Quick Answer: Dew point is the steel-wall temperature at which the air inside your container can no longer hold its moisture and starts shedding water. For a 20 degrees Celsius, 70 percent relative humidity interior in Ontario, the dew point sits at 14.4 C, so any wall colder than 14.4 C sweats. Use the Magnus formula (Td = 243.04 times alpha divided by (17.625 minus alpha), where alpha = ln(RH/100) + 17.625T/(243.04+T)) and a paired thermo-hygrometer to run the math we plan at every Brantford yard. The LeBlanc family has run Van Blanc Ent. Inc. since 1995. 140+ verified Google reviews at 4.9 stars, 1-3 day delivery.
In This Guide
- Why does dew point math beat gut feel?
- How does the Magnus formula calculate dew point?
- What do real Ontario dew point examples look like?
- How do Ontario climate zones change dew points?
- Where do thermal bridges drop steel below dew point?
- How do you measure temperature and humidity accurately?
- How do you build a dew point spreadsheet?
- How does condensation change by season in Ontario?
- How do you plan insulation and ventilation against the math?
- Why does Van Blanc trust the numbers over feel?
- FAQs
Reading time: about 13 minutes.
Why Does Dew Point Math Beat Gut Feel for Container Condensation?
Dew point math beats gut feel because container condensation is a precise threshold, not a hunch. Calculate the dew point from interior temperature and relative humidity, then compare it to the coldest steel-wall temperature. Whenever a wall drops below that dew point, it sweats. Gut feel cannot predict that crossover; the Magnus equation can.
Most container condensation conversations end at vibes. The buyer says the box is sweating. The supplier says add a vent. Sometimes that works. Sometimes the vent makes it worse because it dumps humid spring air against steel still cold from a 3 a.m. low. Condensation is governed by a precise threshold that moves with every degree and percentage of humidity. At the Brantford yard, Van Blanc runs the full condensation playbook before recommending a single modification. We calculate dew point from interior temperature and humidity, compare that to the steel-wall temperature at the coldest part of the day, and only then talk about vents, insulation, or desiccants.
“People walk into the yard sure their box will not sweat because the contents are cold. Cold air still carries water, and a north wall on a clear night gets colder than the air does. I would rather run the dew point number with someone in five minutes than sell them a vent kit that makes it worse. The number does not care how confident you feel.”
Christian LeBlanc, second-generation operator, Van Blanc Ent. Inc.
The three numbers that decide condensation
- Interior air temperature (T). Measured at mid-height, away from doors. Drives the air’s moisture capacity.
- Interior relative humidity (RH). Measured at the same point as T. Combined with T, this fixes the dew point.
- Coldest steel-wall temperature (Tw). Usually the north-facing wall an hour before dawn. If Tw drops below the calculated dew point Td, that wall sweats.
Everything else, vents, insulation, vapor barriers, desiccants, is just a way of pushing one of those three numbers in a direction that keeps Tw above Td.
How Does the Magnus Formula Calculate Dew Point?
Dew point is calculated from temperature and relative humidity using the Magnus equation (also called the Magnus-Tetens approximation). It is accurate to within roughly 0.4 C across the range Ontario containers see, from minus 25 winter lows to plus 35 summer highs.
The Magnus dew point equation
alpha = ln(RH / 100) + (17.625 times T) / (243.04 + T)
Td = (243.04 times alpha) / (17.625 minus alpha)
Where T is air temperature in Celsius, RH is relative humidity as a percent (use 70 for 70 percent, not 0.70), and Td is dew point in Celsius. The constants 17.625 and 243.04 are the Alduchov and Eskridge values recommended for water vapor over liquid water from minus 40 to plus 50 C.
That looks ugly until you put it in a spreadsheet cell once. The intuition that matters: dew point goes up when humidity goes up, dew point goes up when temperature goes up, and the curve is non-linear. Going from 50 to 70 percent humidity at 20 C moves dew point from 9.3 to 14.4. Going from 70 to 90 percent at 20 C moves it from 14.4 to 18.3. The last 20 points of humidity push dew point harder than the first 20. The Alduchov and Eskridge constants (17.625 and 243.04) are the modern recommended pair.
What Do Real Ontario Dew Point Examples Look Like?
Numbers in tables make this concrete. Each row is a real scenario we have run through the math for a buyer in the last 18 months.
| Scenario | Interior T (C) | Interior RH (%) | Calculated Td (C) | Expected coldest wall T (C) | Will it sweat? |
|---|---|---|---|---|---|
| Brantford garage container, sealed all winter | 5 | 85 | 2.7 | -12 to -2 | Yes, badly, every clear night |
| Norfolk hay storage, late June | 28 | 60 | 19.6 | 15 to 17 | Yes, on a pre-dawn dew morning |
| Burlington bike storage, October | 14 | 72 | 9.2 | 4 to 8 | Yes, on the north wall before sunrise |
| Sudbury insulated bunkhouse, January | 20 | 40 | 6.0 | 14 to 17 | No, insulation holds steel above Td |
| Niagara vineyard storage, August | 26 | 78 | 21.7 | 22 to 24 | Marginal, depends on wind and clear-sky cooling |
The Brantford row is the textbook trap. People assume cold contents in a cold box should not sweat because everything is cold. The math says the opposite. Even at 5 C with 85 percent humidity, dew point is 2.7 C, and a north wall on a clear minus 15 C night drops to roughly minus 10, more than 12 degrees below dew point. Moisture migrates and freezes there, then thaws when the sun hits, then re-freezes the next night. Over a winter the wall accumulates frost that drips into cardboard and fabric every thaw cycle. The Sudbury row shows what proper insulation buys: closed-cell spray foam keeps the steel face within a few degrees of the heated air, and the steel never drops below dew point.
The Niagara marginal case
The vineyard scenario frustrates buyers because the math says marginal and reality says it sweats some days and not others. The reason is sky temperature. A clear August night with light wind drops the roof and upper walls 5 to 8 C below ambient. A cloudy night with the same air temperature does not. Same container, same contents, sweats Tuesday and not Wednesday. Plotting interior conditions against expected wall temperatures gives buyers a probability rather than a yes or no.
How Do Ontario Climate Zones Change Container Dew Points?
Different parts of Ontario produce different dew point profiles, so the same container behaves differently depending on where it sits. We track approximate seasonal ranges by region because the math changes meaningfully from Brantford to Sudbury.
| Ontario climate zone | Summer afternoon Td (C, typical) | Winter morning Td (C, typical) | Container condensation risk |
|---|---|---|---|
| Southwestern (Windsor, Chatham, Sarnia) | 20 to 23 | -10 to -3 | High summer cargo sweat, moderate winter |
| Niagara fruit belt (Welland, NOTL, Fort Erie) | 19 to 22 | -9 to -2 | High, lake humidity drives Td up year-round |
| Grand River Valley (Brantford, KW, Cambridge) | 17 to 20 | -12 to -5 | Moderate, the home turf, well-characterized |
| GTA (Toronto, Mississauga, Brampton) | 18 to 21 | -11 to -4 | Moderate to high, urban heat island lifts Td |
| Halton (Burlington, Oakville) | 18 to 22 | -10 to -3 | High, lake proximity |
| Eastern Ontario (Ottawa, Kingston, Brockville) | 16 to 19 | -15 to -8 | Lower summer, very cold winter, large frost risk |
| Northern Ontario (Sudbury, North Bay, Thunder Bay) | 13 to 17 | -20 to -12 | Severe frost cycles, lower absolute summer humidity |
| Cottage country (Muskoka, Haliburton, Bruce) | 15 to 18 | -15 to -8 | Lake-driven summer dew, severe winter frost |
The bigger the gap between dew point and coldest wall temperature, the more aggressive the condensation. Northern Ontario buyers assume their dry winter air protects them. The math says no. Dry outside air does not mean dry inside air, because contents release moisture continuously inside the sealed steel box. The interior climate is its own thing, and running the numbers honestly is the only way to tell a Sudbury buyer whether their setup will sweat in February. When the answer is yes, the fix usually starts with how we spec spray foam and rigid panels for cold northern routes.
Where Do Thermal Bridges Drop Steel Below Dew Point?
A thermal bridge is metal that connects outside to inside, conducting cold across an otherwise insulated barrier. Shipping containers are built almost entirely of thermal bridges because the corrugated walls, corner castings, hinges, roof joists, and floor cross-members are continuous outside-to-inside. Interior air is roughly uniform in temperature and humidity. The wall surface temperature is not. At a thermal bridge, the steel face is often 5 to 10 C colder than the insulated panel a foot away.
Critical thermal bridges on a standard shipping container
- Corner castings. Solid steel blocks, 7 by 7 inches roughly, continuous from outside corner to inside corner. The coldest spots on the container in winter.
- Roof crossmembers. Steel beams running the width of the roof every few feet, visible as ridges inside. Always 3 to 6 C colder than the roof panel between them.
- Floor cross-members. Steel C-channels under the plywood floor, exposed below. In winter they conduct cold up through the floor.
- Door hinges and locking-rod brackets. Solid steel through the wall, often the visible-from-inside frost spots.
- Cargo-door header and threshold. Doubled steel construction, twice the conductive area of the wall.
A buyer can insulate every flat panel and still see condensation forming in a grid pattern that mirrors the roof crossmembers, because each crossmember stays below dew point even when the panel between members is well above it. Closed-cell spray foam is the common Ontario solution because it covers corrugations and crossmembers in one continuous layer, breaking every thermal bridge. Batt insulation between framing leaves the framing itself as bare thermal bridges. A finished conversion done under engineered drawings handles this correctly: it costs more upfront but eliminates the problem rather than spreading it across the panels. Crossmember spacing also shifts with the box, and our dimensions breakdown for every container length shows how it differs between 20ft, 40ft, and 45ft high cubes.
How Do You Measure Container Temperature and Humidity Accurately?
You cannot calculate dew point from bad data. Most buyers who say they have measured their container interior were reading a single hardware-store thermo-hygrometer hanging by the door. That number is not what you need. The setup that works: two calibrated thermo-hygrometers (Sensirion SHT41, Honeywell HIH8121, or consumer Govee H5102 for budget setups), one mid-container at chest height, one near the floor against the coldest wall, both logged at 15-minute intervals for at least a week.
Measurement gotchas we see at the yard
- Single-point readings lie. A 6 a.m. door check says 12 C, 65 percent. The far corner is 4 C, 88 percent. Different math.
- Hygrometer drift. Cheap RH sensors drift 5 to 8 percent in six months. Calibrate or replace yearly.
- Sun-exposed probes read too hot. A probe near a south wall in afternoon sun reads 35 C when bulk air is 22 C.
- Battery dropouts. Cheap loggers stop in cold weather. The data you need most (minus 15 C nights) is what you do not have.
For most Van Blanc buyers the answer is simpler than running their own monitoring. They tell us the use case, location, contents, and typical hours the box is open. We run the math against Environment Canada climate data for their region. That gets them an answer in 20 minutes instead of a six-month measurement campaign.
How Do You Build the Dew Point Spreadsheet We Use at the Yard?
Six columns wide. Anyone can build it in 10 minutes. Same tool we run for every customer who asks about condensation risk.
| Column | What goes in it | Example value |
|---|---|---|
| A: Interior T (C) | Measured or estimated air temperature inside container | 15 |
| B: Interior RH (%) | Measured or estimated relative humidity | 75 |
| C: alpha | =LN(B1/100) + (17.625 * A1) / (243.04 + A1) | 2.144 |
| D: Dew point Td (C) | =(243.04 * C1) / (17.625 – C1) | 10.6 |
| E: Expected coldest wall T (C) | Estimated steel temp at coldest spot, coldest time | 3 |
| F: Margin (C) | =E1 – D1, positive means safe, negative means condensation | -7.6 (will sweat) |
Columns C and D are the Magnus equation in spreadsheet syntax (LN is natural logarithm). Negative margin in column F means the wall is below dew point and condensation will form there. Extend with month-by-month rows for an annual risk map. For ESA-certified offices, climate-controlled storage, or humidity-sensitive contents, the annual map justifies the insulation spend.
How Does Container Condensation Change by Season in Ontario?
Three Ontario seasons produce different condensation patterns. The math explains why.
Spring shock (March to May)
The interior is still cold from winter (5 to 10 C). Warm humid spring air enters through any opening and meets cold surfaces. Dew point of incoming air is often 8 to 12 C while the steel sits at 4 C. Massive condensation event in the first hour after doors open. Buyers pulling farm equipment out in April see the whole interior fog over within minutes.
Summer sweat (June to August)
Interior temperature rises to 25 to 35 C during the day with the door closed and sun on the box. Overnight the box cools but stays warmer than the dew point of the saturated interior air, so morning sweat is mostly on the north wall and roof crossmembers. Cargo sweat (moisture forming directly on cold goods) is more common than container rain this season.
Fall trap (September to November)
Warm humid days with cool clear nights. The interior holds summer humidity. Clear-sky cooling drops the steel roof 5 to 8 C below ambient overnight. Roof condensation is severe. Buyers who put cottage equipment away in late September see musty smells by Thanksgiving.
Why December 26 is the worst single date
For unheated containers in Ontario, the worst single day is usually late December or early January. Interior temperature has dropped to outside ambient (no thermal mass left to buffer), accumulated moisture pushes interior humidity high, and a sudden warm front raises outside dew point above the steel temperature. Result: condensation on every surface simultaneously, dripping audibly from the ceiling. We get the calls on December 26 every year.
How Do You Plan Insulation and Ventilation Against the Dew Point Math?
The point of running dew point calculations is making decisions that move one of the three numbers: interior T, interior RH, or wall T. Each intervention shifts a different one.
| Intervention | Which number it moves | How much | Best suited for |
|---|---|---|---|
| Passive vent kit (4 louvres) | Lowers interior RH | 5 to 15 percent | Low-stakes storage where outside air is usually drier than inside |
| Whirlybird turbine vents | Lowers interior RH | 10 to 25 percent | Wind-exposed sites needing passive air exchange with no power run |
| Powered exhaust on humidistat | Lowers interior RH | 20 to 40 percent | Sites with hydro available and a steady moisture load to clear |
| Calcium chloride desiccant buckets | Lowers interior RH | 5 to 15 percent | Short-term spikes and sealed boxes with no power, replaced each cycle |
| 2 inch closed-cell spray foam | Raises wall T toward interior | 15 to 25 C lift on coldest wall | Permanent fixes where the wall sits well below dew point in winter |
| Heated box (heat tape or small heater) | Raises interior T and wall T | Whatever heater can sustain | Heated offices or freeze-sensitive contents, with ongoing hydro draw |
| Dehumidifier (residential) | Lowers interior RH dramatically | 30 to 60 percent | Humidity-sensitive electronics and paperwork on a powered, drained site |
The right combination depends on which margin you are trying to close. A box that sweats in spring because the wall is 6 C below dew point needs spray foam. A box that sweats from cargo moisture release needs ventilation and desiccants. Humidity-sensitive electronics need a dehumidifier. Buyers who skip the math sometimes spend on insulation when a vent kit would have closed the margin, or vice versa. Matching the intervention to the margin is exactly what our crew scopes when you order venting, insulation, and other build-outs done at the yard before delivery, so the box arrives ready instead of needing a second round.
Why Does Van Blanc Trust the Numbers Over Feel?
“I have watched buyers spend thousands on insulation packages that did not solve their condensation problem because they never measured what they were dealing with. The math is not hard. A container in Brantford with 70 percent humidity at 15 C has a dew point of 9.6 degrees. If your wall ever gets below 9.6 in the coldest hour of the night, you will see water there. That number is the same whether you are storing tools, hay, motorcycles, or paperwork. Calculate it first. Then decide what to spend. I have been in the container industry for 19 years and the buyers who run the numbers always end up happier than the buyers who go on feel.”
Paul LeBlanc, owner, Van Blanc Ent. Inc.
Paul’s point is operational, not academic. We see buyers come back six months after a wrong fix asking for a second round of modifications. The Magnus equation has been around since the 1840s and the constants were refined by Alduchov and Eskridge in 1996. The math is settled. The application is just discipline, and our guide to stopping container condensation turns that settled math into a working storage plan.
Frequently asked questions
What is the dew point of a typical Ontario container interior?
Most Ontario container interiors in spring and fall sit at 12 to 18 C with 60 to 80 percent relative humidity. That gives a dew point in the 5 to 14 C range. Winter interiors with cold contents and trapped moisture can have dew points as low as 2 C with frost forming below that. Summer interiors with warm humid air can have dew points up to 22 C.
How do I calculate dew point without a spreadsheet?
For quick estimates, dew point in Celsius equals roughly T minus (100 minus RH) divided by 5, valid above 50 percent humidity and between 0 and 30 C. At 20 C and 70 percent RH, that gives 20 minus 6 equals 14, very close to the 14.4 from the Magnus equation. For lower humidity or temperature ranges outside this band, use the Magnus formula or a phone app like Psychrometric Calculator.
What humidity level prevents condensation in a container?
Below 50 percent relative humidity at typical Ontario interior temperatures, dew point sits low enough that most steel walls stay above it. Below 40 percent is the practical safe zone for unheated storage. Most container interiors run 65 to 85 percent without intervention, which is why condensation is the default outcome unless you act.
Does dew point change between summer and winter for the same container?
Yes, substantially. Interior dew point depends on interior air conditions, which track outside conditions but lag and modify them. A Brantford container in July with closed doors might have interior dew point of 18 C. The same container in January has interior dew point of 0 C or below. The wall temperatures change by even more, which is why winter frost cycles differ from summer sweat events.
What is the difference between cargo sweat and container rain?
Container rain is moisture from interior air condensing on the cold steel walls and roof, then dripping down onto cargo. Cargo sweat is moisture from warm humid air condensing directly on cold cargo surfaces, like pulling chrome motorcycles from cold storage into a humid garage. Both are dew point events, but the cold surface is different. Cargo sweat is more common in transit. Container rain is more common in stationary Ontario storage.
How accurate is the Magnus formula compared to lab measurements?
The Magnus formula with the 17.625 and 243.04 constants is accurate to about 0.4 C across the full Ontario range. That accuracy is more than sufficient for container planning. Reference chilled-mirror hygrometers in calibration labs do better than 0.1 C but cost thousands. For working decisions about ventilation, insulation, and desiccants, the Magnus equation is exact enough.
Will adding a vent always lower interior humidity?
Not if outside dew point is higher than interior dew point. On a humid August morning when outside air carries more moisture than interior air, opening vents adds humidity rather than removing it. Ventilation works when outside air is drier than interior air. That is most of the time in Ontario but not always. A humidistat-controlled fan that runs only when outside dew point is lower is the proper engineering solution.
Why do my thermal bridges sweat even after insulation?
Because thermal bridges (corner castings, crossmembers, hinges) conduct cold across the insulated wall. Their interior surface temperature stays close to outside ambient even when the panel between them is warm. Solving this requires continuous insulation that covers the bridges, typically closed-cell spray foam at 2 inches or more, or thermal-break construction at major connection points.
Can I prevent condensation without insulating?
Partially. Aggressive ventilation, desiccants, and humidity-controlled heating can keep interior dew point low enough that uninsulated walls stay above it most of the time. The trade-off is energy cost and accepting occasional condensation during weather extremes. For valuable contents, insulation is usually more cost-effective long-term than running dehumidification continuously.
Where do I get reliable Ontario climate data for these calculations?
Environment Canada’s Canadian Climate Normals service publishes 30-year average temperature, humidity, and dew point data for every weather station in the country. For Brantford, the Brantford airport station provides hourly historical data. For container planning, the monthly average dew points and the extreme high and low temperatures give you the working envelope.
Sources
- Alduchov, O. A., and Eskridge, R. E. (1996). Improved Magnus Form Approximation of Saturation Vapor Pressure. Journal of Applied Meteorology, 35(4), 601 to 609. American Meteorological Society. journals.ametsoc.org
- National Physical Laboratory (UK). (2024). How do I convert between dew point and relative humidity? NPL Reference Q&A on humidity measurement. npl.co.uk
- Environment and Climate Change Canada. (2025). Canadian Climate Normals 1991-2020. Brantford and Hamilton airport station data. climate.weather.gc.ca
- International Organization for Standardization. (2022). ISO 6346:2022 Freight containers, Coding, identification and marking. iso.org/standard/82754.html
- Cargo Intelligence. (2024). Container Rain and Dew Point Physics, Why It Happens and How to Prevent It. desiccant.com
Reach Van Blanc in Brantford
We have been supplying shipping containers across Ontario since 1995. Our yard is at 90 Morton Avenue E in Brantford, and we deliver across the province in 1 to 3 days from our four Brantford yards on a cash-on-delivery basis. Every quote comes with a real lead time, not a hopeful one.
Van Blanc Ent. Inc., 90 Morton Ave E Unit 1B, Brantford, ON N3R 7J7, +1 888-509-6658
If you are sizing insulation or ventilation against your specific dew point math, call the yard. Paul or Christian will walk through your interior conditions, your contents, and the wall temperature envelope for your part of Ontario before quoting modifications. Worth the drive for unbeatable quality, family customer service with 30 years of experience.
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