Insulating Below-Ambient Indoor Systems in Warm, Humid Climates

William A. Lotz, P.E.

March 1, 1998

This is a first in a series of regular columns on technical aspects of insulation. The author will attempt to answer questions from engineers, contractors, or anyone else with an interest in commercial or industrial insulation.

Q: What insulation system should engineers specify indoors for cold ducts and pipes in Florida-Gulf Coast area of the United States?

A: For an engineer who is experienced in specifying insulation on chilled water piping or air conditioning ducts in New England or Ohio or Oregon, it’s a whole different world in New Orleans or Miami or Charleston, South Carolina. Whereas in most colder climates air conditioning is used only seasonally, in the hot, humid areas of the United States the air conditioning is running most of the time.

Fibrous insulations with factory applied reinforced foil facers work well in Northern climates on chilled water piping and air conditioning ducts. Due to the higher (and year-round) humidity in the Southeast, fibrous insulation frequently is wet and moldy within a year of installation if it is not sealed vapor tight.

The prudent engineer will specify foam insulations for cold applications in humid climates. Hangers are critical in cold applications, so the engineer should give special consideration to hangers that do not penetrate the insulation.

If you specify cellular glass, be sure you have enough thickness to prevent condensation. The sections must be carefully sealed with a suitable bedding compound in all insulation joints. (My specifications contain specific brand names and numbers for insulation accessories). Staggering the half sections is a good idea. Fasten the sections with ½” wide aluminum or stainless bands on 9″ centers.

As for foam insulations, elastomerics are quick-just seal the joints with the manufacturer’s recommended adhesive. Urethanes, isocyanurates, and styrenes are not themselves vapor barriers, so a sheet vapor barrier (such as all service jacketing or 20-mil PVC) needs to be sealed air/vapor tight.

Florida Duct Reinsulation

Foam insulations are better able to withstand “feet” than fibrous insulations. When Fibrous insulations faced with thin foil-kraft laminates (such as ASJ or FSK) are walked upon, the foil cracks and tears and the perm rate of the vapor barrier is lost.

The insulation in the photos on page 20 became wet and moldy within a year. The insulation was 8 pcf density mineral fiber with an ASJ facer. The workmanship was good, but the application failed, partly because various trades walked on the ducts instead of using ladders.

The ducts (located on the central East Coast of Florida) are being reinsulated with black elastomeric foam sheets. This is the second reinsulation in 18 months; the original insulation fell off due to a failure of both the adhesive and the clips.

In summary, the keys to the successful insulation of cold pipes and ducts in hot, humid climates are good specifications and good workmanship.