Help with building up wall under windows

[Bill the Builder]

Hi,

I am in the process of renovating my hoose and have taken out the auld concrete floor and about to install a limecrete floor system. Before I do that I am looking to build up underneath the windows. Both used to have wood underneath, but due to rot, this was all removed. I am now looking to build up these areas with blockwork and looking for advice as to best practice way to do this.



Window 1. I intend to build up to right under the window as the window is only partially supported by the existing sandstone sill.

I intend to insulate with lime plaster and woodfibre boards.

Can I just use concrete block and NHL 3.5 as mortar which will give it a certain degree of breathability.

What would be the best way to construct it?



Window 2. I intend to build up to a level where I can use it as a seat at the window. Also to be insulated as above.

Can I again just use concrete block work? What would be the best way to construct this?

I want to make sure that there is no impediment on breathability. Will using concrete with NHL mortar still give plenty of breathability?

All advice and recommendations on how best to complete these tasks will be greatly appreciated.

Cheers!

 

[fernicarry]

Blockwork won't have much insulating value and will take up valuable space that could be filled with insulation especially with the wall being thinner in the window reveal. I'd frame it out in timber and fill the voids with insulation, something with a bit of give like Indinature given that the wall won't be completely flat. I parge the stonework with an insulating plaster first to fill gaps and smooth it off somewhat but it still won't really be flat enough for rigid boards. You can attach the studwork to the stone by hammering wooden wedges (dooks) into gaps between stones which is probably how it was before being stripped out. The studwork can be made beefy enough for a window seat if that's what you want.

If the window cills need a bit of help then you could tie them into the studwork but if the whole case is quite secure I wouldn't bother. I try to avoid attaching anything directly to the wooden cills as they tend to be consumable items around here and I'd like to be able to get them back out in the future without destroying the internal joinery (again). The last one I did I cut a groove into the back of the cill and put a tongue on the window board so that it would fit snuggly into the cill without needing any fixings between them (yours look like they already have a groove). I also make my own T&G bead board that matches the originals to line the reveals. If the boards had previously rotted then that tends to indicate there is water getting past the window somehow so you should get that sorted if not already done, especially as you have unobstructed access at the moment.

Whereabouts are you?

 

[Bill the Builder]

Thank you for your reply. Yes it was as you described. However, Im not bothered about the insulating effect of the blockwork as the whole wall will then be insulated with a plaster leveling coat, 60mm of woodfibre boards, then a finishing coat of plaster. Approx 85mm in total.

My reasoning for using concrete blockwork is to make the wall more uniform, add support to the existing window and I also need to raise the height a bit anyways so it is above the level of the new limecrete floor and thought easier to then just continue with the block work. Using NHL 3.5 to allow some breathability between the blockwork. However I've never built this sort of thing before and really looking to make sure I do it properly, the materials I'm using are appropriate for the building and will still give suffucient breathability to the wall.

I've installed French drains outside and made good the external of the windows by stripping them back to wood. Then painted with linseed paint. Bedded in with lime mortar and burnt sand mastic to seal the joint between the window and the external reveals. All breathable material. Window are watertight.

I'm up in the Highlands.

 

[stuart45]

For that small area you could use blocks or bricks, although in theory the top job when building a wall is to use the same materials as the existing, so the movement is the same.

 

[fernicarry]

to make the wall more uniform

In what sense? For practical purposes like getting to the window latch to open and close it you really need to be able to stand in the reveal close to the window. Unless your windows are very short. So if you mean to square off the wall at the bottom that will make it awkward.

 

[Cubist]

I'm thinking that rather than building up the walls masonry why not consider letting a wooden seat/step directly into the recess using a chunky piece of timber?

 

[Bill the Builder]

I was thinking about using stone for that very reason. However, for ease and speed would prefer to use block and brick. So for a small area like this it will fine as long as I use NHL mortar?

Ref the windows i see what you mean. Fortunately the one where I propose to have the seat is a sealed unit with no openings. The other window only opens outwards at the top section and will still be easily reached.