Heat loss around windows

I've just surveyed a lovely old Welsh stone cottage.  It was interesting in being warm and dry - and had just had UPVC windows fitted all round.  Now I know I don't like these, but in this case, they are doing a good job - the windows are warm, and are not losing heat.  What is very obvious though is that heat is being lost AROUND the windows, through the reveals. This is very typical of such solid walled buildings.  Most heat actually does get lost around the windows, not through them.  You can see here that where the wall is very thin - heat is able to leak around the frames.  This is the cause of the very common mould and mildew patches around windows - often we see little areas of mould in the corners or on the reveals at the bottom. 

How do we stop this?

The best way to prevent heat loss in window reveals is to insulate them.  In an old solid walled building, it's almost certain that modern gypsum or cement plaster will now be present - so we first chop everything off - take it back to clean stone or brick.  Then we re-plaster using a hemp lime mix - hemp is a really good insulator, and added to the lime, gives it good qualities.  Thicken it as much as possible into the window frame, and you will find heat loss is then minimised.

This is actually a major issue in old houses - heat is not lost through the main walls - but window openings are a big contributor to heat loss.  The devil is in the detail - and in this case, going back to old technology like hemp lime, is a really good way to keep your solid walled 'old' house cosy and warm.