Window weatherstripping: stop draughts without blocking home ventilation

Adhesive weatherstripping can reduce a window draught when it is fitted only in the gap between the opening sash and its frame, never over trickle vents, drainage holes or water-runoff joints. It is a straightforward fix for a window that has a slight rattle, lets cold air in around the latch or makes a curtain move on a windy day. It will not repair rotten timber, a warped frame, loose glazing or rain entering through the façade. First identify the path of the air; buying tape before doing that usually leads to condensation, a window that will not close or a larger repair being hidden.

Contents

1. What weatherstripping can fix — and what it cannot
2. How to locate the gap without taking the window apart
3. Which seal should you choose for your window?
4. Fit it step by step without damaging the latch
5. How to keep ventilation working after reducing draughts
6. When to stop the DIY work and call a professional

What weatherstripping can fix — and what it cannot

Weatherstripping is intended to reduce small air paths between moving parts: an opening casement and its frame, or two leaves that do not meet tightly. The Save Energy Together project, involving ADENE, promotes simple energy-efficiency measures in homes; sealing a local gap is one low-complexity measure where the window itself is sound.

That does not turn every old window into a new one. The aim is to correct a local, reversible gap. Before measuring, separate three situations. If you feel air only near the latch and the window closes easily, a seal may work. If the sash scrapes, drops when opened, has very different gaps around its perimeter or the latch does not engage, start with adjustment or frame repair.

If there is staining, lifted paint, dark timber, a damp smell or water on the sill after rain, do not tape over the symptom: find the cause in the outer joint, sill, drainage route or façade.

• Draught with no water: a typical case for a weatherstrip trial.
• Noise or vibration in wind: first check for a gap or loose hardware.
• Damp or water ingress: address weatherproofing and drainage, not only air movement.

In Lisbon, north-facing windows or windows exposed to wind can make a small gap feel particularly noticeable in winter. The home still needs regular air renewal.

How to locate the gap without taking the window apart

Test on a breezy day without rain. With the window shut, move your hand slowly a few centimetres from the frame, especially around corners, hinges and the latch side. A light strip of tissue paper can make air movement easier to see, but never use a candle, lighter or flame. Then open the window and inspect the meeting surfaces: look for dust gathered in a narrow line, rubbing marks and old gaskets that are flattened or coming loose.

1. Clean the frame and sash with a lightly damp cloth, then dry them fully.
2. Close the window over a sheet of paper at several points. If it slides out freely in one place but grips in another, the closing pressure is uneven.
3. Check for small air inlets in the frame, grilles or slots at the bottom. Mark them so they are not covered.
4. Look for drainage holes on the outer face or lower track of sliding windows. They must stay clear.

Ordinance No. 138-I/2021 in the Diário da República treats natural ventilation as a system of openings, internal air passages and ducts. Do not assume that a grille or slot is accidental: check whether it forms part of the home’s ventilation before covering it.

Which seal should you choose for your window?

Choose the profile based on the actual gap, rather than the thickness that looks most insulating on the packet. A strip that is too thick forces the window, distorts the seal and may prevent the latch from closing; one that is too thin achieves very little. Measure in several places because older frames rarely have an even gap all round.

A thin foam or rubber strip will often suit a small, even gap in a well-adjusted window. Where the meeting edges have a slight irregularity, a flexible rubber profile of compatible height may adapt better. For sliding windows, use a brush seal or a manufacturer-specified solution, leaving tracks, slots and drainage holes clear. If a modern frame has a dry original gasket, seek a compatible replacement and confirm its profile before removing the old one.

Avoid mastic, silicone or expanding foam where the sash needs to open, slide or drain water. Do not tape the outside face of a frame to seal rain either: it can hold water in the wrong place. For a timber window with failing paint, assess whether substrate repair comes before a seal; adhesive will only bond properly to a sound, clean and dry surface.

Fit it step by step without damaging the latch

Allow time to prepare the surface. Most failures come from dust, grease, flaking paint or fitting the strip to damp timber. Work with a dry window at a moderate temperature; adhesive applied to a cold or wet frame is likely to release quickly.

1. Clean and dry. Remove dust from corners with a soft brush. Wipe the bonding line and let it dry completely.
2. Run a short trial. Apply 15 to 20 cm in an unobtrusive area, close without force and open again. Make sure the latch engages.
3. Mark the route. The strip belongs where the sash compresses against the frame. Do not place it on an edge that sweeps against another part.
4. Apply without stretching. Peel the backing gradually. Stretching causes the strip to shrink later, especially at corners.
5. Make clean corners. Butt ends together without a thick overlap. A badly folded corner creates a spot where the window cannot close.
6. Test operation. Open, close and lock several times. If you need to push hard, remove that section and choose a lower profile.

After two or three days, repeat the paper test and check for morning condensation on the glass. The DGS recommends airing homes daily and during moisture-producing activities, including cooking, bathing, washing and drying laundry indoors. Sealing should reduce uncomfortable infiltration, not replace that routine.

How to keep ventilation working after reducing draughts

Sealing and ventilation solve different problems. The first reduces uncontrolled air entry through gaps; the second deliberately replaces indoor air. Do not seal grilles, trickle vents, inlets in shutter boxes, permanent openings in rooms with gas appliances or any feature identified as part of the ventilation system.

Use a simple routine: air rooms more intensively for a shorter period when outside air is acceptable, use kitchen and bathroom extract when available, and manage internal doors according to the home’s air path. During wildfire smoke or poor outdoor air quality, the strategy changes temporarily; keeping windows shut can then be sensible. A weatherstrip helps because it gives you more control over when and where air enters.

Watch for signs after fitting:

• glass that fogs every morning;
• a stale smell that was not there before;
• mould behind furniture against outside walls;
• weak extraction from the bathroom fan;
• gas appliances in a space without adequate air admission.

In these cases, check whether you covered an air inlet, whether extraction works and whether excess indoor moisture is being produced. If the cold feeling comes from the floor rather than the window, it may also be useful to review underlay choices for floating floors.

When to stop the DIY work and call a professional

This small repair has clear limits. Call a professional if the window has cracked or loose glass, damaged hardware, soft timber, significant corrosion, a warped sash, repeated water ingress or requires frame dismantling. Do not attempt to correct a heavy window with improvised packers, random screws or increasingly thick strips.

In an apartment, replacing a window or changing a visible external colour, pattern or profile may affect the façade and condominium rules. Lisbon City Council’s information on public-space occupation for works exempt from prior control explains that work involving scaffolding, materials or containers may require its own procedure. Confirm the planning and condominium position before ordering new windows.

If the issue is a draught around the entrance door, the same principle applies, although threshold details and security need their own assessment. Where this is part of a broader renovation, also see how to prepare a pre-renovation condition record in an apartment building: photographs of the existing condition help distinguish pre-existing defects from later damage.

Key takeaways

• Find the actual gap before buying a seal; adhesive tape will not repair a misaligned window.
• Fit the strip only on the compression line between sash and frame, leaving vents and drainage clear.
• Test a short section and make sure the window closes without force.
• Stop and diagnose the cause if water, decayed timber or persistent condensation appears.

If the window needs more than adjustment or a seal, include the frame, ventilation and external junctions in the renovation survey. Contact Obra Lisboa to place that decision within the real scope of the work.

Sources and further reading

• Save Energy Together project, involving ADENE
• DGS recommends airing homes daily
• Ordinance No. 138-I/2021 in the Diário da República
• public-space occupation for works exempt from prior control

Editorial image; it does not depict an Obra Lisboa project.