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Moisture and the Tight House

Paul Kando

The house was less than 5 years old. A “tight, well insulated house”, its builder proclaimed even as he called for help. The heating bills were indeed reasonable but the occupants kept complaining about mold and moisture damage. There were water stained ceilings, black mold on moist sheetrock and, where damaged wallboard had been removed, wet insulation and wood rot inside the wall. What went wrong?

Moisture problems (NOT the house under discussion)
photo credit: Green Building Advisor

Our builder, after putting R-19 batts of fiberglass in the 2x6 frame walls, added 1” foam board to the outside, just under the siding, boosting the R value to close to 30 and creating a tight house. For this he asked 18% less than the net zero energy bid his customers got from another builder.

In retrospect the net zero option would have been by far the better, more economical choice. True, the owners got an insulated, tight house on the cheap, but now they and their builder faced thousands of dollars worth of repairs. While the foam board on the outside of the walls added insulation and air-tightness, it was insufficient to keep the inner surface of the outer sheathing above the dew point. Warm air trapped here would lose its moisture as it cooled, and the moisture would condense into liquid water.

A 19th century builder knew that his houses “had to breathe”, even at the price of burning lots of fire wood. There was no insulation as such, but he applied generous layers of horsehair plaster, to minimize indoor breezes. A 21st century builder should have known that it is not enough to insulate and tighten a house. Heat, air and moisture interact – both in the global climate and inside a house. Moisture evaporates when heated and warming air absorbs more and more of the vapor. Since warming air expands, pressure builds in the house, forcing the moist, warm air to pass through holes and cracks into the building envelope (e.g. through electrical outlets). So, in addition to insulating, the builder must also prevent air and airborne moisture to penetrate the envelope by applying a continuous air barrier on the warm side of wall, ceiling and floor assemblies.

From a tight house air cannot escape, so moisture accumulates. This is why a properly built house (e.g.: a passive house) is not merely tight and super-insulated. It also has heat retaining ventilation to remove excess moisture and provide fresh air. Our builder did not know any of this. Sadly, many contractors don’t.

Buyer beware, the saying goes. You get what you pay for. Ask your builder to explain in detail how the house will deal with not just heat losses, but air movement and indoor moisture. Better yet, don’t build or buy a new house that does not meet passive house standards. The basics of how a heated house works are not difficult to learn, so ignorance on the part of builders is inexcusable. On the part of home buyers it is merely foolish and often expensive.