Detailed Instructions for old method of secondary glazing
This the old I first did secondary glazing before I got the proper aluminium spacere used in double glazed units. The new method is described here

The biggest problem with secondary glazing is moisture. After rain, water will seep through the wooden frame and emerge in the gap. It will evaporate of the frame and condense on the cold glass. This effect is minimised by placing a plastic seal over the wooden bead on the primary window then a rubber seal against the glass. Problems can also occur if moisture lies against the rubber seal. This can occur at the top and bottom of the window. The top is normally well protected by the eaves; the bottom can be addressed by a small breather gap.
Step 1. Painting the plastic barrier
The idea is to fully cover the existing beading with a right angle plastic cover. This plastic comes in white, so you normally want to paint it before use. It is best to lay sheets of paper on a bench, place the strips on top and then use a spray can to paint it. Normally start spraying beyond the plastic and move over the strip with an even motion. It will normally require about 5 coats to cover it. It is much better to do multiple light coats than try to do one heavy coat. Make sure that you do it in a well ventilated area and be careful that you do not paint the wall or floor. A carport is the ideal location.
Step 2. Attaching the plastic barrier
Cut the plastic strips to the correct length and ensure they will fit. If the beading has a bevelled finish, add a piece of 6 mm angled dowel to make it up to a right angle. Add silicone over the surface to hold the plastic. Also add a small bead of silicone where the window meets the frame. Place the plastic in place and allow to dry. You can smooth the silicone on the glass but be careful you donít make it worse. Dip your finger in dishwashing detergent so that the silicone does not stick to your finger. Excess silicone can be removed using turpentine on a rag. Alternatively you can allow it to try and remove it with a razor blade and paint scraper. Allow 48 hours for the silicon to set before adding the external window. Often the plastic is slightly bent, so you need small nails to hold the plastic in place while drying, or you can use a staple gun. It is advisable to have two people, one to hold the plastic the other to secure it.
Step 2b. Alternative plastic barrier
Sometimes the beading is the wrong shape and it is not possible to add the plastic capping. In this case, you can remove the beading and replace it with a U shaped piece of plastic. Add a 3 mm piece of dowel inside the U shape to give it strength. The U shaped plastic can be attached to the glass using silicone.
Step 3. Clean the glass
It is most important to ensure that the two glass surfaces in the gap are clean. Remove any paint etc and use a good quality window cleaner.
Step 4. Build up the frame (Overhang)
If the the glass unit extends beyond the frame, you must build up the frame so that it is level with the window when the window is inserted and the rubber seal compressed. You can buy 4 or 8 mm rectangular wood to achieve this.
Step 5. Adding the seal
Attach the rubber seal on all four sides. If the frame is uneven, you may need to add two strips in places.
Step 6. Insert the second sheet of glass
Lift the external sheet of glass into place using suction cups. It is recommended that you use a proper glazier's suction cup because it has a marker that indicates when the seal is failing. Otherwise use two suction cups. If you are using low-e glass, ensure the low-e surface is internal. It is desirable that you have two people for this, especially if the area of glass is large.
Step 7. Cut external beading (recessed)
In most cases the window frame will extend beyond the glass unit. Use the 12 mm angle beading. Use a mitre board to cut the wood at 45 degrees. It can be confusing as to which way it should be cut. When you look at the cut, you find that one edge is at right angles, the other at 45 degrees. The right angle cut should be on the longest side.
Step 8. Add external beading (recessed)
Place tape on the glass so that the wood will just cover it. On the bottom of the frame, add a 4 cm piece of rubber seal where each screw is placed. This allows air movement to dry out any moisture that forms at the base of the window. It also stops the water seeping up the joint. Push firmly on the glass when inserting screws.
Step 9. Add external capping (Overhang)
Use a 30 mm cap to cover the frame. Place a layer of rubber seal on the glass before adding the capping. If you are capping all the way around, cut the capping at 45 degrees. If you are using capping on only one side, join it at 90 degrees. It is suggested that you use screws so that it can be removed in case moisture forms between the glass.
Step 10. Paint the frame
You will normally need to paint the whole window frame on completion. Depending on your skill as a painter, you may wish to paint the beading before attaching it then just touch up any imperfections. If you get paint on the glass, allow it to dry and then use a razor blade and paint scraper to remove it

Details on removing glass
Details on adding a double glazed unit

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