It’s ironic that as we super insulate houses we have created unintended moisture consequences that in many ways may cancel out any energy savings benefit we have actually gained. Over the past 20 years moisture caused issues in new construction have skyrocketed. These issues include condensation, mildew and mold, extractive staining (tannin bleed), peeling paint coats, and the presence of rot in doors, windows, trim, framing, siding, and sheathing sometimes within only a few years after construction.
According to an article by Stephen Smulski in the Forest Products Journal, natural leakage of air through the nooks and crannies of wood frame houses worked well for centuries in allowing moisture to escape the building envelope. Gradually through the 20th century, insulation began to be added to walls of houses. This lowered the temperature of the inside of the sheathing to below the dew point during cold weather and mildew and mold began to appear on the inside of the sheathing. Increased insulation began to reduce the flow of air through the sheathing and the trend towards tighter buildings continued up to today. From the 1960’s on, the introduction of electric heat, high efficiency low draft furnaces, power vents, sealed combustion appliances, and heat pumps all signaled the demise of the active chimney, the traditional air exchange device in a house. No longer were large amounts of moist air expelled from the house via the chimney, and less replacement air got drawn in through the leaks in the tighter building envelope. Insulating glass, Low-E glass coatings, ice and water barriers, house wraps, tapes, caulkings and sealants all worked to make a tighter envelope that didn’t breathe the way the older house could.
Pre-energy crisis houses with much more air flow have always been much more forgiving when they got wet. Felt paper acted as a blotter, held the moisture, and then allowed it to evaporate over time. Simpler roof lines of older houses, without all of today’s cheeks and valleys tended to shed water better and keep it out of the house. Perhaps the good old days really were the good old days in terms of levels of craftsmanship and tradespeople’s experience. The demise of the apprenticeship system in our country means that the novice carpenter doesn’t often have the benefit of an experienced master carpenter teaching the rights and wrongs of construction.
What can you do to improve your building envelope?
- Employ design features and practices that promote water shedding and channel water away from the house structure
- Back prime and end seal exterior siding and trim
- Check the permeability rating of your building paper, housewrap, and roofing underlayment. Give moisture a way out
- Use top quality exterior finishes and educate your homeowner as to the importance of maintenance
- Design wood frame walls to dry from either the inside or the outside
- Apply a continuous vapor retarder to the warm side of walls and ceilings
- Dehumidify basements during cooling season
- Vent clothes dryers, bath and kitchen range exhaust fans directly outdoors
- Keep the relative humidity in the house below 40% using dehumidification or air conditioning
- Augment low rates of air exchange with mechanical ventilation and makeup air- work with your HVAC contractor to give your house the means to breathe mechanically, if it is too tight to do so on its own
- Vent roof and attic spaces
Without proper moisture management practices, we create a “pay me now or pay me later” conundrum. Isn’t it ironic that we potentially waste more energy and money solving moisture problems that we may unintentionally create than we might actually save on heat loss? Be careful, as you make your choices, not to solve one issue and start another. Consider what items work well with others. Don’t void the warranty of your roofing or siding products, for example, by putting them over a product with which they are not designed to work. Remember, we get what we inspect, not what we expect.