Window Weatherstripping: Foam Tape, V-Strip, or Both?

Stand near an old window on a windy day and you can usually feel it before you can see it — a faint, cool current that seems to come from nowhere in particular. The instinct is to blame the glass, or the frame, or the whole window as a lost cause. Most of the time, though, the draft isn't coming through the window at all. It's sneaking through the gaps where moving parts meet stationary parts, and once you know where to look, sealing it is a genuinely cheap, genuinely doable afternoon project.

Where drafts actually come from

A window is not one sealed unit; it's several parts that have to move against each other to open and close, and every one of those moving joints is a potential gap. On a double-hung window — the classic type with an upper and lower sash that slide vertically — the biggest culprit is usually the meeting rail, the horizontal bar where the top and bottom sashes overlap in the middle when the window is closed. That's a joint that gets latched but was never really designed to be airtight on its own. The second big source is where each sash rides in its track along the side jambs — those channels loosen with age and paint buildup, leaving play that lets air slip through on both sides.

On a casement window — the kind that cranks outward like a door — the story is different. Those seal by compression: the sash presses against a strip around the entire frame when it's cranked shut, and drafts there almost always mean that compression seal has gone flat, hardened, or gotten gummed up with old paint or grime so it can't compress evenly anymore.

The frame itself is a source too, but it's a different kind of problem — a gap between the window frame and the surrounding wall calls for caulk on the exterior trim, not weatherstripping, since weatherstripping is only for joints between two parts that still need to move.

Finding the draft before you seal it

Don't guess. On a cold or windy day, run the back of your hand slowly along the meeting rail, then down each side track, then along the bottom sash where it meets the sill. A genuine draft usually announces itself as a distinct cool line rather than a general chill in the room. If you want a more sensitive test, hold a lit candle or a stick of incense a couple inches from the same spots — a steady flame that flickers or leans in one particular spot, rather than wavering generally, is pointing right at the leak. It sounds old-fashioned, but it's more precise than your hand for a hairline gap.

Foam tape: cheap, fast, and right for compression joints

Self-adhesive foam tape is the simplest option and it's genuinely well suited to casement windows and to the bottom sash where it meets the sill on a double-hung — anywhere a flat surface presses against another flat surface when the window closes. Clean the surface well before applying, since foam tape's adhesive won't hold on grime or old paint dust, and pick a thickness that matches the actual gap: too thin and it won't compress enough to seal, too thick and the window won't latch properly. Foam tape's weakness is durability — it compresses a little more each season and tends to need replacing every year or two, which is a fair trade for how cheap and fast it is to redo.

V-strip (tension seal): the better answer for sliding joints

V-strip, sometimes sold as tension seal, is a thin strip of plastic or metal folded into a V shape that you press into the track or along the sash edge. Instead of compressing flat like foam, it works by spring tension — the two sides of the V flex slightly and press outward against both surfaces as the sash slides past, which is exactly what a moving joint like a double-hung's side tracks or meeting rail needs. It holds up far longer than foam tape, often five to ten years, because it's not relying on foam cells that permanently flatten under repeated pressure. The tradeoff is that it takes a bit more care to install straight and flush, and it's a slightly fussier product to find and cut cleanly than a roll of foam tape.

The honest answer: both, in different spots

Most double-hung windows do best with a combination — V-strip along the side tracks and at the meeting rail, where the sashes actually slide against each other repeatedly, and a strip of foam tape along the bottom sash where it presses flat against the sill. Casement windows are simpler: a full loop of foam tape or a purpose-made compression gasket around the frame usually does the whole job on its own. Either way, this is a genuinely inexpensive fix — a few dollars in materials per window — and it's worth doing before assuming an old window needs full replacement. A tight seal on the moving joints solves the draft; it just won't fix a window whose glass or frame has failed in other ways, which is a separate and bigger conversation.

Don't overlook the lock

On a double-hung window, the sash lock at the meeting rail does more than keep the window from being opened from outside — pulling it fully closed also draws the two sashes together snugly against whatever weatherstripping is in that joint. A window that's pulled shut but left unlocked often isn't seated as tightly as it should be, which can look like a weatherstripping failure when it's really just a habit worth changing. Get in the habit of locking every window you weatherstrip, not just the ones you're worried about someone opening, and you'll get noticeably more out of the seal you just installed.

One last materials note: Both foam tape and V-strip are sold in a range of widths and thicknesses at any hardware store, usually near the caulk and insulation aisle rather than with tools, and neither requires anything beyond scissors or a utility knife to install. Measure your gap before buying rather than guessing — a strip that's noticeably too thick will keep a sash from closing or latching fully, which defeats the purpose entirely.