Effects of airflow infiltration on the thermal performance of internally insulated ducts

<p>Air flowing through a supply duct infiltrates perviously faced, porous, internal duct insulation, degrading its thermal performance. Encapsulating the insulation's air-facing surface with an impervious barrier prevents infiltration, increasing the capacity of the conditioned supply air to heat or cool the space to which it is delivered.</p><p>This study determined the air-speed dependence of the thermal conductivity of fiberglass insulation by measuring the inlet-to-outlet temperature drop of heated air flowing through a long, insulated flexible duct. The conductivity of a flexible duct's low-density, internal, fiberglass-blanket insulation increased with the square of the duct air speed, rising by 140% as the duct air speed increased from 0 to 15 m s⁻¹. At air speeds recommended for branch ducts, the conductivity of such insulation would increase by 6% above its still-air value in a residential system and by 16% in a commercial system. Results partially agreed with those reported by an earlier study.</p><p>Simulations indicate that encapsulating the air-stream surface of internal fiberglass duct insulation with an impervious barrier increases the effectiveness with which a duct delivers the thermal capacity of supply air by 0.15%–0.9% in typical duct systems.</p>