Our Approach to Restoring Historic Windows

We restore a lot of wood windows and doors here at Kronenberger & Sons, so we’ve developed a pretty comprehensive approach to bringing them back to life – to “like new” condition if you will.

One thing that makes us unique here at KSR is that we have divisions dedicated to the different components of a restoration project. One of those dedicated areas is our windows and doors shop. Our thorough restoration practices, starting with assessment and ending with precise paint thicknesses, allows us to offer expert, consistent work on every window we work on. Because we have a focused team that works just on windows and doors, they represent vast experience with all types of window styles, have proven methods and materials they use, and are truly experts in the preservation and restoration of old wood windows.

It All Begins with a Thorough Assessment

The key to successful planning for window treatments is a careful evaluation of the existing physical conditions of each window unit individually. We use either a drawn graphic or photographic system to record existing conditions, and to illustrate the need and scope of any repairs.

We use a window schematic listing all of the parts of a window, to note existing conditions and repair instructions. Spaces by each part allow notes on. When this form is completed for each window, we have a precise list of repair tasks to be performed for each window.

We note the following:

  1. Window and/or door location
  2. Condition of the paint (Usually lead based paint)
  3. Condition of the frame and sill
  4. Condition of the sash (rails, stiles and muntins)
  5. Hardware, and
  6. Overall condition of the window

Many Factors Impact Window Condition

Many factors contribute to window deterioration. They include poor design, moisture infiltration, insect infestation, vandalism, and lack of maintenance. Moisture is consistently the primary contributing factor in wooden window decay. When we are working on a window restoration project, we inspect all window to see if water is entering around the edges of the frame. If so, the joints or seams are caulked to eliminate the danger of moisture permeation. Additionally, glazing putty is checked for cracked, loose, or missing sections which allow water to saturate the wood, especially at the joints. The back putty or the bedding on the interior side of the pane should also be inspected because it creates a seal which prevents condensation from running down into the joinery. The sill should be examined to ensure that it slopes away from the sill.

Paint Failure Often Indicates Moisture

One clue indicating excessive moisture is the condition of the paint. Since excessive moisture is detrimental to the paint bond, areas of paint blistering, cracking, flaking, and peeling usually identify points of water penetration, moisture saturation, and potential deterioration. Each window is examined for areas of paint failure. Paint failure, however, should not be mistaken for failing wood. Wood is frequently in sound physical condition beneath unsightly paint. After noting areas of paint failure, the next step is to inspect the condition of the wood, particularly at the points identified during the paint examination.

Inspecting Window Sash & Joints

Each window is examined for operational soundness beginning with the lower portions of the frame and sash. The sill, joints between the sill and jamb, corners of the bottom rails, and muntin joints are typical points where water collects and deterioration begins. The operation of the window through continuous opening and closing over the years, as well as seasonal temperature changes, weakens the joints. When the joints weaken, they move and usually separate. This weakening makes the joints particularly vulnerable to water which is readily absorbed into the end grain of the wood. If severe deterioration exists in these areas, it will usually be apparent on visual inspection, but other less severely deteriorated areas of the wood may be tested by two traditional methods using a small ice pick or knife.

An ice pick or a knife may be used to test wood for soundness. The technique is simply to jab the pick into a wetted wood surface at an angle and pry up a small section of the wood. Sound wood will separate in long fibrous splinters, but decayed wood will lift up in short irregular pieces due to the breakdown of fiber strength.

Window Assessment Determines the Extent of Restoration Necessary

After each window and/or door has been thoroughly evaluated, we can then determine the extent of repairs necessary to fully restore the unit.

Generally, the repairs required to return a window to “like new” condition will fall into three basic categories:

  1. Routine Maintenance Procedures
  2. Structural Stabilization – Epoxy Consolidation
  3. Parts Replacement – Sash Replacement

What Do We Do When We Restore a Window?

Fundamental Window Restoration

When a window unit is operationally sound, the repairs and maintenance required to restore a window to “like new” condition normally includes the following steps:

  1. Stripping lead-based paint and glazing in our Clean Room with our steam strippers;
  2. Removal and repair of sash (including re-glazing);
  3. Repairs to the frame (including marine epoxy consolidation and sanding);
  4. Weatherstripping and reinstallation of the sash;
  5. Priming, bedding, glazing and repainting (this is done in our professional spray booth with high quality exterior paint).

Intermediate Window Restoration

Many wood windows will show some additional degree of physical deterioration, especially in the vulnerable areas like the bottom rail corners and the tenons. Even badly damaged windows can be repaired using simple processes. Partially decayed wood can be “hardened” with thinned marine epoxy.  We inject and brush the thinned epoxy on and into the decayed wood and let it cure. Once cured, we can then build up or consolidate the affected area. After they are painted, they will be in sound condition, look great, and have a greatly extended life. Three techniques for repairing partially decayed or weathered wood are discussed in this section.

One of our established techniques for repairing wood which is split, checked or shows signs of rot, is to:

  1. Dry the wood;
  2. Remove any decayed wood and replace it with a wood Dutchman which is secured with marine epoxy;
  3. Hand plane and sand to match the existing profile;
  4. Fill any divots, open seams, cracks and imperfections with thickened marine epoxy;
  5. After the epoxy has cured, sand and level to make the repair homogenous with the surrounding wood;
  6. Priming, bedding, glazing and painting.

Major Window Restoration – Sash and Parts Replacement

When parts of the frame or sash are so badly deteriorated that they cannot be stabilized, we have methods which replace the failed areas and permit the retention of some of the existing or original fabric. These methods involve replacing the deteriorated parts with new matching pieces, or splicing new wood into existing members. These techniques require more skill, and are more expensive than any of the previously discussed alternatives.

It is necessary to remove the sash and/or the affected parts of the frame and bring them to our woodworking mill shop where damaged or missing parts will be reproduced. Our shop can duplicate any and all window and door parts, including muntins, rails, trim, doors and all other architectural components associated with window and door reproduction.

Our Shop Can Reproduce Any Sash or Window

There are times when the sash is too far gone and we will have to make a duplicate sash as a replacement. We make every attempt to duplicate the sash to the exact dimensions including height and width, and all profiles to match the adjacent windows. Like-and-kind wood for reproductions as well. We can also duplicate unique window characteristics such as arches, decorative elements, and pane configurations. When a reproduction is necessary, we see that through right to the glass, using restoration glass which has bubbles, waviness and other imperfections to match historic glass.

When our window restoration projects are complete – original wood windows are brought back to life and will last another 100 years with proper maintenance!

If you have questions about your historic windows, or would like a tour of our Window Shop – please CONTACT US!


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