What Is The Difference Between Foam Core And Solid Core PVC?

Upgraded Home Team
by Upgraded Home Team

Foam core (or “cellular core”) polyvinyl chloride (PVC) has small air pockets embedded in the composite, whereas solid core PVC is solid plastic. If you run your thumbnail over the end of foam core PVC pipe, you can actually etch it. You can’t do this with solid core.

Whereas foam core PVC can only be used for drain, waste, and venting (DWV) systems, plumbers can use solid core PVC for DWV as well as:

  • Pressurized potable water distribution up to a building.
  • Irrigation and sprinkler systems.
  • Gas appliance direct air intake and exhaust connections.
  • Condensation waste disposal for heating and air conditioning systems.

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Why Foam Core PVC Is Limited to DWV

Foam Core PVC doesn’t have the strength to withstand the high pressures water distribution systems typically contain. Charlotte Pipe, a leading pipe manufacturer, states that “the use of cellular core pipe in pressure applications may result in system failure and property damage.” This notice applies to the type of fittings plumbers use as well.

PVC Fittings: Pressure Fittings vs DWV

Think of a “90” fitting, which redirects the flow of its contents at 90 degrees (as the name implies).

Manufacturers design DWV 90s to discourage the accumulation of solid waste. They have a longer bend to help greywater (from laundries, sinks, and showers) and black water (from toilets) flow through the DWV system to a septic tank or sewer. Since they don’t handle pressure, companies don’t design DWV fittings to be particularly strong – they don’t have to be.

In contrast, a pressure 90 is made of a high-tensile, high-impact PVC mold. They may handle pressures as high as 80 pounds per square inch (PSI) in most residential applications. They don’t need the longer “sweeps” characteristic of DWV fittings because they handle constant pressure – water’s being forced through them.

The same principle applies to foam core and solid core PVC. The latter is simply a stronger material.

Can Foam Core PVC Be Used For Underground Piping?

Unless otherwise specified by your local jurisdiction, yes: you can use foam core for an underground DWV system. However, when doing so, there are some best practices one must follow:

  • Dig a trench three times wider than the diameter of the pipe to ensure surrounding debris – gravel, clay, cobblestone, etc. – won’t cause harm.
  • Lay the pipe in a bed of 8 inches of sand – four inches below it and four inches on top. This ensures the pipe will be well protected in the future. The sand will act as a cushion for any natural pressures the surrounding area may impose on the buried pipe.

Apply this method when burying solid core PVC as well. It may be a stronger material, but that doesn’t mean you should willfully subject it to harsh conditions.

Solid Core’s Applications In Gas Appliance Venting

Depending on the jurisdiction, technicians may use solid core PVC to vent exhaust gases from Category IV gas appliances. According to the International Fuel Gas Code, a category IV appliance is one that operates with a positive vent static pressure, and possesses a vent gas temperature capable of producing condensation.

Without getting into the weeds, a category IV appliance is the most efficient gas-burning appliance available. Wall-hung tankless water heaters, condensing boilers, and Carrier’s Infinity furnaces are examples of Category IV appliances. Jason Obrzut, Director of Industry Standards and Relations at ESCO Institute, noted category IV units often achieve efficiencies 95 percent or greater.

What does this have to do with solid core PVC? It can withstand the heat and the condensation accumulating in the exhaust vents. Foam core would deteriorate quickly under these conditions.

As mentioned, there are some jurisdictions that do not permit solid core PVC as a means to vent category IV appliances. Massachusetts, for example, instituted a new ruling in April 2021 stating that gasfitters may only use CPVC, polypropylene, and “other pipes which have been Product-accepted” by the Board of State Examiners of Plumbers and Gas Fitters.

PVC Manufacturing Process

According to the Plastic Pipe and Fittings Association, PVC is made from natural gas and rock salt. How do manufacturers take these two materials to produce PVC?

  • Natural gas is heated under pressure in a process called “cracking,” which turns the gas into ethylene.
  • Electrolysis splits the rock salt to produce chlorine and lye (sodium hydroxide).
  • Manufacturers combine chlorine and ethylene to form vinyl chloride monomer (VCM).
  • Once the VCM molecules form, manufacturers join molecules together in an end-to-end (i.e. “polymerized”) fashion, creating long chains of PVC.
  • The final step involves compounding, melting, and extruding the PVC powder into lengths of pipe.  

The difference between foam core and solid core PVC lies in the extrusion process.

How The PVC Pipe Extrusion Process Works

The pipe manufacturing process is facilitated by an extruder machine, which melts raw plastic pellets and forms the material into a continuous length. An extruder consists of a hopper, barrel, gear box, screw, heaters, cooling fans, drive motor, and die.

Here’s how an extruder machine makes a length of PVC:

  • The hopper, which is filled with plastic pellets, granules, or powder, sits on top of the barrel.
  • Heaters surround the first section of the barrel, which is a long cylinder in the shape of a pipe.
  • A thick screw resembling an auger rotates inside of the barrel as the pellets melt.
  • As the plastic cools, the auger forces the plastic through a die, which forms the precursor of the finished product (in this case, a length of PVC pipe).

The aforementioned process describes how manufacturers produce solid core PVC. Charlotte, NIBCO, and other companies make foam core PVC through a process known as co-extrusion.

Co-extrusion involves extruding two or more plastics through a single die. This allows the plastics to weld together into a single piece of pipe before it cools down. Thus, you can bond solid PVC molds with PVC casts containing air-bubbles, producing foam-core PVC.

Schedule 40 Vs Schedule 80 PVC

The amount of plastic in a cast of pipe isn’t the only difference between foam core and solid core PVC. Each type of pipe has a specific wall thickness, or “schedule.” There are two types of schedules: schedule 40 and schedule 80.

The chief difference between schedule 80 and schedule 40 PVC is that the former has thicker walls than the latter. Schedule 80 is typically dyed a dark grey, whereas schedule 40 is colored white.

As solid core is designed for higher pressures and gas appliance venting, manufacturers fabricate it in both schedule 40 and schedule 80. Economically, it doesn’t make much sense for them to make schedule 80 foam core, given that it’s only used for DWV systems.

Many plumbers are actually starting to use schedule 80 pipe when venting category IV appliances. Personally, I’d install schedule 80 instead of schedule 40 in this application. I believe the larger wall thickness and higher strength will withstand the corrosive condensation much better over time.

My trust in the material isn’t based on an assumption of what it can do, either. Schedule 80 is approved for use in temperatures up to 140 degrees Fahrenheit, according to PVC Pipe Supplies. It is not only designed to handle high-pressure water systems, but also corrosive chemicals.

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Related Questions

How Come I Have Metal Pipe Instead of PVC In My Home?

If you live in an older house, the pipe you’re describing is likely hub-and-spigot cast iron. As far back as the 1960s, plumbers were using this material for DWV installations.Nowadays, many plumbers use compression joints to fit hub-and-spigot systems together, but the one in your home probably looks a little different. More than likely, it’s a lead-and-oakum joint. A lead-and-oakum joint consists of hemp rope treated with tar with molten lead poured over it. Pouring these joints is considered a dying art within the trade.If the cast iron in your home is leaking, isolate the source of the leak and call a plumber to fix it. They should recommend replacing it with another length of cast iron. Never install PVC below cast iron – it can’t handle the weight. That doesn’t mean you can’t use PVC with cast-iron systems at all. It’s perfectly fine to replace the vents in your attic with PVC. The cast iron drainage system should have no problem supporting it. There are several ways to adapt between the two materials but I prefer using a Fernco clamp.  If you’re planning a major remodel (adding a bathroom, gutting the kitchen, etc.) I’d recommend replacing as much of the cast iron as possible. While it’s stood the test of time for now, it’s only a matter of time before it starts to expand with rust and clog up your system. You won’t get this problem with PVC.

Can I Use Foam Core PVC On Drainage Systems?

Yes, you can. It’s a common material among professionals installing perimeter drains, roof leaders, and septic systems. As noted earlier, ensure the PVC is well protected if you’re installing it underground.

Upgraded Home Team
Upgraded Home Team

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