An ice dam is a mound of ice that forms along the bottom of a sloped roof or in a valley where two roof planes meet. Like a real dam, it can hold back water, which can lead to roof damage or structural damage to your house. Once formed, it is nearly impossible to get rid of until it melts in warmer weather. Most people, however, don't realize that ice dams usually are caused by what's going on inside the house.
Ice dams occur on houses both with and without gutters. Warm air creates warm spots on the underside of the roof that melt the underside of the snow on top. The water flows to the cold eaves and refreezes, layer by layer, resulting in water backing up underneath the shingles.
Keeping warm air out of the attic is the key to avoiding ice dams. A properly ventilated attic should be close to the outdoor temperature. In theory, the insulation in the ceiling should keep the warmth below. However, careful investigation of insulated houses shows that warm air rises into the attic anyway, bypassing the insulation and even sneaking through it. These flows, called "attic bypasses," allow the warmed air into the attic while also allowing cold air to seep down through the gaps.
Hands - on experience
Although I'm an architect, I operated a hands - on "house doctor" business in the '80s that applied the findings of research done at Princeton University.
Our two - person crew would arrive at a house early in the morning and install a big fan unit, called a blower door, in the entry door to lower the indoor air pressure so cold outside air would pour through holes and cracks. Instruments connected to the fan allowed us to measure the amount of air leakage in the house. After reversing the fan to blow into the house, we would go to the attic and observe where heat was escaping with an infrared scanner. When we finished our diagnostics, we would fix the defects in the home's thermal boundary and then repeat the blower door test to see how we'd done.
You probably can find the key air leaks in your home without the expensive equipment we used. It's a little like the family doctor diagnosing ills with some poking, prodding and questions rather than putting the patient through a battery of high - tech tests. Although not as fancy, it still works in most cases.
Is blocking these hidden gaps, whether they're large or small, worth the effort? Consider this: Researchers at Princeton University studied hundreds of buildings in the 1970s and discovered numerous ways that heat escapes supposedly well - insulated houses. On average, they concluded, blocking these voids can reduce energy needs by 15 to 20 percent - much more if you include insulating an attic stairway. Remember, too, that a lot of little gaps can add up to one or two large ones, so seal them all.
Don't worry about working in your attic in the winter; it will be more comfortable now than in the summer. Besides, if your house has many or large bypasses, the attic might be fairly warm.
Understanding air movement
Assuming your home already is insulated, the doors and windows properly weather - stripped and the trim edges caulked, your first goal is to locate the gaps behind ceilings and walls that allow air to sneak through or around the insulation. Bigger, more complex houses tend to have more problems, but even a simple ranch - style home can have major bypasses.
Keep in mind that insulating material alone doesn't keep the heat in. You need a seal, such as a ceiling or polyethylene sheeting, in contact with the insulation.
Most of the problems can be found in the attic and basement. The stack effect - cold air entering the basement and warm air rising through the house and escaping through the attic - drives much of the heat leakage. This makes the attic the most important place to look for leaks.
The basement is the second place to look. Drafts there may indicate cracks in the foundation walls or even an opening around a window.
However, be careful you don't tighten the basement too much or your home's gas - fired furnace, water heater and clothes dryer could draw so much air out of the area that your chimney backdrafts toxic combustion gases into the basement. Install a carbon monoxide (CO) detector to monitor the CO levels. They're not that expensive (prices start around $25) but worth every penny if it saves your life.
Finding the leaks
Start by scrutinizing each room. Close all the windows and doors and turn up the heat. Check for drafts at electrical receptacles and switches. You could use a smoke stick, which gives off a light smoke trail that's easily disturbed by even a slight draft. Or you can use the tool that's attached to the end of your arm - your hand. Place the back of your hand against outlets, switches, fixtures and trim. If there's a cold draft, you need to figure out where it's coming from.
Indications of an air gap include:
- A draft from a wall outlet or switch. This means that the wall is full of cold air that is pouring down from the attic.
- Cold joist spaces in a garrison - style house. These suggest that outdoor air is leaking through the unblocked cantilevered framing (similar to knee walls).
- A blast of cold air from the under - sink cabinet. This will often be cold air from the soffit over the wall cabinets slipping on through the wall and exiting the cabinet via the openings around the plumbing pipes. The air can get through the insulation because there is no drywall or other air barrier backing the attic insulation at that point in the soffit.
Plugging the holes
When your attic is unfinished, fixing air leaks is a breeze. Otherwise, you have to remove some floorboards to get at the problems.
Accessways - Start with the access itself. Whether it's a small hatch in a closet ceiling or a large pull - down stairway in a hall, the opening must be well insulated and weather - stripped. You can build an insulated attic access cover using 1 - 1/2 - in. rigid foil - faced foam insulation and lumber. Weather - strip the bottom and weight down the top with drywall to compress the gasket.
When you have a regular stairway with a door at the bottom, cover the opening in the attic floor with rigid foam insulation. Glue 1/4 - in. hardboard to both sides for weight and stability. Then weather - strip the bottom where it meets the floor. Don't make the hatch cover too heavy. It could be too cumbersome or dangerous to move.
- Wires and vents - Inspect the top plates of interior and exterior walls between the attic floor joists and plug any openings. A black spot on the insulation is a good indication that there's a leak. Warm air picks up and deposits dirt as it flows through.
- Use any type of caulk to seal openings around wires and pipes that go through the top plates. Use a foam backer rod or tightly wadded fiberglass insulation in the opening before caulking. Stuff scraps of insulation into larger openings around plumbing vent stacks. Check along the joist bays, too. Make sure the insulation is snug against the ceiling below and the sides of the joists.
- Eaves - Pay special attention to the insulation fit at the eaves. While it's important that the living space's ceiling insulation not block the air intake vents in the soffits, it must extend fully to the top plate so cold air from the soffits cannot hook down against the ceiling. Short, bay - wide (16 - in. - OC or 24 - in. - OC) baffles of waxed cardboard, plastic or rigid foam insulation ensure that insulation and ventilation needs are met.
- Knee walls - When attics are finished, the joist spaces can conduct air from the cold attic to under the knee wall. - The remedy is to install rigid foam blocking between the floor joists just below the knee wall. Again, make sure the insulation between the floor joists fills the cavities.
- Interior soffits - Soffits built above kitchen cabinets on interior partition walls often are open to the ceiling joists and wall studs behind them. Seal off the bottom of the joists with plastic sheeting from the attic to block cold air from migrating down the wall. Staple the plastic and replace the insulation.
It pays to be thorough
Beauty isn't a big deal with house doctoring because most of the work is concealed. But if you want your efforts to be effective, be thorough. Sealing a large bypass, for example, won't be effective if you close off only 85 percent of it. Even if it's torture and takes three times longer, get that last 15 percent sealed.
Although tightening a house can trap contaminants and increase indoor air pollution, it makes no sense to accept energy - wasting gaps in place of proper ventilation. Dedicated ventilation systems will give you far better control with far fewer problems. For the most part, an older home will not be too tight no matter what you do to it. Experience has shown, however, that you can overtighten a newer home that is small and simple. Again, effective and fairly inexpensive ventilation systems are available and advisable.
Tom Blandy is an architect in Troy, NY. He specializes in adaptive reuse of older buildings and is the author of All Through the House (McGraw - Hill, 1980), a guide to home weatherization.