Double glazing in roof windows

[Feltwell]

I've managed to pick up a batch of 8 roof windows, very similar to the "Replica Rooflight" - i.e. copies of the original cast iron Victorian windows. These particular ones were made a few years ago as a one-off - someone doing a conversion on a old dairy building had links with a foundry and had them made up. They appear to be in cast aluminium, but despite that are still extremely heavy!

The question is - they aren't fantastically well-designed, the glazing units sit in a rebate and are held in place with (lots of) mastic. Internally they look fine, there is a smear of clear silicone between the glazing units and the frame that can hardly be seen. Externally though is a different matter, there is a very heavy fillet of a black silicon-type sealant holding them in, in much the same way as you would use linseed putty on a single-glazed wooden window.

This black sealant has not been very well applied and is badly discoloured and in places peeling away, after 8 years exposure.

Rather than just cutting out the sealant to replace it with fresh, I was thinking of gluing in a hardwood fillet and then painting it with linseed paint, overlapping onto the glass to help hide the double glazing unit seal (having first put some shellac on to keep the linseed away from the DG seals). Has anyone any better ideas, or know of a black sealant which will not discolour? I had thought about using ordinary linseed putty and painting it, but am concerned about the effect on the DG seals if the linseed reaches them.

Also, when scraping off a small area of sealant, a foil type tape that surrounds the glazing unit comes away with it. What does this do, and do I need to worry about replacing it? (Biff !)

I wouldn't put these on a house, there is no thermal lining of any kind and I reckon they'd suffer from condensation, but in my case they are for an unheated outbuilding so it's not a problem. I could replace the DG units with single glazing but it seems a waste as there's nothing wrong with them, plus the depth of the window is designed to take a DG unit.

 

[Lime]

The tape seals the edge of the double glazing keeping water out.
Between the glass edges is a fillet with holes in filled with silica gel (or similar) that keeps the trapped air dry.
The silica gel can only absorb a limited amount of water before it becomes saturated which will then allow condensation to occur and failure of the double glazing.

If these units were mine ~and I was going to the bother of installing them~ I'd remove the sealed glass units from the aluminium frames and replace the mastic (actually it should be silicone) with ordinary putty for metal windows.
While the silicone may be better if correctly applied it is not easy to do and never looks as good as smoothed putty.

I'd take that opportunity to alter/improve the frames if possible.
Even a decent coat of paint would improve the condensation problem on the frame.
(don't forget to use acid etch for the primer/base coat so that the paint sticks to the aluminium. It can be bought in an aerosol.)

Other than that they look a good find, ideal for the job you have in mind.

 

[Feltwell]

Thanks Lime.

I agree about putty versus silicone, putty is far easier to smooth and get looking good. But I thought that you couldn't use Linseed putty next to DG units, as the linseed oil affects the seals? Or am I getting confused with laminated glass?

 

[Lime]

With regard to the double glazing seal being damaged by linseed oil, it very much depends on how the glazing is sealed.
Many double glazing sheets are fabricated using a silicone seal covered over with the aluminium tape.
It is usually reasonably resistant to any linseed oil being present but you could have a point with some types of seal.

 

[Penners]

Stand by for blasphemy....

Actually, I would choose clear silicone. The final location of these units means that they'll be difficult to maintain, so I would be doing everything I could to make them as maintenance-free as possible. Painted putty will need repainting regularly - particularly in such an exposed location, and the putty risks being dried out by the sun (as it's facing the sky).

I agree that silicone rarely looks as good as well knifed putty, but on the roof are you really going to see the difference?

By the way, if you need to remove the black silicone I think you'll find cutting it out pretty difficult to do cleanly. I find this stuff very good: http://www.u-diy.co.uk/index.aspx?s=Product&p=874

 

[Feltwell]

No blasphemy Penners - it's a new building, styled to blend in with the old one next to it but built unashamedly using modern materials & methods. Much as I'd love a traditionally constructed building, I don't have the time or money to build something from scratch in the traditional way. So there - I'm the blasphemer, for building a pastiche!

Something that will last is very high on the list. The poor appearance of the current silicone is due in part to the cack-handedness of whoever put it in, but mostly to the dis-colouration. I'm confident that I can make a neater job, certainly good enough when viewed from ground level, but the dis-colouration is a problem.

I wouldn't want to use clear - I reckon that would look manky quite quickly - but glazing silicones don't seem to come in black. Of course I've no idea what the original sealant was, so for all I know it may have had "internal use only" on it. Best thing I can find is this:-

http://www.screwfix.com/prods/62431...s/Dow-Corning-791-Weatherproof-Silicone-Black