By Tim Ledden - Armacell Technical Manager-Insulation

Elastomeric foam duct liner is effective at sound attenuator and supports high IAQ standards

Hospitals are noisy places.  Staff, patients, medical equipment and mechanical systems combine to create a steady stream of sound which is transported from space to space through the mechanical ductwork.  So itis no surprise that mechanical engineers frequently advocate for duct liner to help deaden all this noise.

Elastomeric foam duct liner is ideally suited for healthcare environments because it supports high IAQ standards, but confusion exists over the material’s effectiveness as a sound attenuator and if and when it can be applied in hospitals. 

Effective at Critical Low to Mid-Frequencies 

Elastomeric foam reacts to sound waves by converting sound energy into movement of the foam. This response makes elastomeric foam a superb sound attenuator at low to mid-frequency ranges, which tend to be the most problematic in a mechanical system. The sweet spot of elastomeric foam’s acoustic performance is between 125 Hz and 500 Hz, though it is effective up to 1000 Hz. 

While health care facilities benefit from the use of duct liner as a sound attenuator, its use in these environments has been largely restricted in the past. This is because duct liners have been historically associated with fiberglass, which has the ability to trap dirt and moisture and potentially support mold growth. Fiberglass also degrades over time and may infiltrate the air stream, clogging filters.  These problems do not apply to elastomeric foam, which is fiber-free with near zero moisture permeability. Still, it has been prohibited along with all other duct liners in certain critical care areas of hospitals.  

Fortunately, many of the restrictions on duct liner in hospitals are beginning to lift, depending on a state’s adoption status of the latest guidelines from the Facility Guideline Institute (FGI). 

ANSI/ASHRAE/ASHE 170 Relaxes Restrictions on Duct Liners

Most states adhere to the FGI’s Guidelines for Design and Construction of Health Care Facilities as code or reference. When it comes to HVAC matters FGI typically defers to ASHRAE and that’s where some of the most promising changes have occurred with regard to duct liner, specifically in ANSI/ASHRAE/ASHE 170: Ventilation of Health Care Facilities. 

The latest version of this standard is incorporated into FGI’s 2014 Guidelines and is significantly less restrictive than it has been in the past. The standard now states:

“Duct lining shall not be used in ductwork located downstream 

of Filter Bank No. 2. Duct lining with an impervious 

cover may be allowed in terminal units, sound attenuators, 

and air distribution devices downstream of Filter Bank No. 

2. This lining and cover shall be factory installed. 

d. Duct lining shall not be installed within 15 ft (4.57 m) 

downstream of humidifiers.” 

Previous guidelines prohibited the use duct liner in any duct serving operating rooms, delivery rooms, LDR rooms, nurseries, protective environmental rooms and critical care units.  Now the use of duct liner is limited more by its location within the duct rather than the space the duct is serving. This gives engineers substantially more leeway for sound attenuation. 

Some states, including Georgia, Louisiana, and Pennsylvania have already accepted the 2014 guidelines and others are sure to follow. In the meantime, designers must check the local governing authorities to confirm the applicability of duct liner in hospitals. The same applies to various outpatient facilities, although restrictions in these types of facilities have been far less prohibitive.