problem with upvc windows sealant

A question about : problem with upvc windows sealant

My large living room window has developed a problem in the past year. The right hand side of the frame from the top to about half way down is pulling away from the wall.
It has actually cracked the sealant and I have had to temporarily fix cellotape to stop
the draughts.
The funny thing is,when its warmer weather the crack closes up and when its cold it opens up again to about 2cm. How can window frames or walls contract?
The frames and sealant on the outside seems intact.
Can someone shed light on what is causing the problem?

Best answers:

• 2cm??? Really? Or do you mean 2mm? I've never come across such a huge gap as 2cm...
  Russ
• Ha ha sorry, yes I meant 2 mm. Any ideas what is causing this?
• Try putting a small amount of expanding foam into the gap when opened up and be careful not to get it everywhere as it is a nightmare to remove from pvc, From the years I fitted windows the occasional one here and there may have a small gap but this was not weather related more like a wonky wall.
  Wood does shrink and enlarge in different weather but I have never heard of pvc doing this, I presume it is the wall not being square and then filled with silicone which of course dries over time and will shrink back and need replacing every now and then.
• Upvc will shrink and expand, that's why a lot of people hear creaks and cracks when the sun comes out.
  I suggest that you re seal the inside caulk lines when it's cold and that should take care of the expansion when it gets hotter.
  HTH
  Russ
• Thanks! Ruski. Which caulk is best to use? I would assume the flexible one!
  Could you recommend a good one I could buy on the high street or the diy places? Thanks.