I would like to stress that secondary glazing is only a cheap option if you already have the second sheet of glass. If you need to buy the second sheet of glass it is usually cheaper to buy a double glazed unit and then used the piece of glass you have removed to secondary glaze another window.
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. My first approach to minimise this was by placing a plastic seal over the wooden bead on the primary window then a rubber seal against the glass (see here ). My improved method is closer to proper double glazing. I have purchased a large amount of aluminium spacer which is what is used in proper double glazing. This is formed into a rectangle and siliconed to the sheet of glass that is to be added.
The beading holding the primary sheet of glass is removed, a rubber seal applied and the second sheet of glass applied, making a pretty good seal. An alternative is to actually remove the primary sheet of glass and silicon it to the secondary sheet. This is then a cheap double glazed unity which is installed the same way as a normal double glazed unit. This gives a much better seal, but involves more work and you risk breaking the primary sheet of glass.
In each case, you can insert silicon crystals to the frame to absorbe moisture. Note: Silicon crystals only absorb oisture once, so theu may well stop absorbing after several months, depending on how good a seal you have.
Compared to double glazing, secondary glazing is not as good as a proper doubel glazed unit because:-
1. It risks condensation
2. It doesn't have argon in the gap
3. It will normally only be 3 mm glass
4. it will not be as clean
5. It won't be Low/E glass
6. You need to purchase the aluminium spacer, plastic joiners and silicon crystals. We provide these for around $12 per window.
Detailed Instructions for secondary glazing
Step 1. Assemble the aluminium spacer
Cut 2 spacers 5 mm shorter than the width and 2 spacers 15 mm shorter than the height. Assemble them in a rectangle using the joiners that we provide. Holes should be drilled on the inside at the top of the vertical sections. These are used later to add the silicon crystals.
Step 2. Attach the aluminium spacer to the secondary window
This is best done with a little bit of silicone. It can be held in place with paper clips or just weights. Once set, apply silicone liberally all the way around.
Step 3. Remove the existing beading
If it is a wooden bead or putty, this is normally quite easy. However sometimes a type of cement is used which can be difficult to remove. The glass should then be carefully cleaned firstly to provide a goood seal at the edge, but secondly to ensure no marks are internal to the window since once assembled it cannot be removed.
Step 4. Apply a rubber seal
The rubber seal is applied to the sheet of glass that you are adding. This makes it easier to align with the edge of the aluminium spacer. You could try silicon rather than the rubber seal. However getting the silicon in can be messy and makes it very difficult to remove if you ever need to.
Step 4b. Remove the existing glass and build your own Double Glazed Unit
An alternatiove approach is to fully remove the existing window and silicon the sheets together to make your own double glazed unit. This is more work but will give a better result since you can apply lots of silicon and get a really good seal. Once you have built the double glazed unit, follow instructions for normal double glazed units
Step 5. Add Silicon crystals
Silicon crystals are added to the frame using a funnel. You can almost fill up the whole section. These crystal will absorb moisture only once. If your seal is good, they will keep working for a long time. If poor, they may only last a matter of months. Unfortunately I have no accuate information on how long they will last, but they are better than nothing!
Step 6. Insert the secondary sheet of glass and hold it with beading
If the window rebate was 16 mm, the glass will finish flush with the frame and can be held in place with capping. If it is over 25 mm, you can use normal beading to hold it in place. Otherwise you will need to shape the capping to hold it in place.
Step 7. 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