The Bostik T635 AIRSEAL SEALING TAPE 3D UF Compressed Multi-Functional Underside Window Sealing Foamtape is supplied as a pre-compressed open-cell polyurethane foam tape for the underside connection of exterior window and door units. The 3D UF designation identifies a three-dimensional compression response in the lower frame joint, where vertical frame loading, horizontal thermal movement, and longitudinal building tolerance act in separate planes. The tape is manufactured in factory-compressed roll form and installed by liner removal, followed by controlled expansion into the prepared joint. The primary specification is combined air, moisture-vapour, and acoustic separation for junctions where the lower frame member bears on masonry, concrete, coated timber, or pre-formed sill profiles. The product is not formulated as a structural adhesive, and sealing contact relies on constrained expansion and recovery force rather than adhesive peel or mastic flow. The roll is supplied in widths from 15 mm to 100 mm depending on configuration, with compressed thickness selected for joint depths from 5 mm to 20 mm; published data for this specific configuration is limited for non-standard joint depths and for joints with continuous hydrostatic loading. The product is evaluated under DIN 18542 for joint sealing of exterior components, and the installed joint may form part of a window or door assembly assessed according to EN 1026 and EN 12207 for air leakage classification. The foam matrix is open-cell and vapour-permeable; it is not intended as a bulk water barrier or as a standing-water seal.

In underside window applications, the tape is placed between the underside of the frame and the structural sill. The lower frame joint is a critical hygrothermal plane because it receives run-off from the glazing and, on poorly drained sills, can retain capillary moisture behind the outer profile. The 3D UF construction resists compression set under repeated frame movement and maintains contact pressure after cyclic loading. When the inner sill air barrier must be maintained, the tape is combined with an inner sealant meeting EN 15651-1. The exposed lower edge must remain drained, and the tape must not be installed across weepholes, drainage grooves, or sill slots. Where the sill has a positive slope of less than 5°, water may pond behind the tape; a drained sill profile is then specified. The product is placed on the interior side of the drainage plane in exposed facades and is covered by a sill flashing or external seal line where direct rain impingement occurs.

What Limits the Functional Recovery of a Pre-Compressed Open-Cell Foam Tape in Underside Joints?

Functional recovery is governed by the viscoelastic response of the cellular polyurethane network. After liner removal, the open-cell structure expands in two stages: an initial elastic recovery controlled by cell-wall buckling recovery, followed by slower diffusion-limited recovery of the compressed cell ribs. At 23 °C and 50 % relative humidity, the tape typically attains 70 % of final thickness within 10 min and stabilises within 24 h. At 5 °C, the same recovery may require more than 48 h; below 5 °C, the glass-transition-related stiffening of the polyurethane matrix reduces immediate recovery and increases installation stress. The expansion force measured according to ISO 3386-1 on a specimen compressed to 50 % of its expanded thickness is in the range of 8–25 kPa for the expanded product, depending on density and cell orientation. The tape should not be compressed beyond 80 % of its initial foam thickness, because permanent cell collapse can occur. Joint width variation greater than ±3 mm can create local areas where contact pressure falls below the sealing threshold. In cyclic joint movement testing, performance is influenced by the ratio of installed thickness to maximum joint width; for a joint of 10 mm, the installed expanded thickness should be at least 15 mm to maintain positive contact after movement. Recovery force is typically recorded on a universal testing machine equipped with a 2.5 kN load cell and a crosshead speed of 50 mm/min. Published data for this specific configuration is limited for combined thermal cycling and wind-induced frame deflection beyond 10,000 cycles.

Surface preparation on production-scale window installation lines follows a constrained sequence. Sill surfaces are brushed or vacuumed to remove dust; oil and form-release residues are removed with a solvent cleaner that does not soften the sill coating. The tape backing is not a pressure-sensitive adhesive and must be pressed into a clean, dry joint where the tape is mechanically retained by frame load and adjacent surfaces. In high-wind locations where external wind suction exceeds 300 Pa, mechanical fastening or a compatible adhesive must prevent displacement before full expansion. Roll packaging is polyethylene and should remain sealed until the installation shift. After opening, partial rolls may be held for up to 4 h if re-rolled tightly and placed in a temperature-controlled area at 15–25 °C. Re-packing after full expansion is not effective because the recovery set can be lost. Liner removal is performed in a continuous motion, and the tape is pressed into the joint to 50 % of the expanded thickness so that residual recovery force holds it in place. Joint intersections are butt-cut, not overlapped; overlaps create double thickness that can prevent the frame from seating level. In corner details, the tape is folded at 90° without stretching, because stretched foam loses recovery force. Published data for this specific configuration is limited for complex multi-corner details with folded intersections.

Hygrothermal, Acoustic, and Air Permeability Performance Data

Property values vary with compressed thickness, expanded density, and joint depth. The table below consolidates key material parameters evaluated by the listed test methods. Air permeability values refer to a finished joint specimen with a single tape line and no additional wet sealant. Acoustic insertion loss is measured in a laboratory partition assembly according to ISO 10140-2; the variation is joint-depth dependent.

Thermal conductivity of the expanded foam is typically in the range 0.035–0.040 W/(m·K) according to ISO 8302; the contribution of the tape to the linear thermal transmittance of the window sill is limited because the joint depth is small. The product is not an approved thermal break. In condensation assessment according to EN ISO 13788, the low diffusion resistance does not create a vapour barrier; therefore the risk of interstitial condensation in the sill is controlled by the exterior drainage plane and the inner air seal. Acoustic performance is influenced by seal continuity rather than by the mass of the joint fill; partial gaps reduce insertion loss significantly. A continuous tape line with compressed foam at a width of 20 mm and joint depth of 10 mm may provide a weighted acoustic joint insertion loss of 3–6 dB in the frequency range 100–3150 Hz; published data for this specific configuration is limited. The tape is not regarded as an acoustic absorber in free field; its function is joint-fill damping and flanking reduction. Air leakage performance in the finished window assembly is verified at the component level according to EN 14351-1; the tape alone is not a window classification.

Substrate compatibility has caused more installation failures than incomplete foam recovery on some exposed production lines. The open-cell polyurethane matrix is compatible with PVC-U, aluminium, coated timber, concrete, and masonry substrates. It must not be installed against bitumen-rich coatings or solvent-borne primers that have not fully cured, because plasticiser migration can soften the cell walls. Avoid contact with aromatic hydrocarbon solvents, ketones, and ester-based cleaning agents; these can swell the polymer and create dimensional instability. The tape is not resistant to prolonged exposure to strong alkaline bedding mortars while wet; installation into fresh mortar joints is not recommended until the surface pH is below 10 and free moisture content has fallen below 5 % by mass. The product is not a closed-cell seal; capillary water can pass through the open cell structure under continuous water pressure. In sill configurations where a standing water head of more than 20 mm can occur, a closed-cell EPDM or butyl-based exterior seal is used in combination with the T635 as the inner air and vapour-permeable stabiliser.

Compared with PES-laminated sealing strips and butyl cords, the Bostik T635 AIRSEAL 3D UF differs in supply format, sealing mechanism, and hygrothermal function. Butyl cords seal by plastic deformation and adhesion; the T635 seals by recovery force and does not rely on surface adhesion. Pre-expanded EPDM gaskets provide closed-cell water resistance and can be installed at lower temperatures, but they are less conformable to irregular masonry sills. Polyethylene backer rods do not provide air sealing or moisture-vapour control. The 3D UF tape is selected when the underside joint requires simultaneous air sealing, acoustic isolation, and vapour permeability, but not bulk water resistance. Other Bostik Airseal tapes may be configured for lateral frame joints; the T635 is specified for underside compression where vertical frame loading and joint movement occur together.

When the Joinery Underside Joint Is Exposed to Driving Rain or Standing Water

Driving rain and standing water change the tape’s performance from air and vapour control to liquid-water exclusion. Because the polyurethane foam is open-cell, it is not a liquid-water barrier under hydrostatic pressure. The underside joint must have a drained exterior plane, a sill flashing, or an external sealant bead meeting EN 15651-1. In exposed facades, the tape is placed on the interior side of the drainage cavity or behind the pressure-equalised chamber. The joint geometry is designed so that water is not trapped against the foam; if water remains in contact with the tape for prolonged periods, algal growth and salt crystallisation in the cell structure can reduce recovery force. Under frost conditions, trapped water in the foam may cause surface spalling; the product should not be used as an exposed external joint seal without a protective cover. Installation is validated by site water-spray testing of the complete sill assembly under DIN 18542 or by window testing according to EN 1027 and EN 12208. The tape does not replace a wet seal where the lower frame joint is subject to direct rain impingement at a wind pressure above 300 Pa.

Compliance status for the product is established through factory production control and type testing. The following checklist consolidates the relevant standards and classification fields.

Roll packaging is resealable only if the roll has not begun to expand. On production lines, cutting should be performed with a guillotine or scissors after measuring the joint length; torn cuts create uneven edges and local contact pressure anomalies. The tape is not self-adhesive in all configurations; where a self-adhesive backing is specified, the adhesive side is protected by a release liner. The product is not specified where the underside joint is fully immersed or where the sill is subjected to continuous hydrostatic pressure; in those situations, a closed-cell exterior seal and positive drainage are required.