How To Prevent Ice Dams



Ice dam inspections, thats a good one. Clear, to the point and timely, and applicable to Montreal now too.

One caution though, there are situations where the venting air flow can be out of balance and too high a velocity at the soffits can suck in falling snow.

In the high open volume spaces of the attic this snow falls out as the air slows down resulting in a patch of snow on top of the insulation deep inside the attic.

This snow will eventually melt and cause problems. As inspectors we have to be aware of this possibility and recognize the residual effects of this damage, even when we see attics zones in the summer months.



Hi Reuben,

It’s not so much a case of too much soffit venting but situations where high speed air movement is induced.

This most often occurs where there is high vertical rise between the intake vents (soffits) and the exhausts (here that’s usually ‘Maxi’ vents). The cause can be too many Maxis on the ridge line.

One place had 4 where one would have been sufficient. Some re-roofers think they are doing a good thing and put in too much without doing the math ratio calculations.

The first place I saw this problem had only one Maxi, but it had been converted from the original ceramic tile to an asphalt shingled roof. (Classic ceramic tile roofs are sieves. They are open to the wind, tiles on furring or purlins, no need to vent them.) When I opened that attic hatch, my hair stood on end, not from fright but from  the blast of air going up to that attic. This was stack effect in a 4 story building.

When the hatch is closed the pull from the Maxi at the peak drew air from 4 vents below dormer windows at floor level of the apartment below. The air was sucked up the rafter bays at high velocity because they were slim spaces filled 80% with insulation. This air carried snow with it and as soon as it hit the open volume of the attic above the ceiling it fell out. in other words it snowed.

The snow accumulated in small 2 or 3 square foot patches by the rafter bays and when it melted the water ran back down the bottom edges of the rafters to soak into the walls. (I had been called in to inspect and consult on the visible mold growing in the wall and ceiling corners.)

Other places where I’ve seen snow in attics are where garage (or one story wing) roofs attach to and are continuous with main higher attic space of the house. Again stack effect and big volume changes between the lower and upper vent positions are the common factors.

I hope that gives you a better picture of the sequence for attic snow.


We’ve received over 34″ of snow in Minnesota in the last 25 days, which makes this the snowiest December on record.  While the snow turns our state in to a beautiful white winter wonderland, it also causes nasty ice dams that wreak havoc on homes like never before.


The heavy amount of snowfall and temperatures in the teens have been the perfect conditions to create ice dams.  Water leakage from ice dams is an epidemic in Minnesota right now.  I have friends, neighbors, and family members with water leaking in to their homes.  It’s everywhere.  I’ve taken all of the photos below within the last two weeks.

Water in wall

Ice Dam

Ice on wall

Wet Insulation

Damaged Ceiling

Water at ceiling

The two things that everyone wants to know is how to get rid of ice dams and how to prevent ice dams.  As I mentioned in a previous blog, the only completely safe and effective way to get rid of ice dams is to hire a professional ice dam removal company; they’ll use steam, which will completely remove the ice dam.  Anything else is a hack method.

The best way to prevent ice dams from forming is to address the three factors in your attic that contribute to ice dams; insulation, ventilation, and attic bypasses.

Attic Bypasses (air leakage)

This is the largest contributor to ice dams.  In almost every house that I inspect to determine the cause of ice dams, I find attic bypasses directly below the beginnings of the ice dams.  Attic bypasses are passageways for warmed air to enter in to the attic space, and traditional insulation won’t fix this.  The photos below show some common attic bypasses that I can find in just about any older house.

This first photo shows one of the largest and most common bypasses – the space around the furnace and / or water heater vent.  Sometimes these are huge. The one shown below is quite small.

Bypass at furnace vent

In the photo below, you can see several holes in the top plate of a wall that were drilled for wires to pass through.  These holes could all be easily filled with spray foam, but finding these holes all over the attic would be a challenge without first removing the insulation.

Bypasses at bore holes

With additions, the transitions between the ‘new’ and ‘old’ construction seem to always be sources of attic bypasses.  I had to dig through a lot of insulation to find this gap, but I wasn’t surprised at what I found.

Bypass at addition

When plumbing vents enter in to the attic, the space around the vents needs to be sealed.  This one obviously wasn’t.

Bypass at plumbing vent

Some older houses have whole-house fans that are designed to run on hot summer nights; these fans are gigantic sources of heat loss, because they’re usually not insulated or sealed up.  I took the photo below from inside the attic without a flash on my camera.  There’s some crazy heat loss occurring there, and as you might imagine, there was a huge ice dam nearby.

Bypass at attic fan
What makes most of these attic bypasses so difficult to locate is that they’re almost always buried in insulation.  Finding these buried air leaks turns in to an educated guessing game.  Lately I’ve been using an infrared camera while doing ice dam inspections, and I’ve found it to be very useful for finding these hidden passageways.

If you plan to have more insulation added to your attic, it’s probably a worthwhile investment to have all of the existing insulation completely removed and the bypasses sealed up before you re-insulate.  There’s just no way to find all the bypasses if you don’t do this.  If your home is over 20 years old, it’s almost a guarantee that you’ll have attic bypasses that need to be sealed.  It’s a shame that so many insulation contractors just add insulation on top of what’s already there without sealing the bypasses.

I also practice what I preach; I’m such a firm believer in removing insulation and sealing bypasses before re-insulating that I did this at my own house, my parents had this done at their house, and my sister had this done at her house.  It makes a big difference.


This is a basic concept that everyone understands; you need insulation in your attic.  If there are voids in the insulation, they need to be fixed.

My first choice would be to have about two inches of closed-cell foam insulation applied to the attic floor, and then have several inches of cellulose installed on top of that.  The closed cell foam would seal up every bypass, and the cellulose would be a cost effective way to get the insulation level up to current requirements.  The drawback with this method is cost; closed cell foam ain’t cheap.

My next choice of insulation would be all cellulose insulation, with all of the bypasses sealed first.  I prefer cellulose to loose-fill fiberglass because it has a higher insulating value per inch, it seems to do a better job of stopping air leakage, and it’s not itchy fiberglass.  I’ve been digging through a lot of fiberglass covered attics lately, and my arms start getting itchy just thinking about it.

My last choice of insulation would be loose-fill fiberglass insulation… not that there’s anything wrong with it.  I personally just don’t like dealing with fiberglass.

What about fiberglass batts?  You know, those big rolls of fiberglass?  No way, Jose.  That stuff is impossible to install properly.  Fiberglass batts leave gaps all over the place that add up to an exponential level of heat loss.  I’m pretty sure that fiberglass batts are installed in attics exclusively by handy homeowners.


Having adequate ventilation for the attic space will help to keep the roof surface cold, which will help to prevent snow from melting, which will help to prevent ice dams.  The best way to ventilate an attic space is with continuous soffit vents and continuous ridge vents, but this isn’t always possible.

Reuben inspecting ice dams

If you don’t have enough soffit vents, add more.  If you don’t have enough roof vents, add more.  You can’t have too much ventilation.  If your soffit vents are dirty, clean ’em or replace the grills if they’re painted shut.  Grills are cheap.  If your soffit vents are blocked with insulation, you need air chutes installed at the eaves to prevent the insulation from blocking the vents.

When all else fails…

If you’ve already done everything you can think of to fix your ice dams but they keep coming back, or you’ve hired a contractor to fix your ice dams but they seem to be scratching their head a lot or coming up with a bunch of different guesses, call a home inspector.  We look at this stuff every day, and some of us even specialize in ice dam inspections.

In some cases, it’s not cost effective to control ice dams from inside.  Next week I’ll be blogging about controlling ice dams from the exterior.


Reuben Saltzman, Structure Tech Home Inspections – EmailMinneapolis Home Inspections

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Original blog post on ActiveRain: Link to Blog Post