Capabilities and Variations

Spunstrand® Inc. has been manufacturing underslab HVAC ductwork for over forty years in both insulated and non-insulated forms. Insulated and non-insulated ductwork is available in but not limited to standard sizes 2″¯ through 120″¯. Custom sizes, larger and smaller, round or rectangular are also available. Rectangular or square duct systems are not usually designed and laid out for underground use except for vertical boot risers. Boot risers are typically sheet metal that can be made out of fiberglass if extremely high water table or corrosive soil conditions exist.

Insulation values are available in ICC listed and code approved standard insulation values of R-4, R-6, R-8, R-10 and R-12. Greater values required can be easily accommodated. Wall thickness may also be varied to attain greater depths of burial, or to provide better structure for pipe racks and other equipment used for conveyances through the duct system.

Spunstrand® Inc. does offer a complete line of pre-insulated or non-insulated fittings. If fittings other than standard or variations from standard are required, these can be handled upon request and normally at no additional cost. Pre-manifold sections or factory-attached fittings are also available.

Capabilities List

  • Watertight joints
  • Direct burial
  • Special shapes and sizes
  • Complete line of pre-insulated or heavier wall thickness non-insulated fittings
  • Pre-drilled duct for solar bed aeration
  • Greater insulating values
  • Factory manifolding
  • Custom lengths
  • Alternate joint systems
  • Custom fittings, round or rect.
  • Deep burial engineering square to round
  • Consulting
  • FRP boots

Spunstrand® fiberglass reinforced plastic duct (FRP) is rated Class 1 and code approved for direct underground burial serving any commercial application which has the supply and/or return air system under the building slab. Typical applications include the following.

Underslab HVAC/Slab on Grade Construction

  • Banks
  • Carbon Monoxide Exhaust
  • Churches
  • High Rise Construction
  • Libraries
  • Hotels
  • Housing Projects
  • Hospitals (HVAC & Garage Exhaust)
  • Restaurants
  • Schools/Gymnasiums
  • Supermarkets
  • Swimming Pools (HVAC & Heat Recovery Systems)
  • Theaters/Auditoriums

Spunstrand® Inc. duct for these applications is manufactured using the filament wound method, where continuous fiberglass strands are impregnated with resin and machine wound on a mandrel which has first received a foil scrim kraft surface liner. Winding continues until the desired wall thickness is reached and then the mandrel with the duct laminate is moved to a heated curing station where it is rotated until the resin is set. The approximate glass content of the filament wound duct is 70%.

The internal surface liner provides the Class 1 rating for this product, while the fiberglass fibers furnish strength, dimensional stability, and temperature resistance. The type of resin is chosen to provide chemical and electrical resistance, color, finish hardness and thermal properties.

Commercial/Residential Documents