Home Ducts Sealing & Insulation

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Home Ducts Sealing & Insulation

Image Above Credit: Energy Star; A= Leaky connections; B= Leaky return; C= Furniture blocking registers D= Leaks at furnace and air filter slot and duct tape failures E=Fallen duct insulation; F=Leaku Supply Ducts G= Kinks restricting airflow

Our home’s ducts are a common source of energy losses.

Check your ducts for air leaks. Look for sections that are separated (when they should be joined), and look for possible cracks and holes.

Duct insulation is a very cost effective investment, namely in the case of ducts located in unconditioned areas such as attics or basements.

Savings through duct sealing and insulation

Heated and cooled air escapes through unsealed duct joints and holes, increasing the house’s energy costs. On the other hand, unconditioned air (hot summer attic air, for instance) can also infiltrate into ducts through unsealed points with the consequent increasing of energy bills.

If your duct system is poorly sealed and insulated, sealing and improving its insulation can reduce your heating and cooling costs. Statistics state average losses of 20-30%.

Existing Home Ducts

How to improve existing house's duct systems?

All depends on where the ducts are located, their condition and the costs involved. But there are two main types of measures:

- Sealing and improving the duct’s insulation
- Relocating the ducts within the house thermal system

Ducts located within conditioned spaces – heated and cooled spaces - are more efficient than ducts placed in exterior walls, garages, crawlspaces or attics.

If possible, new ductwork should be located within the main house space since losses are regained by the building space (that’s why insulating ducts in a basement or garage may make those spaces colder)…

Anyway, relocation may also be difficult or too expensive, and in this case the best solution for poorly insulated and leaky ducts is sealing and insulation.

Choosing the duct types

When choosing ducts for you home, take into account the three main types of ducts and their insulation value:

Sheet Metal Ducts

It’s the most common type of ducts, allowing high durability, versatility and efficiency at low costs. The drawbacks? They involve many joints, seams and connections which are potential source of leaking. Besides, they must be insulated when used in unconditioned spaces

Flexduct

Flex-duct involves a plastic inner liner within an insulated tube covered with a vinyl vapor barrier. Flexduct allows few duct connections and low costs, but they too have some drawbacks: they can be easily damaged, and they offer a higher resistance to air flow than metal ducts. Standard round flexduct may easily be damaged, mainly if used in lengths over 15 feet.

Ductboard

Ductboard is made of high-density sheets of fiberglass with foil facing on one side. The result is a lightweight material with vapor barrier, wholly insulated, but also with some drawbacks: it is rather damageable and its durability depends strongly on tapes and mastics used on closure methods. Its cost is relatively high, though very close to that of metal ducts with complete insulation. Ductboard is particular suitable for attic ducts.

The importance of sealing the home's ductwork

Without a proper sealing, insulation doesn’t prevent air leakage. Ducts should be sealed before insulating.

Sealing the ducts is crucial and involves the use of special duct "mastic" and fiberglass mesh. These materials will allow a long-lasting and effective sealing.

Duct Sealants

Mastic, fiberglass mash and butyl-backed foil tape are the main sealant products. Foil tape is a bad option, since it may come loose after some time, and is inappropriate for many joints.

Mastic is a much better option, due to its durability and resistance. Mastic is used on cracks up to ¼ inch (0.65 cm) wide, and is applied to connections and joints.

Fiberglass mesh is used to seal holes larger than ¼ inch (0.65 cm) wide.

Butyl-backed foil tape is a relatively recent product, with a rather unknown durability. It is used do seal cracks and holes.

When sealing, look for top tapes, as those qualified by the Underwriters Laboratories (logo on left)

Materials for Home Duct Insulation

Duct insulation (in attics and other open spaces) can be improved by placing common batt or blown insulation over them. But there are very specific insulation materials for ducts, namely:

- Rigid Fiberglass Duct Liner and
- Fiberglass Wrap insulation

Fiberglass Wrap insulation (with or without a vapor barrier) is used to cover the outside of ducts in unconditioned spaces. Their typical R-values range between 3.6 and 4.2 per inch.

Fiberglass Duct Liner is used to line the inside of rectangular metal ducts. Their R-values are the same of fiberglass wrap insulation.

Flexduct and Ductboard do not require insulation: insulation is part of the duct itself, and its R-values are usually higher than hose of fiberglass insulation: R.4.3, R-6.5, R-8 and R-11 are common or available values.

For metal ducts, outside insulation should include a vapor retarder to prevent condensation on the duct and to avoid consequent damages in wall, in the insulation or in other components of the house.