I have had quite a bit of experience with various UV and visible light
control at our museum and other historical house museums I advise. In
general, I do not recommend the use of window films. They should not be
applied to historic window glass because they are difficult to remove
without harming the painted wood window components and old fragile
glass.Their quality is inconsistent and some fade or loose their filtering
capability as a result of extended exposure to high light levels, especially
on south windows. 

Instead, I recommend hanging tinted UV filtering Plexiglas or Acrylite
acrylic or polycarbonate (Lexan) sheet material in front of the window on
the inside, allowing air to circulate between the acrylic sheet and the
window. It is also possible to make acrylic interior storm windows mounted
against a wood stop applied to the inside of the window frame. A silicon
weatherstripping material can be used between the acrylic sheet and the wood
stop to prevent moisture from leaking into the space between the window. In
northern climates, the interior storm window should be the tightest so that
any moisture that does get between the windows finds its way outside through
the looser original window structure. Such interior storm windows tighten
the lease efficient portion of the building envelope, saving energy while
controling light. If the windows need to be opened in the summer, individual
panes of sheeting can be cut to fit the window panes and placed directly
against each pane, using one glazers point on each edge to hold the sheet in
place. 

The UV absorbing material is cast into the body of the plastic sheets and I
have seen no degredation in UV filtering capability with these materials
even after 15+ years. 

I recommend using warm bronze-tinted acrylic or polycarbonate (vs. cool
gray)  to block out at least 50% of the visible light as well as all of the
UV. Clear UV blocking sheeting gives a false sense of security since
significant fading is still caused by the shorter wavelengths of visible
light. No one will notice the 50% tinting once the sheet is in place as you
view out the window. In fact, even 75% filtering does not significantly
affect the views out the window. We have used 95% tinting in gallery
windows, and although the windows appear dark, one can still enjoy the view
out. Such tinting also reduces glare throughout the room, especially on
glass case fronts or glazed paintings and prints. However, since you have
cut out a significant portion of the total room illumination the room will
be darker inside, so sufficient auxillary interior lighting is important if
you are going to use this method of light control. 

Even with tinted sheeting on the windows, I still recommend using
room-darkening window blinds or drapes when the museum is closed. Since
light damage is cumulative, blocking all light when the museum is closed
significantly extends the life of any light sensitive artifacts in the room.
Simple roller blinds are inexpensive and easy to pull and can be mounted
unobtrusively inside window casings even in historic interiors. 

Tinted acrylic or polycarbonate sheets must usually be special ordered, and
manufacturers or distributors may require a minimum order of several sheets.
Check web sites for Plexiglas, Acrylite, Lexan, or polycarbonate sheet for
distributors in your area. A local glass supply house can often order such
sheeting.

IMLS and NEH Preservation Assistance Grants will often fund purchase and
installation of tinted UV sheeting.

Richard L. Kerschner
Director of Conservation
Shelburne Museum
PO Box 10
Shelburne, Vermont  05482
(802) 985-3346  x3361

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