Building Science-Ice Dams

This year seems to be a good (or bad depending on how you look at it) year for ice dam problems in my area, the intensity of ice dams seems to change year to year.  I was recently on an ice dam diagnostic with an insulating contractor and a general contractor, the home was built in the early 1990’s and there is evidence there have been issues from the beginning.  We spent a couple hours testing this home, I’ll get into what we found in a bit.

First, how does an ice dam form?  It’s really pretty simple, in areas that receive snow fall, the snow will accumulate on the roof.  Some mechanism (we will get into those in a bit as well) causes the roof sheathing below the snow to warm which causes the snow to melt.  As the now liquid water runs down the roof, it comes in contact with the roof eave, which is typically cooler and causes the liquid water freezes at this location.  The ice builds up over time until it eventually forms an “ice dam”.  Often this accumulation of ice forms icicles which hang on the eave, this can be beautiful and dangerous at the same time.

So, what are those mechanisms that form the ice dams?  There can be many causes that form ice dams.  Dark colored siding on the South facing side of a home can create some heat that moves up to the roof deck and melts snow.  Mechanical ventilation and combustion exhaust exiting the home out a wall near a roof eave could also create enough heat to melt snow.  Both these are less common.  Another issue that can be more common, forced air ductwork located in an unconditioned attic space, but the biggest reasons ice dams will form is due to air leaks at the ceiling or upper wall, attics with inadequate insulation levels, especially near the eave, or lack of attic ventilation, though increasing attic ventilation probably won’t cure ice dam issues.  The air leaks and lack of insulation are typically the problems.

The current diagnostic.  The ice dam diagnostic I was recently on (top photo) had a very convoluted roof design, lots of changes in pitch and several valleys.  The entire interior ceiling was vaulted and finished using wood paneling.  The day before we arrived, the homeowners had water dripping into the home under the area of the ice dams.  The roof was new last year, I don’t think there was a roof leak, I believe air leaks in the vaulted ceiling caused frost to form on the roof sheathing.  The day of the water leaks, the temperature had risen to the upper 20’s, warm enough to allow the frost that had accumulated over the winter to melt.  This melting frost also managed to find its way to the soffit and eave ventilation where it formed small icicles.

My diagnostic process for ice dams.  To start I typically have a chat with the homeowner to gather any information on the home; the year it was built, if they know any of the construction details such as amount and type of insulation used, and if there is any window frost or moisture during the winter, (gives me an indication of how tight the home might be).  I also give them a printed copy of Building Science Corporation’s BSI-97, De-Icing Ice Dams, Dr. Joe Lstiburek does a great job describing the science behind the issues. 

I then take out my thermal imaging camera and conduct a quick scan around the home before I start the blower door.  In the case of this home, I believe we identified an area of moisture in the ceiling paneling, this moisture was close to where the customer was having the water dripping into the home the day before.  I then start the blower door and scan again with the camera.  The blower door was pulling 4,000 CFM of air into the home, I did not measure the home for volume, but I would estimate that to be around 8 ACH50, very leaky for a very cold climate.  Below are a couple thermal images I took; these are directly under the area where water was leaking into the home the previous day.

The ceiling directly above these areas has attic knee walls used as storage.  This ceiling terminates at the floor of the second level.  There was a gap between the floor and ceiling that we could get our fingers up into and touch the fiberglass insulation, the air control product, which is bubble wrap, (not a product I recommend using for air/vapor control) ended before the roof met the floor, the air control was not continuous, this is where the air was leaking into the roof and warming the sheathing to form the ice dams.  The hand framed roof is not deep enough to accommodate a high level of insulation, this will also contribute to the warm roof sheathing.

What’s the fix?  In this home, its air sealing.  The least costly, and also the least likely to succeed is air sealing from the interior at the ceiling to floor intersection.  The insulating contractor is planning on using a box kit (a portable two part closed cell spray foam kit that typically comes in two 35 lb. cylinders) to seal the air leaks.  If you are trying to accomplish air sealing this way, I recommend using a blower door to check the results as the air sealing progresses.

A better way, which is also very expensive.  Remove the roofing materials and roof sheathing to apply closed cell spray foam against the interior ceiling finish, then reinstall the roof sheathing and replace shingles.  This will not only air seal the ceiling, but also increase insulation levels from the roughly R-3.5 per inch of the fiberglass insulation to the R-7 per inch of the closed cell spray foam.  I would recommend leaving a space under the roof deck for ventilation.  Why is this a better way?  Below are a couple thermal imaging photos of this roof in other locations.  There are many more areas of air leakage than just where the roof meets the upper-level floor in this home.

The best way, add insulation to the top side of the roof deck.  This option will warm the original roof sheathing so it is no longer a condensing surface, it will also keep the roofing material colder, eliminating the ice dams.  This option can be very complicated, it requires a specific ratio of inside roof insulation to exterior insulation, in my area, climate zone 7, the ratio is 61% exterior insulation.  This home has R-30 fiberglass that is existing in the roof, we would need to add R-49 above the roof deck to assure we warm the existing roof deck enough so there will be no condensing surface, that’s 10 inches of XPS foam and a total roof R-value of 79!  There can be no roof ventilation at the original fiberglass roof insulation, the existing ventilation need to be sealed off.  The existing roof sheathing would become the main air control layer.  A second layer of roof sheathing would be installed above the exterior insulation.  This all results in a very thick eave and facia, a detail that can look out of place.  This is a very complex and expensive assembly.  I’ve only seen this once in my market, and it was done incorrectly, the contractor didn’t use enough exterior insulation and did not seal the original roof/attic ventilation. 

My hope with this project is the insulating contractor can be successful with the plan to air seal from the interior.  It might work if they use a blower door to direct the air sealing.

Ice dams are common in my climate, especially in the older homes.  A winter walk around my neighborhood will result in the number of homes with ice dam issues quickly increasing into the double digits.  Most choose to live with the ice dams, trying to reduce the risk by raking the snow off their roofs.  For others, they want the conditioned corrected.  Understanding how they form will determine the best way to correct the issues, most of the time, the fix is expensive and complicated.



3 Replies to “Building Science-Ice Dams”

  1. Hi Randy,

    My neighbors across the street had ice dams, the home was built in 1977. The majority of the ceilings were vaulted and the roof had recently been replaced. My solution for this house was to work from the inside. I added a dedicated ventilation space (1 1/2″) under the roof decking, increased the fiberglass insulation in the remaining 2×10 rafter space and added 3″ of polyiso under the rafters. I then added wood strips below the rafters for the ceiling finish connection. The new configuration added up to an R-50 and was very airtight due to the polyiso with the seams foil taped. Originally the insulation was an R-22 in a 9 1/4″ rafter space. The roof is ventilated at the eave and ridge.

    The house tested at 7.3 ACH50 before the work and 4.1 ACH50 after the insulation upgrade. I only addressed the ceilings, there are other areas that could use some air sealing measures to further reduce infiltration. The house had a heating usage of about 5.5 Btu/sf/hdd before the work and 3.82 Btu/sf/hdd after so we did improve the efficiency by a fair amount. I worked with a local energy rater to document the progress. To learn from improvements you need an established baseline.

  2. You are so right about each winter being different with regard to ice dams. The house I referred to above also has leaf guards, an aluminum cover over the gutters, rounded on the edge. These leaf guards seem to want to collect ice on all sides of the house but the south. This year to date they are ice free on all sides. In previous winters they would hold ice in a thickness commensurate with the pitch of the gutters. Thinner on the high end and thicker on the low end. After the ceiling insulation project, the ice did not extend up on the roof for any distance, just on top of the metal leaf guards. We discussed removing the leaf guards and screening the gutters instead, we left them as is, for now all is well.

    Your photo does show a complicated roof connection, it might perform with full insulation and an airtight ceiling. When installing a new roof I use a lot of ice and water shield. A layer running under the valleys and then beyond code from the roof edge. In potential problem areas I will use all ice and water protection for the underlayment. Have seen too many times the bath fans exhausting into the attic space further adding to the attic frost buildup.

Leave a Reply

Your email address will not be published. Required fields are marked *