Internal Gaps Around Windows in a Victorian Home

[magdalena]

Concern 1: The Visible Gap Around the Stone Bay Window Cill


We discovered a 12cm high gap that varies between 3-5cm in depth, where the original stone cill is exposed. The demo work revealed some thin tiles that had been placed here, but unfortunately, these crumbled away when the render was stripped (only two timber battens remained in place). Now, we’re looking at options to fill this gap before applying our lime-based render.

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I’m considering a few possible solutions for the gap:

1. Timber Frame with Sheep Wool & Mesh – A timber frame could be constructed to fit the gap, with sheep wool inside and mesh to hold it in place. I thought of mesh because we want to use the lime basecoat (x2 layers) and lime plaster for breathability, but not sure how basecoat will react with sheepwool.
2. Timber or Thin Bricks – Another option could be to simply pack the gap with timber or thin bricks, which would give a solid base for the lime render to bond to. However, I’m concerned about the long-term performance of the timber in terms of stability and moisture control.
3. Wood Wool – I’ve also come across wood wool, which could be an interesting option for filling the gap. It’s breathable and offers some insulation properties, so I’m curious if someone used it?

Concern 2: Slots Along the Window Sides


There are additional, more regular slots along the sides of the windows. My initial thought is to pack these up with sheep wool as well. The main concern here is how well the lime render will adhere to the wool. The gaps are quite thin but deep (2cm wide x 12cm deep).
To prevent direct contact with the lime basecoat, I’m considering tucking a piece of timber in between the wool and the basecoat to ensure the integrity of the render and avoid any issues with adhesion.

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Could Paper Work?
While not as common these days, using paper could be a nod to the original materials ... less durable, but it could be useful in areas where you’re not expecting too much movement or stress?


Would love to hear if anyone else has worked on similar issues or has experience with any of these materials. Any tips or insights - I would really appreciate it!

 

[fernicarry]

I'd fill the gaps under the windows with insulating plaster. That will give a solid base for your plaster finish and a tiny bit of insulation as well. A strip of wood wool would work too if you have offcuts around but that offers almost no insulation value.

The bits of wood may be to support the window board so might be an idea to reinstate those all along.

Are those still sliding sashes? The weight pockets will be either side of the windows so if they still operate you can't fill in those spaces. If they don't need to operate then I think I'd maneuver some sheep's wool insulation in there as you can get a lot of air movement in that space.

For bringing the wall out to meet the window casings I'd tack a strip of wood wool of a suitable thickness onto the bricks. That will give you a rigid edge to plaster up to.

There's quite a large depth difference, was the original plaster 3cm deep? I'd also use thermal plaster to build it up to a level where you can apply your finishing plaster and end up flush with the joinery.

 

[magdalena]

I'd fill the gaps under the windows with insulating plaster. That will give a solid base for your plaster finish and a tiny bit of insulation as well. A strip of wood wool would work too if you have offcuts around but that offers almost no insulation value.

The bits of wood may be to support the window board so might be an idea to reinstate those all along.

Are those still sliding sashes? The weight pockets will be either side of the windows so if they still operate you can't fill in those spaces. If they don't need to operate then I think I'd maneuver some sheep's wool insulation in there as you can get a lot of air movement in that space.

For bringing the wall out to meet the window casings I'd tack a strip of wood wool of a suitable thickness onto the bricks. That will give you a rigid edge to plaster up to.

There's quite a large depth difference, was the original plaster 3cm deep? I'd also use thermal plaster to build it up to a level where you can apply your finishing plaster and end up flush with the joinery.

Thank you for your input! I was completely overwhelmed with what to do.
Unfortunately, the windows are not original, so the gaps on the sides do not need to be maintained.

When we removed the original render in some areas, we found that it was even thicker than 3 cm (judging by the protrusion of the cornices, as we haven’t touched the ceiling yet).

I didn’t consider insulating plaster—only two layers of base coat and a finish—but I may look into it.

How would you adhere the insulating plaster or wood wool below the window?

I read about lime-based adhesive, but if there is a specific product you have used successfully in the past, I would really appreciate a recommendation. The plasterer is coming to help us tomorrow, so I don’t have much time to prepare everything.

Thank you once again!

 

[Cubist]

Before you do anything else, if I were you I would turn a pressure washer on the outdoor perimeter - top, bottom and sides - of that window frame to check just how water-tight it may be. This is a very frequent water ingress route that can damage plaster and woodwork alike - ten minutes now might save you a small fortune later.

You should also avoid using any material that has the ability to absorb and, more importantly, stubbornly hold on to, moisture from any source. So best bin the idea of stuffing those vertical gaps with paper. Firmly packed it will hold onto any water it absorbs for months. Other than that, I'd be following @fernicarry 's advice.

 

[magdalena]

I’ll check the pressure washer—thanks!

If I could pick your brains on how to fix wood wool panels, that would be great. I don’t think washers are ideal since the backing I’m fixing them to is a stone sill. Would you consider using a dot-and-dab method with lime adhesive?

 

[Cubist]

There have been a number of discussions concerning internal insulation and the use of wood-wool panels recently and these contain some excellent guidance concerning who to fix them to masonry/stone. Use the 'Search' function in the menu bar above to get a comprehensive list.

 

[fernicarry]

I had a quick check of the thermal conductivity of wood wool (Savolit 0.066) and thermal plaster (Breathaplasta 0.127) and the wood wool is actually better (lower). I always thought its insulating value was negligible. So in terms of thermal performance you would be better off with wood wool to build up as much of the depth as you can. Insulating plaster is good for filling irregular spaces and getting to a level surface suitable for the boards so you might still want to have some on hand. Sorry for the about face on approach.