Air Ducts

By Kenton Shepard

Insulating Ducts

It’s important that heating ducts run through unheated space be sealed and insulated. Heat loss resulting form routing leaky, uninsulated ducts through unheated spaces such as crawlspaces can increase heating costs by 25% to 40%. Even better are designs in which ducts are run only through heated space and are not routed through exterior walls.

Sealing Ducts

To minimize leakage from air ducts, ducts should be sealed with a sealant such as mastic or a spray sealant, not duct or aluminum tape.

Cold air Returns

Cold air returns are registers connected to ducts which provide a pathway for air to return to the furnace to be reheated. Ideally, each commonly-occupied room should have its own cold air return with the exception of bathrooms, kitchen, closets, pantry and laundry rooms. Supply registers and cold air return registers should not be located too close to each other.

If too few returns are installed in the home the furnace will not be able to operate efficiently. Warm air should flow from supply ducts at the same rate at which cooled air is returned to the furnace through return-air ducts.

Rooms with supply registers and no return registers which have no provision for allowing air to circulate freely (such as undercut doors) may feel cold during the heating season and may not cool sufficiently during the cooling season.

Restricting circulation between supply and return registers may also depressurize areas of the home excessively, drawing unhealthy gases into the space such as Radon or toxic products of combustion such as carbon monoxide.

Ducts for Cooling

Because cool air is denser than warm air, ducts for delivering cool air must be larger than those designed for distributing warm air. This can be a problem with homes which were originally designed without an air-conditioning system. The retrofit usually involves using existing heating ducts for cool air distribution and the results may not be satisfactory. Inspectors should note this in their reports.

Interstitial Spaces

Cold air returns should be sealed with caulk where ducts penetrate walls in order to avoid including interstitial spaces (spaces between walls or inside walls) as part of the home conditioned-air circulation system. Interstitial spaces are spaces between conditioned areas Wall and ceiling cavities and ceiling, wall and utility chases are examples of interstitial spaces.