How do you deal with window sills during External Wall Insulation?
One of the most common questions we were asked when providing a quote for External Wall Insulation was:
How do you deal with the window sill when it is too short to extend out over the insulation?
This is a very understandable question because your window sills have to stick out from the wall to enable the water to run off properly when it rains. Without a window sill, the rainwater can pool and cause damp problems, or cause stains down the wall from where it doesn’t run off properly.
If you are replacing your windows before the EWI install, it is really worth getting the sills installed with the correct length for the EWI to fit underneath. This is the ideal solution as the window and sill will have the same style and the finish intended by the window installer and yourself. If you already have good windows that you do not plan to replace, then you will need to extend the window sills in one of the following ways:
Plastic UPVC Sills
The simplest and most often used way to extend the sill is to add a new UPVC sill over the top of the existing one, with a bigger width to allow the water to run off properly. If the windows or the sill are plastic already, then this is the most common option an installer will use. They are easy to work with and won’t look out of place, although some people find plastic to look a bit cheap.
This is not untrue, as plastic sills are very cheap to buy and very simple to install. If you are installing EWI on a budget, it is going to be the best option.
For those that do not want plastic over sills, or where it would look out of place, there are metal over sills that you can purchase. These come in a few different colours, and come pre-painted to prevent rust. These sills will provide a slightly different look, but at a similar price to plastic.
Many solid wall pre 1930s properties have stone or tile sills. This means a solid piece of stone or concrete across the length of the window, and it is often referred to as a ‘period feature’ and something that many installers find tricky to extend and make sure that the end result is in keeping with the style of the property.
XPS sill extensions offer an innovative and stylish finish to closely mimic the existing sill style – perfect for conservation areas or properties where you need to keep the appearance as similar as possible. So how does it work?
A waterproof high density extruded polystyrene (XPS) board is essentially wrapped around the existing sill. A mesh and cement layer is then added to strengthen and support the sill. Then finally either a coloured render or silicon paint is added to finish.
What you end up with is a sill that from just a few meters away will be easily taken for a stone sill. It is very strong also (as much as the wall itself), and can therefore support your usual windowsill paraphernalia like flower pots.