Secondary double glazing

[RichD1]

I have a nice leaded light window on my stair landing which I'm preserving having restored the frame by getting rid of the rotten wood and scarfing in new sections.

As the window always suffered from condensation I've decided to make a secondary frame to take a dg unit. We do not intend to have the window opening so I've made a fixed frame.

I'm now ready to fit the frame but my question is: do I seal the original window so to have further insulation or should I allow it to breath?

 

[JohnD]

seal it from the house and let the natural expansion and contraction of the air, due to temperature changes, vent to the outside. The pocket of still air will give the necessary insulation.

You can drill a tiny hole to the outside and plug it with a scrap of loft quilt to keep out dust and insects.

Yes, this is the correct way, and no, it will not lead to condensation.

 

[RichD1]

Hi John, when I refitted the restored opening light I had allowed a 2mm gap so that I could fit some EPDM ribbed draught excluder. Could leave a small gap in this filled with with a bit of fibreglass as you suggested.

Richard

 

[JohnD]

Is this sealing the jamb between outer sash and its frame?

 

[RichD1]

Yes.

Out of interest, why is it necessary to vent the outer cavity rather than sealing it? What would happen if it was totally sealed?

Richard

 

[JohnD]

the temperature of the air in the cavity will vary. it is not dehydrated so it will contain an amount of water vapour. When the air is cooled this will condense on a cold surface.

You will not be able to totally seal the cavity. The air inside will expand and contract depending on the temperature, and when it contracts, it will draw in air.

External air holds less water vapour than internal air, so the cavity should be vented to the outside.

 

[RichD1]

Thats brilliant John, I now understand. Just fitted draught excluder to the outside jamb/sash closures and left a small gap between one of the joints.

Richard