Experimental Evaluation of Gas Filled Plenum (GFP) Insulation for Ducts Page: 2 of 14
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DRAFT 01/26/03
pressure would have to rely on another structural element for lofting (e.g., a standard flex duct core) and the
insulation itself would need to define its volume with some springy stiffness.
Figure 1. Example of multilayered GFP (Photo: Goudey, LBNL)
Initial samples of GFP duct were constructed by hand in various geometries, including longitudinal sections,
concentric rings and a continuous helix. All of the samples were round ducts, rather than rectangular, because the
ducts used for comparison and most ducts installed outside conditioned spaces (where insulation is most important)
have a round cross section.
For the longitudinal construction, the seams separating the gas filled spaces are parallel to the axis of the duct.
Figure 2 shows a sample duct section constructed using this technique. A flat panel is constructed and two sides
joined to form a hollow tube about 20 inches (50 cm) in length. The width of the flat panel, and therefore the
number of gas filled spaces, determines the diameter of the resulting tube. Building panels of different widths allow
different diameter tubes to be nested (as shown in Figures 2 and 3), so as to build up several concentric layers of gas
filled panel. The length of each section was limited by the machine used to manufacture the GFP and the
dimensions of the raw material used in the panel. Although GFP construction requires metallized surfaces to reduce
radiant heat exchange between the GFP surfaces, we also used some plain materials without metallization, as shown
in the right-hand illustration in Figure 3. The non-metallized samples allowed us to vary the spacing of the sealed
strips that separate each gas filled space to change the flexibility of the panel. This flexibility is important because
closer spacing between sealing strips means that there is more seam area compared to gas filled area. The seams
have negligible heat flow resistance compared to the gas filled sections, thus additional seams lead to lower overall
insulation value for the GFP. Too large a spacing led to the resulting panels being very difficult to form into round
cross section ducts. For example, if only five seams were used the duct tends to form a square cross section with a
seam at each corner. Lastly, we also tried to form the panels into a concentric spiral, but the material was difficult to
form into this shape. It required wider sections than we were able to manufacture because enough material for three
or four layers needs to be made in one continuous piece. Lastly, the changing internal and external diameters
(required using this technique) made it difficult to line-up the non-metallized sections when forming the individual
air spaces.
Using this longitudinal construction method would require some sort of duct liner for any bends in the duct
because the duct tends to fold sharply rather than bend smoothly. Thus this construction method is suitable only as
an insulation replacement rather than a stand-alone duct. An advantage of the longitudinal construction method is
that the fused sections that have little insulation value can be arranged so that they are always covered by a non-
fused section, otherwise thermal bridging could occur if a single layer of GFP were used.2
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Walker, Iain S. & Guillot, Cyril. Experimental Evaluation of Gas Filled Plenum (GFP) Insulation for Ducts, report, January 26, 2003; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc736146/m1/2/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.