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Ice damming, attic temperatures and insulation

Most people in the world will think of home insulation only in terms of energy savings. However those that live in very cold and extreme climates have other concerns too. Heat leaking onto your roof can damage the house.

Attic ventilation

I am proving here a bit of background information because this is a very Canadian problem and mostly unknown to people in less extreme climates such as e.g Europe. The temperature difference between summer and winter are here about 70'C. The summers are very warm and the winters cold. To live comfortable in those extremes we have heatings systems that use air. Such a forced air system allows you to both heat and cool the house using the same air ducts. The attic is not an extra room that could be used for storage. The attic is only for ventilation and insulation of the home. The roof has a vent at the top and metal plates with small holes known as soffits function as air intakes. The insulation does not sit directly under the roof. Its on the attic floor and there is a lot of insulation. Typically 30cm to 40cm of glas fiber wool. The air flow in the attic is supposed to look like this:


How attic ventilation is supposed to work

A cold attic and a cold roof

The main point of this type of ventilation is to keep the roof cool in winter. It has as well the added effect that the building material can breath. Should ever any drop of water or condensation or humid air get in then it can evaporate again and escape through the vents. Glas fiber wool is excellent for this application because it can breath too and does not trap water. The main point is however to keep the roof surface cold such that the snow on top of the roof does not melt. The snow remains often a powder and wind will blow some of it off after a couple of days. This prevents excessive snow accumulation on the roof and "ice damming" at the same time.

Note that this kind of attic ventilation works well in cold and mainly dry climates. It can not be used in hot areas with a lot of humidity for long periods of time.

What are ice dams?

Of course in reality there could be all kinds of reasons why the attic ventilation and insulation does not work as expected. Many factors play a role and I get to those in the troubleshooting section. Let's first understand what ice dams are and why you should care. If the attic is warm enough to melt some of the snow on top of the roof then the warm water will run down the below the snow towards the eaves. The eaves are always cold and the water will freeze again. A block of ice forms. The block of ice is normally below the snow and difficult to see but in most cases houses with ice dams will as well have icicles and they are easy to see.


Ice dams on the eaves (the snow on top of the roof is not shown).

When the weather gets later in the season much warmer then all the snow will melt. A big puddle can form behind the ice dam. Roofs with a slope and shingles work only if the water can run down the roof. Standing water on such a roof will back-up behind the shingles and run into the attic and from there it can run into the house, stain the walls and cause other damage. The amount of damage depends on the amount of water that enters the home.

How cold should the attic be?

This is the main reason why I wrote this article. I could not find any information as to how cold and attic should be for it to work properly. If you try to find informaiton on this subject you will find many posts from people that think that the attic will have the same temperature as outside once it's ventilated. That is obviously wrong. Anybody with an unheated garage attached to the house will know that it is most of the time above freezing in that garage even when the garage window is slightly open to allow for ventilation. There will always be some heat leak from the house into the attic and it is important to have the attic a bit warmer otherwise condensation and mold would form. My home had problems with ice dams for many years and it took me a while to fix them. I had sensors installed in the attic to monitor what was going on. This allowed me to collect data, something that nobody seems to have done before.


Attic temperatures as a function of the outside temperature.
Red: temperature at the top near the ridge of the roof. Green: about half way between ridge and eaves. All temperaturs in 'C.

The above graph shows temperatures at a point where I had the insulation just good enough to prevent ice dams. This is the limit. In general you want to have a slightly colder attic but this would be just good enough.

The curve is approximately linear down to -10'C. Theoretically it should be totally linear. I am not sure what the non-linear effect is. You can use the following formula to calculate the max. attic temperature (Ta) given a known outside temperature (Toutside).

Attic temperature calculator

Outside temp.:
Max attic temp:
Ta = 5 + 11/13 * Toutside The formula is valid for outside temperatures from around 0'C down to -10'C. All temperatures in 'C.

An attic temperature lower than Ta is better. However your attic should alwasy be at least a degree warmer than the outside to avoid mold and condensation in wet weather conditions.

The above measurements were done using a roof with a slope of 1/3 (roofers call this for some reason a 4/12 slope). Ice dams are obviously less of a concern the more slop you have since a higher slope will reduce the size of the water puddles that can form behind an ice dam. The temperature measurements where always taken on days with no wind. Strong winds can influence the temperature in vented attics significantly.
Climate/Weather conditions: Temperatures are throughout the winter below -5'C with the exception of very few days. Those periods with daytime temperatures above -4'C are no longer than 3 days.

What does an ice dam look like?

Most ice dams are not visible because they are forming below a cover of snow. It's however possible to see the actual ice in very special weather conditions where not a lot of snow had fallen and most of the snow did already melt. The big block of ice will melt last and you can see it.


An ice dam, visible after a few warm days. The dam melts last.

Icicles hanging from your gutters are normally the only way to know that you have an ice dam. The icicles fall off first when the snow starts to melt and the ice dam disappears last.

Ice dam troubleshooting, find the cause

Most contractors will tell you that you just need to add more insulation and improve the ventilation of the attic. It sounds simple but the problem might have other causes. It is alwasy nice to get "expert advice" but in the end there is nobody that knows your house better than you. You are observing it throughout the year and in different weather conditions. Nobody will be a better expert than you, the home owner. You can find the cause of the problem if you keep your eyes open and observe carefully. Here are is a list of things to investigate.

Ventilation problems

Ventilation is however only in very few situations the cause of the problem. You don't need much ventilation if you have proper insulation.

Heat leaks and insulation problems

Can I use an infrared thermal imaging camera to locate the problem?

Infrared cameras are very good to locate surface temperature differences. Snow is a good insulation material and the surface temperature does not change much when the temperature further down changes. Ice dams form only when some snow deep down melts because the upper layers of snow trap the heat. The top snow layers don't really change their temperature in any significant way. If there is no snow on the roof then the temperature differences will be very small since most heat can radiate immendiately (the insulating snow layer is missing).

In other words thermal imaging will only spot a huge problem when used on the outside of the house. Here is the same area where you saw the ice dam in the photo further up shown on a infrared camera.


infrared image of the area prone to ice dams, you don't see anything suspicious.

As you can see the infrared thermal imaging is not capable of locating heat loss when used on the outside of the house because the temperature differences are very small. If you want to see those heat patters on your roof then you have to wait for a day in fall or early spring where the roof is just covered with a very thin layer of snow and the outside temperature is just around zero. On those days you will then see a snow pattern that corresponds to the heat pattern and you are looking for areas where the snow melts faster. This is where you have problems with insulation.


Heat loss pattern visible with a thin layer of snow on the roof on a "warm winter day".


Infrared thermal imaging is not very useful to locate ice dams on the outside of the house but you can locate the problem by using such a camera on a cold winter day on the inside of the attic.

It allows you to find the areas where you have heat leaks below the insulation. Here is a nice example. The camera is looking towards the eaves. You see heat leaking through the insulation just below the styro foam baffle.


Just in front you see between the rafters a styro foam baffle and there is a heat leak visible on the insulation below the baffle.

Heating ducts in the attic

It is not the best idea to put the heating ducts into the attic since there is always a chance that they leak air but it is commonly done because it is easy to install them in the attic. Glas fiber wool insulation can not seal a stream of hot air. It is complicated to fix this situation but it can be done.

I have heating ducts in the attic and that is why I worked on this problem over several seasons and I managed to fix the problem such that I will only get very small ice dams from time to time and in most winters, especially the colder ones, I will have none.

You need to find all the leaking joints on your ducts. The most efficient way to find them is to use an infrared camera. Get such a camera on a cold winter day and climb up in your attic. Take notes of all the leaks and mark them. When it is a little bit warmer (not too warm) you go back into your attic. You dig out areas of the duct that you marked from under the insulation and you fix the leaking joints with aluminum duct tape.


A duct joint leaking air from under the insulation.

While doing the survey with the infrared camera you will as well see ducts that have too little insulation around them. They are not leaking but they are too high above the attic floor and the insulation around them is too thin. Top-up the insulation around those ducts.

I needed two seasons to fix everything. In the first winter I searched for leaks. Towards the end of that winter, when it is more comfortable to work in the attic, I fix those leaks and I toped-up insulation where needed. In the next winter I went back to check again. I fix any remaining problems. You need multiple seasons because the infrared camera shows the leaks only on very cold days. It does not see anything if the background temperature is too hight. It would be like taking a photo of a white paper on a white background. Such a photo would not be very good. You need a different background color.

Heat leaking from the living space into the attic

The most common cause of ice dams despite "proper insulation" is a heat leak from the living space. It can be a small hole of some kind. Here is a case where the hot air vent from the heating system is located in the ceiling. It is already suspicious to see no mouting screws on the sides. Somebody did a quick job with a bit of glue.


Forced air vent in the ceiling looks suspicious, let's take it off.



The duct sits loose in a much too big opening.


Pressurized air from the duct will not only come down into the room. About 10% will leak up into the attic (yellow arrows).


Proper duct installation: close all the gaps.



The cover is back on and mounted properly.

Icicles and ice dams because of the sun?

The sun could melt the snow on your roof and cause ice dams even when everything else is working perfectly. This is however a very rare weather situation in Canada. The sunny days in winter are usually the coldest. It is the dry air from the north that pushes away the clouds during the winters in Canada and it is therefore very cold during those sunny days.

In other words ice dams do not form in Canada because of heat from the sun. It's a problem that does not exist here but it might exist in other areas that have sunny warm days during the winter month.

What about a foil under the roof shingles?

A strong foil such as "grace ice and water shield" is always an extra protection but the foil comes in roles of a fixed width. Water can eventually get behind the foil. Remember as well that all the nails holding the shingles will go through the foil and damage it a bit. Those special foils have kind of an asphalt glue on the underside and they will seal the area around nails but after a number of years the glue will get brittle. It is always good to fix the root cause of the problem. Remove the cause of the ice dam and use "grace ice and water shield" in addition.


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