How To Fill Gaps Around Windows

How do you fill gaps around windows to stop draughts?

Draughts are one of the main reasons a room feels cold even when the heating is on, and window frames are where most of them come from. Sealing those gaps is cheap, quick and one of the few DIY jobs that pays you back on the power bill. The method is: find the leaks, clean the joint back to sound material, cut the nozzle to the size of the gap, run one continuous bead with steady pressure, then tool it smooth and wipe up immediately.

For gaps on the inside of the house, the product this guide calls for is Selleys No More Gaps Interior Multipurpose. If the gap is on the outside of the house, use Selleys No More Gaps Exterior instead — the interior product's technical data sheet states it is not recommended for exterior use or areas exposed to constant water. That distinction matters more than it sounds, and the section on limitations below explains why.

Step 1 — find the leaks before you seal anything

Light a match or candle near any window or door where you suspect a leak. If the flame dances around, air is flowing through the area. The movement should intensify — or the flame will blow out — as you get closer to the source. Work along the frame slowly; the leak is often at the head or the sill rather than the obvious side joints.

This step is worth doing properly rather than sealing every joint on principle. A window that is not leaking does not need a bead, and a bead applied where none is needed just makes the next repaint harder.

Step 2 — clean the area to be sealed

Remove any old caulk and paint before you start filling, and make sure the area is free from dust, grease and loose or flaking paint. The whole strength of a caulked joint is adhesion to two clean faces; a bead laid over flaking paint is bonded to the flakes, not the frame.

A putty knife handles most of the old caulk. For old paint, Selleys suggests a paint stripper such as Selleys Kwik Strip. Kwik Strip is a solvent-based stripper and its data sheet is emphatic that it must be used outdoors only, with gloves, and never with a hot air gun or torch. It is also not suitable for application to fibreglass, PVC, acrylics, linoleum, rubber or plastics — which rules it out on many modern window frames.

Step 3 — prep the caulking gun

Cut the seal at the top of the cartridge with a knife or secateurs. Cut the nozzle at a 45° angle, with an opening approximately the size of the gap being filled. Fit the nozzle to the top of the cartridge and insert it into the caulking gun by pushing the base of the canister into the gun.

Size the opening conservatively. You can always cut the nozzle wider; you cannot make it narrower again, and an over-large opening puts far more product on the frame than the joint needs. There is a fuller guide to using a caulking gun if this is your first time with one.

Step 4 — caulk the leak

Apply steady pressure to the trigger to force the gap filler into the area causing the leak, and move the nozzle along the joint in one smooth action. At the end of each section, wipe the excess away with a finger, spoon or spatula dipped in a 50/50 mix of water and detergent. The detergent stops the filler dragging on the tool.

Two things matter here. The first is forcing the product into the gap rather than laying it on the surface — a bead sitting on top has almost no depth of bond and will peel. The second is doing each run in one pass. Stops and restarts leave weak points that show up as hairline splits later.

Step 5 — finish

Clean off any excess with a damp cloth immediately. Then allow the filler to skin before painting. No More Gaps Interior Multipurpose has a skin time of 20 minutes at 23°C and 50% humidity, and Selleys' guidance is to allow roughly 30 minutes to dry to the touch before painting or sanding. Allow 24 hours before painting with oil-based paints.

For a neater finish and easier clean-up you can tape both sides of the gap before you apply the product, then pull the tape while the bead is still wet.

Technical specification — No More Gaps Interior Multipurpose

Technology	Water-based
Skin time	20 minutes (23°C, 50% RH)
Movement capability	Up to 15%
Maximum gap for paint crack resistance	10 mm wide × 5 mm deep
Paintable	Yes, when cured; allow 24 hours for oil-based paints
Water resistance	Low

Indicative data only, not intended for the preparation of specifications.

What the 10 mm limit means in practice

The gap size limit is the figure most people ignore, and it is the one that causes the visible failures. For optimum paint crack resistance, filled gaps should not exceed 10 mm wide and 5 mm deep. The reason is shrinkage: the effects of a sealant shrinking as it dries are far more evident in a large gap, and the paint film over a large bead cracks as the bead moves under it.

So a hairline gap around an architrave is exactly what this product is for. A 20 mm cavity behind a window reveal is not. For gaps greater than 2 cm, use an expanding foam such as Selleys No More Big Gaps, which is a polyurethane foam formulated to fill large or irregular cavities — around pipes, behind skirting boards and architraves, and around window and door frames.

The 15% movement capability is the other number worth understanding. Window frames move a little with temperature and with the building; 15% of a 5 mm joint is well within what a well-made joint sees. It is not enough for a joint that moves substantially, and Selleys directs you to an appropriate silicone sealant where movement is excessive or where water contact is frequent.

Limitations — where No More Gaps Interior Multipurpose should not be used

• Not recommended for exterior use or areas exposed to constant water. Use No More Gaps Exterior outside — it is formulated with a titanium compound to protect against UV and weather damage, has a movement capability of ±25% and should be protected from water for 24 hours after application to let it strengthen.
• Low water resistance — unsuitable for permanent water immersion or constantly damp areas.
• Not suitable for polyethylene, polypropylene or Teflon.
• Not for gaps between tongue-and-groove boards or weatherboards — use No More Gaps Exterior for those.
• Maximum gap 10 mm wide by 5 mm deep for optimum paint crack resistance.

If you follow the common advice to seal window frames on both the inside and the outside of the house, use the correct product on each side. The interior product outside will not survive UV and weather; the exterior product is the one made for it.

Clean up

Uncured product comes off with a damp cloth or water. Once cured, No More Gaps has to be cut out; remaining residue can be softened with acetone and then removed.

Common questions about filling gaps around windows

How do I find where the draught is coming from?

Light a match or candle near the frame. If the flame dances or blows out, air is moving through that point.

How long before I can paint over the filler?

No More Gaps Interior Multipurpose has a skin time of 20 minutes, and Selleys suggests allowing about 30 minutes to dry to the touch before painting or sanding. Allow 24 hours before painting with oil-based paints.

What is the biggest gap I can fill with it?

10 mm wide and 5 mm deep for optimum paint crack resistance. For gaps greater than 2 cm, use an expanding foam such as Selleys No More Big Gaps.

Can I use the interior product on the outside of the house?

No. Its data sheet says it is not recommended for exterior use or for areas exposed to constant water. Use No More Gaps Exterior outside.

What is different about the exterior product?

It uses a titanium compound to resist UV and weather, has a movement capability of ±25%, a skin time of 30 ± 5 minutes, and should be protected from water for 24 hours after application. It should not be stored or applied below 10°C.

How do I get a neat bead?

Cut the nozzle at 45° to roughly the width of the gap, move in one smooth action, and wipe with a finger or spatula dipped in a 50/50 water and detergent mix. Taping both sides of the gap first makes it neater still.

Do I have to remove the old caulk?

Yes. Remove old caulk and paint, and clear dust, grease and flaking paint, so the new bead bonds to sound material.

Can I use a heat gun to strip the old paint first?

Not in combination with paint stripper. Kwik Strip's data sheet says do not use hot air guns or torches with paint stripper, and to use it outdoors only.

Will the filler stick to a PVC or aluminium window frame?

No More Gaps Interior Multipurpose is not suitable for polyethylene, polypropylene or Teflon. On other frame materials, the requirement is the same as anywhere: clean, dry and free of grease and flaking paint.

Is this a permanent fix for a leaking window?

It seals air gaps. Where movement is excessive, or where the joint sees frequent water, Selleys directs you to an appropriate silicone sealant instead of a gap filler.

What else should I seal to stop draughts?

Skirting boards and door frames are the other common culprits — see how to prevent draughts in your home and how to fill gaps between skirting boards and walls.