Condensation: lower indoor humidity, insulate cold pipes and surfaces, stop venting moisture indoors
Homeowner guide · Two days, mostly waiting
How to tell condensation from water coming through the wall.
Both leave a wet wall. One is fixed with dehumidification and insulation, the other with drainage and waterproofing. Guessing wrong is expensive.
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 and the outdoor weather. This gives context to whatever the test shows.
- 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.
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.
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Want someone to look at what you found?
Describe the result and we will inspect the property.