Sealant over expandable foam

17 Aug 2023
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United Kingdom
Hi there
Complete amateur so excuse the possibly silly question. 1850s cottage, fairly new windows and frames, cracked sealant causing leak if wind drives rain against the house. I want to reseal the window frame with silicone sealant but after removing g old cracked sealant I am faced with quite a gap partially filled with expandable foam (I guess).
A: can I apply the silicone directly over the foam?
B: should I add more foam to reduce amount of silicone?
C: should I do something completely different?
Thanks a lot
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Large holes can be filled with mortar. There's nothing to stop you to use as much silicone as you want. But you have to do that in a single application because silicone won't stick to itself. For multiple applications at the same place, hybrid polymer sealants would work better.
If it's a mid-nineteenth century cottage then you need to fill that gap with limestone mortar. Anything else is a sin.
That’s fair enough. So remove the old expandable foam and then mortar? Sounds like a job for someone that knows what they are doing and not a quick DIY by myself
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I don't think you can take out the foam around the window without removing it so best get someone in to take a look at the job on site.
Speaking an amateur, and having learnt a small amount about using lime putty from reading this: and by watching a few of the Mike Wye videos on youtube, and then having done quite a lot of both lime putty and hydraulic lime (NOT hydrated!) work on my own house - I'd fill that gap with mortar made up of NHL (Naturally Hydraulic Lime) (NOT hydrated) and sand, then once it had completely gone off, maybe a few weeks later, I'd probably use a bead of silicone beteween the lime and the frame.
I'd most likely use NHL 3.5, but if all I could get hold of is NHL 2, or NHL 5 then I'd use that.

The reason I'd use NHL lime rather than cement is that it's slightly easier to use, in that you've more time till it sets, but more importantly, it is slightly flexible, and allows a little moisture to move out of the structure and into the air outside. Cement is likely to seal moisture in, could possibly cause a little cracking in the lime mortar around it, and is unlikely to seal as well around the window frame.

It's an easy job - mix it up and slap it in.

...and don't be tempted to mix a little cement in with the lime - there's been a sound piece of research that demonstrates that it just causes problems.

Personally I wouldn't bother removing the foam at all - it ain't broke!

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