Solid wall insulation – Window Sills

Solid wall insulation – Window Sills

When solid wall insu­la­tion is installed on the exter­nal walls of a prop­erty it is nor­mally 10cm thick or more. What this means is that the sill of pre­vi­ously installed win­dows will stop well before the edge of the insu­la­tion. This doesn’t look great to be hon­est and could cause issues with water runoff, so the sills need to be extended out beyond the edge of the exter­nal solid wall insu­la­tion to pro­vide an attrac­tive end fin­ish. Click on the image below to learn more about water ingress and damp.

In an ideal world, we rec­om­mend replac­ing the win­dows at the same time as get­ting the insu­la­tion works done. The rea­son for this is twofold; the first is that if you come to replace the win­dows in the future, the process of remov­ing and refit­ting them doesn’t com­pro­mise the integrity of the solid wall insu­la­tion. The sec­ond rea­son is that the sills can been ‘pre-extended’ when the win­dows are made, so if for exam­ple you have wooden sash win­dows being installed – the sills on the new win­dow already have taken into account the addi­tional 10cm of insu­la­tion, so when they are in and the insu­la­tion is installed they look great.

For many of us though, the cost involved with replac­ing win­dows and doing exter­nal solid wall insu­la­tion at the same time is just too great. So when it comes to extend­ing win­dow sills what options do you have?

XPS or uPVC sills to match existing window profile

Well the first thing to con­sider is what the exist­ing sill looks like, the idea is to keep the look of the extended sill as close to the orig­i­nal as pos­si­ble. If the exist­ing sill is made of con­crete, by far the best way of extend­ing it is to use XPS. XPS is a really strong type of poly­styrene so can take the addi­tional impacts and loads that you might expect from nor­mal win­dow sills. The XPS is treated in the same way as the insu­la­tion, but many of our cus­tomers do decide to go with a slightly dif­fer­ent colour ren­der to enhance the fact the sill pro­trudes out of the solid wall insu­la­tion!

If you have uPVC win­dows in place, the sills tend to be much thin­ner com­pared to con­crete ones. In this case we use a uPVC sill exten­sion board, which is attached on top of the exist­ing sill using a com­bi­na­tion of sill exten­sion board fix­ings and adhe­sive.

The final option and def­i­nitely the best in our opin­ion is to use the pow­der coated stain­less steel sills. The great thing about these is that they can be pro­duced to match the exist­ing look of the sill. Have a wooden win­dow fit­ted in your prop­erty? No Prob­lem! We have pow­der-coated sills that per­fectly mimic the look and feel of real wood.

Another advan­tage of the stain­less steel sill is that they are incred­i­bly strong. The sills will out­last the win­dows in most cases, and the pro­tec­tive pow­der coat­ing will ensure that it never weath­ers over time.

Our partners at EWI Store discuss the function of sills, beads, and verge trims here, so click through and have a read!

At Be Con­struc­tive we give our clients the option to install any type of win­dow sill when we insu­late the exter­nal walls of the prop­erty. We obvi­ously will give guid­ance where we can, but at the end of the process we want the cus­tomer to be happy with their new-look home!