Spray Foam Insulation Condensation

Basil Pallone lives just south of me in Sudbury, MA. But he's got a problem with his older home he's trying to improve. Here's what he wrote to me:

"I recently installed spray foam insulation in the attic of my two story colonial home, built in 1968.   Since I installed the spray foam insulation, the windows on the second floor are experiencing excessive window condensation.   I checked the humidity levels throughout the house, and the humidity levels are fine.   I am at loss to explain the cause of the situation and have no idea for a solution.

My HVAC contractor suggests replacing the bathroom fans.  I was thinking that I might need some sort of air exchange unit, but a consultant that I hired thought that might make the problem worse.   The insulation contractor has no clue.

Any suggestions on the source of the problem or remedy?

Thank you in advance."

Well, Basil, I'm willing to bet dollars to doughnuts I know the problem. I've got an entire category here at my AsktheBuilder.com website about Condensation. CLICK HERE to see all my condensation columns.

The above email was one of the subjects in the December 12, 2014 AsktheBuilder Weekend Warrior.

Water vapor is in every house, unless you live in the Atacama Desert.

In the winter, this water vapor usually travels right through regular insulation, plaster, wood, etc. and makes its way into your attic. If you have great ventilation, then in gets outside into the air where it belongs.

Water vapor can and does turn into liquid water when it contacts cool or cold surfaces that are at or below the water vapor's (air in your house) dew point. You see that happening on your windows.

Older homes have a tendency to be more drafty and this is not a bad thing when it comes to water vapor. The incoming cold air has much less water vapor in it and it helps to absorb some of your interior water vapor as this incoming air is heated.

In addition, leaky houses have plenty of places for the humid air to escape to the outside air.

If your foam insulation is closed cell, it's a very effective vapor BARRIER. It's blocking the escape of the water vapor. This allows the relative humidity to climb inside the house and with that up goes the dew point.

As the dew point goes UP, condensation gets worse because the vapor turns to liquid faster on the cold window surfaces.

You need to LOWER your relative humidity in the house by finding out all of the things that are contributing to the addition of water into the air in your house.

It could be from excessive boiling of water, aquariums, hanging laundry indoors, lots of indoor plants, lots of showers with no one using the exhaust fans, etc.

The big danger is what's happening inside the walls where you can't see?