Forgiving

It is said, to err is human to forgive is divine. While that quote may sum up the most common meaning that one thinks of when hearing the word forgiving, that meaning does not apply to buildings. If it did we all are living in divine structures. Houses are quite forgiving in that when something is done wrong, the house functions pretty much okay. From my experience, sometimes surprisingly so. In reality this is a good thing for all homeowners.

Living and inspecting in a cold climate, a common issue I find is condensation in attics. Conventional wisdom has been the cause for moisture in attics is due to inadequate ventilation. Building science and of course time have proven that ventilation or lack of is not the root cause for this phenomenon.

Houses are complex systems that are influenced by numerous variables. Some are controllable, such as the design or systems. Others are not. The weather being the most obvious. Temperature, wind, rain, snow and sun all effect the structure. So incorporating design features that lessen the influence of the uncontrollable has been the goal of home designers and builders for as long as there have been homes. I'm certain cavemen knew that covering the front of the cave opening or a slope away from the entrance would be more desirable and comfortable. Basic air infiltration and water control.

What's funny, as in strange, is that as houses have evolved to better withstand these uncontrollable influences, new problems have arisen. Although old houses were drafty, creaky and cold, they didn't have the moisture related issues of today's tightly built dwellings. While insulation, vapor barriers and air sealing have made houses energy efficient, these improvements have also prevented the modern house from breathing and drying. The influence and understanding of these new dynamics is still being learned. Change is often a hard thing to understand and embrace.

Recently I was contacted by a condo management company regarding a unit where the homeowner had discovered water dripping from the roofing nails in his attic. Needless to say the man was concerned and was looking for answers. The woman from the management company had been told by a few other "contractors" with whom she had spoken, the problem was inadequate attic ventilation. After giving her a quick lesson and list of potential possibilities, she agreed to have me go out to the unit for a look.

Upon arriving, I immediately noted the presence of both ridge and soffit venting for the attic, which was expected. Upon closer examination, I saw that the ridge vent was short (that's the bump in the roof peak). A ridge vent should extend the entire length of the ridge save for the last few inches from each end. This particular vent and the other on the lower roof appeared to be one length of cobra vent. No doubt a cheat by the roofing company to save money.

Entering the attic I found further issues with the attic ventilation installation. At least half the rafter vents had been incorrectly or I should say, shoddily installed. The purpose of the vents is basically two things, one to provide an unrestricted air way for the soffit vent and second to allow for good ceiling insulation coverage. In particular over the entire width of the exterior wall (the diagram above depicts a proper install).

With these ventilation issues present, the attic showed absolutely no signs that the ventilation is inadequate. Looking around I did see the tell tale tiny rusty red pock marks on the yellow field of fiberglass ceiling insulation, left from the dripping water the homeowner had discovered a few weeks before. However the roof sheathing was clean, displaying no sign of a chronic condensation issue. 

So what caused the condensation the homeowner found?

Several conditions that already exist that came together with some uncontrollable conditions to form a "perfect storm".  For moisture to enter the attic it must have a way to get in there in the first place. Modern construction practices provide plenty of those access points.

The ceiling plane at the attic for all intents and purposes is holier than the pope. The photo shows just one such opening present in this attic. Has a theme begun to appear? The placement of the insulation is on par with the rafter vent installation. 

An infrared image taken of one of these gaps shows the heat from the living space bleeding through into the attic. That space also allows for air and moisture to diffuse into the attic. There are also holes in the ceiling made for wiring, plumbing and duct work. Covering them with a piece of batt fiberglass insulation does not stop the air movement! Further the ducts are leaky, which was easily detected by the infrared camera. And the HVAC system has a humidifier, so now we have ducts that leak moist air.

And in spite of all that condensation has probably occurred, based on what I saw, maybe just once or twice before. The reason I believe for the most recent brief rain storm in this attic, unusual cold temperatures coupled with a roof covered in snow.

The sun beating on the roof warms the sheathing. Warm sheathing can not cause moisture to condense. Further, the radiant heat does slightly raise the attic temperature. A few degrees can mean just enough of difference in the dew point to avert condensation.

So in spite of the plethora of conspiring conditions, the house overall is so forgiving of all the errors, perhaps it should be up for sainthood.