SIGA Fentrim Tape: Complete Guide for Air & Weather Sealing

Why high-performance builders choose Fentrim for window flashing, foundation transitions, and the toughest air-sealing details on the job site.

Air leakage accounts for 25% to 40% of heating and cooling energy loss in a typical building. The weakest points are transitions — where the window meets the wall, where the sill plate meets the foundation, where dissimilar materials join. These are the details that standard construction tapes fail at within 5 to 10 years.

SIGA Fentrim is a family of flexible, self-adhesive sealing tapes manufactured in Switzerland, engineered specifically for these critical transitions. It bonds to rough concrete, treated wood, plastic membranes, metal flashing, and OSB — even at temperatures below freezing. This guide covers every Fentrim variant, where each one fits in a building assembly, and how to install it correctly.

What Makes SIGA Fentrim Different

Most construction tapes use a pressure-sensitive acrylic adhesive on a polyethylene or polypropylene carrier. They work well on smooth, clean, dry surfaces at room temperature. On a real construction site, those conditions rarely exist.

SIGA Fentrim uses a solid acrylic adhesive system (not solvent-based, not hot-melt) on a flexible, split-backing carrier. The key differences:

• Adhesion to rough substrates: The adhesive is thick enough (approximately 0.2 mm) to flow into the pores of concrete, cinder block, and rough-sawn lumber. Most tapes sit on top of these surfaces and peel off within months.
• Cold-weather application: Fentrim can be applied at temperatures as low as 14 degrees F (-10 degrees C). The adhesive remains tacky and builds bond strength even in cold conditions. Most competitor tapes require 40 degrees F minimum.
• Split-release liner: The backing paper splits down the center, allowing you to position one half of the tape, then peel and press the second half. This is critical for tight corners, curves, and window sills where you need precision.
• Long-term durability: SIGA tapes are tested to maintain adhesion for 100+ years under accelerated aging protocols per RAL standards. The adhesive cross-links over time, getting stronger rather than degrading.
• Vapor-variable permeability: Fentrim IS 20 specifically allows moisture to pass through at rates that adapt to humidity conditions, preventing moisture trapping in wall assemblies.

Fentrim Product Lineup: 430 vs IS 20 vs 230

SIGA makes several Fentrim variants. Each has a specific use case. Here is how they compare:

Product	Width	Key Feature	Primary Application
Fentrim 430	150 mm (5.9")	Interior-side air sealing, vapor-variable	Window-to-wall interior flashing, sill plate sealing
Fentrim IS 20	100 mm & 200 mm	Exterior weather barrier, UV-resistant for 90 days	Window rough opening flashing, exterior air barrier transitions
Fentrim 230	100 mm (3.9")	Narrow profile, high flexibility	Tight corners, pipe penetrations, electrical box sealing
Fentrim F	Various	Pre-formed corner pieces	Window sill corners, inside/outside corner transitions

Fentrim 430: The Interior Air Seal Workhorse

Fentrim 430 is the tape you reach for when sealing the interior side of window and door installations. At 150 mm (nearly 6 inches) wide, it bridges the gap between the window frame and the rough opening framing, creating a continuous interior air seal.

The vapor-variable membrane carrier adapts its permeability based on ambient humidity. In winter (dry interior conditions), permeability drops below 1 perm, preventing interior moisture from entering the wall cavity. In summer (humid conditions), permeability rises above 10 perms, allowing any trapped moisture to dry inward. This smart-vapor behavior prevents the moisture trapping that causes mold and rot in wall assemblies.

Common uses for Fentrim 430:

• Interior window frame to stud sealing
• Interior door frame to rough opening sealing
• Sill plate to subfloor air barrier connection
• Electrical panel penetration sealing
• Any interior junction where the air barrier must transition between materials

Fentrim IS 20: Exterior Flashing and Weather Sealing

Fentrim IS 20 is the exterior counterpart. It is designed to be exposed to weather during the construction process (up to 90 days of UV exposure) and to serve as a permanent weather-resistive flashing at window and door rough openings.

IS 20 comes in two widths: 100 mm for standard joints and 200 mm for wide sill pans and larger transitions. The adhesive bonds aggressively to OSB, plywood, Zip System sheathing, concrete, and metal flashing. On porous surfaces like concrete block, SIGA recommends priming with Dockskin primer for maximum adhesion.

Common uses for Fentrim IS 20:

• Window rough opening sill pan flashing
• Head and jamb flashing over exterior sheathing
• Foundation-to-framing transition at the sill plate (exterior side)
• Membrane-to-concrete connections at grade
• Flashing integration with exterior air/water barrier membranes like SIGA Majvest

Fentrim 230: Precision Sealing for Tight Details

At 100 mm wide with exceptional flexibility, Fentrim 230 handles the small, fiddly details that wider tapes cannot reach. Pipe penetrations, electrical conduit entries, J-box sealing, and inside corners all benefit from 230's ability to conform to irregular shapes without wrinkling.

Window Flashing with Fentrim: Step-by-Step

Window rough opening flashing is the most common Fentrim application and the one where proper technique matters most. A poorly flashed window is the number-one source of water intrusion in residential construction. Here is the correct sequence:

Exterior Flashing (Fentrim IS 20)

1. Prepare the rough opening. Ensure OSB or plywood sheathing extends to the edge of the rough opening. The surface must be clean and dry. In cold weather (below 32 degrees F), warm the substrate with a heat gun. Do not apply tape to frost.
2. Flash the sill first. Cut a piece of Fentrim IS 20 (200 mm width) to the width of the rough opening plus 8 inches (4" overlap on each side). Peel the center split of the release liner. Press the tape into the sill corner, extending down the face of the sheathing by at least 3 inches.
3. Flash the jambs. Cut Fentrim IS 20 (100 mm width) to the height of the rough opening plus 4 inches. Apply over the sill flashing at the bottom (shingle-lap principle — upper pieces always overlap lower pieces). Extend down over the sill flashing by at least 2 inches.
4. Flash the head last. Apply Fentrim IS 20 across the top of the rough opening, overlapping the jamb flashing by at least 2 inches on each side. The head flashing goes on last so it sits on top of all other layers.
5. Roll all seams. Use a SIGA Pressfix roller (or any J-roller) to apply firm, consistent pressure to every inch of tape. This activates the adhesive bond. Skipping this step reduces adhesion by 30% to 50%.

Interior Air Sealing (Fentrim 430)

1. After the window is installed and shimmed, apply a continuous bead of Fentrim 430 around the entire perimeter of the window frame, bridging from the frame to the stud face.
2. At corners, use pre-formed Fentrim F corner pieces or fold the tape carefully to avoid wrinkles. Any wrinkle is a potential air leak path.
3. The tape should overlap itself by at least 2 inches at all joints.
4. Roll every seam with a J-roller.

Foundation and Sill Plate Sealing

The junction between the concrete foundation and the wood sill plate is one of the leakiest points in any building. Cold air infiltrates through the gap between the concrete and the pressure-treated sill, and moisture wicks up from the concrete into the wood.

Fentrim IS 20 solves both problems when applied correctly:

1. Before the sill plate is set, apply a sill gasket (foam or EPDM) to the top of the foundation wall.
2. After the sill plate and subfloor are installed, apply Fentrim IS 20 (200 mm) on the exterior. Half the tape bonds to the concrete foundation; half bonds to the sheathing or rim joist above.
3. For concrete surfaces that are rough or dusty, brush on SIGA Dockskin primer and let it dry for 30 minutes before applying the tape.
4. On the interior, apply Fentrim 430 from the sill plate down to the foundation wall, creating a continuous interior air seal at the floor line.

Why Fentrim Outperforms Competitors

The North American market has many flashing tapes: Zip System flashing tape, 3M 8067, Huber ZIP Stretch Tape, Grace Vycor Plus, and dozens of generic butyl tapes. Here is where Fentrim stands apart:

• Adhesion to concrete: Butyl tapes and most acrylics do not stick to concrete without primers. Fentrim IS 20 bonds directly to most concrete surfaces, and the optional Dockskin primer handles the worst cases.
• Cold-weather performance: Most flashing tapes require application temperatures above 40 degrees F. Fentrim works down to 14 degrees F. In Canada and the northern US, this extends the taping season by 2 to 3 months.
• Conformability without wrinkling: The split-liner design and flexible carrier allow Fentrim to wrap into corners and around curves without bunching. Stiffer tapes (like Zip flashing tape) require relief cuts at every corner, creating potential leak points.
• Aging performance: Independent testing (ift Rosenheim in Germany) shows SIGA tapes maintaining over 90% of initial adhesion after accelerated aging equivalent to 100 years. Butyl tapes typically lose 40% to 60% of adhesion in the same tests.
• System compatibility: Fentrim is designed to integrate with the full SIGA system — Majvest exterior membrane, Majpell interior membrane, Wigluv sheathing tape — creating a tested, warrantied air and weather barrier assembly.

Surface Preparation and Primers

Fentrim sticks to most construction surfaces without primer. However, some substrates benefit from preparation:

Substrate	Primer Needed?	Notes
OSB (clean, dry)	No	Direct application. Best on freshly cut surfaces.
Plywood	No	Direct application.
Zip System sheathing	No	Compatible. Bonds well to the coated surface.
Smooth concrete	Recommended	SIGA Dockskin primer for best results.
Rough/porous concrete	Yes	Dockskin primer required. Brush into pores.
Concrete block (CMU)	Yes	Dockskin primer required on the block face.
Metal flashing	No	Clean with solvent first. Remove oils and coatings.
PVC window frames	No	Clean with isopropyl alcohol before application.
Fiberglass window frames	No	Direct application to clean, dry surface.
Dusty or weathered wood	Recommended	Brush off loose dust. Dockskin primer if heavily weathered.

Storage and Handling

SIGA tapes perform best when stored between 50 and 77 degrees F. In winter, keep rolls inside a heated space until ready to use. Cold tape (below 40 degrees F) unrolls stiffly and the adhesive is less immediately tacky, even though the long-term bond will develop fully.

Shelf life is 24 months from manufacture when stored in original packaging. Check the date code printed on each roll. Rolls stored beyond 24 months may have reduced initial tack, though the adhesive chemistry remains sound.

Integrating Fentrim with the Full SIGA System

For maximum performance and warranty coverage, SIGA recommends using Fentrim as part of a complete air and weather barrier system:

• Exterior air/water barrier: SIGA Majvest 700 SA (self-adhered) or Majvest 500 (mechanically fastened) as the primary exterior membrane.
• Sheathing tape: SIGA Wigluv 60 for taping sheathing panel joints.
• Exterior transitions: Fentrim IS 20 for all window, door, and penetration flashing.
• Interior air barrier: SIGA Majpell 5 as the interior vapor-control layer.
• Interior transitions: Fentrim 430 for all interior window, door, and penetration sealing.

This system approach ensures all components are chemically compatible and tested together. Using SIGA tapes on a competitor's membrane (or vice versa) may work, but it voids the system warranty and introduces uncertainty about long-term adhesion compatibility.