by: Paul Kando
Heat, air and moisture interact. As moisture - from basements, crawlspaces, kitchens, showers, potted plants - warms, it evaporates. The vapor is absorbed by the warming air, and the warmer the air the more moisture it holds. The air-held vapor (relative humidity) also contains all the heat the water absorbed by evaporating. Warming air expands, becoming lighter, so it rises and is replaced by cooler air from below -- a convective pattern. Since air-sealing houses is a relatively new idea, in most Maine houses the rising warm air and the moisture it contains are free to leave the heated space through cracks and holes into the attic and through cracks, electrical outlets and switches into the wall cavity.
Most houses are insulated with fiberglass, which allows air to flow through, so the warm air proceeds until it reaches a cold surface - the outer sheathing of the wall or the underside of the roof. There the air cools, releasing the vapor it can no longer hold. The vapor condenses, causing moisture damage and releasing its heat of evaporation. (The heat of evaporation of water is 970 BTU/lb or 60.625 BTU/oz. Research has shown that through a tiny, 1 mm (~1/32") hole, 360 grams of air-held water can pass per day. That's 12.3 oz, just under 746 BTUs lost. Consider that a single leaky duplex outlet comprises an over 20 mm hole and that a typical Maine house can have as many as 40 such outlets. Do the numbers: that's an awful lot of heat energy to lose every day.
This information was not common knowledge when, in the wake of World War II, Europe embarked on programs to ease the post-war housing shortage. They knew about the need to better insulate and minimize the wind blowing through, but not much about indoor moisture. In Scandinavia thousands of highly insulated houses were built or upgraded, only to suffer mold, mildew and rot damage within a few years. Remediation proved costly. The need to find answers spawned "building science", a whole new engineering discipline, and a new trend to design and build houses for maximum energy performance. Whole new building systems were developed as a result, culminating in thousands of Passivhaus certified homes, schools and other buildings in all climate zones, including a growing number in Maine.
Much good building research was done in the U.S. as well, for especially in national laboratories like Lawrence Berkeley and Oak Ridge. Then, in the early 1980s, domestic research programs were drastically cut. Money for applied research and results-commercialization dried up first, on the rationale that these belong in the private sector. But the building industry failed to pick up the slack. Much unfinished research was shelved or found its way to Europe where, under pressure to address climate change and rising energy costs, governments and industry were eager to act. In contrast, even today, inclusive of all subjects, only 1¢ of each U.S. federal income tax dollar is being spent on research.
We live with the results. Unaware of what happened in post-war Europe, thousands of U.S. houses have been built incorporating similar moisture problems, especially in the wake of the 1970s "energy crises", when tighter construction and more insulation became the norm. The result: much potential damage, depending on how leaky the house is, how much indoor moisture is available, the amount and quality of insulation, and even the amount of night-time setback temperature. Compounding the problem are misguided remediation attempts, like adding a moisture barrier to the cold side of an assembly - ½" insulating foam board under clapboards or wall shingles, or a plastic sheet on top of existing attic insulation, for example. Such additions only ensure that water is trapped and condenses there. Stained and dripping ceilings are visible results, but in walls there may be no visible sign until much damage is done. Similarly, insulation stapled up "in the usual way" to basement or crawl space ceiling joists adds a vapor barrier on the cold side.
What to do? To prevent water damage, not to mention heat losses by means of air-borne moisture, and to correct existing moisture problems, have an independent energy audit done. You don't want to repeat the mistake of acting on guesswork, anecdote, custom, wrong or partial information. Follow the audit report's recommendations. Seal all leaks, including electrical outlets, switches, around pipes, wires, chimneys and access hatches that lead to the attic or behind knee walls. Only an energy audit can identify all leaks. Don't skimp on installing heat recovery ventilation. It not only provides fresh air, but distributes heat and prevents moisture buildup. It may save your house, not to mention your health. Make sure that anyone working on your house understands the basic physics and follows your instructions. It is better not to have an air/vapor barrier, than to have one on the cold side of an assembly. A base coat of shellac-based primer (such as BIN) will add an inexpensive air/moisture barrier to otherwise leak-free walls and ceilings.