Aluminium foil tape is a soft aluminium foil coated with a pressure-sensitive adhesive, used to seal HVAC duct joints, close insulation seams and protect surfaces from heat and moisture. The right tape depends on three choices: foil thickness, adhesive chemistry and, for ductwork, whether the job requires a UL 181 listed product. This guide covers each, with sizes, application practice and the mistakes that cause tape to peel.
Foil tape has two layers: an annealed aluminium foil backing, usually an alloy such as 1235, 8011 or 1145 in the O temper, and an adhesive mass protected by a release liner until use. The foil gives a backing that does not stretch, rot, absorb water or soften with heat, and it reflects radiant heat. The adhesive does the sealing.
The name "duct tape" causes confusion. The grey cloth-backed tape sold under that name has a rubber adhesive that dries out and loses grip on warm metal ducts within months. Laboratory testing at Lawrence Berkeley National Laboratory in the late 1990s found cloth duct tape to be the least durable of the duct sealants it aged, while foil tapes and mastic held up far better. That is why most mechanical codes and energy codes now call for listed foil tape or mastic on duct joints, not cloth tape.
Beyond ductwork, foil tape is used to seal the facing of insulation boards and blankets, protect cables and hoses from radiant heat, mask parts during plasma spraying and powder coating, repair vapour barriers, and provide electromagnetic shielding where a conductive adhesive grade is used.
Soft foil with a solvent or water-based acrylic adhesive. The general-purpose choice for sheet-metal duct joints, insulation seams and repairs. Acrylic resists ageing and UV and keeps its bond across a wide temperature range.
A thick, soft butyl rubber mass on foil. Very high tack and gap-filling ability, fully waterproof; used on roof flashings, outdoor duct, gutters, condensate lines and building envelope seams.
Silicone adhesive that keeps its strength well above the range of acrylics. Used around exhausts, ovens, industrial dryers, masking for thermal processes and aerospace or automotive heat shields.
Foil laminated to glass scrim and kraft paper (FSK) or to glass cloth for tear resistance. Matches FSK-faced insulation and resists puncture during installation.
Most foil tapes use a paper or film release liner. Linerless tapes have a release coating on the foil itself; they are faster to apply but harder to handle in cold weather.
Adhesive filled with conductive particles so the tape bonds electrically to the substrate. Used for EMI shielding of enclosures, grounding and cable wraps.
| Adhesive | Initial tack | Long-term hold | Water resistance | Typical service range* | Best for |
|---|---|---|---|---|---|
| Acrylic | Medium | Good to excellent; bond strengthens over 24 – 72 h | Good | About -30 to +120 °C, some grades higher | HVAC duct, insulation seams, general use |
| Butyl rubber | High | Excellent | Excellent | About -40 to +90 °C | Waterproofing, outdoor duct, uneven surfaces |
| Silicone | Medium | Excellent at temperature | Good | About -50 to +250 °C | Exhausts, ovens, thermal masking |
| Rubber (natural or synthetic) | Very high | Poor to fair; dries out when warm | Fair | Low-temperature indoor only | Temporary holding; not recommended for ducts |
*Service ranges vary widely between products; always use the range on the tape manufacturer’s data sheet. Application temperature (the temperature at which the tape can be applied) is usually much narrower than service temperature.
A useful rule is that acrylic is the default, butyl is chosen when the surface is wet, outdoors or irregular, and silicone is chosen only when temperature demands it, since silicone tapes cost more and can contaminate surfaces that will later be painted.
| Foil thickness | Imperial equivalent | Typical use |
|---|---|---|
| 25 – 30 µm | About 1 – 1.2 mil | Insulation seams, light repairs, vapour-barrier patches; conforms easily to curves |
| 35 – 50 µm | About 1.4 – 2 mil | Standard HVAC duct tape gauge; the most common UL 181 listed construction is about 2 mil foil |
| 60 – 80 µm | About 2.4 – 3.2 mil | Heavy-duty duct work, dryer vents, abrasion-prone areas |
| 100 µm and above | About 4 mil and above | High-temperature and industrial service, heat shields, masking that must survive grit blasting |
Total tape thickness is the foil plus the adhesive, typically adding 20 – 60 µm for acrylic and much more for butyl. Thicker foil is stronger and resists puncture but is harder to press into corners and over rivet heads; on round flexible duct, a thinner foil often seals better because it conforms without creasing.
The foil itself is a soft, fully annealed product. Hard-temper foil would spring back, open creases and lift at the edges. Alloys 1235 and 8011 dominate because they combine softness with good resistance to pinholes and are readily available in the required gauges.
For HVAC ductwork in North America and in many projects that follow US practice, the key standards are the UL 181 family. A tape listed and marked to these standards has passed tests for adhesion, shear, temperature, mould growth and surface burning characteristics.
| Marking | Standard | Application |
|---|---|---|
| 181A-P | UL 181A, pressure-sensitive tape | Closure of rigid fibrous glass duct board |
| 181B-FX | UL 181B, pressure-sensitive tape | Closure of flexible air ducts and connectors |
| 181A-M / 181B-M | UL 181A / 181B, mastic | Mastic closures; not tape, but the usual alternative |
| UL 723 / ASTM E84 rating | Surface burning characteristics | Flame-spread and smoke-developed indices; mechanical codes commonly limit duct materials to 25 / 50 |
Many building inspectors look for the printed listing mark on the tape backing, so tapes for duct work are usually printed with the marking at intervals along the roll. For sheet-metal duct, energy codes typically accept listed tape or mastic, and installers often apply mastic to longitudinal seams and listed foil tape to transverse joints.
In Europe and other regions, duct airtightness is specified at system level (for example EN 12237 for circular sheet-metal ducts and EN 1507 for rectangular ducts), and tapes are chosen by the duct supplier to meet the required airtightness class. Fire reaction of insulation systems including tapes is classified under EN 13501-1.
Tape converters buy the aluminium foil as wide coils, coat it with adhesive and slit it into finished rolls. For converters, the important foil properties are uniform gauge, a clean annealed surface that the adhesive primer wets reliably, and low pinhole count, because a pinhole becomes a leak path once the tape is on a pressurised duct.
Most tape failures come from skipping the first and fourth steps. Tape that peels from a duct a year after installation was usually applied to a dirty or oily surface or was laid on lightly without burnishing.
It degrades quickly on warm metal and is not accepted by most codes for duct closure.
For duct board and flexible duct, an unlisted tape may fail inspection even if it performs well.
Silicone is for heat; it costs more, has lower tack, and can leave residue that stops paint adhering.
Mill oil and dust are the main causes of peeling; clean first.
Below the application range the adhesive cannot flow into the surface and never develops full bond.
Tape seals joints; mechanical fasteners or clamps must hold ducts together.
Yes, and it is the preferred tape for duct joints. For fibreglass duct board use a tape marked UL 181A-P, and for flexible duct a tape marked UL 181B-FX. On sheet-metal duct, listed foil tape or mastic is normally accepted by energy codes.
The foil itself is unaffected well beyond any HVAC temperature; the limit is the adhesive. Acrylic tapes typically handle around 100 – 150 °C depending on grade, while silicone-adhesive tapes are used up to about 250 °C. Check the data sheet for the exact product.
The foil is impermeable to water and vapour. Waterproofing depends on the adhesive and application: butyl foil tape is the usual choice outdoors and on wet or uneven surfaces, while acrylic tape is water resistant once fully bonded.
Duct tape has a cloth and polyethylene backing with a rubber adhesive; it is versatile but dries out and loses grip on warm ducts. Foil tape has a metal backing that does not degrade and acrylic, butyl or silicone adhesives that hold for years.
Yes. Metal foil tape is commonly used to seal joints in metal dryer exhaust duct, partly because many codes prohibit screws that project into the duct and catch lint. Never use cloth duct tape there.
Soft, fully annealed foil, usually 1235, 8011 or 1145 in the O temper, in gauges from about 25 to 100 µm. Metline supplies this foil in wide coils or slit widths for tape converters, who coat, laminate and slit it into finished tape.
Tell us the foil gauge, adhesive, width, roll length and destination. We reply with options and a delivered price.
