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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.
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
Attic temperatures as a function of the outside
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
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
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 is however only in very few situations the cause of the problem. You don't need
much ventilation if you have proper insulation.
- Are your soffits acutally functioning and can the air get intro the
- Is the insulation inside the attic too high and therefore blocking
the air near the eaves? -> You can buy styro-foam rafter vents that
can guide the air along the bottom of the roof.
- Was spray-in insulation (loose glas fiber or cellulose/paper) used in the attic? If yes then take an
inspection camera and look from inside onto the soffits. Your contractor might
have been spraying all over the place and did accidently put a nice layer
of insulation on the inside of the soffits which reduces the efficency of those
- In older houses where soffits had been retro-fitted: Are the soffits
really open or is there a board behind them with just a few small openings
in the board?
- How can the air get out of the attic? You should have
"Maximum"-vents on top of the roof for optimal ventilation. Older houses might be ventilated
on the sides. Those side vents can not be combined with vents on the
top of the roof since this would just short circuit the air flow.
Heat leaks and insulation problems
- More insulation is not always better. With glass fiber insulation
you need in Canada a layer of about 30cm to 40cm. If you have already
about 30cm and you get icicles then adding more insulation is not going
after the root cause of the problem.
- Can air from the rooms below the attic leak into the attic?
- Are electrical boxes where lights are installed in the rooms below
properly covered? Pot lights are especially a concern and they might
guide a lot of warm air from the living space into the attic.
Those electrical boxes and pot lights can be covered with an air tight box or wraped with aluminum foil.
- Is the access to the attic (hatch) air tight and insulated?
- Can air from the heating system leak into the attic?
- Do you have heating ducts leaking air into the attic?
- Are the air vents properly installed in the ceiling or can they blow air into the attic ? (see further down for an example from my house).
- Go on a cold day with an infrared thermometer into the attic
and check the surface temperature of the insulation around the heating
ducts to find leaks. Even better if you can get an infrared camera but they
are very expensive.
- Can air from bath-room vents leak into the attic?
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.
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.
Copyright © 2004-2017 Guido Socher