Compressible Expanding Tape: The Window Sealing Detail That Determines Your Air Test Result

Your new windows arrive on a Tuesday. By Tuesday lunchtime your fitter has them in, foamed around with a single bead of expanding PU, and is moving on to the next opening. Eighteen months later your air test fails by 1.5 m3/(h.m2), the blower-door survey traces most of the leakage to the window perimeters, and you discover the forty pounds of compressible tape that would have saved you would also have taken the fitter an extra ninety minutes. Window perimeter sealing is one of the most predictably mishandled details on a UK extension, and it is the single biggest controllable contributor to your air permeability result.

What it is and what it's for

Compressible expanding tape is open-cell polyurethane foam that has been impregnated with a bituminous or acrylic resin, then pre-compressed into a tightly wound roll wrapped on a release paper. Apply it to the side of a window or door frame, peel the paper away, and the foam slowly expands back to its uncompressed thickness. By the time the frame is set in the opening, the tape has filled the gap between the frame and the masonry reveal, sealing it against air, water, and sound.

It is not the same as the thin black draught-excluder strip you can buy at B&Q. It is not the same as expanding PU foam from a can. It is a separate product category with its own performance ratings and its own role in the window installation. Confusing these three is the most common reason UK fitters get the perimeter detail wrong.

The standard performance figures for a quality tape, taken from the Tremco illbruck Compriband TP600 datasheet:

• Thermal conductivity (lambda): less than 0.048 W/mK
• Air permeability: less than 0.1 m3/(h.m.daPa)2/3
• Driving rain resistance: 600 Pa minimum (1050 Pa for premium tier)
• Fire class: DIN 4102 B1 (flame retardant)
• Temperature range: -30 to +90 degrees C in service
• Life expectancy: more than 25 years (15-year manufacturer guarantee)
• Certification: BBA Cert. 96/3309 (TP600 NG); EC1 Plus (low emission)

Those numbers say one thing in plain English: a properly specified compressible tape, correctly installed, gives you a window perimeter that will not leak air, will not leak water in driving rain, will not transmit sound, and will not need redoing for a quarter of a century. None of those guarantees apply to a single bead of expanding foam.

Why this matters for Approved Document L

Since June 2022, every new dwelling in England (and Wales under the equivalent regulations) must pass an air permeability test. The maximum permitted air leakage is 8 m3/(h.m2) at 50 Pa. The notional dwelling figure (used for SAP calculations) is 5 m3/(h.m2). Future revisions are expected to push the maximum down to 5. Your extension may not be a new dwelling but if you are doing a substantial renovation or your building control body is testing post-completion, the same junction-sealing principles apply.

Window and door perimeters are one of the three biggest leakage paths in any house, alongside service penetrations and floor-to-wall junctions. A poorly sealed window perimeter on a 1200 by 1500mm opening leaks about as much air as an open letterbox. Sealed correctly with a three-plane system, the same opening contributes near-zero to the leakage figure. The difference at your blower-door test is the difference between passing and failing, and the cost difference is in the low hundreds of pounds across an entire extension.

The three-plane sealing principle

The German DIN 18542 standard, adopted by every major European tape manufacturer, treats a window perimeter as three separate problems that need three separate solutions, working in three concentric layers from outside to inside.

Outer plane: weather seal. The outermost layer faces driving rain, UV, frost, and wind. It must shed water but allow any moisture trapped behind it to diffuse outwards. This is the BG R or BG 1 (outer) classification under DIN 18542. Compressible tape products like Compriband TP600 (600 Pa rain rating) or TP654 TRIO 1050 (1050 Pa) sit here.

Middle plane: thermal and acoustic fill. The middle layer fills the bulk of the gap with insulation, eliminating the cold bridge between frame and masonry. This is where bulk-fill products earn their keep. illbruck's i3 system uses FM330 Pro Foam Air Seal (a low-expansion gun-applied PU foam) for this layer; alternatives include mineral wool stuffing or wider compressible tapes that span all three planes.

Inner plane: airtight and vapour control. The innermost layer is the airtight line and vapour barrier. It sits on the warm, dry side of the assembly. This is BG 2 or BG 1 (inner) classification, and it must have a higher vapour resistance than the outer layer. The illbruck guidance condenses this into one rule: "inside tighter than outside." Get the rule wrong (a vapour-tight outer layer over a leaky inner layer) and warm interior air drives moisture into the joint, where it condenses and rots the frame from the inside.

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The reason this principle exists is moisture management. A window perimeter is a thermal weak point. Warm interior air will always try to push outwards through any gap. If it meets a cold outer surface inside the joint, it deposits its moisture as condensation. Over years, this rots timber frames, corrodes steel reinforcement, and grows mould inside the reveal. The three-plane system stops the warm air at the inner layer before it ever reaches a cold surface.

UK window installers rarely think about it this way. The German trade does, because their building regs have required it for decades. Adopting the principle on a UK extension takes one extra material (the tape) and about ninety minutes of fitter time per window.

Brand and product comparison

Two manufacturers dominate the UK specialist market and a handful of others compete on price. The brand attribution gets confused in trade conversation, so here is the correct mapping:

The honest summary: for a UK extension where you are aiming to comfortably pass Approved Document L without claiming Passivhaus performance, Compriband TP600 in the correct gap-range size is the right answer. It is widely stocked, BBA certified, and well understood by the small minority of UK fitters who already use tape. For a certified low-energy build (Passivhaus, EnerPHit, AECB Carbonlite), you need a 1050 Pa product with the correct DIN 18542 BG R rating: TP654 TRIO 1050, Partel CONPRESeal 3V BG1, or ISO-Bloco One.

For a one-off DIY job where you just want to seal a single replacement window properly, the budget tier (Lynvale, Xpanda, or other BBA-certified generic) at around two to three pounds per linear metre is genuinely adequate. Pay for the certified premium products only when the spec or the warranty requires them.

Sizing the tape: the practical problem

The most common installation failure is buying the wrong tape size. Compressible tape works inside a designed expansion range, and outside that range it either fails to seal (gap too big) or pushes the frame out of position (gap too small). Worse, a tape's "working" expansion range and its "weathertight" expansion range are different. A tape rated 5 to 17mm may only be weathertight up to 12mm.

The naming convention on a typical Compriband product is width / gap range, for example 20mm / 8-15mm. Width is the dimension that runs along the frame edge (visible on the perimeter); the gap range is the perpendicular gap between the frame and the masonry reveal that the tape will fill.

Practical sizing rule:

1. Measure the actual gap between frame and reveal at four points around each opening (top, bottom, both sides). These can vary by 5mm or more on a single window.
2. Take the largest gap.
3. Subtract 1mm to land inside the weathertight portion of the expansion range, not at its edge.
4. Buy tape whose gap range covers the resulting figure, preferably with the figure sitting in the middle of the range, not at the top.

For most UK extension windows, gaps are 8-15mm and the 20mm by 8-15mm Compriband TP600 is the right size. If your bricklayer has been generous on the opening, you may have 15-20mm gaps and need the 25mm by 8-24mm size. If the opening is tight, you need the 15mm by 3-15mm size. Buy multiple sizes if your gaps vary across the project.

Cost: budget through Passivhaus

£11 – £14 for a small Compriband TP600 roll (10mm width, 2-15mm gap, approximately 8m). This is the smallest pack size and suits a single small replacement window or a spot repair.

£19 – £27 for the most commonly specified size: 20mm width, 8-15mm gap, 3.3m roll. This is the workhorse size for UK extension windows. A typical 1200 by 1500mm window has a perimeter of 5.4 linear metres, so allow two rolls per medium-sized window if you intend to apply tape to the full perimeter.

£170 – £180 for a roll of the Tremco illbruck TP654 illmod TRIO 1050 (66mm wide, 6-15mm gap, 11.5m). This is the Passivhaus-certified premium tier with 1050 Pa rain resistance and three-layer construction.

£131 – £135 for a roll of Partel CONPRESeal 3V BG1 (64mm wide, 6-15mm gap, 14.1m). The Partel equivalent in the same Passivhaus tier as TP654.

£8 – £15 per roll for budget BBA-certified tape (typical 5-8m roll). Brands include Lynvale, Xpanda, and various generic specialists. Suitable where the spec does not require Passivhaus certification.

For a typical domestic extension with four to six windows and a set of bifold or French doors, expect to spend 80 to 200 pounds on TP600 standard tape across the project, or 400 to 800 pounds if specifying a Passivhaus tier. Compared to the cost of failing an air test and reworking the perimeters after plaster, this is trivial. Compared to the cost of redoing rotted timber frames after ten years of interstitial condensation, it is a rounding error.

How to install it

The official Tremco illbruck installation sequence for TP600 around a typical window. Read it once before opening any roll.

1. Clean and degrease the frame edge where the tape will sit. Wipe with a lint-free cloth and isopropyl alcohol if needed. Dust or release agents prevent the self-adhesive backing from gripping.
2. On absorbent or porous surfaces (raw timber frame, untreated masonry), apply the manufacturer's primer first and let it flash off.
3. With the window still on the trestle (not yet lifted into the opening), peel the first 200mm of release paper from the roll and press the adhesive face onto the frame edge. Start from a top corner and work along the head, then the jambs, then the cill. Do not stretch the tape: it must sit relaxed on the frame, not under tension.
4. At each corner, leave a slight excess (5mm or so) of unexpanded tape and press the two ends together to butt-seal them. A gap at a corner is the most common leakage point.
5. Trim the end of each tape run with a sharp knife (a Stanley with a fresh blade) once the tape is in position.
6. Lift the window into the opening and shim it level and plumb in the normal way. The tape will continue to expand against the reveal as you set the frame.
7. Fix the frame mechanically (screws into the masonry through the frame, per the manufacturer's instructions) before the tape has fully expanded. If you wait until the tape is fully expanded, you may not be able to shift the frame far enough to get screws in.

For windows installed in cold conditions (below 10 degrees C), the tape's expansion slows considerably. Compriband's own guidance allows applying indirect heat (a hot air gun on low, kept moving) to encourage expansion. Conversely, in hot weather (above 25 degrees C), put the unopened roll in a fridge for an hour before use to slow the expansion enough to get the tape applied before it sets.

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Combining tape with the inner air seal

One Compriband tape on its own is not a complete window perimeter detail. It is the outer layer (or in some products the all-in-one solution). For the canonical three-plane build-up on a UK extension where you care about the air test:

• Outside: Compriband TP600 (or equivalent) along the full perimeter, applied to the frame before installation. Provides the weather seal.
• Middle: Low-expansion gun-applied PU foam (illbruck FM330, Soudal Flexifoam, or similar low-expansion product) injected into the gap from the inside after the frame is fixed but before the inner sealant goes on. Provides bulk insulation and acoustic damping. Use low-expansion only: standard expanding foam will bow PVCu frames inwards and ruin the alignment.
• Inside: Either an airtight membrane tape (illbruck ME350, Pro Clima Tescon, Siga Fentrim) lapped from the frame onto the inner plaster, or a high-quality permanent-elastic frame sealant (Siga Primur or similar) applied as a continuous bead. Provides the airtight line.

Do not skip the inner air seal. The compressible tape outside is the weather defence; the inner seal is the airtightness defence. Without both, you are using only half the system.

For Passivhaus or EnerPHit specs, the all-in-one tapes (TP652 Trio+, TP654 TRIO 1050, ISO-Bloco One, CONPRESeal 3V) condense all three planes into a single product, with a high-density airtight closed-cell foam on the inner face and a slow-recovery vapour-open foam on the outer face. They simplify the installation but cost three to ten times the basic TP600.

How to make sure your installer actually uses it

This is the section that matters most. UK window fitters default to expanding PU foam for the same reason painters default to magnolia: it is what they know, it is what they have on the van, and it is faster than the alternative. A fitter who has never used compressible tape will resist using it on your job. A fitter who has used it once will charge you a labour uplift for the time it takes.

The Carbon Co-op Community has documented cases where homeowners specifying Compriband had installers turn up with thin black draught-excluder strip instead, claiming it was "the same thing." It is not the same thing. The thin strip is a finishing weatherseal, not a structural perimeter seal. It will not pass an air test.

Practical defences against installer resistance:

Specify the product in writing. Your quote and contract should name the tape product (not just "compressible tape"), the size range needed, and the three-plane build-up if relevant. Example clause: "Window perimeter sealing to use Tremco illbruck Compriband TP600 (or BBA-equivalent) at outer face, low-expansion PU foam fill in body of gap, and Pro Clima Tescon Profil airtight tape lapped to plaster at inner face." Vague language ("sealed properly") is worth nothing.

Ask to see the rolls. On the day of installation, ask the fitter to show you the tape they have brought before they unwrap any of it. Check the product label. If they have brought generic foam strip or nothing at all, stop the install.

Request photographic evidence. Ask the fitter to take photos of the tape applied to each frame before the frame is lifted into position, and photos of the perimeter before the inner trim or plaster is applied. Make this a contractual condition. The photos take thirty seconds each and they create an enforceable record.

Pay for the labour uplift. A competent fitter will quote an extra hour or two per window for the three-plane detail. This is a fair charge. Refusing to pay it incentivises corner-cutting. Build the uplift into the quote rather than discovering it as a variation.

Use a fitter who has done it before. Ask any window contractor whether they regularly install with compressible tape and a three-plane sealing system, and whether they have done a Passivhaus or EnerPHit project. Even one project's experience is the difference between a fitter who knows the sequence and one learning on your job.

Storage and shelf life

Unopened Compriband rolls have a 24-month shelf life when stored at 1-25 degrees C, kept dry, and laid flat. Once a roll is opened, the tape begins expanding immediately on exposure to air and ambient humidity, and the unused remainder cannot be reliably sealed for later use. Buy roll sizes that match the job you are about to do, not the project as a whole.

Storage in direct sunlight degrades both the foam and the impregnant. Storage in damp conditions degrades the adhesive backing. Storage on edge (rather than flat) flattens the roll asymmetrically and makes installation messier. Cool, dry, dark, flat: those four conditions cover it.

Where you'll need this

• Windows and doors - applied to every window and external door perimeter at installation, before the frame is lifted into the opening
• Insulation - part of the airtight detailing strategy at the building envelope, alongside membrane tapes and vapour control layers
• Walls and blockwork - occasionally used at large expansion joints in masonry where movement and weather sealing are both required

Compressible expanding tape is part of the air-sealing detail on any extension or new dwelling where building control are testing for air permeability. Its use spans the structure phase (windows and doors going in) and the first fix phase (the inner airtight line being completed before plaster). It is not project-specific to kitchen extensions.

Common mistakes

Trusting "the fitter knows what they're doing." Most UK window fitters are good at fitting windows and bad at perimeter sealing because the trade has never required them to be good at it. A fitter who has done two thousand replacement double-glazing installs with a single bead of foam each time is a fitter with two thousand bad perimeter seals behind them. Past experience is not a substitute for spec.

Buying one tape size for the whole job. Gap sizes vary across openings on the same project. A single roll of one width will be the wrong size for at least one window. Buy a small range (typically two adjacent gap sizes) for any project with more than two windows.

Opening the roll before the window is ready. Once exposed, expansion begins. A roll that has been open on the bench for fifteen minutes in a warm room is partly self-set and useless on the next window.

Using standard expanding foam as the middle layer. Standard PU foam expands aggressively and bows PVCu and aluminium frames. For the middle layer, only low-expansion gun foam (FM330, Soudal Flexifoam, or product specifically labelled "low expansion" or "frame foam") is acceptable.

Skipping the inner airtight layer. Compressible tape outside without an inner seal is half a system. Warm interior air will push outwards through the perimeter, condense at the cold outer line, and rot the frame from inside over five to fifteen years. The inner seal is not optional.

Specifying without certification. A non-BBA-certified tape may perform identically to a BBA-certified one, but it cannot be used to claim Approved Document L compliance in your air-test paperwork. If your build control are auditing the spec, the certification matters as much as the performance.

Leaving the tape exposed long-term. UV degrades PU foam. Tape that is not covered by trim, capping strip, or sealant within a few months of installation will degrade noticeably within five years. A timber bead or aluminium trim over the outer face is the durable detail.