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


How do you deal with window sills on an External Insulation job?

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

A plas­tic sill exten­sion

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

7b465032-a895-456f-a1db-45b5885af3e9Many 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.


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      1. What can you do about the drip sills above the win­dows. As a period fea­ture I want to keep/mimic them in order to keep the nat­ural appear­ance of the house.

        • Agnes Rouselle
        • 1.05.2017
        • Reply

        Some of the window sill extensions I see on these EWI jobs do not look very good. Is there a premium option to get better looking windows after insulating, or am I stuck with these metal trays that I have seen so much of on my street?


          • Alan Bouquet
          • 21.06.2017
          • Reply

          Hi Agnes,
          There are a range of different options but that will be reflected in the cost. Best to speak to one of our sales team when they visit your property to see what we can offer.


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