Cold Air Coming Under the Window in Winter — Fix Without Removing the Sill

Do you remember how last winter cold drew in from under the windowsill? Condensation built up on the sill, and in hard frosts ice even appeared in the corners. Many people put it down to "old windows" or "a draught from the radiator". In reality the cause is almost always the same — voids in the installation seam under the window and inside the support (sub-sill) profile. I'm Dmitry, a window repair master in Minsk working since 2003, and I'll explain why this happens and why exactly now (in spring and summer) is the time to deal with such problems, rather than waiting for the next cold spell.

Why a window freezes at the bottom and draughts from under the sill

Many people wrongly think the sill itself or poor sealing of the joints is to blame. In fact the cause is deeper, inside the load-bearing structure.

Poor installation. During fitting the surfaces of the opening weren't cleared of dust. As a result the installation foam bonded badly to the concrete or brick, came away and left gaps.
Settling and subsidence of the support profile. Cold gets in directly under the sill.
No foam under the sill. Often only the vertical seams are filled with foam, while the space under the sill is left empty. The wind moves freely there — hence the draught.

The result: you're heating the street, the lower part of the window stays icy, mould appears on the slopes, along with condensation and ice.

Why now is the time to deal with this problem

In winter such work can't be done. Why? Working with installation foam and sealants requires above-freezing temperatures (from +5 °C and up). Outdoor work (removing the drip cap, re-foaming) requires dry weather and no snow. Mortars and sealants for filling chases and junctions also don't set in frost. Spring, summer and early autumn are the ideal time: warm, dry, and you can do everything properly without rushing.

How I fix bottom draughting without touching the interior finish

The main advantage of the method is that the interior finish is preserved — there's no need to remove the sill, slopes or wallpaper. Everything is done from outside.

I remove the external drip cap carefully, so as not to damage it.
I re-foam the seam under the sill and the support profile with quality foam.
Where there are large gaps or deep chases, I use cement mortar or acrylic sealant.

The result: the window stops draughting from below, the sill becomes warm, and condensation and ice no longer appear — all without a single scratch on the slopes inside the flat.

Important points

Don't seal the drainage holes in the bottom of the frame! They're needed to drain off condensation. I carefully work around them or leave them free after foaming.
If the gap between the wall and the drip cap is very large (more than a couple of centimetres), it's filled with mortar first so the foam doesn't fall out.
After the foam cures, the excess is carefully trimmed, and the junction of the drip cap with the wall is sealed (with a weatherproof sealant).

How much it costs, and whether it's worth doing

The cost of the work depends on the size of the window and the difficulty of access. On average: comprehensive insulation of the bottom of the window (sill + support profile + drip cap) — from 150 BYN. Compared with constant heat loss and the risk of the slopes being destroyed by mould, that's a trifle.

Don't wait for the next winter

If in winter your window "wept", draughted from under the sill, and water or ice collected on the sill — don't put off the solution. While it's warm, I can fix the problem in a single visit, without mess and without removing the sill. Call me — I'll come, inspect, show you the problem spots (where there's no foam), name the exact price and bring the warmth back to your window.

📞 Phone: +375 (29) 135-50-50
🌐 Website: rpo.by
Author: Dmitry, window repair master, more than 20 years' experience. Brand: Spravnae Akenca