I'm just getting ready to install my Windows, and I want to be sure they're right, regarding building control. 

I've got 100mm cavity, which I'm using cavity closers to be compliant, I'm also strapping them in. The new windows have a 150mm sill, now, can I fit lining up to the inside of the outer block, or do I need to use a 180mm sill, and bridge over the cavity 30mm. I've been told both so have no idea what I'm suppose to do. Hope someone can help..... Bob 

4 minutes ago, 80BSY said:

I'm just getting ready to install my Windows, and I want to be sure they're right, regarding building control. 

I've got 100mm cavity, which I'm using cavity closers to be compliant, I'm also strapping them in. The new windows have a 150mm sill, now, can I fit lining up to the inside of the outer block, or do I need to use a 180mm sill, and bridge over the cavity 30mm. I've been told both so have no idea what I'm suppose to do. Hope someone can help..... Bob 

 

I'm not sure what the perfect answer is but in the past I've always sat the window to the inside edge of the outer leaf so 150mm cill is fine.

As you say then cavity closers all round. 

 

I've seen a good few new builds where they scrimp on the cavity closer on the inside cill, just stuffing with rock wool (if you're lucky!) and slapping a timber cill down.  

Thermally, the best place for the windows is smack bang in the middle of the insulation layer, usually, as highest thermal conductivity element of a window is almost always the frame.  Having insulation abutting the frame all around reduces heat loss in the peripheral, (radial) direction, leaving just the thermal path perpendicular to the wall.  This can make a significant difference over having the window further out, where the periphery of the frame is in a cold layer, like the external skin.