Windowsill and radiator: how not to lose heat and not to damage the sill

A radiator warms the air next to it. Warm air is light, so it rises along the wall and the window, cools at the cold glass, sinks and returns to the radiator. This loop is called convection and it is what heats the room. The warm stream along the window also keeps the glass from misting.

The sill lies right in the path of this stream. If it covers the radiator completely and tightly, the warm air hits the slab and spreads out under it: less heat reaches the room and the glass.

• The room warms up more slowly and the radiator has to work longer (and you pay for heat that is stuck under the slab).
• The glass gets less warm air, so the risk of misting and condensation grows, and with it mould on the reveals.
• Warm air collects under the sill and dust and moisture settle on the underside of the slab.

1. Wait for the heating season and a cold day.
2. Hold your palm above the sill right at the window: you should feel a warm movement of air at the glass.
3. Or hang a light paper strip on a thread above the sill: if it sways upwards, the air is moving.
4. If it is cold at the glass but very hot under the sill, the flow is blocked: you need a gap or grilles.

1. A gap above the radiator. Leave a noticeable gap between the top of the radiator and the sill: usually at least 5 cm is advised, check the exact value in the heating design or ask the installer.
2. Ventilation cut-outs and grilles. If the sill is wide and covers the radiator, cut-outs for grilles are made in the slab above the radiator: warm air passes through them. A grille cut-out costs 50 UAH each.
3. The width of the sill. Do not take a width larger than needed for a 30–50 mm overhang: the narrower the sill, the freer the path for air. How to calculate the width is in the article “How to measure a window and choose a windowsill”.

Fitter’s tipIf you really want the sill wide (for example as a table), make a long grille cut-out above the radiator: the heat goes up while the slab stays in place.

If you are unsure what radiator you have and how best to fit the sill, send the manager a photo.

If heating pipes come up through the slab, cut-outs are needed. To make them fit perfectly, do this:

1. Measure the distance from the wall and from the side reveals to the axes of the pipes and their diameters.
2. Make a paper template and test it in place.
3. Allow a small gap around the pipe (a few millimetres): it is needed for expansion and installation and is later hidden with a decorative cover or sealant.
4. Order the cut-out while the sill is being prepared: this way the cut is clean, without chipping.

For complex shapes there is a template cut-out (40 UAH per side) and a U-shaped cut-out (100 UAH each). All of this is part of the cutting services.

Even a perfect sill will not help if long heavy curtains hang below or level with it: they cover the radiator and block the flow just like a slab. Choose curtains above the sill or roller blinds and do not let tulle hang onto the radiator.

A Werzalit sill is designed for heat: the manufacturer states that brief exposure up to 180 °C leaves no marks. You do not have to fear the heat of a radiator. The point of the gaps and grilles is different: not to protect the sill but to keep the heat in the room and protect the glass from condensation.