Filling in half done cavity wall insulation

Hi there - new to this space so apologies if this isn’t posted in the right place/way! This may also be quite long and unclear & I can only upload one photo!

I have a question about cavity wall insulation in a 1990s kitchen extension on my Edwardian terrace. We recently knocked down internal walls between the extension and dining room and small loft ceiling in the extension, exposing (and damaging!) the internal concrete blocks of the extension. We need to rebuild the concrete block wall at the top and then plaster the whole space. FYI we also put in the skylight window.

Between the concrete blocks and the external brick wall there is a 3inch cavity. There is insulation up to the height of where the old ceiling was. Looking in there’s lots of rubble in there, ties between the concrete and brick and I think the existing insulation is fibreglass and there is no vapour barrier. Attached photos for reference !

These are external north/west facing walls so we really want to make sure we can insulate them as well as possible. We can get into the space a bit as the concrete block wall is broken and lots of gaps. Couple questions and please appreciate we are complete newbies to this!

  1. What would be the best way to insulate this area? Just stuff it with more fibreglass and hope for the best?
  2. Do we need to worry about vapour barrier or moisture in this area?
  3. There are air bricks in the top corners, which were there to provide ventilation to the old loft space) should we keep these in? We have other air bricks at ground level throughout the space.

Any advice at all is greatly appreciated!

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Just replying to add a photo !

To answer the second, and easier, two first, no, you don’t need to worry about a vapour barrier but yes you might need to worry about moisture.
And yes, if you are incorporating the space into the habitable area you do need to remove the airbricks.

Given the quality of what you have uncovered I think it would be wise to check the mortar and pointing outside. Poor finishing can lead to damp ingress.
If there is penetrating damp you will be very limited in the type of insulation you can use and then the insulation must prevent the damp from transferring between leaves of the wall.

You will never have a well insulated wall if there is rubble in the cavity. It can also act as a bridge for moisture. Ideally it needs to be removed, which might involve dismantling one leaf of the wall, so a major project and expense. Otherwise just fill the rest of the cavity with a pourable insulation from holes drilled at the top. “Stuffing fibreglass” is not a solution. It won’t reach everywhere and will not insulate where compressed by attempts to force it in. I am thinking of polystyrene beads. Without access from above you will unavoidably get gaps at the top of the insulation due to the method of application. A band of internal wall insulation (IWI) along the top would help a little.
Once complete, consider either IWI or EWI to bring the wall up to a decent standard. You will find discussions on the pros and cons elsewhere on this forum.

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Hi Tim - thanks so much for the response, incredibly useful. We managed to pull the rubble out of the cavity, the internal wall has now been rebuilt and we are arranging to fill it with the pourable polystyrene insulation. Thanks again for the help!

You are welcome. As an afterthought, you can add a very slow set glue to the beads, so that the insulation doesn’t escape from any future holes in the wall (intended or otherwise). Be sure not to use a glue that dissolves the beads; test a small sample.

For example, see