What to use to fill gap under uPVC Window?

2 May 2004
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United Kingdom
I have a new uPVC window sitting on a single brick wall (ie no cavity). In actual fact it's sitting on a couple of plastic window packers. The resultant gap under the cill that the window sits on varies about 5-10mm along the entire length, which is actually noticeably large when the wind blows through :) .

The interior side of the wall is going to be completely replastered so before getting this done I need to fill the gap.

Since the window sits on an external stub cill I would imagine the gap is best sealed from outside to prevent filling material going outside under the cill where I can't get at it to clean it up.

So my question is what should I use to close the gap (varying 5-10mm high x 65mm window frame depth)? Should I use a frame sealant or expanding foam or mortar or what?

My main concern is towards ensuring best practice from an insulation and weatherproofing point of view.

Appreciate any replies.

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You can use expanding foam, but will need a flexible sealant (or paint) at the exposed edges (it will not stand up to sun and rain).

It is amazingly sticky and will bulge out when it expands. Mask off the brickwork with newspaper and tape. Spray a bit of water into the gap to dampen it before injecting the foam to help it expand and set.

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