How to reduce heat loss to windows

Want to reduce heat loss? Then you’ll need to eliminate thermal bridges!

A thermal bridge is where there is a hole or discontinuity in an insulation layer, where heat is then sucked out. Pipes, junctions or changes in direction of walls and floor and roofs are the usual culprits. They are often misunderstood and ignored. 

One area that I’ve observed is often overlooked, is the base of a window. Where the wall insulation stops and a window opening starts is a key area to consider properly insulating. If not, there will likely be cold spots [and potential for mould and damp growth] if this area is not designed and built properly. 

In our example [pictured below] an old solid stone walled property has had breathable cork insulation [about 50mm thick] applied as internal drylining. It was then finished with lime and hemp plaster.

As the wall is so thick there would likely be a considerable gap between the internal cork board insulation layer and the window unit itself, leading to – you’ve guessed it – a thermal bridge. 

To get round this, we had to use a slightly thinner insulation layer [about 20mm thick] from the main wall, all the way up to the window frame. This was applied on all four sides of the wall opening – top, sides and base.

It’s necessary to use a thinner insulation layer or else the plaster will oversail the window frame. Though thinner, this insulation layer still really helps prevent cold spots.

Normally, the top and sides of the opening are usually easily completed and plastered over, but often the base is missed, as a timber window board is usually fixed straight to the top of the wall. 

For this particular project I drew a specific detail and reminded the contractors several times on site how important it was, until the base layer of insulation was eventually installed.

When it was completed, the window sill was at the right height to enable the granite kitchen worktop to be installed on top of the base insulation layer, right up to the window frame, so there is no sill upstand between worktop and sill. We find this provides a neater detail and also give a bit more usuable worktop space. 

If we want to ensure we get energy efficient buildings built correctly, it will usually require a bit more time during the design and construction stages. This is to ensure things aren’t forgotten about, but also to explain the reasons why and ensure things aren’t lost in translation. 

Another top tip on how to keep toasty warm during this cold winter!