Preservation Tech Notes: Windows 4 Replacement Wooden Frames And Sash: Protecting Woodwork Against Decay

Procedure code:
Preservation Tech Notes, National Park Service, Pad
Doors And Windows
Wood Windows
Last Modified:



William C. Feist
Forest Products Laboratory
U.S. Department of Agriculture

This standard includes the bulk of information contained in the
original Preservation Tech Notes developed by the National Park
Service and the Center for Architectural Conservation at Georgia
Tech.  The Preservation Tech Notes are case studies of exemplary
projects designed to provide specific examples of sound
preservation techniques.  To obtain a complete copy of The Window
publications, including figures and illustrations, please contact:

         Historic Preservation Education Foundation
         P.O. Box 77160
         Washington, DC  20013-7160

The Window Handbook, jointly prepared by the National Park Service,
Preservation Assistance Division and the Center for Architectural
Conservation at Georgia Tech, also contains all of the Tech Notes
on Windows and is available for purchase from the Historic
Preservation Education Foundation for $32.00.  The Window Workbook
is available for $49.00.  The two publications together can be
purchased for $72.00.



The survival of millions of historic wooden windows is a testament
to their long useful life.  Faced with windows that are beyond
repair, however, many owners are reluctant to install wooden
replacement widows, in part due to the belief that without constant
maintenance the windows will quickly decay.  Studies undertaken by
the Forest Products Laboratory (FPL), U.S. Department of
Agriculture, have convincingly shown that when wooden elements in
windows are treated with a water repellent very little decay will
occur in the new windows even if many years of maintenance neglect
follow.  This important finding was an outgrowth of a research
project to determine alternatives to potentially toxic chemical
wood preservatives.


When old wooden windows in historic buildings have to be repaired
or replaced, it is always advisable to incorporate treatments that
will extend the useful life of the new wood.  Application of a
water-repellant chemical preservative, such as pentachlorophenol,
to new wood prior to painting traditionally has been recommended.
The toxicity of some formulations, however, pose potential health
problems.  A treatment to prolong the useful life of the new wood -
and therefore the windows - is needed which avoids certain
potential health hazards.


A 20-year test on wooden windows by the FPL in Madison, Wisconsin
has concluded that there is a safer alternative to traditional
water-repellent chemical preservatives for treating wood in order
to prevent decay.  It was found that the easiest way to prevent
decay in woodwork items such as frames and sash is the application
of small amounts of wax to the surface.  The wax, in the absence of
chemical preservatives, protects the wood from excessive moisture
and provides good long-term protection to window units and other
wood exposed above ground.

Twenty years ago, test window units at FPL were dipped for 3
minutes in either a solution of water-repellent with a chemical
preservative or a water-repellent without chemical preservatives.
Some units were left untreated as comparison controls.  After only
6 years' exposure on an outdoor test in Madison, the untreated
samples were so badly decayed that they fell apart as they were
being removed.  

All test units were painted originally, but never repainted.  Most
paint was gone from exposed surfaces after 10 to 12 years'
exposure.  The water-repellent with a chemical preservative
treatment was very effective in protecting the window unit long
after all the paint had weathered away.

But the most surprising result in the 20-year test was that window
units treated with a simple water repellent (1.5 percent paraffin
wax in mineral spirits plus 10 percent exterior varnish resin with
no chemical preservative) performed as well as did the water-
repellent preservative (which contained both wax and a chemical
preservative).  This showed that a non-chemical water repellent
like paraffin wax with a small amount of resin, such as exterior
varnish, was capable of providing protection to wood exposed above
ground to the elements for 20 years in a northern climate.

A water-repellent treatment alone can provide excellent decay
resistance to outdoor painted woodwork without the addition of a
chemical preservative.  This can represent a saving of money and
resources and judicious avoidance of chemical preservatives in
items such as windows, sheds, porch and fence rails, and other
above-ground wood products.

The water-repellent treatment is easily done before or after
construction and before painting.  A simple formula, easily
prepared is:

-    Exterior varnish    3 cups

-    Paraffin wax        1 ounce

-    Mineral Spirits, or
    paint thinner, or
    turpentine          Add to make 1 gallon

Treatment is best done by dipping the wood for 1 to 3 minutes in
the solution.  If dipping is inconvenient, liberal brush
application can be made - paying particular attention to heavy
treatment of all board ends and joints.  The treated surface can be
painted after 2 or 3 days of warm weather.  In fact, paint should
last longer over the treated surface than over untreated wood.


The field test conducted by the Forest Products Laboratory showed
that there are safer treatments for protecting woodwork in northern
climates than many commonly used.  The combination of pretreating
and painting provides good long-term protection against decay.  Of
equal interest, the test showed that there are effective ways to
prevent decay in wooden window elements even where the windows are
exposed to long periods of maintenance neglect.

In the southeastern states and in the Pacific Northwest where there
is a high decay potential due to the combination of higher moisture
and moderate to warm temperatures, it is still recommended that
wooden windows be treated with both a water-repellent and a
chemical preservative.  A number of new, less toxic chemical
preservatives are now commonly available and will provide similar
long-term protection.

                             END OF SECTION
Last Reviewed 2012-02-24