cracks in rendering


My house has an exterior of plain bricks around the ground floor and white rendering around the upper floor. There are a few cracks in the rendering. It could be subsidance I suppose but I have no problems with the lower brickwork and from what I can see of the interior uncovered walls there does not seem to be anything too bad. The rendering cracks seem to get worse in the dry hot summer months with nothing similar seeming to happen in the winter. Also there are a few identical houses in my area and I have noticed the odd crack in their rendering also. Hopefully assuming it is not something serious like subsidance what can I do to improve things?
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To prevent moisture entering a silicone sealer.
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