Why Do Some Homes Test Low Once and Higher Another Time?

If you’ve ever looked at a radon report and thought, “How can it be low one time and higher the next?” you’re not alone. This is one of the most common (and genuinely interesting) homeowner questions we hear across Pennsylvania—especially in Montgomery County and Bucks County communities like Harleysville, Lansdale, Hatfield, Souderton, North Wales, and Blue Bell.

Radon is invisible. You can’t smell it, see it, or “sense” it the way you might notice a drafty room or a musty basement. That’s why it can feel confusing when numbers change. But fluctuating results are often a normal outcome of how radon enters a home and how a home “breathes” day to day.

This guide explains why radon levels change, what influences test results, and how to approach testing with confidence—without scare tactics. If you’re a homeowner, buyer, seller, or real estate professional, the goal is simple: replace guessing with clear information you can act on.

Quick refresher: what radon is (and why it varies)

Radon is a naturally occurring gas that comes from the breakdown of uranium in soil and rock. It can move through the ground and enter buildings through pathways such as:

• Cracks in basement slabs or foundation walls
• Construction joints and sump pits
• Gaps around utility penetrations and floor drains
• Crawlspaces and porous materials

Once inside, radon levels can rise or fall based on how easily the gas is drawn in and how quickly indoor air is exchanged with outdoor air. In other words: radon is not “set and forget.” It responds to pressure changes, weather, and how the home is being used.

Why radon levels change: the most common causes

When one test reads low and another reads higher, it’s typically because one or more of the factors below changed between test periods. In Pennsylvania homes, it’s often a combination.

1) Weather and barometric pressure shifts

Weather can strongly influence indoor radon. A low-pressure system (often associated with storms) can increase the movement of soil gases into a home. Higher barometric pressure can sometimes reduce that draw.

In Montgomery County and Bucks County, where weather patterns can swing quickly, it’s not unusual to see different readings from one week to the next—especially in basement-level tests.

2) The “stack effect” and seasonal changes

During cooler months, heated air rises and exits through upper levels of a home. That upward movement can create a slight suction at lower levels, drawing in soil gases—including radon—through foundation openings. This is part of what’s known as the stack effect.

As a result, radon levels may be higher in winter than in mild seasons when windows are cracked more often and the HVAC system cycles differently. Summer can bring its own patterns, especially if air conditioning changes how much fresh air enters the home.

3) Natural ventilation: windows, doors, and lifestyle patterns

Radon readings are influenced by how “open” or “closed” a home is during a test. A period when windows were frequently open, doors were used often, or exhaust fans ran heavily may create more air exchange and temporarily lower readings.

Later, if the home is more closed up—typical during very hot, very cold, or rainy stretches—radon can build more easily. This is a major reason two tests can disagree even when the home itself hasn’t changed.

4) HVAC operation, air balancing, and exhaust fans

Your HVAC system, bathroom fans, kitchen range hood, and even a clothes dryer can affect indoor pressure. Some devices remove indoor air faster than fresh air replaces it, which can increase the slight vacuum that draws soil gas into the home.

Changes like these can happen without you noticing:

• A new high-powered range hood
• Bathroom fan used more frequently
• Different thermostat settings or runtime
• More laundry loads during a busy week

None of these automatically means “high radon,” but they can help explain why a second test might read differently.

5) Ground moisture and soil conditions

Soil moisture affects how gases move underground. After heavy rains, melting snow, or prolonged wet conditions, the ground can become more sealed, potentially directing more soil gas toward foundation openings. In other scenarios, changing moisture can open or close pathways in the soil.

Because Pennsylvania weather varies, the same house in Hatfield or Blue Bell might test differently after a wet week than after a dry, stable stretch.

6) Foundation changes: settling, cracks, and small openings

Homes shift over time. Tiny cracks or gaps can appear around utility penetrations, slab joints, sump lids, or along a basement wall. You might not notice anything visually significant, but airflow pathways can still change. Even a small change in the “path of least resistance” can affect radon entry.

Similarly, work done in the home can change air movement and pressure relationships. Examples include finishing a basement, adding insulation, sealing drafts, replacing windows, or updating doors.

7) Test placement and test type differences

Two tests aren’t always equivalent. Variations can come from:

• Location: A device placed near a draft, sump pit, or exterior door may read differently than one placed more centrally.
• Level of the home: Radon is usually tested on the lowest livable level. A test on a basement can differ from a first-floor test.
• Duration: Short-term tests capture a snapshot; long-term tests reveal averages across changing conditions.
• Device type: Different approved methods can produce slightly different results, especially across short timeframes.

This doesn’t mean a test was “wrong.” It means the testing method and conditions matter—and it’s worth aligning the approach with your decision (buying, selling, or long-term occupancy).

Short-term vs. long-term radon tests: why the timeline matters

A short-term radon test can be very useful—particularly for real estate timelines. But it’s important to understand what it represents: a specific window of time influenced by weather, ventilation, and pressure conditions.

Long-term tests, on the other hand, provide a more stable picture of typical exposure because they capture multiple seasons and living patterns.

• Short-term testing: Helpful for quick decisions; sensitive to day-to-day changes.
• Long-term testing: Best for understanding average levels over time; reduces “snapshot” variability.

If your goal is curiosity and clarity for your household—especially if you plan to stay in the home—longer-term data can be especially valuable. If your goal is a transaction deadline, a properly conducted short-term test can still provide meaningful information.

To schedule professional testing with a clear plan (not guesswork), visit Radon Testing.

Real-life scenarios we see around Montgomery & Bucks Counties

Here are a few common situations where homeowners in places like Lansdale, North Wales, and Souderton notice changing numbers:

• “We tested during a mild week with windows open.” A later test during a rainy or cold stretch reads higher.
• “The basement was finished last year.” Changes to airflow, doors, insulation, or new HVAC returns alter pressure and ventilation patterns.
• “The sump lid was upgraded or the pit was opened for maintenance.” Even small changes around sump pits can influence air movement.
• “We had a big storm.” Barometric pressure and ground moisture shift, affecting soil gas entry.

These aren’t reasons to panic—they’re reasons to test thoughtfully and interpret results in context.

How to get more consistent, meaningful radon results

If you’re trying to compare tests or reduce the chances of “one low, one high,” focus on consistency and best practices. Here are practical tips homeowners and real estate professionals can use.

Testing best practices

• Test in the right location: Typically the lowest livable level (follow professional guidance for unique layouts).
• Place the device correctly: Away from drafts, exterior doors, windows, and high-humidity areas; avoid direct airflow from vents.
• Keep conditions steady: Avoid major ventilation changes during the test window if possible.
• Document conditions: Note weather, HVAC settings, and unusual events (power outages, gatherings with frequent door use, etc.).
• Repeat with a plan: If a result is surprising, follow up with a consistent method—don’t just “try again whenever.”

Professional testing helps ensure the device type, placement, and interpretation fit the reason you’re testing (curiosity, planning, or real estate). If you’re ready, use Request Service or call A1 House Prep at 215.853.8226.

Radon numbers and decision-making: stay calm and stay data-driven

Because the topic can feel sensitive, it helps to separate the emotional response from the practical next step. A changing radon number is information—not a verdict.

If you see different results across tests, consider:

• Was one short-term and one long-term?
• Were they in the same location and level?
• Did weather or home use differ?
• Is the home seasonally different (heating vs. mild weather)?

For buyers and sellers, a clear testing approach supports smoother conversations. For homeowners, it replaces uncertainty with a repeatable plan.

Related “invisible home facts” that are worth testing (without guessing)

Radon is one invisible variable. There are others that can influence comfort, odor, and confidence in a home—especially in older housing stock and areas with basements, wells, or seasonal humidity swings.

• Air Quality Testing for practical insight into indoor conditions and potential contributors to discomfort or odor
• Water Testing for homeowners on private wells or anyone wanting baseline information
• Water Heater Flushing for routine property preparation and performance support

Odors after a vacancy or renovation? Ozone can help—when used correctly

Sometimes homeowners notice a musty, smoky, or stale odor after a home has been closed up (or after a renovation). If you’re preparing a property for listing or move-in, odor removal can be part of the process—but it should be handled safely and appropriately.

A1 House Prep offers Professional Ozone Machine Rentals intended for odor treatment in unoccupied spaces only. Ozone use requires careful setup and timing.

• Only use ozone machines in spaces that are completely unoccupied (no people, no pets, no plants).
• Follow all provided instructions for run time, sealing/ventilation, and re-entry timing.
• After treatment, ventilate the space thoroughly before re-occupying.

Ozone is not a “cover-up.” It’s a tool that can be used responsibly for odor situations—especially when paired with good property prep and a clear understanding of what you’re treating.

FAQ: Changing radon levels in Pennsylvania homes

1) Is it normal for radon results to change from one test to another?

Yes. Radon can fluctuate based on weather, pressure differences, ventilation, HVAC operation, and seasonal changes. That’s a major reason professionals focus on correct test type, placement, and timing.

2) Why would my home test low in summer but higher in winter?

Winter conditions often increase the stack effect (warm air rising and exiting the home), which can increase suction at lower levels and draw in more soil gas. Homes also tend to be more sealed in winter, reducing fresh air exchange.

3) If I got one low test, do I still need to test again?

It depends on your goal. A single short-term test provides a snapshot. If you want a broader understanding of typical levels over time, a longer-term test or a planned re-test in a different season can be useful—especially if the first test occurred during unusual weather or ventilation patterns.

4) Can opening windows or running fans change radon readings?

Yes. Increased ventilation can lower radon temporarily by diluting indoor air. Exhaust fans and dryers can also affect indoor pressure, which may influence how much soil gas is drawn into the home.

5) Where should a radon test be placed for the most meaningful result?

Typically, on the lowest livable level of the home, in a regularly used area, away from drafts, exterior doors, windows, and direct airflow from vents. Professional testing helps ensure placement aligns with recognized best practices.

6) I’m buying/selling a home in Montgomery or Bucks County. What’s the best approach?

For real estate, a properly conducted short-term test is commonly used due to timeline needs. If results raise questions, a follow-up plan can help clarify whether conditions during the test may have influenced the reading.

7) Do new windows, finishing a basement, or air sealing affect radon?

They can. Changes that tighten the building envelope or alter airflow may change indoor pressure and ventilation patterns. This doesn’t automatically mean radon will rise, but it’s a good reason to test after significant renovations or energy-efficiency upgrades.

Next step: If you’re curious about your home’s “invisible facts” and want a clear, calm plan for radon testing, visit /services/radon-testing/ or request service at /request-service/. You can also reach A1 House Prep at 215.853.8226.