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Lintel decorated with Gothic scroolwork, serving as a habitat, nest and all.
Examine windows systems and surrounding areas during rain storms.

The parts that make up a window unit begin to deteriorate from the moment the unit is installed. A window straddles two environments, the interior and exterior, which are often extremely different with regard to temperature and moisture content. Window failure begins at the joints, where the different parts meet. If these areas are left unattended, deterioration of the main components themselves sets in.

The primary causes of window deterioration come from three main sources:

  1. exterior rainwater driven against exterior surfaces or into the joints of an ill-fitting window;
  2. standing rainwater on poorly sloped surfaces;
  3. condensation formed on interior surfaces.

Window joints become inoperabel, becoming either stuck or seformed, because of poor maintenance practice, notably excessive paint buildup. Other factors beyond the design and construction of the window unit itself cause problems such as the building's settling or jacking caused by the corrosion and subsequent expansion of structural iron in the window opening.

New York Landmarks Conservancy, Repairing Old and Historic Windows : A Manual for Architects and Homeowners, Hoboken, NJ: John Wiley & Sons, Inc., 1992, p.63

A detailed inspection of windows should be carried out regularly, probing vulnerable areas with a sharp instrument for signs of decay. This will usually take the form of wet rot, which affects both softwoods and hardwoods, and occurs where:

  1. water is allowed to stand on horizontal planes, such as sills
  2. paintwork has cracked, and at joints
  3. moisture is attracted by capillary action and becomes trapped, for instance between a timber sill and the masonry below
  4. adjoining masonry is damp for long periods
  5. condensation persistently forms on the inside face of the glass especially in bathrooms and unheated rooms.

Wet rot is recognisable by slight ripples and discoloration of the paintwork, and the underlying timber becomes soft, breaking up when probed. However, paint deterioration has many causes, and does not necessarily indicate rotten wood.

Other forms of decay include the wood-boring larvae of certain types of beetle [not prevalent in the U.S.], and dry rot, a specific type of fungus.

Dry rot rarely affects windows, but is sometimes found behind sealed shutters and in the boxes of sash windows when there has been an outbreak of fungus elsewhere in the building. Treatment is more complex than for wet rot and is best dealt with by a historic building specialist.

Wood-boring insects pose less of a problem than fungal attack. . .

Wrightson, David. The Conservation and Renewal of Timber Windows, Public Information Leaflet, Cathedral Communications, Wiltshire, England [PDF file]

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