Window Sills for Plastic Windows

But replacing the old glazing unit is only half the job. To give the window opening a neat and aesthetic look, you also need to finish the window sills. Usually, the contracting company handles this part as well, doing the entire job as a complete package.

However, if you believe that choosing the method of sill finishing and the material for it is less important than selecting the plastic windows themselves, you are deeply mistaken. Window sills are just as crucial, and before choosing a material, you should know its advantages and, most importantly, its drawbacks.

Let’s examine the different types of window sills for plastic windows.

Sill Panels Made from Sandwich Panels

One of the most common methods for finishing window sills uses single-sided and double-sided sandwich panels. While single-sided panels are not ideal for wide sills due to insufficient rigidity, double-sided sandwich panels are almost perfect for this purpose.

Advantages of this construction material include:

• high strength,
• rigidity,
• durability,
• reliability,
• excellent aesthetic appearance,
• superior thermal insulation.

Double-sided sandwich panels can be used in modern new-builds, panel buildings with sills up to 12 cm wide, and brick structures with wall thickness up to 60 cm.

They are easy to process, require no special skills during installation, and are simple to maintain. Plus, the appearance of plastic panels remains nearly unchanged over time—they are not sensitive to sudden temperature changes or high humidity.

Sills Made from Sheet Plastic

Window sills can also be finished with sheet plastic. This method is widely used because sheet plastic is one of the most reliable and high-quality building materials.

Key advantages include:

• excellent thermal insulation, enhanced by adding an extra layer of mineral wool;
• high strength;
• long lifespan;
• simple processing and installation;
• excellent visual appearance;
• compatibility with homes of any type.

The only drawback is the relatively high cost, which is significantly higher than other finishing methods.

Sills Made from Panelite (Cellular Plastic)

If the sills are not too wide, you can use cellular plastic – Panelite – for cladding. This material is affordable, easy to process and maintain.

However, Panelite is only suitable when the window sill width does not exceed 25 cm. This is because the maximum width of a single plastic panel is limited to that size, and joining them together diminishes the visual appeal. Moreover, Panelite surfaces tend to fade and yellow over time, which is a major disadvantage.

Therefore, if you truly want your windows and sills to keep their neat appearance for years, it’s better not to use Panelite.

Plaster Sills

Until recently, most window sills were made using traditional plaster mixtures. The main advantages of this method are moderate cost and the ability to complete the work independently without hiring professionals.

However, plaster sills have several serious drawbacks, making them impractical when used with plastic windows.

First, no plaster mixture can be firmly attached to the plastic window frame. As a result, separation will eventually occur, gaps will form, and the window’s airtightness will be compromised.

Second, temperature fluctuations cause significant condensation on the plaster surface, leading to mold growth.

Third, plaster finishing takes considerable time, as it must be applied in multiple layers, waiting for each layer to dry completely before applying the next.

In short, installing plastic windows and finishing the sills using traditional plaster is neither cost-effective nor logical.

Adhesive Sills (Plastic Film)

This is likely the simplest and most affordable way to finish window sills – by applying thin plastic film. The process is very easy: the film sticks well to smooth surfaces and requires minimal time.

Advantages of adhesive sills include simplicity and low cost. However, a major drawback is that in winter, due to temperature fluctuations, the thin plastic layer may peel off. The material lacks thermal insulation, so the wall behind it may freeze. Condensation often forms on the plastic surface, leading to mold and mildew.

Finally, the quality and appearance of plastic window sills depend not only on the chosen finishing method but also on the expertise of the installers. Always trust the installation of plastic glazing units and sill finishing to experienced, qualified professionals.

If you decide to do it yourself, be extremely careful and precise. Follow all instructions step by step, and always adhere to expert recommendations.