Minnesota got dumped on last weekend with nearly fifteen inches of snow in some areas. With hardly any snow last winter, we almost forgot what a real snow storm looked like. I haven't heard about any concerns over ice dams yet, but I suspect they'll be coming very soon. The perfect conditions for ice dams are large amounts of snow and temperatures in the teens and twenties, which is what we're expected to have this week.
Two years ago, many Minnesota homeowners experienced ice dams like never before. The two things that everyone wanted to know was how to get rid of ice dams and how to prevent ice dams. Today, the focus is going to be on prevention.
How Ice Dams Form
Because it's been a couple of years since I've blogged on the topic of ice dams, here's a quick refresher: ice dams are literally dams of ice that form on roofs and cause water to back up. The dams form when the snow that touches the roof melts, and then that water freezes again before running off the roof. This usually happens at the edges of roofs, but not always. Here's a great diagram showing how this works, courtesy of Steve Kuhl.
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 air leaks.
Attic Air Leaks (aka - attic bypasses)
This is the largest contributor to ice dams. In almost every house with ice dams, there will be attic air leaks directly below the beginnings of the ice dam. 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 can be found in just about any older house. The image series below shows how an infrared camera can be used by a home inspector or energy auditor to locate these bypasses.
The photo below 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 relatively small, but allows a lot of air to leak up in to the attic.
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, or performing an infrared inspection with a thermal imaging camera. The insulation had to be pushed aside to find these and take this photo.
With additions, the transitions between the 'new' and 'old' construction seem to always be sources of attic bypasses. The gap below was easily identified with the use of an infrared camera, but a lot of insulation had to be moved to get to the bottom of it.
When plumbing vents enter in to the attic, the space around the vents needs to be sealed. This one obviously wasn't.
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. The photo below was taken from inside the attic without a flash. There's some crazy heat loss occurring there, and as you might imagine, there was a huge ice dam nearby. No infrared camera needed to find this.
Old ramblers often have stairwells with nothing covering the top - you could fit an entire family in this dead space. It looked fine in the attic until the insulation was pulled away to show that this area was completely open.
The space around masonry chimneys is also a notorious location for attic air leakage.
What makes many of these attic bypasses so difficult to locate is that they're almost always buried in insulation. Finding these buried air leaks can turn in to a guessing game for someone without a lot of experience in digging through attics.
If you have these types of bypasses in your attic, my recommendation is to have an insulation contractor seal the air leaks. They'll know where to look and how to seal them properly. If you want to do the work yourself, download this guide from the Minnesota Department of Commerce - Attic Bypasses, and this guide from Building Science.com - Attic Air Sealing Guide and Details. These guides both give some excellent information on how to properly seal all of these air leaks yourself, and they discuss other concerns that need to be considered when performing attic air sealing, such as combustion appliance safety, knob & tube wiring, and vermiculite insulation.
If you plan to have more insulation added to your attic, have the air leaks sealed first. This can't be stressed enough. This is the driving force behind ice dams, and is even more important than having insulation added. If your home was built before about 1990, it's almost a guarantee that you'll have attic bypasses that need to be sealed all over the attic. Unfortunately, many insulation contractors just add insulation on top of what's already there without sealing the air leaks.
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. If there isn't enough insulation, add more. The current minimum requirement for new homes in Minnesota is insulation values between R-38 and R-44. This means about 10" - 12" of cellulose, or 12" - 16" of loose fill fiberglass, depending on the manufacturer.
I'll follow up with another post on different attic insulation methods and challenges with insulating older attics in the near future.
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. Ventilation is required for attics, but it's the last thing that should be considered when troubleshooting the causes of ice dams.
The traditional way to ventilate an attic was to have half of the ventilation provided by vents installed low, such as soffit vents, and the other half at the top, such as ridge vents. According to information shared at a recent seminar put on by Dr. Lstiburek here in Minnesota, a better way to ventilate the attic is to make the ratio about 1/3 high and 2/3 low. This means way more soffit venting than ridge venting. If the soffit vents are dirty, clean them or replace the grills if they're painted shut. Grills are cheap. If the soffit vents are blocked with insulation, install air chutes at the eaves inside to prevent the insulation from blocking the vents.
If you've already done everything you can think of to fix your ice dams but they keep coming back, or you hired a contractor to fix your ice dams two years ago but the ice dams have returned, call a home inspector or an energy auditor. We look at this stuff every day, and some of us even specialize in ice dam inspections.
Reuben Saltzman, Structure Tech Home Inspections