duct mastic

Duct mastic is an excellent duct sealing material, contrary to most duct tapes.

Some heat-activated tapes and some pressure-sensitive tapes can be good duct sealing options, but only for specific locations and types of ducts.

See: Duct Tapes

Duct Mastic

Duct mastic is cheap and the best general solution for duct sealing. Just pay attention to two details: the name mastic suggests adhesive properties that mastic doesn’t have: mastic doesn’t hold ducts together. To that job you need mechanical fasteners (screws, straps, etc.). The other important detail is that there are many mastics, with rather different properties.

See: Duct fasteners

What’s mastic?

Mastic is a very flexible sealant that never fully hardens: it expands and contracts as the duct expands and contracts (allowing longer lifespan and higher effectiveness), and it withstands condensation.

Modern mastics do not use petroleum as a solvent base; they are water-based products, which is safer and make them easier to clean. They may come in cartridges, tubs and buckets and you can apply them with a brush, a caulking gun or a trowel.

Mesh Tape

Mastic is used on cracks up to ¼ inch (0.65 cm) wide. To seal wider joints and gaps you should also use mesh tape (fiberglass reinforced). In this case, apply a thick layer of mastic, and then one or more layers of fiberglass mesh tape, topped by another layer of mastic. The mastic should cover the mesh entirely.

The mastic and the mesh make a particularly strong sealing.

Mastic types

Be aware when choosing mastic. The properties of mastic aren’t standardized. Adhesion, cohesion, water and fire resistance or non-toxicity are important properties, that may differ widely from one manufacturer to another. Color is also a property you may want to consider, if ducts are going to be visible… Pay attention to the label of the package.

Prefer mastics listed and labeled in accordance with Underwriters Laboratories/UL standards.

Duct Application

Mastic should be properly installed. Pay attentions to some general guidelines. Clean diligently the surfaces where the duct is to be applied, and secure the joints properly (see Mechanical Duct Fasteners). Also use a fiberglass mesh to gaps and openings wider than 1/4 '' (0,5-0,7cm).