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 - Vinyl stripping may be too thick for use in some
situations.
- Compressible foam is best for large windows with only
slight bending or distortion or with a gap of no more than
1/4". However, the foam must be replaced more frequently
than spring-metal or vinyl.
- Sealant beads are good all-around weatherstripping for all
types of metal windows and air infiltration problems.
4.10.5 Double Glazing. This process adds a second layer of
glazing to the window glass. This will approximately double the
original insulating value of the window. Options include:
o
Glass or Acrylic Panels. Panels secured to the frame over
the entire window are not suitable for windows needed for
ventilation or fire exit since the panels make the sash
inoperable. Magnetic interior panels can be attached to
the sash. With acrylic panels, provide l/8" vapor bleeds
to control condensation and prevent moisture damage.
(The
bleeds will increase energy loss slightly). Panels must
be removable to allow excess moisture to be wiped away as
needed. Considerations: cost , ability of window to
support additional weight, location and size of window,
long-term maintenance.
o
Interior or Exterior Storm Windows.. May have to be
specially fabricated to match original sash configuration
(e.g., casement storms). Generally not suitable for
operating windows that project on either side of the
window frame when open, such as pivot and austral windows.
Considerations: visual impact on building. Exterior
storms can protect against damage to ornamental windows
from air pollutants, vandalism, etc.
o Replacement Thermal Glass. The best but most expensive
way to upgrade thermal efficiency is to replace the
original glass with thermal or low-E glass. The new glass
can usually fit in the existing metal sash and does not
affect the appearance of the historic building.
4.10.6 Glass Block Windows. Glass block windows should be
preserved and repaired with new glass blocks. The blocks are
commercially available. For further Information:
Myers, John H., Preservation Briefs: #9, The Repair of
Historic Wooden Windows; and Parks, Sharon C., AIA,
Preservation Briefs: #13, The Repair and Thermal Upgrading of
Historic Steel Windows.
Prepared by Technical Preservation Services, Preservation
Assistance Division, National Park Service. Available from
Superintendent of Documents, U.S. Government Printing Office,
Washington, DC 20402, or from your SHPO office.
4-44
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