Ice Damming – What is it and why does it matter?
Ice damming is a problem in many homes. It occurs when the roof of a building becomes snow covered. If the roof deck is warmed from the inside to a point where the snow on top of it melts, that melted water then flows down the roof deck. Ice dams form when that water reaches a point where the roof deck is below freezing. This causes the water to freeze on the deck. More water continues down the roof and freezes at about the same point. Over time, this freezing of the melt water causes a dam to occur, and the melt water builds up behind it where the deck is above the freezing point.
If the roof covering is impermeable, such as on a flat roof, then this is not a problem. However, most roofs are covered with asphalt-based shingles, or some other layered covering, like shakes or shingles. These types of coverings are only designed to shed water down the roof. They are not waterproof. Therefore, if the dam builds up high enough, the water can get in underneath the covering and leak into the building. This can cause a great deal of damage to the insulation, structure and wall finishes in the home. In severe cases, where the problem is ongoing, mold can begin to develop.
Not only can ice damming cause interior damage, but it can also damage the shingles themselves. It can also damage gutters. The weight of ice build up can bend or twist the gutters. In extreme cases, ice build up has been known to tear gutters right off the building.
This diagram shows typical ice dam formation:
As you can see in this case, the ice dam occurs where the roof deck extends out over the side of the building. This is where the roof deck is no longer warmed by heat escaping the house into the attic. It is here that we begin to see the solution to ice damming issues.
It seems simple enough. All you have to do is keep the roof deck at the ambient outside temperature and you shouldn’t get ice dams forming. If the air is above freezing, the deck is also, all the way to the gutters. Any melt water flows down the roof, falls into the gutters and is taken away. Below freezing, there is no melt water forming. Simple as that! As with most things in a complex system such as a building, it is never that simple!
The fact is, heat escapes from buildings. Whether you put heat into a building with natural gas, solar, geothermal, a fireplace, or some other method, it will find a way out of your house. If that heat hits the roof deck, it may cause the snow on the roof to melt. It turns out that keeping this from happening requires a multi pronged approach. Let’s work on it from the top down:
One solution to preventing interior damage is to make the roof covering impermeable to water. Roll on, torch applied, tar and gravel, and peel and stick roof coverings are all impermeable when properly applied, but are generally not good choices for a sloped roof.
Some people have had good success with metal roofing reducing their issues with ice damming. This is mostly due to metal roofs tendency to more easily shed snow. Metal roofs still have joints however, and these joints are susceptible to leaking under ice damming conditions. Placed up on battens, with good ridge venting, metal roofing can also provide a layer of thermal separation between the roof deck and the roofing, lowering heat transmission and therefore melting. It is not the cure-all some would have you believe though.
On overlapping style roofing coverings, such as shingles or shakes, a water proof membrane or two layers of roofing paper is supposed to be applied as a backup layer of protection against water penetration to the roof deck. This protection should be applied from the edge of the roof, up to 12″ from the inside of the exterior wall. Depending on the slope of the roof, a single run of standard width membrane may not offer enough protection. The intent is to provide a water proof layer against the roof deck, should water back up behind the lower shingles on the deck. Depending on the severity of the ice dam, this may not be enough protection.
Regardless of protection, damage from ice dam formation can still occur to the roof covering itself, and to the gutters.
Ok, so let’s keep the heat out of the attic. We’ll just insulate it.
That sounds great, and indeed, at one time it was touted as the answer to the issue. However, there are a couple of problems with that idea:
Insulation does not stop heat flow. Insulation resists a transfer of heat, but never stops it completely. (R Value, that number you see on insulation bales and is much touted when talking about how insulated a building is, is defined as resistance to heat transfer) From a practical standpoint, there is only so much insulation you can put into a given space. Complicated structures, such as semi–bungalows, or homes with vaulted ceilings offer particular challenges to using insulation to solve ice damming problems.
Many older homes were not built as air tight as homes are built now. Ceiling light fixtures and other penetrations allow conditioned air to escape into the attic as well. If you can access these holes, sealing them can go a long way towards lessening the amount of heat being lost to your attic.
Using the correct type of insulation is important as well:
Fiberglass insulation does not offer a great deal of resistance to air flow. Blown in fibreglass, used extensively in attics due to its ease of installation when compared to batt insulation, is particularly poor at reducing heating loss due to leaking air. Fiberglass also has a particularly poor performance curve as the temperature lowers. Studies have shown that at 0 Degrees C, fiberglass insulation offers about half of its rated R value. Just when you need it most is when it performs the worst.
Blown in Cellulose insulation, at greater than 3″ thicknesses, has been proven to be effective at reducing heating losses due to air flow. It also holds up better in the cold, with its R value not too dependant on air temperature. This makes it a great choice for attics, especially attics in older, more leaky homes.
By far the best type of insulation for attics, indeed for your entire home, is spray foam. This type of insulation offers a great R value per given thickness. It also seals insulated surfaces tight against air flow. Many people have found great success spraying the bottom side of their roof deck with foam. Not only does this help keep heat from reaching the roof deck, but it also can create more conditioned space in the home, a decided advantage over other methods of insulation. Given the right thickness of insulation, the rate of heat transfer to the deck may be low enough that melting does not occur. It can be quite expensive to have this work done however.
No matter what type of insulation you choose, heat is eventually going to make its way to the roof deck. Taking that heat away from the deck leads us to what is, in most cases, the key to solving ice damming problems.
Roof ventilation is one of the least understood dynamics in home building and maintenance. This is mostly because of the fact that roof ventilation is usually handled by roofing contractors. Many, if not most roofing contractors get their information primarily from companies which are based in the USA. In most of the States, the biggest roof ventilation concern is around getting rid of heat build up in the summer. The extreme heat that can build up in an attic and on a roof deck under a blazing sun, at high ambient temperatures can really damage the roof covering, to say nothing of what all that heat trapped in your attic can do to your air conditioning bill!
The challenge is that we have exactly the opposite problem in most of Canada. Granted, our summers can get hot, and the sun still bakes our roofs, but when we really need to get the heat out of our attics is in the winter. Very little of the literature from the major roofing manufacturers talks about this important dynamic. Holding the heat in the house sounds like a great idea from an energy savings perspective, but holding that heat in the attic warms up the roof deck, and causes ice dams to form.
In a perfect world, the attic of every house would be ventilated like this:
Ambient temperature air enters the attic through the vented soffits, and flows through the joist space above the edge of the building. As the air in the attic picks up heat from the interior of the house, through the insulation or air leakage, it warms up and rises. At the highest point in the roof, there is an opening, in this case a ridge vent, which allows the heated air in the attic space to exit. This keeps the roof deck cool and voila! No Ice dams!
Of course, the real world seldom works this way. Add snow cover, and the ridge vent might not work. Often, soffit vents are missing or blocked. Improper installation of attic insulation can block the rafter space so that air cannot travel from the soffit space to the attic space. Those ubiquitous “mushroom” style air vents, which are most commonly used on roofs, often get buried in snow and are rendered non-functional. Again, just when we need them the most is when they fail to perform. If you add on additions and other changes in structure, in many cases this type of ventilation is impossible.
Homes with vaulted ceilings, and Semi-bungalows in particular, present additional challenges. Here are the best solutions to properly venting those spaces:
Once again, the real world can interfere with this perfect application of theory. Typically, in vaulted ceilings, or the sloped portion of a semi-bungalow ceiling, the builder will pack the space with as much insulation as possible, to gain the most R value they can. And I have seen it packed! Packing fiberglass into areas actually decreases its insulating qualities. If you stuff R20 insulation, normally for a 2×6 wall or ceiling, into a 2×4 wall or ceiling, you’ll actually end up with less of an R value than you would have had you used the standard R-12 insulation. So often, in an attempt to handle a difficult to insulate area, the builder will actually make things worse, to say nothing about the fact that there is no way air can get to the roof deck to cool it off.
The real world tends to be about compromising, and coming up with the best solution, recognising all of the factors involved.
One of the mistakes I see contractors and homeowners make is assuming that because they put in a couple of vents, or a big Whirlybird on their home, their attic space is vented. The style and placement of those vents is critical to proper ventilation. The laws of physics do apply!
Warm air rises. Many people correctly figure that in order to let warm air to escape, they should place vents as high as they can. What they often forget is that in order for warm air to escape, cool air must get in. In our perfect example, that cool air comes from the soffit vents. In the real world, we try to get as close to that ideal as possible, by providing intake air as low as possible, and placing vents for exhausting air as high as possible in the space. Typically, you should have one square foot of open space ventilation, both intake and exhaust for every 300 square feet of floor space. For instance, if your home is 1800 sq. ft., you should have about 6 sq. ft. total of both intake and exhaust ventilation.
In semi-bungalows, I’ve had good success placing gable vents as low as possible, and as close to the eaves side of the knee wall space as possible. Combined with roof deck vents placed as high as possible and closer to the centre of the roof, this has been effective in reducing ice damming problems.
I really like the ventilation products produced by the company Maximum Ventilation:
They are solidly built, and, because they are produced by a company situated in Quebec, are engineered and tested for real world Canadian conditions. Their roof top vents are more efficient than anything else on the market at ventilating attic spaces, are designed to sit above snow pack, and are practically impervious to water infiltration, unlike whirlybird style ventilators. They have a line of vents designed specifically to address the issue of getting air intake ventilation low on the roof deck, as well as many other unique situations.
So, let’s recap:
Ice damming is caused by heat warming the roof deck and causing snow on the roof to melt. Solutions to ice damming should mostly be concerned with keeping the roof deck cool. This can be accomplished by reducing heat escaping into the area below the roof deck as much as possible by sealing air leaks and insulating properly. Proper ventilation of the space under the roof deck is critical to removing heat. Placement of vents is central to allowing air to enter and exit the space, facilitating heat transfer from under the roof deck. The standard rule of thumb for the amount of ventilation required is 1 sq.ft. of both intake and exhaust ventilation for every 300 sq. ft. of floor area.
This may seem like a lot, and the vents on the roof may not be the most attractive, depending on where they need to be placed, and how many separate areas need to be ventilated, but vents are essential in preventing the significant damage that ice damming can cause, both to the interior and the exterior of your home.