The difference between vapor intrusion and vapor encroachment is where the vapors are. Vapor encroachment means chemical vapors are present, or likely present, in the soil beneath or near a property. Vapor intrusion means those vapors have actually entered the air inside a building. One is a condition of the ground. The other is a condition of the building.
That distinction is not academic. It decides which standard applies, who does the work, what the finding obligates you to do, and whether anyone is currently being exposed. Getting it wrong in either direction is expensive.
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What a vapor encroachment condition actually means
ASTM E2600 defines a vapor encroachment condition, almost always shortened to VEC, as the presence or likely presence of chemical of concern vapors in the vadose zone of the target property, caused by a release of vapors from contaminated soil or groundwater either on or near that property.
Two words in that definition carry most of the weight. Likely means a VEC can be identified without anyone measuring a vapor, on the strength of records alone. Near means the release does not have to be on the property being screened.
A screen produces one of four outcomes:
- A VEC exists
- A VEC likely exists
- A VEC cannot be ruled out
- A VEC can be ruled out
Notice that three of the four are not clean. Much of the professional judgment in this area sits in the difference between cannot be ruled out and likely exists, and that judgment is what a good environmental professional is actually being paid for.
| Vapor encroachment | Vapor intrusion | |
| What it describes | Vapors present or likely present in the vadose zone on or near the property | Vapors that have entered indoor air in a building |
| Where it is assessed | The property, including undeveloped land | A specific building |
| Governing document | ASTM E2600-22, a voluntary guide | EPA vapor intrusion guidance and, in Wisconsin, ch. NR 700 with RR-800 |
| Context | Real estate transaction and due diligence | Environmental investigation and public health |
| Purpose | Managing transactional and liability risk | Determining and stopping exposure |
| Typical output | A VEC is identified, or it is ruled out | Measured concentrations compared against screening levels |
| Who performs it | Environmental professional as part of, or alongside, a Phase I ESA | Environmental consultant sampling under a regulatory framework |
| Can it exist with no building? | Yes, undeveloped land can have a VEC | No, intrusion requires a building to intrude into |
How ASTM E2600 screening works
The guide uses two tiers, and most screens never leave the first one.
Tier 1: records and distance
Tier 1 is non-invasive. The environmental professional identifies known or suspected contaminated sites within a search radius of the property, using the same database and historical records work that supports a Phase I.
The radius differs by contaminant type, roughly a third of a mile for non-petroleum releases such as chlorinated solvents, and roughly a tenth of a mile for petroleum releases. That gap exists because petroleum vapors biodegrade in soil and chlorinated solvents largely do not, so fuel releases have to be much closer to matter.
Tier 2: narrowing it down
If Tier 1 cannot rule out a VEC, Tier 2 gathers more detail. It does not start with drilling. It starts with regulatory files, since state agencies often hold plume delineation reports that show exactly where the contamination sits and which direction groundwater moves. That alone frequently rules a VEC out.
Where the files do not settle it, Tier 2 can extend to actual sampling of soil gas or groundwater. That is the point at which a vapor encroachment screen starts to resemble an environmental investigation, and the cost profile changes accordingly.
The reason those search distances are set where they are, and why building size and contaminant type change the analysis, is covered in our article on what causes vapor intrusion.
Is vapor encroachment screening required in a Phase I ESA?
No, and this is worth stating plainly because a fair amount of published material online gets it wrong.
ASTM E2600 says so in its own scope. The guide is intended for voluntary use. It may be used alongside E1527 but does not alter or define the scope of that practice, and performing it is explicitly not a requirement of all appropriate inquiries and does not expand or define AAI as approved by EPA under CERCLA.
What is required is more subtle, and it trips people up. Under the current Phase I standard, vapor is treated as a migration pathway like soil and groundwater, which means a vapor condition can constitute a recognized environmental condition. The environmental professional has to consider the vapor pathway. They are simply not obliged to use E2600 to do it.
The practical consequence: if a consultant does not follow E2600, the methodology they did use to evaluate vapor should be documented in the report. A Phase I that never mentions the vapor pathway at all is the one worth questioning.
Standards currency check: ASTM E1527-21 has been the governing Phase I standard for AAI since February 2023, and the previous edition was fully retired in February 2024. ASTM E2600-22 is the current vapor encroachment guide. If a report in front of you cites an older edition of either, that is a reasonable thing to ask about.
Why a VEC is not a finding of vapor intrusion
A vapor encroachment condition says vapors are, or are probably, in the ground near the property. It says nothing about whether they are getting into a building, and often there is no building involved at all.
Several things have to be true before encroachment becomes intrusion:
- A building has to exist over or near the affected area
- A pathway has to connect the soil gas to the building interior, through the slab, foundation, or a utility route
- The building has to be pulling rather than pushing, which depends on pressure, HVAC operation, and season
- Concentrations have to arrive at levels that exceed health-based screening values
Any one of those failing breaks the chain. A property can carry a VEC and have completely clean indoor air, which is a common and perfectly legitimate outcome. The reverse is rarer but possible, since a screen based on records can miss a source that was never reported.
This is also why a VEC identified on vacant land still matters. There is no intrusion today because there is no building. Put one up without controls and there may be.
That is the same argument behind designing soil gas controls into new construction rather than retrofitting later. Our work on radon-resistant new construction in Wisconsin covers the design principles, which carry over almost directly to chemical vapor.
What happens after a VEC is identified
A VEC is a trigger for a decision, not an automatic obligation to do anything specific. Where it goes next depends on the transaction and on whether a regulatory file already exists.
- Do nothing further, and price or allocate the risk in the deal. Common where the VEC is marginal and the property is undeveloped.
- Order a Phase II investigation. Soil gas, sub-slab, or groundwater sampling to find out whether there is a real vapor problem.
- Sample the building directly. Where a structure is occupied, sub-slab and indoor air sampling answers the exposure question rather than the property question.
- Design controls into a planned build. Where redevelopment is the plan, vapor controls go into the slab design rather than a retrofit budget.
- Install mitigation preemptively. Sometimes chosen where sampling cost, deal timing, and future liability make a system the cheaper certainty.
How vapor intrusion testing works covers what the sampling in options two and three actually involves.
Which framework applies in Wisconsin?
Both can apply to the same property at the same time, for different reasons, and they do not talk to each other automatically.
The transaction framework is ASTM. It is voluntary, driven by the deal, and its purpose is liability protection for the buyer or lender. Nobody at a state agency is waiting for the result.
The regulatory framework is ch. NR 700 with the DNR’s RR-800 vapor guidance. It is mandatory, driven by a known release, and its purpose is stopping exposure. Wisconsin requires screening for vapor intrusion at every contaminated site in the state, which the DNR states directly on its vapor intrusion resources page.
Where a property sits inside an open regulatory file, the regulatory obligation runs regardless of what any Phase I concluded. Where it does not, a VEC in a Phase I creates no reporting duty by itself. Understanding which situation you are in is usually the first useful question after reading the report.
Anything touching disclosure, allocation between parties, or who is liable for a release is a question for an environmental attorney rather than a contractor or a consultant. This article describes how the technical frameworks work, not what any specific deal requires.
FAQ
Can a VEC be ruled out without any sampling?
Often yes. Most screens resolve on records alone. If the nearest release sits outside the applicable search distance, or if regulatory files show a delineated plume moving away from the property, the environmental professional can rule out a VEC without anyone entering the field. Sampling becomes necessary when the records are incomplete or the plume boundary is genuinely uncertain.
Will a VEC appear on the environmental database report I ordered?
No. A database report lists nearby sites, distances, and regulatory status. It is an input to the screen, not the screen itself. Turning that list into a VEC determination requires an environmental professional to weigh contaminant type, distance, groundwater direction, and geology. Buying the database report and reading it yourself gives you the raw material without the conclusion.
Does a vapor encroachment condition have to be disclosed to a buyer or lender?
That depends on the contract, the jurisdiction, and the parties, and it is a legal question rather than a technical one. As a practical matter, lenders on commercial transactions routinely ask, and a VEC identified in a Phase I is normally visible to anyone who receives the report. Get advice from an environmental attorney before making a disclosure decision in either direction.
Can a building have vapor intrusion even if no VEC was identified?
Yes, though it is uncommon. A screen built on records can only find releases that were reported. An unreported spill, an undocumented former use, or a preferential pathway such as a sewer line carrying vapor from further away can all produce indoor air impacts that no records-based screen would have predicted. A ruled-out VEC lowers the probability. It does not prove absence.
Does a VEC affect financing or insurance?
It can. Commercial lenders vary in how they treat vapor findings, and some require resolution or mitigation before closing while others price the risk instead. Environmental insurance underwriting also looks at vapor conditions. Neither outcome is automatic, and both are negotiated rather than fixed, which is one reason the timing of a screen within a deal matters as much as the result.
If the existing building is being demolished, does the VEC still matter?
More than people expect, and arguably the timing gets better rather than worse. Demolition removes the current receptor but not the source, and whatever replaces the building will sit on the same ground. Designing vapor controls into a new slab costs a fraction of retrofitting an occupied structure, so a VEC identified before demolition is a scheduling advantage if it is acted on.
Conclusion
At some point the paperwork ends and someone has to decide whether a building needs a system. That decision rests on measured concentrations compared to Wisconsin screening values, not on a VEC determination.
If you have reached that point, what matters is a system designed for the building’s actual footprint and the actual contaminant, installed to the ANSI/AARST soil gas standards, with the verification documentation a DNR project manager will expect to see.
The vapor intrusion pillar guide covers Wisconsin requirements, testing, and system design in full.
Milwaukee Radon Mitigation works with environmental consultants, property owners, and developers across Milwaukee, Waukesha, and Ozaukee counties on vapor mitigation design and installation, including systems built into new construction. See our vapor intrusion mitigation services, get in touch, or call (414) 455-7279.
