Plasterboard Fixing Failure at Window Reveal

[Spinny]

Could do with some suggestions on how to fix a problem boarding up a window reveal.

 

The window is 2.4m high running up from floor level. The window has been fixed to the inside of the outside blockwork leaf so that the inside frame is level with the inside edge of the outer leaf. It is an unusual detail as we had the architect remove the inner blockwork leaf so we could have a narrower window reveal. Hence there is no conventional cavity closer. In insulating and boarding the area we decided to fix some 30mm XPS to the inside of the outer block and then also fix the reveal board to this. So we used resin to fix the XPS to the concrete block and resin to fix the plasterboard to the orthogonal edge of the XPS. We used the XPS as it is a good insulator, rather than fix a strip of timber against the outside leaf.

 

The plaster board fixing to the XPS has just failed. And so has the XPS fixing to the concrete block. What seems to have happened with the plasterboard resin fixing is that XPS itself has failed in tension so the resin is still stuck to the board along with a thin layer of the XPS. What seems to have happened with the XPS resin fixing to the concrete blockwork is that it has failed to bond well to the blockwork possibly not helped by some mortar residue on the block.

 

Now the problem is how can we redo this reveal to get secure fixings ?

(Especially since the front face has now been insulated and boarded limiting options.)

 

See the attached pics...

 

 

 

 

[crispy_wafer]

Ooh, now…. Wait for the experts to appear, but my initial thoughts would be bonding 25mm angle to the frame top to bottom and using this to fit the reveal board to.

[Spinny]

The pencil line shows the chosen plaster line to be achieved, so the plaster board edge will cover the edge of the frame and the 25mm angle would need to be fixed to the concrete rather than the frame.

 

How would I attach the 25mm angle to the concrete block ? It would be adjacent to the edge of the concrete block so screws would be likely to break out. Is there some other adhesive that could be used and be stronger than the resin ?

[crispy_wafer]

Right, I’ve a thought, but you might not like it…. Can the inner habito section be dismantled?  If so, get that out the way so you can get a good access to the block work, then mechanically fix a length of timber to it.  Or perhaps a 15mm*25mm angle but flipped inwards so you have the 15mm facing inwards, bond or mechanically fix to the frame.  The reveal board when plastered will cover the metal.

 

me, I’d look to my first option.

 

I think @Iceverge may have had solutions for tricky reveals in the past?  And @Nickfromwales may have seen a situation like this out in the wild.  Sorry for tagging you chaps, this one needs more than my willing head!

[Spinny]

@Iceverge @Nickfromwales @crispy_wafer or anyone

 

The habito face of the adjacent wall section is screwed into the vertical timber at that corner, but on the other side has been fixed to the end of the inner leaf blockwork using PU010 panel adhesive, and the other side/corner just bonded out ready for skimming. We did have access from that side before that which we used to put PIR insulation in the void behind. So it would be gutting to rip that all off again, likely require new board to be cut to size with apertures etc.

 

I was reluctant to put timber against the inside of the outside leaf originally on the basis it would have a lower insulation value, and if by any chance moisture ever made it through the block onto the timber it might rot or mold etc.

 

XPS is strong in compression, but clearly not if you put it in tension.

 

Unfortunately we did the same on another two reveals, so now concerned we will have to redo those with a different solution too.

 

The outer blocks are rendered on the outside face so there is reluctance to start drilling into the back face close to the edge by the window opening for fear of the block breaking out.

I don't know if there is another adhesive that would work better. The plasterboard piece to close the reveal seems to need pulling in tight somewhat to stop it bowing out towards the window in the centre.

 

More photos attached.

 

 

[Nickfromwales]

I've been holding off as it's taken a while to digest, between the other things I have to do to keep 6 people alive lol.

 

The black (XPS?) is friable, so cannot be bonded to the way you have tried, as the surface area just isn't sufficient for any decent kind of permanent purchase.

 

@Spinny

I see (and assume) you've tried to thermally disconnect the Habito from the frame, but this just isn't robust. Instead, I would mask the frame off (Frog tape or PVC electricians tape) so the surface of the frame doesn't get damaged, and pump a load of clear CT1 in the gap between the black XPS and the frame; I'd open the gap with some packers, then pump the goop in, and then push the XPS back towards the frame to encourage them to form a "Spice Girls" connection (two become one).

 

The ideal would be to have the Habito cut and ready, and to fit it into the then still wet CT1, and brace it with a length of 4x2 to maintain its form during the curing process. As it'll be screwed top the inboard timber batten one side, the 4x2 should only need to offer a little restraint overnight.

 

You'll need to work pretty quickly though, and control the excess amount of goop and it's subsequent displacement onto the masking tape. You will need to clean this away with some stout cardboard and some baby wipes, and then use the excess to coat the surface of the XPS, by spreading the displaced goop instead of binning it.

 

Once you have a boded XPS board, and that and the frame are stuck together forever, whilst this is still all wet you then remove the masking tape, slowly and carefully, and dispose of.

 

Then you apply a nice fat bead of CT1 all the way from top to bottom, at the outboard edge of the XPS where it meets the frame. Press the Habito into place, put the screws in the far end, and set the 4x2 in place and leave for 12hrs minimum to cure.

 

Each of the side reveals can be done in the same sitting if you want, be left to cure, and then the head if that needs sorting (but would be a different situation with the lintel obvs).

 

Take 2 of those and call me in the morning  

[Nickfromwales]

@Spinny

 

Was it you being over-zealous with the FR plasterboard or your BCO? I had to sober one such BCO up recently for exactly the same thing, and plenty more before him, about that being absolutely NOT a requirement. He said "it needs to be there", and I said "prove it". It did not go in.

 

Too many dicks out there with red pens afaic.

[Gus Potter]

On 30/01/2026 at 18:13, Spinny said:

Unusual detail as we had the architect remove the inner blockwork leaf so we could have a narrower window reveal.

Did you check with your SE? What loads are coming from above? 

 

Are you one of these Clients that are stubborn and it's you way or the highway?