Timber frame with stone cladding / render - Window details

[HanleeHouse]

HI All, 

I've just received planning permission for a replacement dwelling. Timber frame build - 140mm frame + 9mm OSB.

 

Windows are triple glazed aluminium, window company has asked for details on position in frame and how the stone / render will return to the openings. 

 

We have 120mm PIR insulation in the frame and a further 30mm internally. Aluminium frames are 70mm. 

 

Please could you have a look at the attached sketch? Should I continue the 30mm PIR, 25mm service batten, 12.5mm plasterboard round into the window reveal, OR.. would it be better to say use 52.5mm insulated plasterboard internally straight to the timber frame? 

 

Appreciate your help. Also, anyone have any tips on software etc to draw these details? 

 

Thanks 

 

 

[JohnMo]

Think you are making things a little difficult as drawn.

 

Your frame needs to straddle the insulation not necessary to fully bedded into it.

 

So if you bring the window frame to outside flush with cement board, you don't need to form an outside return. Which could look a bit bitty.

 

This is mine, cut the stone with a diamond cutter.

 

Inside return is now deeper and easier to form, as you have material available to attach things too without risk of splitting.

[Big Jimbo]

I think the way @JohnMo has done it, is the only way that will look decent. The only other way I can think of is to do it as the picture above , but set the window back further in the frame, and fit say a cladding to the sides and head, that would match the window colour. Bring the cladding out to the edge of the stone. 

The only other thing I would say, is that if you bring the windows forward, you might be bringing them outside the insulated envelope. BRegs say the frames should overlap the insulation by 30mm.

You can combat this by fitting insulated plasterboard to the reveals inside.

You just need to think about not creating thermal cold spots.

[Big Jimbo]

If you were to set them back further, you would need to make sure you were getting deep cills.

[JohnMo]

Forgot to mention, is the external detail robust enough to take the weight of 30mm blocks? Has it been checked structurally?

[HanleeHouse]

On 24/02/2025 at 15:15, JohnMo said:

Think you are making things a little difficult as drawn.

 

Your frame needs to straddle the insulation not necessary to fully bedded into it.

 

So if you bring the window frame to outside flush with cement board, you don't need to form an outside return. Which could look a bit bitty.

 

This is mine, cut the stone with a diamond cutter.

 

Inside return is now deeper and easier to form, as you have material available to attach things too without risk of splitting.

 

Sorry, I missed this reply. 

I was planning on having a finish looking similar to this below. With a 225mm cill. 

 

 

The STS board has a 100kg/m2 rating. The stone is 45kg/m2. STS and timber frame co have confirmed with their SE.