How do I repair misted double glazing?

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Double glazing refers to thermal-paned windows that feature two layers of glass with a pocket of air or other gas between the panes. Once these windows lose their airtight seal and fall prey to condensation, there is only one permanent fix, replacing the double pane. Most manufacturers sell replacement panes for their windows, and some companies make retrofit panes for other makes and models. Consult with a window retailer for specifics on your brand. There are two basic types of seal -- adhesive tape or caulk seals, and rubber gasket seals. The type of seal will determine the repair procedure.

Remove the sash from the window. Most double panes have clips at the top of the sash that allow them to be tilted in for cleaning. Press these to the centre, tipping the top of the sash down until it is level, straight out from the window frame. Grab the sash on either side and lift up firmly to release the pivot hinges and lift the sash out. Pry the stop trim along the side of the window for those that do not tip in. Use a chisel to pry the vertical trim from the jamb on both sides and lift the sash out. Unclip the spring or cable counterweight strap at both top corners to release the window.

Use the chisel to pry the trim from the frame against the pane. The seam in this trim is typically 1/2 inch out from the pane and runs along all four sides. Remove all four pieces carefully. Lift the misted pane from the window, prying up gently with a thin putty knife if needed to release the seal. For stubborn panes, cut the caulking along the outside edge of the pane with a utility knife.

With a razor scraper, scrape the ledge of the sash, where the original pane was installed, to remove old caulk or tape from the frame. Apply a constant bead of 100 per cent silicone caulk around the face of this ledge, or apply adhesive seal tape, in one continuous line around the ledge. Remove the paper backing from the top of the tape, if you are using adhesive tape.

Set the new pane in place, pressing it firmly to the tape or caulk. Reinstall the stop trim around all four edges of the pane. Pull the old brad nails through the back of wood trim to preserve the face and nail it in place with similar nails, at least three per side. Snap aluminium or vinyl trim bead into its groove, by pressing firmly with the front edge of the putty knife. Replace the sash in the reverse order of removal, depending on the type of window.

Remove the sash from the frame as outlined in Section 1. Locate and remove the corner screws from two opposite corners of the sash frame with a screwdriver. Insert the tip of the putty knife into the seams at these two corners and pry the frame gently apart to form two L-shaped frame halves. Gently pull these halves apart, taking care not to stretch the gasket too far.

Remove the gasket from the edges of the pane and fit it to the edges of the new pane. If the new pane came with a new gasket, use it instead. The gasket fits over the edge of the pane, with thin edges folded onto each face of the glass and a wider flat strip running around the edge of the pane.

Fit the pane into one half of the sash frame, and fit the remaining half over the opposite corner of the pane, pressing the halves firmly together until the corner seams are tight. Install the corner screws back into the frame and tighten with a screwdriver. Replace the sash in the window frame in the opposite order of removal.

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