How should I rebuild and insulate these window reveals? DPC/cavity exposed

[adexandros]

Hi all,

I’d appreciate some practical advice before I start rebuilding these upstairs bedroom window reveals (and I completely lose my sanity)
The house was built in the late 1980s and appears to have cavity walls with an aggregate concrete-block inner leaf. The window is uPVC.

The original problem was mould, peeling paint and cracked plaster around the reveals, especially at the lower corners and across the top. It happens in winter only. We often get condensation on the glass and occasionally a small pool of water in the bottom corners, but summer rain does not seem to make the wall wet.
Opening windows during winter and a dehumidifier only partially helps, I also use a window vac everyday but still the walls issue persist.

I repaired one area a couple of years ago using expanding foam, EasiFill and paint, but it failed again after a couple of winters.

I have now stripped the thick hollow plaster from both side reveals. What I found was:

• Very thick backing plaster, in places several centimeters deep.
• Some plaster apparently spanning an unsupported void beside the frame.
• A black membrane, which I assume is the vertical DPC.
• Cavity insulation behind the membrane. (blown insulation)
• Some plaster/mortar appeared to bridge towards the outer brick.
• A few small pieces of the inner concrete block edge broke away during removal.
• The top reveal is plasterboard with plaster over it, apparently supported/reinforced by metal mesh beneath a steel lintel.
• I have only opened a small section of the top. The rest still feels solid.

My current idea is:

1. Remove loose debris and any material bridging to the outside brick.
2. Leave the DPC and cavity insulation in place.
3. Repair any significant damage to the inner block edge with masonry repair mortar.
4. Fill only narrow frame gaps with low-expansion window foam.
5. Fit thin insulated board to the two side reveals.
6. Patch the small opening in the top plasterboard and possibly overboard the full top reveal with thin insulation.
7. Leave a flexible sealed joint where the new finish meets the uPVC frame.

I have considered Marmox Multiboard, although I am open to PIR-backed board, aerogel reveal board or another better option.

My main questions are:

1. Does the black membrane and insulation arrangement look normal and serviceable, or is anything obviously wrong? It doesn't "tuck in" inside the window frame
2. Is insulated board a sensible way to rebuild the side reveals?
3. Should I repair the broken block edge first, or can insulated board/packing bridge the irregular area?
4. For narrow Marmox strips, would MSP360 adhesive be adequate on clean aggregate block, or should they be mechanically fixed? Since there is a lot of leftover plaster on it
5. Can I retain and patch the solid top plasterboard, then insulate over it, rather than removing the entire top reveal?
6. Should I retain the existing external corner beads or remove them and fit new beads over the finished boards?
7. Does this look mainly like condensation/cold bridging, or is there anything suggesting rain penetration? I've checked the external walls around the windows and everything looks sound without gaps

I am trying not to turn a local repair into unnecessary major work, but equally I do not want to cover it up and have it fail again.


Photos attached showing:

• The original mould and failed plaster.
• Both reveals after stripping.
• The DPC and cavity insulation.
• The damaged inner block edge.
• The top plasterboard/metal-mesh construction.

Thanks.

 

[Wayners]

Condensation.
You need a dehumidifier or 2.
We run one upstairs and one down for about 4 hours every evening.
If you dry washing inside in winter use a dehumidifier in the same room.
Deal with condensation and the problem will go away.
Line the inside of windows with 12.5mm plasterboard

 

[adexandros]

Thanks, a bit sad that I might have done all this work for nothing
We use a dehumidifier upstairs, plus open windows for ventilation, don't dry wash inside, etc - the problem is that only (or mostly) windows on the east side are affected, the other 2 on the west side don't have this issue, this is why I wanted to make a proper fix, after this failed twice already - and I'm still not quite sure how to fill those voids at the bottom?

 

[Wayners]

Kirk has a few videos showing how to board around windows.
Pick one and watch.

Here is one...