The Sweat(ing) House – a Condensation Story

We recently investigated a report of water “leakage” at a suburban Boston townhouse. The owner described a history of water infiltration at several locations throughout the home. As it turned out the bigger problem was not water leakage, but condensation.

The Clues

The first clue came in a document from the owner, in which he described an interior wall that was “fully soaked” and attributed it to a leak that “always comes during cold weather”, but was unrelated to storms.

Once on site we took a look at the roof to identify deficiencies that might be contributing to leakage. While we were up there, we saw some something very interesting (to a building envelope geek)… moisture coming out of the roof!

See the dark staining near the ridge? That’s moisture coming out of the ridge vent.

In addition to moist-to-the touch roofing at the ridge vent, there was liquid water coming out from below the shingles. The water below the shingles was especially evident because the shingles were buckled, leaving gaps.

Water trickling out from below the shingles.

Water coming out of a building in a cold climate is a bright red flag to building envelope nerds like us that there’s a condensation issue. But to pin it down we looked for more evidence.

Continuing our building envelope investigation in the attic we found some ugly staining on the underside of the sheathing.

Using an infrared thermometer and handheld psychrometer we determined that the sheathing was below the dew point temperature at the time of our visit. Moisture meter readings showed elevated moisture content of the sheathing (colder) compared to wood framing within the attic space (warmer).

Ugly staining on the underside of the sheathing, which turned out to have elevated moisture content compared to the wood framing within the attic.

We found the last piece of the condensation puzzle down in the basement: a whole-house humidification system. The system was set at 38% relative humidity (RH), and we measured RH in the upstairs living spaces at between 37% and 41%.

So what is happening?

Given the existing building conditions (including air leakage and poor attic venting) and the New England climate, the moisture content of the air in this home as a result of the whole-house humidification system is too high. Moisture from the interior air is condensing on cold surfaces within the building envelope.

Interior air RH is not the only driver of condensation risk, however all else being equal a higher RH means increased risk.

tl;dr moral of this story: don’t use a whole-house humidifier in New England without getting some professional advice.

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