sill, which had been the cause of much of the earlier 

rusting, were infilled. Vinyl weatherstripping was added to the 

frame. 

Fig. 6 d. View looking down towards the sill. The sub frames appeared very rusted, but were in good condition once debris was 

vacuumed and surface rust was removed, in place, with chemical 

compounds. Where necessary, epoxy and steel filler was used to 

patch depressions in order to make the sub frame serviceable again. 

Fig. 6 f View from 

the outside of the 

completely 

refurbished window. 

In addition to the 

steel repair and the 

installation of vinyl 

weatherstripping, 

the exterior was 

caulked with 

polyurethane and 

the single glass was 

replaced with 

individual lights of 

thermal glass. The 

repaired and 

upgraded windows 

have comparable 

energy efficiency 

ratings to new 

replacement units 

while retaining the 

historic steel sash, 

frames and 

subframes. 

Fig. 6. The repair and thermal upgrading oj the historic steel windows at the State Capitol, Lincoln, Nebraska. This early twentieth 

century building, designed by Bertram Goodhue, is a National Historic Landmark. Photos: All photos in this series were provided by 

the State Building Division. 

7 
STEPS FOR CLEANING AND REPAIRING HISTORIC STEEL WINDOWS ________ _ 

Recommended Tools, Products and 

Work Item Techniques Procedures Notes 

*(Must be done in a 

workshop) 

1. Removing General maintenance Vacuum and bristle brushes to Solvents can cause eye and skin irdirt and and chemical cleaning remove dust and dirt; solvents ritation. Operator should wear progrease-from (denatured alcohol, mineral tective gear and work in ventilated 

metal spirits), and clean cloths to area. Solvents should not contact 

remove grease. masonry. Do not flush with water. 

2. Removing 

Rust! 

Corrosion 

Light Manual and mechanical Wire brushes, steel wool, Handsanding will probably be 

abrasion rotary attachments to electric necessary for corners. Safety goggles 

drill, sanding blocks and and masks should be worn. 

disks. 

Chemical cleaning Anti-corrosive jellies and li- Protect glass and metal with plastic 

quids (phosphoric acid prefer- sheets attached with tape. Do not 

red); clean damp cloths. flush with water. Work in ventilated 

area. 

Medium Sandblasting! abrasive Low pressure (80-100 psi) and Removes both paint and rust. Codes 

cleaning small grit (#10-#45); glass should be checked for environ menpeening beads. Pencil blaster tal compliance. Prime exposed 

gives good control. metal promptly. Shield glass and 

masonry. Operator should wear 

safety gear. 

Heavy *Chemical dip tank Metal sections dipped into Glass and hardware should be 

chemical tank (phosphoric removed. Protect operator. Deepset 

acid preferred) from several rust may remain, but paint will be 

hours to 24 hours. removed. 

*Sandblasting! Low pressure (80-100 psi) and Excellent for heavy rust. Remove or 

abrasive cleaning small grit (#10-#45). protect glass. Prime exposed metal 

promptly. Check codes for environmental compliance. Operator 

should wear safety gear. 

3. Removing Chemical method Chemical paint strippers Protect glass and masonry. Do not 

flaking suitable for ferrous metals. flush with water. Have good venpaint. Clean cloths. tilation and protection for operator. 

Mechanical abrasion Pneumatic needle gun chisels, Protect operator; have good ventilasanding disks. tion. Well-bonded paint need not be 

removed if window closes properly. 

4. Aligning Applied pressure Wooden frame as a brace for Remove glass in affected area. 

bent, bowed cables and winch mechanism. Realignment may take several days. 

metal 

sections *Heat and pressure Remove to a workshop. Apply Care should be taken that heat does 

heat and pressure to bend not deform slender sections. 

back. 

S 
Recommended Tools, Products and 

Work Item Techniques Procedures Notes 

*(Must be done in a 

workshop) 

5. Patching Epoxy and steel filler Epoxy fillers with high con- Epoxy patches generally are easy to 

depressions tent of steel fibers; plumber's apply, and can be sanded smooth. 

epoxy or autobody patching Patches should be primed. 

compound. 

Welded patches Weld in patches using steel Prime welded sections after grinding 

rods and oxy-acetylene torch connections smooth. 

or arc welder. 

6. Splicing in *Cut out decayed sec- Torch to cut out bad sections Prime welded sections after grinding 

new metal tions and weld in new back to 45 0 joint. Weld in connection smooth. 

sections or salvaged sections new pieces and grind smooth. 

7. Priming Brush or spray At least one coat of anti-cor- Metal should be primed as soon as 

metal application rosive primer on bare metal. it is exposed. If cleaned metal will 

sections Zinc-rich primers are general- be repaired another day, spot prime 

ly recommended. to protect exposed metal. 

8. Replacing Routine maintenance Pliers to pull out or shear off If new holes have to be tapped into 

missing rusted heads. Replace screws the metal sections, the rusted holes 

screws and and bolts with similar ones, should be cleaned, filled and primed 

bolts readily available. prior to redrilling. 

9. Cleaning, Routine maintenance, Most hinges and closure hard- Replacement hinges and fasteners 

lubricating solvent cleaning ware are bronze. Use solvents may not match the original exactly. 

or rep lac- (mineral spirits), bronze wool If new holes are necessary, old ones 

ing hinges and clean cloths. Spray with should be filled. 

and other non-greasy lubricant containhardware ing anti-corrosive agent. 

10. Replacing Standard method for Pliers and chisels to remove Heavy gloves and other protective 

glass and application old glass, scrape putty out of gear needed for the operator. All 

glazing glazing rabbet, save all clips parts saved should be cleaned prior 

compound and beads for reuse. Use only to reinstallation. 

glazing compound formulated 

for metal windows. 

11. Caulking Standard method for Good quality (10 year or bet- The gap between the metal frame 

masonry application ter) elastomeric caulking com- and the masonry opening should be 

surrounds pound suitable for metal. caulked; keep weepholes in metal 

for condensation run-off clear of 

caulk. 

12. Repainting Spray or brush At least 2 coats of paint com- The final coats of paint and the 

metal patible with the anti-corrosive primer should be from the same 

windows primer. Paint should lap the manufacturer to ensure compatibiliglass about 1/8" to form a ty. If spraying is used, the glass and 

seal over the glazing masonry should be protected. 

compound. 

Fig. 7. STEPS FOR CLEANING AND REPAIRING HISTORIC STEEL WINDOWS. Compiled by Sharon C. Park, AlA. 

9 
WEATHERIZATION 

Historic metal windows are generally not energy efficient; 

this has often led to their wholesale replacement. Metal 

windows can, however, be made more energy efficient in 

several ways, varying in complexity and cost. Caulking 

around the masonry openings and adding weatherstripping, for example, can be do-it-yourself projects and are 

important first steps in reducing air infiltration around the 

windows. They usually have a rapid payback period. 

Other treatments include applying fixed layers of glazing 

over the historic windows, adding operable storm windows, or installing thermal glass in place of the existing 

glass. In combination with caulking and weatherstripping, 

these treatments can produce energy ratings rivaling those 

achieved by new units. 3 

Weatherstripping 

The first step in any weatherization program, caulking, 

has been discussed above under "Routine Maintenance." 

The second step is the installation of weatherstripping 

where the operable portion of the sash, often called the 

ventilator, and the fixed frame come together to reduce 

perimeter air infiltration (see fig. 8). Four types of 

weatherstripping appropriate for metal windows are 

spring-metal, vinyl strips, compressible foam tapes, and 

sealant beads. The spring-metal, with an integral friction 

fit mounting clip, is recommended for steel windows in 

good condition. The clip eliminates the need for an applied glue; the thinness of the material insures a tight 

closure. The weatherstripping is clipped to the inside 

channel of the rolled metal section of the fixed frame. To 

insure against galvanic corrosion between the weatherstripping (often bronze or brass), and the steel window, 

the window must be painted prior to the installation of 

the weatherstripping. This weatherstripping is usually applied to the entire perimeter of the window opening, but 

in some cases, such as casement windows, it may be best 

to avoid weatherstripping the hinge side. The natural 

wedging action of the weatherstripping on the three sides 

of the window often creates an adequate seal. 

Vinyl weatherstripping can alSO be applied to metal windows. Folded into a "V" configuration, the material 

forms a barrier against the wind. Vinyl weatherstripping is 

usually glued to the frame, although some brands have an 

adhesive backing. As the vinyl material and the applied 

glue are relatively thick, this form of weatherstripping 

may not be appropriate for all situations. 

Compressible foam tape weatherstripping is often best 

for large windows where there is a slight bending or 

distortion of the sash. In some very tall windows having 

closure hardware at the sash mid-point, the thin sections 

'One measure of energy efficiency is the U·value (the number of BTUs per hour 

transferred through a square foot of material). The lower the U·value. the better 

the performance. According to ASHRAE HANDBOOK·1977 Fundamentals, the 

U·value of historic rolled steel sash with single glazing is 1.3. Adding storm win· 

dows to the existing units or reglazing with 5/ S" insulating glass produces a 

U·value of .69. These methods of weatherizing historic steel windows compare 

favorably with rolled steel replacement alternatives: with factory installed I" in· 

sulating glass (.67 U·value); with added thermal·break construction and factory 

finish coatings (.62 U·value). 

10 

of the metal window will bow away from the frame near 

the top. If the gap is not more than 114", foam 

weatherstripping can normally fill the space. If the gap exceeds this, the window may need to be realigned to close 

more tightly. The foam weatherstripping comes either 

with an adhesive or plain back; the latter variety requires 

application with glue. Compressible foam requires more 

frequent replacement than either spring-metal or vinyl 

wea therstri pping. 

A fourth type of successful weatherstripping involves 

the use of a caulking or sealant bead and a polyethylene 

bond breaker tape. After the window frame has been 

thoroughly cleaned with solvent, permitted to dry, and 

primed, a neat bead of low modulus (firm setting) caulk, 

such as silicone, is applied. A bond breaker tape is then 

applied to the operable sash covering the metal section 

where contact will occur. The window is then closed until 

the sealant has set (2-7 days, depending on temperature 

and humidity). When the window is opened, the bead will 

have taken the shape of the air infiltration gap and the 

bond breaker tape can be removed. This weatherstripping 

method appears to be successful for all types of metal 

windows with varying degrees of air infiltration. 

Since the several types of weatherstripping are appropriate for different circumstances, it may be necessary 

to use more than one type on any given building. Successful weatherstripping depends upon using the thinnest 

material adequate to fill the space through which air 

enters. Weatherstripping that is too thick can spring the 

hinges, thereby resulting in more gaps. 

Sprlng·metal 

Vinyl Strips 

/ 

Spring·metal com~s in bronze, brass or 

stainless steel with an integral friction·fit 

clip. The weatherstripping is applied after 

the repaired windows are painted to avoid 

galvanic corrosion. This type of thin 

weatherstripping is intended for windows 

in good condition. 

Vinyl strips are scored and fold into a "V" 

configuration. Applied adhesive is necessary 

.. 7A7H which will increase the thickness of the . ? weatherstripping, making it inappropriate 

Fffl1 -z..-.r .',' ' for some situations. The weatherstripping • is generally applied to the window after 

'C.WEATHE'f'>;rI\If" painting. 

Foam Tape <=n~I'JIOI'? 

Sealant Bead EXlel"lloJ<? 

Closed cell foam tape comes either with 

or without an adhesive backing, It is 

effective for windows with a gap of 

approximately 1,4" and is easy to install . 

However, this type of weatherstripping 

will need frequent replacement on 

windows in regular use. The metal section 

should be cleaned of all di rt and grease 

prior to its application. 

This very effective type of weatherstripping 

involves the application of a clean bead of 

firm setting caulk on the primed frame 

with a polyethelene bond breaker tape on 

the operable sash. The window is then 

closed until the bead has set and takes the 

form of the gap. The sash is then opened 

and the tape is removed leaving the set 

caulk as the weatherstripping. 

Fig. 8 APPROPRIA TE TYPES OF WEA THERSTRIPPING 

FOR METAL WINDOWS. Weatherstripping is an important 

part of upgrading the thermal efficiency of historic steel windows. 

The chart above shows the jamb section of the window with the 

weatherstripping in place. Drawings: Sharon C. Park, AlA. 
Thermal Glazing 

The third weatherization treatment is to install an additional layer of glazing to improve the thermal efficiency 

of the existing window. The decision to pursue this treatment should proceed from careful analysis. Each of the 

most common techniques for adding a layer of glazing 

will effect approximately the same energy savings (approximately double the original insulating value of the windows); therefore, cost and aesthetic considerations usually 

determine the choice of method. Methods of adding a 

layer of glazing to improve thermal efficiency include adding a new layer of transparent material to the window; 

adding a separate storm window; and replacing the single 

layer of glass in the window with thermal glass. 

The least expensive of these options is to install a clear 

material (usually rigid sheets of acrylic or glass) over the 

original window. The choice between acrylic and glass is 

generally based on cost, ability of the window to support 

the material, and long-term maintenance outlook. If the 

material is placed over the entire window and secured to 

the frame, the sash will be inoperable. If the continued 

use of the window is important (for ventilation or for fire 

exits), separate panels should be affixed to the sash 

without obstructing operability (see fig. 9). Glass or 

acrylic panels set in frames can be attached using magnetized gaskets, interlocking material strips, screws or 

adhesives. Acrylic panels can be screwed directly to 

the metal windows, but the holes in the acrylic panels 

should allow for the expansion and contraction of this 

material. A compressible gasket between the prime sash 

and the storm panel can be very effective in establishing a 

thermal cavity between glazing layers. To avoid condensation, 1/S" cuts in a cop corner and diagonally opposite 

bottom corner of the gasket will provide a vapor bleed, 

through which moisture can evaporate. (Such cuts, however, reduce thermal performance slightly.) If condensation does occur, however, the panels should be easily removable in order to wipe away moisture before it causes 

corrosion. 

The second method of adding a layer of glazing is to 

have independent storm windows fabricated. (Pivot and 

austral windows, however, which project on either side of 

the window frame when open, cannot easily be fitted with 

storm windows and remain operational.) The storm window should be compatible with the original sash configuration. For example, in paired casement windows, 

either specially fabricated storm casement windows or 

sliding units in which the vertical meeting rail of the slider 

reflects the configuration of the original window should 

be installed. The decision to place storm windows on the 

inside or outside of the window depends on whether the 

historic window opens in or out, and on the visual impact 

the addition of storm windows will have on the building. 

Exterior storm windows, however, can serve another purpose besides saving energy: they add a layer of protection 

against air pollutants and vandals, although they will partially obscure the prime window. For highly ornamental 

windows this protection can determine the choice of exterior rather then interior storm windows. 

The third method of installing an added layer of glazing 

is to replace the original single glazing with thermal glass. 

Except in rare instances in which the original glass is of 

special interest (as with stained or figured glass), the glass 

can be replaced if the hinges can tolerate the weight of the 

additional glass. The rolled metal sections for steel windows are generally from 1" - 1 1/2" thick. Sash of this 

thickness can normally tolerate thermal glass, which 

ranges from 3/S" - 5/S". (Metal glazing beads, readily 

available, are used to reinforce the muntins, which hold 

the glass.) This treatment leaves the window fully operational while preserving the historic appearance. It is, 

however, the most expensive of the treatments discussed 

here. (See fig. 6f). 

A 1\F'P'LlEi7 

ACFlYL I C 

EXlmIOf\. 

EXTEPlIOFI 

I""ANEL? I 

~'" ? 1~",,?A~~ ~~~~:r .... _1-"'-

Fig. 9 Two examples of adding a second layer of glazing in order to impro.ve the thermal performance of historic steel windows. Scheme A 

(showing jamb detail) is of a "acrylic panel with a closed cell foam 

gasket attached with self-tapping stainless steel screws directly to the exterior of the outwardly opening sash. Scheme B (showing jamb detail) is 

of a glass panel in a magnetized frame affixed directly to the interior of 

the historic steel sash. The choice of using glass or acrylic mounted on 

the inside or outside will depend on the ability of the window to tolerate 

additional weight, the location and size of the window, the cost, and the 

long-term maintenance outlook. Drawing: Sharon C. Park, AlA. 

WINDOW REPLACEMENT 

Repair of historic windows is always preferred within a 

rehabilitation project. Replacement should be considered 

only as a last resort. However, when the extent of 

deterioration or the unavailability of replacement sections 

renders repair impossible, replacement of the entire window may be justified. In the case of significant windows, 

replacement in kind is essential in order to maintain the 

historic character of the building. However, for less 

significant windows, replacement with compatible new 

windows may be acceptable. In selecting compatible 

replacement windows, the material, configuration, color, 

operability, number and size of panes, profile and proportion of metal sections, and reflective quality of the 

original glass should be duplicated as closely as possible. 

A number of metal window manufacturing companies 

produce rolled steel windows. While stock modern window designs do not share the multi-pane configuration of 

11 
historic windows, most of these manufacturers can 

reproduce the historic configuration if requested, and the 

cost is not excessive for large orders (see figs. lOa and 

lOb). Some manufacturers still carry the standard preWorld War II multi-light windows using the traditional 

12" x 18" or 14" x 20" glass sizes in industrial, commercial, security, and residential configurations. In addition, 

many of the modern steel windows have integral 

weatherstripping, thermal break construction, durable 

vinyl coatings, insulating glass, and other desirable 

features. 

Fig. 10 a. A six-story concrete manufacturing building prior to 

the replacement of the steel pivot windows. Photo: Charles 

Parrott. 

Fig. 10 b. Close-up view of the new replacement steel windows 

which matched the multi-lighted originals exactly. Photo: Charles 

Parrott. 

Windows manufactured from other materials generally 

cannot mat~the thin profiles of the rolled steel sections. 

AluminulIl, fpr example, is three times weaker than steel 

and must be extruded into a box-like configuration that 

does not reflect the thin historic profiles of most steel 

windows. Wooden and vinyl replacement windows 

generally are not fabricated in the industrial style, nor can 

they reproduce the thin profiles of the rolled steel sections, and consequently are generally not acceptable 

replacements. 

For product information on replacement windows, the 

owner, architect, or contractor should consult manufacturers' catalogues, building trade journals, or the Steel 

Window Institute, 1230 Keith Building, Cleveland, Ohio 

44115. 

SUMMARY 

The National Park Service recommends the retention of 

significant historic metal windows whenever possible. 

Such windows, which can be a character-defining feature 

of a historic building, are too often replaced with inappropriate units that impair rather than complement the 

overall historic appearance. The repair and thermal 

upgrading of historic steel windows is more practicable 

than most people realize. Repaired and properly maintained metal windows have greatly extended service lives. 

They can be made energy efficient while maintaining their 

contribution to the historic character of the building. 

BIBLIOGRAPHY 

ASHRAE Handbook - 1977 Fundamentals. New York: American 

Society of Heating, Refrigerating and Airconditioning Engineers, 

1978. 

Crittal, W. F. A Metal Window Dictionary. London:Curwen Press, 

1926. Reprinted by B.T. Batsford, Ltd., 1953. 

Gayle, Margot; David W. Look, AlA; John G. Waite.Metals in 

America's Historic Buildings: Uses and Preservation Treatments. 

Technical Preservation Services, U.S. Department of the Interior. 

Washington,D.C.: U.S.Government Printing Office, 1980. 

Gillet, William. "Steel Windows." Windows and Glass in the Exterior 

of Buildings. National Academy of Sciences Publication 478. 

Washington, D.C.: 1957,75-78. 

Sarton, R. H. "Selecting and Specifying an Appropriate Type of Steel 

Window." Metalcraft. Vol 6, No. 1 (January, 1931): 43-48, 64-65. 

Sweet's Architectural Catalogue. 13th Edition, New York, Sweets 

Catalogue Service, Inc., 1918. 

The author gratefully acknowledges the invaluable assistance of coworker Michael Auer in preparing this brief for publication. This 

publication is an extension of research initiated by Frederec E. Kleyle. 

Special thanks are given to Hope's Architectural Products, Inc., 

Jamestown, NY, for their generous contribution of historic metal window catalogues which were an invaluable source of information. The 

following individuals are also to be thanked for reviewing the manuscript 

and making suggestions: Hugh Miller, Chief, Park Historic Architecture 

Division, National Park Service; Barclay L. Rogers, Museum Services, 

National Park Service; Susan M. Young, Steel Window Institute, and 

Danny Schlichenmaier, State Building Division, Lincoln, Nebraska. 

Finally, thanks go to Technical Preservation Services Branch staff and to 

cultural resources staff of the National Park Service Regional Offices, 

whose valua~e comments were incorporated into the final text and who 

contributed t the publication of this brief. 

This public tion has been prepared pursuant to the Economic 

Recovery Tax Act of 1981, which directs the Secretary of the Interior to 

certify rehabilitations of historic buildings that are consistent with their 

historic character; the guidance provided in this brief will assist property 

owners in complying with the requirements of this law. 

Preservation Briefs: 13 has been developed under the technical editorship of Lee H. Nelson, AlA, Chief, Preservation Assistance Division, 

National Park Service, U.S. Department of the Interior, Washington, 

D.C. 20240. Comments on the usefulness of this information are 

welcomed and can be sent to Mr. Nelson at the above address.