BCO & Window fitting!

[pilsbury]

I assume your pics are showing that the outer skin is visible inside -a cold spot for condensation? So windows should be fitted flush with the inside of the outer skin?

 

[stuart45]

So windows should be fitted flush with the inside of the outer skin?

Used to be in the good old days, but times have changed and thermal bridging means a change of position.

 

[HammerHappy]

Thanks for all your input.

Yes the BCO said fitting windows in the middle of insulated cavity is all about thermal bridging so reducing chances of condensation issues around the reveals. He did add that this is the ideal and whenever a window is fitted to the outer leaf then insulated plasterboard should be used on internal reveals.

Having looked at all your replys and the extra expense of buying 225mm sills, I'm going to fit them on the outer leaf and set back over the insulated cavity closer as much as possible even if it's 10-20mm making sure there is enough overhang left after rendering for rain drip groove.

The deepest sill I was offered when buying the windows was 185mm. ( So my mistake for not looking into this when ordering it's just I was convinced until now that windows are fitted flush to the inside face of outer block ) This will help a little so I will see if the company will exchange my sills. I do find it strange with this issue that they don't do a 225mm though.

Then when BCO comes out and he wants or we agree it's a good idea to make sure of no future damp problems ( cold bridging condensation) I will insulate reveals with 25mm celotex which I have plenty of anyway.

Thanks again, this is a really helpful site which I have been referring to a lot during my build.

Cheers

 

[Gazman16]

What make of windows are you using? Most do a 225cill (some even do a 250mm). Straps can also be purchased specifically for your make of windows that clip in.

Thermal bridging is a good point and should be considered but good fixings for the windows/doors is also important. Straps only have any support in the vertical plane.
I have just fitted 2 houses of windows (NHBC newbuilds which now state windows must be fitted behind the external brick in coastal areas)
Not a problem for small light windows but doors and large windows this will cause problems in my opinion.

Straps will not support any weight and they wont prevent sideways movement which will cause future problems with adjustments and longevity of the windows IMHO.
Also fitting behind the external reveal is just asking for leaks.

Mechanical fixings would be my recommendation but that generally means fitting at least 30mm forward from the back edge of the brick/block.

Would love to hear others opinions on this, but I really don't like fitting with straps for the reasons above plus a few more.

 

[John D v2.0]

Straps will not support any weight

That's why you put plates across the cavity at the bottom and pack the windows of those. May be a small thermal bridge and some people build ply boxes and all sorts, but you can't float them off sky hooks!

 

[Gazman16]

True but straps and plates still wont stop sideways movement.
Ok expanding foam and being plastered upto will help somewhat but with doors being bashed, Slammed and generally abused I foresee problems

 

[HammerHappy]

What make of windows are you using? Most do a 225cill (some even do a 250mm). Straps can also be purchased specifically for your make of windows that clip in.

The make of all the windows & doors I'm installing is Euramax.

There deepest sill is 185mm. Haven't actually opened the packaging yet to see if there are straps / clips for fitting. If there are none included I will most likely screw through the frame into outer leaf with frame fixers.

I might phone and ask their preferred installation method if there are no instructions or straps included.

 

[^woody^]

Setting windows back too far creates it's own set of problems due to the micro climate formed by the shading of the frame at the reveal. Its bad practice as the thermal bridging issue can be dealt with by other means.

 

[Gazman16]

I take it you have already ordered the windows? 20 odd years in the trade and Ive never heard of them anywhere.
It doesn't say what make of PVC windows they sell on that site so I wouldn't be expect a good quality make

 

[noseall]

Setting windows back too far creates it's own set of problems due to the micro climate formed by the shading of the frame

Thats a canny argument at best. All frames are in the shade at night time, and it would not trump having the frame too far forward.
I think it is important that the frames are far enough back as to alleviate any risk of thermal bridging from the outer leaf components. That means not having anything external visible once the frames are fitted.

Sensible architects were asking for the frames to be lipped at least 10-20mm back into the cavity. Fitters prefer them forward because it makes their lives easier.

 

[Notch7]

Victorian sash windows that were fitted behind the outside brick skin, have lasted so long due to their protection from weather.

So setting back windows from the outer face is a good thing generally.

 

[noseall]

Old style ash windows were positioned so that they could be operated internally i.e. pushed up or down.

 

[^woody^]

Victorian sash windows that were fitted behind the outside brick skin, have lasted so long due to their protection from weather.

So setting back windows from the outer face is a good thing generally.

I suspect they lasted longer because they were not made from a fir tree from Norway grown for five years then cut into knotty planks.

 

[JohnD]

In old London it was made law that timber, including windowframes, had to be set back from the face of the wall, and this style is often used in nearby areas.