Sash Window Cavity Closing

2 Jul 2004
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United Kingdom

I looking for advice on the best way to close a cavity around the outside reveal of a sash window? Also should the concrete cill be allowed to bridge the cavity, be tied into the inner wall etc?


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On my sash windows (1910) the cavity is sealed with mortar, and covered with a piece of wood.
Hi Tom,

I've seen it done this way too, but I'm not 100% sure what will be acceptable today.

I was thinking of closing with polystyrene then mortaring over the gap. The window will sit slightly in front of the inner wall allowing me to fill the remaining gap with mortar.

I’ll have a chat with the building inspector and see what he wants before I start. The existing opening contains French doors and I can hardly wait to see what botch was used to close them.

On the cill front, I was going to hold it short of the inner wall.

I’ll let you know what the inspector says.

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I wouldn't recommend using any modern materials like polystyrene. Look at it this way - when it was built, did they use polystyrene? I'm not sure how polystyrene would interfere with the natural ability of the bricks and mortar to breathe. I doubt there are any advantages. If you're thinking of using something behind the mortar just as a support trust me it really isn't necessary.

I suggest using a mix of 5:1:1 sand:lime:cement. Let it dry for a few days before you operate the window, so it doesn't crack. Lime mortar is better than cement mortar, because normal cement mortar doesn't allow water to permeate, which can mean damage to the face of the bricks during winter - whereas lime mortar allows water to permeate, which reduces the amount of water that freezes in the brick. My windows have been like this for 95 years, and they're working perfectly! :)

I think what you suggest for the cill would be fine. Make sure your building inspector is appreciative of traditional building methods (I'm presuming that your house is old, as you're installing sash windows). For more advice on old houses, try

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