Suggested solutions for massive gaps around bay windows.

[ash_and_iz]

Happy new year all. Would appreciate any input the forum could provide on suggested fixes.

We have had some damp issues high up on an external wall that makes up a small bay window in back of our 1890s Victorian semi-detached. Investigating some potential causes and have pulled the moulding off the window to find just a massive cavity beside the window (pictures attached). There was debris and chunks of mortar/plaster like material and dust that I have cleaned out. This on the left side of the window and measure 35cm wide by 135cm deep. I assume it’s like this all the way around the window but want to have a plan before I remove anymore mouldings.

I’ve read enough of the forum to know that oakum can be used to caulk between to the window frame and masonry but seems like an ineffective solution given the size of the cavity. I have hemp wool insulation I have been using to insulate under the ground floor floorboards but just shoving some of that in there and covering with the architraves seems like a bit of bodge job.

Anyone deal with anything like this or have thoughts on how to approach the fix properly?

Thanks in advance.

 

[Cubist]

Is that a chimney breast to the left? Presumably the exterior wall in which the window is mounted is brick?

 

[ash_and_iz]

Hi @Cubist,

Exterior wall is brick but that’s not a chimney breast to the left it’s just a short stretch of exterior back wall before the corner for the left exterior wall (not sure that makes sense). I can see the exterior brick on the 2 faces of the gap where you would expect to see brick.

 

[Cubist]

Thanks for the prompt reply and I think I understand the wall layout better now. As you've already determined, oakum would be entirely the wrong material to use but, while it would offer some insulation, the Stockholm tar in which the fibres are soaked would quickly leach out into the surrounding plaster to create stains that would resist all efforts to remove. The biggest risk you have there is rainwater penetration through the exterior brickwork, have you checked and verified the condition of the pointing and brickwork outdoors? If not then that needs to be attacked as a priority before making any effort to close that gap, and any others you may later find.

To remedy the gap, once you're sure there is no water penetration you could first firmly pack the gap with a fibre insulation material and over this firmly press into place a rolled and crushed length of stainless steel chicken wire over the exposed face of the insulation and wedged between the outer and inner brick walls to leave a final gap/trench of 5-10mm deep. The mesh must be firmly packed into position as it will be used as an armature/skeleton into which a suitable lime or gypsum plaster can be pressed and to then create the final closure surface mating with the window frame and the internal wall.

 

[ash_and_iz]

Thanks for guidance! That makes sense. The outside is rendered (I do not know enough to say with what but probably something awful and cement from everything else I have seen in this house) so there is no obvious route for water ingress. There is however a lot of salt deposition on the brickwork in the cavity both above and below the damp that is causing the paint to peel. (See photo) so not really sure where or how it is getting in unless it’s the window itself?

 

[a twig]

I used a hemp lime mix to pack in gaps between the window and the stone in a similar situation. You can get away with packing in 30-40mm depth at a time.

You might find it hard to get a smooth finish on the hemp lime so leave yourself a couple of mm for a skim of a normal lime plaster

 

[Cubist]

How is the bay window roofed, what materials? Is there any guttering immediately above the bay and that wall? Are there any joints - for turns, downpipes, et al - in the guttering above the bay and wall? Is the uppermost edge of the exterior render above the top edge of the guttering - could driven rain or overspill from the gutter get to that render edge? Lastly, are there any cracks, even hairline, in the render.

Could be an idea also to check the seal between the window frame and the masonry. It could have perished and failed or simply cracked with expansion contraction of the surrounding materials. If so you may need that oakum after all.

 

[ash_and_iz]

It’s a 2-storey bay (ie the bay is duplicated in the room above and the structure goes up to the roof) and there is no guttering directly above - probably should get that sorted. Have just taken it all apart and it definitely look like the water has been getting in around the joint between the window and the masonry even though from the outside it look pretty tight there is nothing sealing the joint. So as you say Cubist there is a role for oakum after all! How would I go about introducing it? From the inside since that is now exposed or chisel away from of the render for outside? The latter seems like it’s more like to go wrong…

 

[Cubist]

If the joint between the window and masonry is tight - say about 3-5mm then you'd be better with Caulking Cotton rather than oakum. Caulking cotton is supplied in rope balls of around 30 metres or .5kilo and in several diameters, 8mm being the most popular. Pulled taut the cotton reduces in diameter to less than 2mm and can be pressed with the edge of a suitable tool into gaps 1mm or less. You can also braid strands of the cotton into thicker ropes to suit your needs - I've created ropes of over 20mm wide to pack into some spots. You do have to make sure its firmly packed into place and, while some sellers will try to get you to buy some very expensive caulking irons you can get the job done with an old and dull bolster chisel, a stubby broad blade slotted screwdriver and a upholsters tack puller and also a mallet.

The object is to press and hammer the braided cotton in to the gap to leave a shallow rebate into which a suitable overlay filler after painting the surface of the cotton with some linseed oil - a lime based filler or mortar is then used to finally fill the rebate.

Oakum on the other hand can be extremely expensive as it's usually supplied in multi kilo reals that you may not use in a lifetime. It's used in much the same way as Caulking Cotton and is used for gaps above 5mm or above but can be thinned/stripped down for narrower applications. The advantage is that it's pre-soaked in Stockholm Tar and produces a water-tight closure for any gaps and a finishing fillet of lime mortar allows a clean closure of the joint. Your hands will smell nice too.

Never leave Caulking Cotton or Oakum exposed to atmosphere - your local birds will steal it for their nests and drive you nuts while they try to peck it out of the joints.

If you can pack the CC or Oakum into the joint as tightly as possible from the inside then go for it but be careful not to dislodge or damage the render outdoors. Otherwise you may have to wait until the good weather next year.