Bricking up a window. Best insulation for new cavity wall?

Lilt · 27 May 2021

I've currently got a floor to ceiling window which I'm removing, getting the bottom half of the opening bricked up then a new, smaller, window put back in.

The new cavity wall will be approximately 800 x 2000 and I'm wondering what the best form of insulation would be? It seems my choices are either celotex or fibreglass/rockwool. I've no experience with either so have no idea which would be best.

Thanks guys.

cdbe · 27 May 2021

Full fill cavity batts.

^woody^ · 27 May 2021

It's only worth insulating to the same value as the wall either side of it.

Lilt · 28 May 2021

Great thank you. It's the front window of a mid terrace that I'm doing so the walls either side are insulated by my neighbours' central heating!

Last stupid question - presumably I need to leave a gap between the external wall and the insulation to prevent water ingress? The last thing I want is to have damp appear all over my freshly skimmed and painted wall becuase I've done something wrong.

cdbe · 28 May 2021

As per Woody, not much point if the rest isn't insulated but if you must then full fill (clues in the name) cavity batts are designed not to let water cross the cavity.

Lilt · 28 May 2021

cdbe said:
As per Woody, not much point if the rest isn't insulated but if you must then full fill (clues in the name) cavity batts are designed not to let water cross the cavity.

Both the name and the Rockwool website seemed to suggest you could butt it up to the external wall. Thanks for confirming that is the case as it just seemed wrong to me so I assumed i'd misinterpreted.

It's a small house so with the exception of about 200 on either side and the door the window takes up the entire wall which is which I think it's worth sticking something in the to stop heat escaping. There's a small porch extension on the front (accessed via the above mentioned door) which is well insulated and a single storey extension on the back which is also insulated so this seems like the only "weak spot" as I'm guessing that heat loss into the adjoined properties will be minimal.

In any case, given the amount of other work needed in the house, 50ish quid on some insulation is a drop in the ocean so I figure I may as well!

cdbe · 28 May 2021

You would think so but cold(er) areas can encourage condensation. Better to let the whole wall absorb any airborne moisture equally rather than a few smaller parts. That's the theory anyway, but I suppose if the existing window doesn't have a condensation problem it may not be an issue. Have you thought about trying to push some insulation into the cavities either side of the window - it can often be possible.

Lilt · 28 May 2021

cdbe said:
You would think so but cold(er) areas can encourage condensation. Better to let the whole wall absorb any airborne moisture equally rather than a few smaller parts. That's the theory anyway,.

That is something that seems obvious when you say it but hadn't occoured to me - this is way I like talking to a human not just relying on a website/youtube video for advice

I've had a look tonight and the wall to the right of the window is now internal thanks to the extension. The wall to the left is only 50, not the 200 I remembered. Because of this I'm going to risk insulating the new wall and take your advice re. insulating the existing one if I can get access to the cavity.

Thanks very much for your help.

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Bricking up a window. Best insulation for new cavity wall?

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