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Do artstone cills have a structural core?


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A few months back I took delivery of a batch of cast artstone window cills for my self build neighbour.

 

When carrying them into the garage I noticed the cavity facing side of the cill looked like a regular concrete masonry block. I got the impression cills were formed from a concrete block core with a thick outer shell of poured coloured artstone mix.

 

Is this normal?

 

Having seen this type of make up I am now less concerned about the strength of these cills.

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21 hours ago, PeterW said:

They are not structural - they just have a core of concrete as its cheaper than using the mix they have for the outer. 

 

 

Ok good to hear the concrete core is normal. The ones I took delivery of were from a national artstone brand name that sells direct over the internet and I wanted to check the core was not a sign of an inferior manufacturing process.

 

Regarding the structural point, I am looking at a specialist type of cill designed to overhang further into the cavity in order to support a sash window installed with a full brick reveal.  The aperture of the outer brick will be 70mm smaller each to hide the sash box and allow for more glazed area. At the moment I cannot visualise how much of the total window weight will sit on the cill's cavity overhang even so I am reassured by the presence of the concrete core.

 

 

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14 hours ago, epsilonGreedy said:

 

Ok good to hear the concrete core is normal. The ones I took delivery of were from a national artstone brand name that sells direct over the internet and I wanted to check the core was not a sign of an inferior manufacturing process.

 

Regarding the structural point, I am looking at a specialist type of cill designed to overhang further into the cavity in order to support a sash window installed with a full brick reveal.  The aperture of the outer brick will be 70mm smaller each to hide the sash box and allow for more glazed area. At the moment I cannot visualise how much of the total window weight will sit on the cill's cavity overhang even so I am reassured by the presence of the concrete core.

 

 

Does that not make a large cold bridge across the cavity, should the cill not stop short and the window sit on a cavity closer. 

Or are you saying it only projects a small portion into the cavity with the closer sitting behind it. 

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30 minutes ago, Russell griffiths said:

Does that not make a large cold bridge across the cavity, should the cill not stop short and the window sit on a cavity closer. 

Or are you saying it only projects a small portion into the cavity with the closer sitting behind it. 

 

 

Yes it does, and will possibly form a condensation locus I suspect, unless something is included to mitigate the thermal bridge inside the cill. 

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10 hours ago, Russell griffiths said:

Does that not make a large cold bridge across the cavity, should the cill not stop short and the window sit on a cavity closer. 

Or are you saying it only projects a small portion into the cavity with the closer sitting behind it. 

 

 

Not sure of the detail yet but yes I will not position the cill so far back it will touch the inner masonry wall. The special cill for sash windows is 225mm deep which is 35mm deeper than the typical cast cill.

 

I will be hoping for a 25mm cavity gap which I will stuff with a 1" PIR board cut off. I also intend to enhance my wall u-value with PIR backed plasterboard. With a combined 57mm of PIR at this potential cold bridge I hope the problem is minimized. It should also be possible to reduce the area of the cold bridge by letting the inside of the sash window rest on 3 150mm wide cut blocks with the rest filled with insulation.

 

I cannot find any cill installation instructions online that define the depth of the protruding lip of the cill beyond the surface of the facing bricks, hence at this point I am guessing how small the token cavity is between cill and inner wall.

Edited by epsilonGreedy
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