Causes for condensation

There is condensation in between the vapor barrier and insulation in the basement, what would cause this?

Temperature below the dew point.


Was there high humidity in the basement? Was the basement cooled or was it relatively warm? Was the foundation wall cool to the touch?

David is correct. There are several dynamics that can be at play regarding heat transfer, humidity, air flow and vapor barrier issues. It is amazing all the ways that the forces we learned about in “building science” show up in a variety of symptoms that attack a building.

If you can see condensation, there may also be excessive moisture in areas that you cannot see (absorbed into materials). Moisture can be conducive to mould, decay, WDI and corrosion that may be hidden from view as well. Finding the cause and the affect of moisture can be subtle sometimes. I never realized how many different ways it can show up until I started using an IR camera and a moisture meter.

Condensation commonly occurs when a vapor is cooled and/or compressed to its saturation limit

Sometimes, something as simple as cooking can cause a moisture issue that leads to a mould problem.

The battle of hot and cold can cause condensation to be trapped in hidden areas of walls next to masonry products. This is why I cringe when people say all basements are humid and it’s no big deal. The slow migration of moisture through concrete or blocks sometimes makes the issue worse.

It sounds like the vapor barrier is not doing its job…

It sounds like we need more posted information about the problem and his observations.

Parrish, we are kind of “blind” out here!
We don’t even know where you are (fill out your profile).

We need OA conditions.
IA conditions.
wall construction inside.
Wall construction outside.
Heat sources.
Cooling sources.
Sometimes building pressures.

Water on the vapor barrier does not get there in the liquid state.
We need to know about the air movement.

Moisture Intrusion Inspection course](


Keep on the new guys Nick!

Should not have to ask when there is NACHI Training available (for just about anything)!

I regularly see condensation problems in new construction when they do not properly seal around the can lights or have no appropriate vapor barrier to the roof / ceiling area (especially in “townhouses” with flat roofs).

One that I did last winter, the owner (1.9 Mil, 2 year old Lincoln Park style twonhouse, not a rowhouse) replaced the roof when he got leakage from a can light, at the rate of 3 - 4 gallons per day. The next winter, the leakage came right back.

Then the contractor (who is very wise, at least now :mrgreen: ) called me in to actually find the cause of the problem.

The roof (flat) was supported by wooden trusses and the used blown in cellulose between the ceiling and the roof (can you say, mold food!).

After using thermal, I found that the areas around the upper floor can lights were wet. It also did not help that the owner was a concert pianist and his piano tuner advised him that the humidity in the house should be kept at 50% during the winter.

Outside temp: 26 F.
Inside temp: 75 F with a 50% RH.

“attic” area: 63 F with a 89% RH.

And, yes, in picture 3, that is frost on the interior of the masonry in the attic area.

And they wonder where the water was coming from :wink:

Keep the humidity down to around 25 - 30%, especially in huimid (summer) an cold (winter) areas like Chicago.

Hope this helps;