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.

I have been reading lately about the dangers of asbestos insulation in homes. Can you expand on this and explain where we might have asbestos?

Asbestos was used in many forms over the years. In fact Canada still exports asbestos to other countries. The majority of the asbestos in your home is likely either in your attic insulation or your flooring. In order for asbestos to be dangerous it must be friable or floating around the air where it can be breather in. Asbestos only affects the lungs so you must do your best to not breathe in high concentrations of asbestos fibers. If you suspect you have asbestos you can have it tested. I can send samples away to a lab in Ontario where the quantity and type of asbestos can be identified. I usually suggest the customer simply stay away from the asbestos containing material but if this is not possible you need to take precautions. Visit my website for more on this topic.


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It is that time of year we all wait for, an end to winter. With the cold weather disappearance it is also a time for other problems to show up, In the case of today’s topic – Leaking ceilings.

You will notice I didn’t say leaking roofs as a leak in a ceiling could be more than just a leaking roof.  There are three primary reasons you may see water dripping from your ceiling in the spring or in the warm spells in the winter.

  1. Ice damming is the primary cause of roof leaks in the winter when ice is formed on the roof and eventually leaks into the ceiling of the home. Major factors in ice damming include ;
    1. Cold weather
    2. Snow loads on the roof
    3. Lack of insulation and or vapor barrier.
    4. Inadequate ventilation of the attic.
    5. Attic frost is the second most common cause of leaking ceilings in the Midwest. Factors that contribute to frost in the attic include;
      1. improperly vented bathroom vents
      2. large openings in the ceiling structure which allow warm air into the attic
      3. blocked or improperly insulated vent lines
      4. lack of insulation
      5. lack of ventilation
      6. 3.       Blown snow in the attic is the least likely cause of leaks but I do see it from time to time. The most common issues that cause snow to enter the attic is;
        1. Swirling snow on the roof which enters the roof vents
        2. Wind driven snow which gets into the fins of the turbines of the whirly birds
        3. Snow coming in through the gable and ridge vents.

Ok now that we know what the causes are of the leaks we need to discuss why the ceilings eventually leak. Most people have the misconception that insulation stops heat loss so the more insulation the better.  This is not quite right. First of all insulation is meant to slow the rate of heat loss, not eliminate it. This means if we add heat to a room for 15 minutes for example it will make that room comfortable for an hour let’s say.  The heat loss is primarily up through the ceilings as heat rises. In this case the type of heat loss is through conduction. If someone opens a door then the heat loss would be through convection. Radiation is the last type of heat transfer that plays more of a roll in the problems we see above the ceiling. Radiation is primarily from the sun.

Here is generally how the insulation and ventilation all works or doesn’t work together to keep your home healthy and dry. Older homes primarily will have issues with ice damming while newer homes will have more issues with frost forming in the attic. Older homes did not value air tightness and attic ventilation as much as we do today. For instance older homes did not use vapor barrier and many of the homes did not have any soffit vents. Some older homes did not have any eaves at all so adding soffit vents is impossible in that case. The heat in a room in the winter months is usually around 20 degrees while the outside temperature is – 20 or colder. This means the heat in the room is moving up through the insulation until it reaches the cold attic space. In a properly insulated and ventilated attic the warm air mixes with the cold (-20) attic air and eventually is pushed or pulled out of the attic harmlessly. The problem is that sometimes the warm air is not mixing with enough cold attic air above the insulation before it touches the sheathing on the roof. Now the heat energy in the warm air is being transferred to the roof sheathing (wood) where it in turn moves up into the snow above the roof.

Ice damming is now beginning to form on top of the shingles. As the warm air melts the snow on the roof it starts to run down toward the eave where it meets the outside wall.  At this point the water freezes because there is no longer a source of heat feeding the melting process. Each time there is excessive heat loss up through the insulation and the weather conditions are just right you will get more melting. If there is a large buildup of snow on the roof it will act to insulate the heat in and encourage more melting. Now the ice at the eaves gets thicker until it is 4 – 10” thick and large ice cycles will also hang off the eaves. This big chunk of ice at the eave is now acting like a water dam for more melt water that is running down the roof. When the dam is high enough the water behind the dam is high enough to run up under the shingles above it. Next thing you know you have water running in through your dam

Now that you understand ice damming you should know that there are several ways to prevent ice damming. I will assume you have done all you can with the insulation, vapor barrier and ventilation at this point. Once the home is experiencing ice damming it is a bit late to think about adding ice and water shield under the shingles and vapor barrier under the drywall. The suggestions below are ways to deal with ice damming from the outside.

  1.  The easiest thing you can do is pull the snow off the roof near the eaves so it loses its R-value and reduces the rate of melting on the roof.  Be careful doing this.
  2. Another approach is to try to melt the ice that has already formed at the eaves. The critical part is to provide a path for the water to follow off the edge of the roof. One approach is to use heat tape that can melt the ice. Another way is to use calcium or salt to melt it. I have seen some people use a nylon stocking full if road salt to melt a path through the dam but I don’t know how well that would work.

Newer homes with leaking ceilings are generally a result of poor ventilation and not because of vapor barrier or insulation although both those things can contribute to the problem of frost. The problem of frost is very similar to ice damming in that heat enters the attic from the warm conditioned space inside the home. The heat rises up until it reaches the sheathing and forms frost.  Improperly vented bath fans are a usual problem here as the warm moist air from a shower will add a lot of frost in the attic. Another possible cause of frost on the attic sheathing is extreme fluctuations in outside temperatures. This is something that is impossible to control and usually will not cause any problems inside the home. Visit this site as well for a great article on Ice damming.

A friend of mine tells me that the new closed cell spray foam is a very good insulating product. Should I pay a little extra for this type of insulation or just put in conventional insulation?

Spray foam is unmatched in its insulating qualities. The closed cell foam is best as it has trapped inert gases in it that resist most types of energy movement including radiant, convection and conduction. This means it saves a lot of heat energy. The greatest advantage to spray foam is its ability to expand into tight crevices where it would be difficult to insulate using conventional methods. The disadvantage of spray foam is the initial cost of application. Of course you need to research all the benefits of spray foam and decide if the benefits are worth the costs. Spray foam is best in areas such as the rim joists and other hard to reach areas. Often the bottom side of cantilevers is also spray foamed. Call a local foam contractor for more details. 

We are considering changing our kitchen range hood. Should we vent the new one outside or just let it recycle the stale air?

I have never understood recycling the air in a range hood. I know there are high end units out there that have great filtration systems but let’s face it the average range hood you buy today will have a cheap charcoal filter in it. The other problem with recycling the air is that these filters often are not cleaned for months and sometimes years. The fan either pumps the smoke and odors back into air, or the filter is so plugged the fan doesn’t even move the air. I recommend venting the range hood up through the ceiling or out a side wall. In either case be sure that the vent line is sealed well and that you use an insulated duct that will not frost up easily in the cold attic. Installing the vent incorrectly will create moisture and potentially stain the ceiling. The development of mold is even a possibility.


My Front entry is always cold, what is the best approach to fixing this?

Weather stripping is the most likely biggest draft stop component on a door system. If you have a poor seal you will have drafts. You can check the tightness of the door seals by placing a piece of paper between the door and the weather stripping. If the bill is snug but easily removed with a little pressure it should be fine. I always recommend steel insulated doors as they are better doors all the way around. The door jamb needs to be sealed tight as well. If the drafts are coming around the door jamb I suggest removing the trim and spray foaming around the area. Finally make sure the door sweep is in place and in good condition. 

I am installing a gas fireplace in my living room. Are there special rules for the gas connections and vent terminations?

The gas fireplace unit will have installation instructions. You must thoroughly read these instructions as each unit will have different rules about haw they are to be installed. For instance many of these units require a special cement board be placed around the unit and particular setbacks or standoffs must be observed when framing the unit into the wall. Several units I have installed required metal studs as well. In terms of the gas line the unit must have a gas shutoff in the same room as the appliance. On the outside of the home you must observe the minimum clearances to windows and doors and the ground. There are minimums in the code but the manufacturer instructions and minimums take precedence as they are the testing facility and they set the minimums for their unit. These minimums are usually greater than the code specifies as the minimums.

I noticed my bathroom vent is venting into the attic space. I was told this is not good can you explain?

Adding warm moist air into a cold attic is not a good idea as the change in temperature causes the air to condense when it reaches its dew point. This condensation can accumulate on surfaces and cause rot or enough moisture to form running water that will run onto the ceiling or down the walls. Of course once the ceiling or wall materials are damp you may face a mold problem. It is best to vent all fans out through the gable ends or through the roof. Be sure to use insulated vent lines. These photos show what can happen to the sheathing if warm moist air hits the sheathing. This roof was very spongy and almost at the point that would require re-roofing the area.i13 i15

I have frost in my attic, what causes this and how can I fix it?

Frost development in an attic is the result of heat that is moving up through the insulation into the cold attic. More often than not you may find that the heat is coming from a vent that is pumping warm moist air into the attic. On extremely cold days or days of vast swings in temperature you may see some frost. A little frost is inevitable on these types of days and should not create any issues. Excessive frost can cause stains in ceilings and even mold. Another cause of frost and even ice damming is lack of adequate venting in the attic. Make sure your attic has soffit, gable and roof vents if possible. Older homes often didn’t have any soffits so adding gable vents and roof vents is best in that case.Frost in ceiling

There is a switch above my thermostat in the hall. I was told this is for a principle house fan. Can you explain when I should use this fan?

The principle air fan or whole house fan is an inexpensive air exchanger. Most of the time this fan is located near the furnace but pulls air out of the home from a central location in the upper level of the home. Here is the premise of how it works. When you have high humidity in your home or undesirable odors such as smoke from cooking, you can turn this fan on to exchange the stale inside air for fresh outside air. When you turn on the fan it is linked to the blower on the furnace. By using this fan you are removing air from the home so the furnace blower is used as a mechanism to replenish this air. The fresh air is cold in winter so most people only use this fan when necessary. A central air exchanger is a better option but much more expensive.