Hello,
The sash windows on my property have some kind of filler over the foam in the cavity that I’m looking to repair, but I can’t for the life of me work out what it is. Any ideas??!
It goes all the way round the outside of all the windows - photo shows a bit where it has fallen away but it’s generally in fine condition other than a few spots. the cavity is filled with foam and then something over the top. Feels like some sort of putty/ stone mix but haven’t a clue what it could be. The property is 120 years old and the windows are wooden, but I know some of the windows have been repaired/ replaced in the last decade.
Depending where you are, the original window frames may have been set behind the outer bricks. This became the norm in London after 1666.
Readymade modern frames may not fit, though if custom made they should.
I think it is a gap, not a cavity wall.
Expanding foam is often used but decays in sunlight and holds damp. Silicone sealant can be used for cracks and narrow gaps but looks bodgy over about 8mm wide. You must keep water out. Mortar is likely to fall out.
If you can make a wooden batten to cover the gap, you can inject foam in the gap behind it. The wood protects the foam. I can describe a good method if this appeals. Will you be DIYing?
Pointing Mastic or burnt sand mastic is specially formulated sand. When mixed with double boiled linseed oil it creates a thick putty used for joints between timber window and door frames and masonry or render.
Pointing Mastic hardens slowly, but remains sufficiently flexible to absorb differential movement between the two materials. Adheres well to most surfaces and does not require painting.
If you need to paint it should only be done with a linseed oil based paint.
It has been used traditionally in various forms for more than 200 years
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