With winter comes snow. The snow covered landscape is undeniably beautiful. Snow cover for homeowners brings with it the potential for ice dams. Icicles are another one of winters beauties until they line the lower edge of your roof. Awe then turns to anxiety as a homeowner now must be vigilant for leaks. Fortunately not all snow fall will create ice dams that can cause leaks. By and large, the deeper the snow, the greater the likelihood for leaks.
In my experience the understanding of what creates ice dams is not always well understood. One theme I hear fairly often is the gutters are to blame. I have talked with several homeowners who have purchased new gutters after being advised they would solve their ice damming issue. While gutters collect the melt water and eventually the ice that forms, they are not the cause for the formation of ice dams.
Simply, ice dams are formed by the accumulated snow on the roof melting and then refreezing at the edge. Of course all snow melts. The problem is when the snow melts too quickly, often exacerbated by a deep pack. The real problem is not the ice dams, for they are the result of other extremely common building practices. Yes, that's right, building practices. In other words it's the way houses are constructed that is usually the root cause for this problem. Furthering the issue, accessory components such as lighting also contribute.
The problem of ice dams begins inside the house at the ceiling plane common to the attic. Standard practice is to insulate the ceiling, for it is the area where the greatest heat loss occurs. Before the insulation is fitted, many components are first installed that make this surface holier than a piece of Swiss cheese. When all the work of putting in wires, lights, pipes and various ducts is completed, fiberglass insulation, usually in batt form, is finally laid over and around all of these irregularities. The fit, to say the least, is less than ideal.
All of the holes through the ceiling plane are typically referred to as bypasses. The label is appropriate as each hole allows for warm conditioned air to "by pass" the ceiling into the attic. But wait you say, the insulation will halt the air movement. In actuality, usually not, especially when the insulation is fiberglass.
A common sight found on fiberglass insulation are black stains sometimes mistaken for mold. These dark smudges are dirt from air passing by the insulation from the conditioned space. Not convinced? Pull back the insulation at a stain. There will usually be an opening beneath. If not, the batt is poorly fit like in the photo.
The answer to the problem of fitting insulation into cavities and around irregularities has traditionally been to install loose insulation, again usually fiberglass, sometimes cellulose. Loose or blown fiberglass I have found to be marginally better than batt type fiberglass. The insulating ability (R value) of loose fiberglass (2.2/ inch) however is far less than batt type (3.7/ inch) or cellulose (3.8/ inch).
There are two common problems I find regularly with blown fiberglass. First is cheating by installers. R - 38, the standard R value for attics in Connecticut, requires a depth of 15 inches. This assumes an R value of 2.5/ inch. I find with troubling regularity, attics with depths of 8 - 10 inches on newer construction or retro fits.
The second issue is blown fiberglass "insulation" does a very poor job of insulating. The infrared image shows heat leaking through about 8 inches of newer blown fiberglass. The installers cheated on the depth by bending the cardboard gauge used during installation at the 10 inch mark, thus putting in less than half the amount of insulation required by building code. The insulation was mounded near the attic entry to appear deeper.
An independent study done several years ago by the University of Colorado at Denver School of Architecture and Planning found that cellulose insulation significantly out performed fiberglass. Therefore using an insulation that doesn't insulate coupled with a ceiling that closely resembles a sieve is one of the major contributors to ice damming on most houses.
As touched on several times earlier, attic by passes are just the beginning of the problem with heat loss that can cause ice damming. Some of the most common by passes are created by the very design of the house and can be difficult to remedy. The area where the wall and roof join is a known weak point in the thermal protection of the attic insulation. Often the insulation is not or simply can not cover the thermal bridge created by the wall framing. New houses can be built with roof assemblies that allow for insulation to be placed fully over the wall assembly and can accommodate the other important component in preventing ice damming, ventilation.
One other major by pass can be HVAC duct leakage and or uninsulated duct work. Installing HVAC equipment in the attic is in my opinion a very bad idea, but quite often the only option.
Attic ventilation I have found in my experience is too often misunderstood. Ventilation alone is not the cure for ice dams and or moisture issues found in attics. Those issues very often begin at the ceiling. Properly installed ridge and soffit venting has been proven to be a superior attic venting method. However any system, especially a passive one, can be overwhelmed. When attic by passes allow for large quantities of conditioned air, sometimes laden with extra humidity by the homeowners, to enter the attic, the venting is not able to keep up. The result can be moisture issues inside the attic, typically on the roof sheathing and ice dams outside when conditions are right.
To try and neatly wrap all this up, ice dams are created mostly by preventable and often repairable building issues. Solutions should be looked for beginning inside the house, specifically the ceiling common to the attic. Sealing and better insulating the ceiling paired with adequate and properly installed ventilation can significantly reduce or even eliminate ice dams from forming. Lastly the best benefit from having a well sealed and insulated attic is increased energy efficiency. The energy savings should help off set the cost of repairing / upgrading the attic space.
In the end while the evidence may be damning, there is great hope for a reprieve from the winter snows.