Why do ice dams form at 1990 era condos but not on a super-insulated house?

During the snowy and very cold winter many homes develop ice dams, but some do not.

Why is there a difference? We will start with a basic review of how the ice dams form.

Why do ice dams form?

The best building science explanation I have ever read is presented in Bill Rose’s book “Water in Buildings.” I will paraphrase his description here.

Snow buildup on the roof insulates the attic space and can lead to heat buildup in the attic. This heat buildup can melt the bottom layer of snow on the roof. When this melt water reaches a colder part of the roof it can re-freeze.

Traditional drawings of ice dams show a mass of ice at the colder eave only. The figure below is taken from 2009 IRC code commentary.

A more accurate drawing of ice dam formation is shown in Bill Rose’s book. The figure shows that a block of ice does refreeze at the eaves, but there is also a lens of ice that extends up the roof. The pool of liquid water that flows down the slope of the roof is significantly deeper in this scenario. The greater water depth from the enclosing ice lens results in greater water pressure driving the water through the roofing materials.

How are ice dams prevented?

If the snow on the roof can be kept below freezing then melting will not occur and ice dams will not form. There are two basic strategies available to control/ lower attic temperature. Reduce heat entering the attic, and ventilating the attic space to allow any heat entering to escape.

Reducing heat entering the attic is a multipart process. Air sealing the ceiling plane seeks to prevent conditioned air from escaping to the attic. The penetrations into the attic from the conditioned space must be sealed. These penetrations may be wires, lights, ducts, etc. Wall top plates must be sealed to ceiling board. Duct chases and chimneys must be sealed.

Reducing heat in the attic is more complicated if mechanical equipment is located in the attic. In this case, the ductwork within the attic must be tightly sealed and well insulated. High efficiency equipment will lose less heat to the attic. Care must be taken to insure that bath exhaust fans are vented out of the attic. If practical, heating equipment and ductwork should never be located in an attic.

After sealing the attic plane and tightening ductwork, additional insulation is recommended. Current energy code requirement is R49 for attic insulation, but it is not much more expensive to add additional insulation if the contractor is there already. I recommend that R60 be installed based on attending many seminars and personal experience.

The second element to preventing ice dams is ventilation. Typical ventilation will not likely be adequate unless the air sealing and insulation measures described are implemented. Adequate ventilation will allow a minor amount of heat to escape from the attic. Venting can be compromised in winter by snow blocking ridge vents. Over ventilating without air sealing can result in additional conditioned air escaping into the attic.

Why do ice dams form at 1990 era condos but not on a super-insulated house?

Here are two photos taken in February of a condo complex built in the late eighties and early nineties:

Faces southeast

Faces south southeast

Below are two photos of a super-insulated home also taken during February.

Faces south

Faces north

Difference between 1990’s condo and 2010 super-insulated home

The condos shown in the photos above lack sufficient measures designed to minimize ice dams. The ceiling planes are not air sealed and the amount of insulation in the attic is too low (approximately R20). In some of the units there is ductwork in attic, but in units without the ductwork ice dams still form.

Both sides of this super-insulated house roof do not exhibit ice dams. This is due to the meticulous care that went into air sealing the attic plane and installing R60 insulation. There are no ducts or mechanical equipment in attic. The attic is ventilated to allow the minor amount of heat to escape, but the ridge vent is blocked by snow. The heated house roof edges look similar to the adjacent unheated garage shown below.

Faces south

This photo comparison shows that will proper care, ice dams can be eliminated with proper attention to air sealing, insulation and ventilation details.

There are certainly geometrical, access, or other obstacles to completely solving the ice dam problem in some structures. In these cases, more creative solutions will be required. The basic comparison of the two structures examined shows that building a home without ice dam formation is possible. The solutions to the problem are not beyond the skills of skilled and dedicated builders.