The Long story about ice damming and attic ventilation

Ice damming

Ice damming is a real problem in many older homes in particular. I have written about this in several other blogs but it has been a while so I will try to broach the subject once again in a slightly different angle. Let’s start with building construction. In the last 25 – 30 years we learned a lot about building ventilation and attic ventilation in particular. In order for a building to breathe well it must be designed to do so. It is of paramount importance that we build our homes to be healthy if we plan to live in them. If you have ever been to a hunting camp you will know that they breathe really well and don’t seem to have any particular health problems. In a home however we add heat and furniture and close up the gaps as best we can. The air inside our homes can easily become contaminated and stale if we don’t plan to refresh it on a regular basis. It is this process of ventilation and the lack of ventilation that can lead to ice damming.

If an attic is not properly designed and ventilated, problems often arise. The most common problem I see due to improper design and ventilation include molds, moisture and ice damming. There are four very common issues that lead to the above problems;

  1. Lack of insulation,
  2. Lack of vapour barrier,
  3. Low clearance to the roof sheathing near the eaves and
  4. Lack of ventilation.

Lack of insulation and vapour barrier:  Insulation is very important in any heated structure. In the Midwest we experience extreme temperatures, it is the sudden change in temperature that causes the air inside a home to pass through what is referred to as its dew point. When warm air rises it passes through the attic insulation and vapour barrier where it eventually cools to a point where it condenses and any moisture in the air will form frost on the closest building materials that are below freezing. When temperatures rise the frost melts and the resulting moisture can cause all sorts of problems such as mold and rot. Two common solutions to reducing this natural process of cooling and condensation is to reduce the rapid heat loss by adding insulation and  adding a vapour barrier to stop the moisture in the air from rising to the cold attic.

Other causes of moisture and mold in many homes is the design of the roof. If the home is older it may not have much or any eaves. In more modern homes we have discovered the importance of extending the eaves out past the wall of the home and adding vents to provide a pathway for wind driven air to move through the attic space. If your home was built prior to the 1970’s it likely lacks this type of ventilation. Furthermore the distance (slope distance) between the attic ceiling rafters and the roof sheathing near the wall of the home is minimal. This is the area where the warm moist air from the heated space inside the home will often cause problems. When the heat rises through the insulation it begins to cool quickly, because there is little air movement in the attic by design the warm air reaches the sheathing of the roof at the same time it hits its dew point. It is here that the moisture in the air is deposited onto the roof sheathing.

To be clear, it is fairly rare to see frost development near the peak of the attic as the distance between the rafters and the peak is great enough that the heat in the air cools beyond its dew point before it gets that high. By the time the air does rise that high it is too dry to create frost. So we remain with the frost and warmer area of the attic at or near the eaves. One last problem that presents itself in this situation is ice damming. Ice damming occurs when the heat energy from the home rises up and touches the roof sheathing in the cold winter months. Generally it doesn’t matter if the heat is moist or not in this case. What happens is the heat hits the sheathing and transfers through the wood and shingles until it reaches the snow on the roof. When the last of the heat residual hits the snow it begins to melt the snow layer next to the shingles. This melt water begins to run downwards toward the eaves. When the water reaches the outer edge of the exterior wall it freezes again because the heat source below is no longer available to keep it thawed. After a few days of this thaw / freeze cycles occurring,  the ice layer gets thicker and the water from above needs to rise up to get over the dam. When the water behind the dam gets about 3-4” thick it can and often does run into the attic and makes its way into the ceiling and outer wall cavity. Of course the moisture in the wall leads to rot and mold as you might expect.

One last though on this, I always recommend using ice and water shield on the first three feet of the roof eave as it will prevent some of the ice damming. I also recommend adding ventilation as it moves the air around and cools it before it has a chance to create ice damming and frost. Lastly adding more insulation and vapour barrier where possible is a good idea.

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