When buildings close for winter, facility managers worry about frozen pipes. That concern is valid. But the drain risks that come with winter closures go beyond freezing. In heated buildings, warm dry air from HVAC systems accelerates P-trap evaporation. In unheated buildings, the water in P-traps can freeze solid, cracking pipes and destroying seals. Both scenarios leave the building unprotected from sewer gas, pests, and pathogens for weeks or months.
This article covers the dual drain risks of winter closures, explains why common solutions like antifreeze have compliance and environmental drawbacks, and provides a pre-winter checklist for facility managers who need their buildings to reopen without odor, damage, or pest infestations.
The dual risk: freezing and evaporation
Most people associate winter drain problems with freezing. That is only half the story. Winter closures create two distinct failure modes for P-trap water seals, and which one affects your building depends on how the building is conditioned during the closure.
Unheated buildings: freeze risk
When a building loses heat entirely, the water in P-traps is exposed to the same temperatures as the rest of the structure. Water freezes at 32 degrees F (0 degrees C). In an unheated basement or ground floor in a northern climate, sustained sub-freezing temperatures are routine from December through March.
When P-trap water freezes:
- Pipe damage: Water expands approximately 9% when it freezes. This expansion can crack cast iron traps, split PVC fittings, and rupture copper connections. The damage often goes undetected until the thaw, when the cracked trap begins leaking sewage.
- Seal failure: Even if the pipe does not crack, a frozen trap is not a functioning seal. Ice does block gas temporarily, but as the building warms and the ice melts, the water level may be insufficient to maintain the seal, particularly if the pipe has deformed.
- Cascading damage: A cracked P-trap that leaks during thaw can flood surrounding areas, damage flooring, and create mold conditions, all of which compound the cost and delay of reopening.
Heated buildings: accelerated evaporation
Many facility managers keep buildings heated to a setback temperature (typically 55-60 degrees F) during winter closures to prevent pipe freezing. This protects supply pipes and P-traps from ice damage. But it introduces a different problem: the warm, dry air produced by heating systems dramatically accelerates evaporation from P-traps.
Cold outdoor air holds very little moisture. When that air enters a heated building (through ventilation, infiltration, or intentional air exchange), it warms up and its relative humidity drops. The result is indoor air that can be as dry as 15-20% relative humidity, drier than many desert climates. This dry air pulls moisture from every available source, including the water sitting in P-traps.
Under these conditions, a P-trap that would normally take 2-3 weeks to dry out in temperate weather can lose its seal in under a week. The building is protected from frozen pipes, but every floor drain is vulnerable to sewer gas and pest infiltration for the duration of the closure.
The paradox: Heating a building to prevent frozen pipes accelerates the evaporation that causes dry traps. Both conditions, heated and unheated, create drain seal failures during winter closures. The only difference is the failure mode.
Antifreeze in traps: the conventional approach and its problems
The most common recommendation for winterizing floor drains is to pour antifreeze into the P-trap. The antifreeze lowers the freezing point of the liquid in the trap and, depending on the product, may also slow evaporation. But this approach has significant limitations.
Toxicity concerns
Automotive antifreeze (ethylene glycol) is toxic to humans, animals, and aquatic organisms. It should never be poured into building drains. When the building reopens and drains are flushed, the antifreeze enters the municipal wastewater system, where it can interfere with biological treatment processes.
RV-grade antifreeze (propylene glycol) is the non-toxic alternative and is the only type appropriate for plumbing applications. However, "non-toxic" does not mean "no environmental impact." Propylene glycol has a high biological oxygen demand (BOD), meaning it consumes dissolved oxygen in waterways. In jurisdictions with strict discharge limits, even non-toxic antifreeze in drain traps may raise compliance questions.
Evaporation still occurs
Propylene glycol solutions evaporate more slowly than water, but they still evaporate. In a building with very low humidity and active air circulation, antifreeze in a P-trap can lose volume over several weeks. A closure that extends from December through March, common for seasonal facilities, may outlast the antifreeze.
Annual reapplication
Antifreeze must be applied to every floor drain before each winter closure and flushed from every drain at reopening. In a building with dozens or hundreds of floor drains across multiple floors, this represents significant labor. It must also be documented, particularly in regulated environments like schools, healthcare facilities, and food processing plants.
It does not address the underlying problem
Antifreeze treats the symptom (the water in the trap is vulnerable to temperature) without addressing the root cause (the trap seal relies on a liquid that can freeze or evaporate). Every winter, the same drains require the same treatment. The labor, materials, and compliance burden repeat indefinitely.
Pre-winter checklist for facility managers
Whether your building will be unheated, kept at a setback temperature, or fully conditioned, the following checklist addresses drain protection for winter closures. This complements your existing winterization procedures for supply pipes, fire suppression systems, and HVAC equipment.
1. Inventory all floor drains
Create or update a drain map for the building. Include every floor drain in every space: restrooms, mechanical rooms, kitchens, storage areas, stairwells, elevator pits, and loading docks. Note which drains are in heated spaces versus unheated spaces. This inventory is the foundation for everything that follows.
2. Assess current trap seal status
Before the building closes, check each drain. Is there water visible in the trap? Does the drain produce any odor? Has the drain been flowing regularly, or has it been unused for weeks already? A drain that enters winter with a partial seal will fail faster than one with a full trap.
3. Install mechanical trap seals
The single most effective pre-winter intervention is installing waterless trap seals on all floor drains. A mechanical seal cannot freeze (there is no water to expand) and cannot evaporate (the barrier is silicone, not liquid). It provides continuous protection for the entire closure regardless of temperature, humidity, or duration. Installation takes 30 seconds per drain with no tools.
4. Address unheated spaces
For drains in spaces that will drop below freezing and do not have mechanical trap seals:
- Add non-toxic propylene glycol (RV-grade) antifreeze to the P-trap
- Insulate exposed drain pipes where accessible
- Consider temporary heat tape on vulnerable pipe sections
- Document each drain treated with antifreeze for spring flushing
5. Address heated spaces
For drains in spaces that will be heated to a setback temperature and do not have mechanical trap seals:
- Refill all P-traps with water immediately before closure
- Add a thin layer of mineral oil or non-toxic antifreeze to slow evaporation
- Schedule a mid-winter check (if staff access is available) to refill traps that may have dried out
- Understand that in very dry conditions, traps may dry out before the next scheduled check
6. Document and verify at reopening
When the building reopens:
- Inspect all floor drains for damage (especially in unheated areas)
- Check for signs of sewer gas exposure (odor, staining near drains)
- Check for pest evidence (drain fly casings, insect activity near drains)
- Flush all drains with fresh water to restore P-traps and clear any antifreeze
- Verify that mechanical trap seals are seated and functioning
Heated vs. unheated building comparison
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Why mechanical seals eliminate the winter variable
The core problem with winter drain protection is that every traditional approach (water, antifreeze, mineral oil, scheduled flushing) depends on a liquid remaining in the trap for the duration of the closure. Liquids freeze. Liquids evaporate. Liquids need to be replenished.
A waterless trap seal removes liquid from the equation entirely. Green Drain's silicone one-way valve operates at temperatures from -60 degrees F to 400 degrees F. It does not freeze, does not evaporate, and does not degrade over the course of a winter closure. It blocks sewer gas and pests on day 1 of the closure and day 90 with equal effectiveness.
For facility managers who oversee seasonal buildings, summer closures and winter closures present the same fundamental challenge. The building is unoccupied, drains are not flowing, and the trap seal is on its own. A mechanical solution works in both seasons because it does not depend on environmental conditions.
For buildings with existing drain maintenance programs, adding waterless trap seals before the first winter closure reduces the winterization workload permanently. No annual antifreeze application, no mid-winter trap checks, no spring flushing. The drain is sealed mechanically, and the facility team can focus winterization efforts on the supply side of the plumbing system.
View all available sizes or request a quote for your facility's winter preparation.
Frequently asked questions
How do I protect drains during winter closure?
Protecting drains during winter closure requires addressing both freezing and evaporation. For unheated buildings, add RV-grade non-toxic antifreeze to P-traps to prevent ice damage. For heated buildings, understand that warm dry air accelerates evaporation, so traps can dry out faster in winter than summer. The most reliable solution is installing waterless trap seals before shutdown, which create a mechanical barrier that cannot freeze, evaporate, or fail during extended vacancy.
Can P-traps freeze?
Yes. The water in a P-trap will freeze when the surrounding temperature drops below 32 degrees F for a sustained period. Frozen P-traps can crack cast iron and PVC pipe, damage fittings, and break the seal. When the ice thaws, the cracked trap may leak sewage into the building. Floor drains in unheated basements, crawl spaces, and buildings that lose heat during winter are most at risk.
Should I put antifreeze in floor drains?
If your building will be unheated during winter, adding non-toxic propylene glycol antifreeze (RV-grade) to floor drain P-traps can prevent freezing. Never use automotive antifreeze (ethylene glycol), which is toxic and prohibited in plumbing systems. However, antifreeze does not prevent evaporation over long closures and raises environmental compliance concerns in some jurisdictions. Waterless trap seals eliminate the need for antifreeze entirely.
How do I winterize building drains?
Inventory all floor drains and note which are in heated versus unheated spaces. Install waterless trap seals on all floor drains to prevent evaporation and provide a mechanical barrier. For unheated areas without mechanical seals, add non-toxic RV antifreeze to each P-trap. Insulate exposed drain pipes in unheated crawl spaces and basements. Set building heat to at least 55 degrees F if possible. Document all drain locations and treatments for spring reopening.