Window position in cavity

[CH_18]

Hi there,

 

Our sills are not quite deep enough so I have two options below (wood simulating window frame position). I have just placed the sill in position to test fit, DPC's will be going in. See next post for pics...

 

Option 1 (left) Go with a smaller reveal, with a cold bridge and not much overhang on the sill, although still clear of drip groove, which I think looks neater.

 

Or

 

Option 2 (right) Set the window back in the insulation space and utilise a 150mm uPVC sill to cover the flat part of the stone sill. More overhang on the sill but possibly looks a bit naff? (They are just off cuts of slate to replicate colour of sill.)

 

I would really like to have smaller reveals but not sure if the cold bridge is worth the risk? Does it make that much difference? Any input is appreciated.

Thanks

[CH_18]

Option 1

[CH_18]

Option 2

[craig]

Option 2 is my preferred option but slightly further back onto the blockwork - best position thermally.

 

If you can get this type of detail, option 1 but, it is too far out for my liking.

 

 

This drip detail at the bottom negates the need for an external aluminium cill and utilises the stone cill for its "intended purpose" otherwise, you will need a 50mm aluminium stub cill like this.

 

 

Option 2 with an aluminium cill detail (how it looks will depend on the window supplier chosen but unlikely to be "much" different).
 

[CH_18]

Hi @craig

 

Thanks for replying, top man 👍

 

Have you got any examples of the above details for me to see, struggling to picture the setup.

 

Has the first detail you attached just got a normal uPVC stub sill? I was hoping to use a 150mm uPVC sill for option 2, do you think aluminium will be better? Suppose it is slimmer and more pleasing on the eye.

 

One last question, if the window is fully in the insulation layer does the window need supporting from the bottom or can you fit windows with straps only?

 

 

Thanks again for your reply 👍

[Mr Blobby]

10 minutes ago, CH_18 said:

if the window is fully in the insulation layer does the window need supporting from the bottom or can you fit windows with straps only?

 

 

We are planning to use Kilshall Sill supports.  This allows for smaller sills in the cavity, properly supported without any thermal bridge.

 

https://killeshal.com/window-sill-supports-providing-thermal-isolation/

Edited June 16, 2022 by Mr Blobby

[craig]

10 minutes ago, CH_18 said:

Has the first detail you attached just got a normal uPVC stub sill?

 

No, this is a timber aluminium window, with a straight edge profile which sits directly in front of the stone cill upstand.

 

13 minutes ago, CH_18 said:

Have you got any examples of the above details for me to see

 

No, but if I have time, I will do a quick drawing of this tomorrow for you.

 

13 minutes ago, CH_18 said:

I was hoping to use a 150mm uPVC sill for option 2

 

That should work fine.

 

13 minutes ago, CH_18 said:

do you think aluminium will be better?

 

Either or. They both have similar stub cills

1 hour ago, CH_18 said:

if the window is fully in the insulation layer does the window need supporting from the bottom

 

Yes, you'll need to offer support at the bottom, the brackets alone are not enough.

[CH_18]

Thanks again @craig, appreciate your help. 

 

Option 2 (window positioned in the insulation layer) seems to be the way to go. My only issue is now figuring out a way to support the frame at the bottom. I will have the 150mm sill on the bottom but due to the weight and position of the frame (replicated by the piece of wood) the support would not be centered and may cause a cantilever effect (attached pic)

 

Any ideas on how I can support the window? Metal dowels have been mentioned in other threads? Or simply angle iron/brackets?

 

One last question, in normal situations where the frame is sitting on the sill, is this not a massive cold bridge? Or is there some sort of thermal break within the uPVC frame? Not really related but just curious.

 

Everydays a school day 👍

[CH_18]

Been thinking about the best way to support the frame from the bottom, what about if I fix some angle brackets off the inside reveals. I've added a couple of pics to show. Will this be enough support? Obviously there will be straps also.