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Hiya, thank you for reading this 
My question is do I need a vapour barrier/membrane?
The window in the photos has been installed on the outer skin (red brick). I know that's not where it should be but I think it's the original window.
The steel lintel is sitting only on the aerated blockwork - it is 1.5cms above the top of the window and it stops approximately 2cms in front of the inside face of the window frame.
If I put my hand in the gap I can feel a piece of timber approximately 6cms in height and at least the length of the window sitting directly on the back half (ish) of the window frame. On top of the wood I can feel a plastic tray type thing on top of the wood.
The cavity between the aerated block and red brick skin is insulated.
I want to stop any breeze coming through but without using spray foam (yup, I'm one of "those" people who doesn't like the idea of the off-gassing from the foam- please feel free to judge me but then please help me anyway lol).
My current plan is to batten the side and front of the lintel and fill the gaps with rockwool insulation slabs, then plasterboard over the top, using a silicone sealant between the plasterboard edge and window frame.
I'm aware that putting even this small amount of insulation in will presumably cause a slight change in the temperature of the steel lintel and I'm worried about condensation forming on the lintel.
Do you think I need a vapour and/ or air barrier on the warm side of the insulation or do you think would this cause a problem?
My question is do I need a vapour barrier/membrane?
The window in the photos has been installed on the outer skin (red brick). I know that's not where it should be but I think it's the original window.
The steel lintel is sitting only on the aerated blockwork - it is 1.5cms above the top of the window and it stops approximately 2cms in front of the inside face of the window frame.
If I put my hand in the gap I can feel a piece of timber approximately 6cms in height and at least the length of the window sitting directly on the back half (ish) of the window frame. On top of the wood I can feel a plastic tray type thing on top of the wood.
The cavity between the aerated block and red brick skin is insulated.
I want to stop any breeze coming through but without using spray foam (yup, I'm one of "those" people who doesn't like the idea of the off-gassing from the foam- please feel free to judge me but then please help me anyway lol).
My current plan is to batten the side and front of the lintel and fill the gaps with rockwool insulation slabs, then plasterboard over the top, using a silicone sealant between the plasterboard edge and window frame.
I'm aware that putting even this small amount of insulation in will presumably cause a slight change in the temperature of the steel lintel and I'm worried about condensation forming on the lintel.
Do you think I need a vapour and/ or air barrier on the warm side of the insulation or do you think would this cause a problem?
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