Photo of tranite cement-asbestos material used for air ducts in a slab over a sewer pipe (C) Daniel Friedman and Conrad Hazards of Asbestos-containing Transite Pipe HVAC Ducts

  • ASBESTOS TRANSITE DUCTWORK - CONTENTS: Transite pipe cement asbestos HVAC Duct safety hazards. Transite HVAC or Air Duct Asbestos Warnings & Hazard Details. Recommendations where Transite HVAC Ducts are Installed
  • POST a QUESTION or READ FAQs about transite air ducts: asbestos cement air duct issues: identification, sealing, removing or abandoning procedures

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Transite HVAC air ducts: this article explains the potential hazards of transite (cement asbestos) air ducts - asbestos fiber release, radon, and indoor air quality concerns, and duct collapse when transite air ducts are is found in buildings. Transite pipe, an asbestos-cement product, was used for HVAC ducts and for chimney or flue material to vent gas-fired appliances.

Cement-asbestos transite pipe may also have been used for water piping in some communities. We discuss how to identify cement asbestos transite air ducts, what the safety & health hazards are, how to seal or abandon the ductwork, & alternative approaches. This document assists building buyers, owners or inspectors who need to identify asbestos materials (or probable-asbestos) in buildings by simple visual inspection.

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Transite Pipe HVAC Ducts: Asbestos Heating or Air Conditioning Duct Material Warnings

Photograph of transite cement asbestos heating duct

Transite pipe, which contains significant percentage of asbestos fibers, was often used for heating ducts and on occasion heating and cooling ducts in older buildings. Transite pipe used as HVAC ducts or air ducts for heating and air conditioning was often installed buried in a concrete floor slab - methods that placed the asbestos-cement transite piping below or in a building floor slab.

Asbestos-containing transite pipe HVAC ducts were also used in exposed areas such as shown in the crawl space photograph at the top of this page. And transite pipes were also used as flues or chimneys for some heating equipment, usually where gas fired heaters were installed.

Article Contents

Transite air ducts in slabs often collect water, mold, pathogens. We provide photographs and descriptive text of asbestos insulation and other asbestos-containing products to permit identification of definite, probable, or possible asbestos materials in buildings. The photo (above-left) of a sewer line routed immediately below a transite asbestos in-slab floor heating duct was provided courtesy of reader Conrad.

This case is illustrated in more detail

While an expert lab test using polarized light microscopy may be needed to identify the specific type of asbestos fiber, or to identify the presence of asbestos in air or dust or water samples, many asbestos-containing building products not only are obvious and easy to recognize, but since there were not other look-alike products that were not asbestos, a visual identification of this material can be virtually a certainty in many cases.


Photo of tranite cement-asbestos material used for air ducts and for heating vents (C) Daniel FriedmanReaders should also see:

Also see Micro-Photographs of Dust from the World Trade Center collapse following the 9/11/01 attack. Links to U.S. government and other authoritative research and advice are included. [Page top photo of transite duct material courtesy of Thomas Hauswirth, a Connecticut home inspector.

Transite HVAC or Air Duct Asbestos Warnings & Hazard Details

Transite ducts used for HVAC air flow, especially when used buried in building concrete floors or slabs, may break, collapse, leak water in (forming a mold and bacterial reservoir in the HVAC system, or may release asbestos and other particles in building air when the HVAC system is operating.

An up-flow or down flow furnace in a building with concrete slab and with perimeter duct work raises some important health and cost questions:

The photograph above shows a transite cement asbestos heating duct in a carpeted floor slab. We recommend that in-slab heating or air conditioning ducts made of transite be sealed and abandoned, and alternate heat sources installed. This improvement removes an asbestos hazard, a flooded duct and mold hazard, and in some locales, also a radon gas entry point.

The photograph shows the edges as well as surface of the transite material. Transite pipe HVAC ducts get quite dirty and are not always easy to identify. [Photo and comments on transite in-slab HVAC ducts courtesy of Roger Hankey, a Minnesota home inspector.]

  • Asbestos hazards of transite ducts: Cementious duct material may contain asbestos. What is this "cement" duct work made of Cement and asbestos fibers.

    How much asbestos is in Transite pipe? While it's cementious, transite ducts or even transite pipe used for heating flue vents is a potential asbestos hazard in buildings. Transite pipe typically contains about 15% to 25% asbestos fibers, typically fibrous chrysotile asbestos.

    A careful asbestos testing lab may report both fibrous and fragmented asbestos which can occur in still smaller pieces (thus more easily remaining airborne and increasing human exposure to asbestos).

    The balance will be cement and possibly other fibers or binders. If transite pipe is damaged or is cut mechanically (such as by using power equipment), friable, airborne asbestos fibers may be generated - a health and costly cleanup concern.
Loose blower assembly pulley or belt reduces airflow Carson Dunlop Associates
  • HVAC air ducts located inside concrete slab floors invite flooding, mold, insects, and where transite - cement asbestos - ductwork was used, asbestos particle contamination or collapsed ductwork.

    Sketch at left courtesy Carson Dunlop Associates.

SLAB DUCTWORK - catalogs the functional and environmental problems found when HVAC air ducts are routed in or below floor slabs

  • Air quality hazards of in-slab duct systems: include water leaks into the duct system which can in turn generate a mold or bacterial hazard or can cause softening, collapse, and blockage of the in-slab transite pipe duct.

    While there are companies offering duct cleaning and duct sealing services, we remain cautious that such a "sealing" project creates a false sense of confidence that no remaining duct issues exist, causing the occupants to miss the discovery of future leaks and in-duct problems.

  • Radon entry through in-slab duct systems: can occur in areas where radon is present at problem levels in the soils. In particular, because a return air duct is often at negative pressure (when the blower is operating), the movement of radon gas from the soil into the building air through a leaky in-slab duct can be significant, certainly greater than the movement into the building from other openings such as through a basement slab crack.
Photo of tranite cement-asbestos material used for air ducts in a slab over a sewer pipe (C) Daniel Friedman and Conrad
  • Sewer Gas entry into the HVAC duct system through in-slab transite asbestos ductwork: as reader Conrad discovered while tracking down the odor of sewer gas that was appearing in a home's in-slab ductwork, placing in-slab ductsclose to sewer piping (or septic system piping ) invites any future sewer gases leaking out of the piping right into the HVAC duct system.

    The owner tracked the sewer gas odor that was coming out of the building's heating ducts to a break in the sewer piping located in the same floor.

    The repair of the sewer gas odor involved jack-hammering out the floor slab, removing and replacing the leaky cast iron sewer pipe, and repairing the floor.

    Because of the inconvenience and cost of relocating these air ducts or converting to an alternate heating distribution method, the owner elected to retain the transite in-slab floor ducts.

    This case of cast iron drain leaking sewer gas into a transite asbestos heating air duct is illustrated in more detail at CAST IRON DRAIN PIPING.


    Additional measures useful in tracking down the source of odors coming from duct work can be found
  • Asbestos fiber release hazards during removal of demolition of transite piping are discussed

Incorrect spellings of transite asbestos piping or transite duct material that we've seen include transit pipe, transit ducts, transite chimneys, transide pipe, transide ducts, and transight pipe or transight ducts. "Transite" is the correct spelling.

Recommendations where Transite Asbestos HVAC Ducts are Installed

Apply an internal transite HVAC air duct sealant

There are also duct-sealants that some contractors offer as an in-duct sealant/spray. The contractor extends a spray wand into the HVAC ducts to deliver a coating that, if perfectly successful, can prevent or at least reduce the risk of asbestos fiber release into the building air. And Andrew Oberta has described standards methods for repairing asbestos-cement products including underground transite piping.[5]

Water and rodents in air duct (C) D FriedmanA down-side with in-slab ductwork is the difficulty in accessing for application of the spray and difficulty in inspection in the future to see what's going on inside the duct: collapsing walls, sealant falling off of duct interior, flooding, mold, asbestos-releasing scraps, rodents, etc.

Our in-slab air duct photo (left) shows evidence of a history of floods in the duct system as well as rodents (the rodent poison).

A second concern is that even if the coated transite air duct interior surfaces appear to have been treated successfully, especially with in-slab ducts we are not assured that the in-slab ducts remain clean, dry, and undamaged in the future nor that the transite duct interior coating remains bonded to the duct surfaces.

But given the history of concerns with the product, in particular with in-slab ducts, we would give strong consideration to abandoning in-slab ductwork entirely.

Abandon in-slab and other transite asbestos HVAC air ducts

In-slab air duct abandoned & sealed (C) Daniel Friedman

We recommend abandoning in-slab HVAC air ducts, including transite asbestos cement HVAC ducts, reasoning that there are multiple indoor air quality and potential health as well as functional concerns with such installations.

We described concerns with ductwork run in floor slabs in the article above, including risks of air duct collapse that interferes with air flow through the system, water leaks into the in-slab duct system (not a problem unique to transite ducts), and rodent or insect infestations or even mold contamination.

Odor complaints may be traced to the duct system because of these problems (DUCT & AIR HANDLER ODORS).

At SLAB DUCTWORK we give details about abandoning in-slab HVAC ductwork. That article also catalogs the functional and environmental problems found when HVAC air ducts are routed in or below floor slabs.

When abandoned, in-slab ductwork of any material can usually be left in place. However where radon gas is an issue, we seal the ducts at the air delivery registers in the building floor slab, as well as sealing any slab cracks that may allow radon gas to enter the building at increased levels.


Continue reading at TRANSITE WATER PIPES or select a topic from the More Reading links shown below.


Also see ASBESTOS DUCTS, HVAC where we describe other sources of asbestos in HVAC duct systems.

Readers concerned with ice or water leaks into or out of HVAC ductwork should also see WET CORRODED DUCT WORK

Suggested citation for this web page

TRANSITE PIPE AIR DUCTS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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