How do you deal with window sills during External Wall Insulation?

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.

Metal Sills

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.

XPS Sills

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.