In the previous article, we explained why elastic PU foam (flex foam) is the only sensible choice over standard rigid foam. But what if we told you that at the cutting edge of European construction technology there is a material that can completely replace both the foam and the exterior silicone?
Welcome to the world of Self-Expanding Joint Tapes, widely known by the technical term Compriband. This is the "Holy Grail" of sealing - the material that lets Passive House engineers sleep soundly during storms. Let's see how this "smart" tape works.
Visually, before use, it's unimpressive. It looks like a simple, thin roll of black or grey sponge (just 2–3 mm thick), with one self-adhesive side. In reality, it is a special open-cell polyurethane foam, impregnated with synthetic resins (acrylic polymers) and factory-compressed to the maximum degree.
The installer sticks the thin tape around the frame perimeter before the window is placed in the wall. Once the window is fitted into the opening, the tape "wakes up": it begins expanding on its own, very slowly and steadily. Within a few hours (or 1–2 days in winter), it has swollen enough to "embrace" the wall, hermetically filling the gap. The reaction is temperature-controlled - slower in cold, faster in warmth.
Compriband tapes are sold in specific working ranges: the tape expands up to a defined thickness. You must choose a tape that matches the width of your joint (e.g. a 10–15 mm tape for a 12 mm joint). Too thin and it won't reach the wall. Too wide and it will be over-compressed, losing its breathability.
Why pay more for tapes instead of letting the installer run a bead of silicone on the outside? The answer lies in three fundamental properties that silicone simply cannot offer.
Silicone is 100% impermeable. If moisture gets trapped behind it, it can't escape - it will rot the plaster and breed mould. Compriband, by contrast, works like Gore-Tex fabric. Its resins repel rainwater (withstanding pressures above 600 Pa - equivalent to hurricane-driven rain!) yet allow water vapour to escape outward. The home breathes; rain stays out.
Silicone, after 5–6 years under the Greek sun, "burns", hardens and detaches, leaving a gap. Compriband tape sits inside the joint, permanently compressed (like a spring trying to open). It follows the aluminium's expansion 100%. It never tears and never detaches.
For certified RAL installation, there are special, wide Compriband tapes called "3-in-1" (Multifunctional tapes). A single tape covers the entire frame depth, simultaneously replacing all three sealing levels.
The interior side of the tape acts as a vapour barrier, preventing room moisture from entering the joint. It fully replaces the classic EPDM or Fleece membrane on the indoor side.
The dense open-cell foam structure provides top-tier thermal and acoustic insulation, completely replacing elastic PU foam. Thermal conductivity stays below 0.040 W/(m·K).
The exterior side stops rain yet lets the wall breathe. The tape withstands water pressures above 600 Pa - outperforming any silicone.
Compriband tape is the ultimate tool, but it has one basic prerequisite: the wall opening must be level and flat. If you're building new or have fitted proper insulating subframes (Purenit/EPS), the opening is perfect and the tape will work wonders.
New builds with level openings, buildings with insulating subframes, Passive House applications. The tape expands uniformly and fills the joint perfectly - ideal results with minimal effort.
In renovations of old stone houses with uneven, "rough" walls, elastic foam combined with liquid membranes may be safer, since it adapts better to large stone irregularities. Foam "fills" what the tape cannot "reach".
The era of sealing windows with a dab of mastic and silicone is over for good. Self-expanding tapes are the "invisible shield" of your home against extreme weather. When you receive a quote for new windows, ask the manufacturer: "Do you use Compriband tapes for exterior sealing?" It is the mark of a true professional.
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