Condensation: lower indoor humidity, insulate cold pipes and surfaces, stop venting moisture indoors
Homeowner guide · Two days, mostly waiting
How to tell condensation from seepage: water coming through the wall
Condensation and seepage both leave a wet wall. One is fixed with dehumidification and insulation, the other with drainage and waterproofing. Guessing wrong is expensive.
Short answer
Tape a square of aluminium foil or clear plastic tightly to a dry section of the wall, sealing all four edges, and leave it for two days. If moisture appears on the room-facing surface, the problem is condensation forming from humid indoor air. If moisture is trapped between the sheet and the masonry, water is moving through the wall and it is seepage. If both surfaces are wet, you have both conditions at once, which is common.
Before you start
What you need
- A square of aluminium foil or clear plastic, roughly 12 by 12 inches
- Strong tape that will hold on masonry
- A hygrometer to measure humidity
- A towel to dry the test area
Step by step
7 steps.
- 01
Choose a representative spot. Pick a section of wall where you have seen moisture, but that is dry at the moment you start. Testing a currently wet area tells you nothing, because it is already wet from whichever cause.
- 02
Dry the surface completely. Wipe the area with a towel and let it dry fully. Any moisture left behind will confuse the result on both sides of the sheet.
- 03
Seal the sheet to the wall. Tape the foil or plastic flat against the masonry and seal all four edges completely. An unsealed edge lets room air behind the sheet and ruins the test.
- 04
Measure the room humidity. Note the relative humidity in the space with a hygrometer, along with the date, the indoor temperature and the outdoor weather at the time, since all of that context matters for interpreting whatever the sheet test shows a day or two later.
- 05
Wait two days. Leave the sheet undisturbed. In summer, when humid air meets cool masonry, a condensation result usually appears quickly. A seepage result may need the full period or a rain event.
- 06
Read both surfaces. Peel the sheet back carefully and look at both faces. Moisture on the room side means condensation. Moisture on the wall side, or damp masonry beneath, means water moving through the wall. Wet on both means both conditions are present.
- 07
Repeat in another season if unsure. Conditions change through the year. A test in July and another in April will tell you more than either alone, particularly where the basement is only wet at certain times.
Reading the foil test and what else to check
The foil or plastic test answers one question: is the moisture coming from the room air or from the wall. It does not say how much water there is or whether the problem is getting worse, so pair it with a few other observations. One test is never enough.
Run the test on more than one wall, on a dry day as well as a damp one, and again after a storm, so that you can see whether the answer stays the same when the weather and the humidity in the room both change. Note the date and the outdoor conditions beside each result. A wall that reads dry after a week of sun and wet after a storm behaves very differently from one that is damp all summer.
| Result | Meaning | Also check |
|---|---|---|
| Beads on the room side of the foil | Condensation from indoor air | Humidity, dryer venting and uninsulated cold pipes |
| Moisture behind the foil against the wall | Water moving through the masonry | Grading, gutters and downspouts on that side |
| Both sides damp | More than one source at once | Treat the water first, then the humidity |
| Dry on both sides | No moisture at this spot in this weather | Repeat after heavy rain or in a humid week |
After the check
What to do with the result
Seepage: trace the water path outside and below grade before choosing a repair
Both: correct the bulk water first, then manage the remaining humidity
Keep the dated photographs either way; they are your baseline
Common questions
How to Tell Condensation From Seepage
How big should the test square be?
Around 12 by 12 inches is enough to give a clear result and small enough to seal properly. What matters more than size is that all four edges are sealed tightly so room air cannot get behind the sheet.
What if both sides come back wet?
That means you have both conditions, which is common in Vermont basements. Correct the bulk water first, because that is the larger and more damaging source, then reassess the humidity once the wall has had a chance to dry.
Does a dehumidifier fix seepage?
No. A dehumidifier lowers the moisture content of the air and can help with condensation, odour and comfort. It does nothing about water arriving through the wall, and running one against an active leak simply raises your electricity bill.