Icicles hanging over the edges of roofs might look pretty, but once enough melted water gets trapped behind that ice, water can leak in to the house. Minnesota saw a ridiculous amount of ice dams two and three winters ago, which prompted many homeowners to start looking for ways to prevent ice dams.
The two main driving factors behind ice dams are attic air leaks and insufficient insulation. If those two are fixed, the potential for ice dams is quite minimal. A third component to ice dams is attic ventilation. Attic ventilation is required by code, but it’s not an ice dam elixir. As long as attic air leaks are sealed and insulation is sufficient, only small amount of ventilation are needed to keep the attic space cold.
Unfortunately, lots of people claim that inadequate attic ventilation causes ice dams, which leads people to do all kinds of crazy stuff to get more air flowing in to their attic.
While the photo above of a desk fan installed in someone’s attic is a little silly, the person who installed it obviously thought it was a good idea. There are actually fans made specifically for attics. Most need to be connected to a 120 volt power source, but there are also solar powered fans available, such as the one pictured below. This one was sold at Costco for a while.
My advice is to skip these fans entirely. While attic fans will certainly suck air out of your attic, the problem is that this air needs to be replaced. Where does the air come from? Some of it will come from the soffit vents, and some will come from attic air leaks. If you have problems with ice dams, a LOT of air will come from attic air leaks. This means you’ll actually be sucking warm air IN to your attic.
If you have problems with ice dams, focus on fixing your air leaks and insufficient insulation. Once your attic lid is perfectly air sealed and you have enough insulation, the main causes of ice dams will have already been corrected. Don’t waste your money on an attic fan. If you already have an attic fan installed, your best option would be to have it removed and have proper passive ventilation added if not already present.
What about using these fans during the summer? Nope. Check out any of the links below for more info on why you’re better off not using attic fans… ever.
Related links, mostly pertaining to attic fans running during the summer:
- Advanced Energy: The Scientific Spin on Powered Attic Ventilators
- Optimal Building: The Dangers of Powered Attic Ventilators
- Energy Star: About Attic Ventilation
- Home Energy: Drawbacks of Powered Attic Ventilators
- ASHI Reporter: Don’t Let Your Attic Suck
Reuben Saltzman, Structure Tech Home Inspections
Chris
January 15, 2013, 8:27 am
How about small solar attic fans in attic crawl spaces? We have a long low 3′ tall ish attic crawl space in a finished attic that gets incredibly hot in the summer since air doesn’t seem to want to “stack” well and rise from the soffits to the vent. It is sealed up pretty well, and with the solar fan running it cuts the heat in there from around 120? to 105? wich matches the temp in our other more traditional attic crawl space that is peaked and taller. I feel the mechanical ventilation is necessary to reproduce what would be considered a normal airflow.
I have experimented leaving it on and off in the winter, but it doesn’t change the temperature or humidity that dramatically, probably because there is not much sun! The temp will drop 2 degrees and the humidity will drop 4-5% but it bounces back quickly so I leave it off. No ice dams due to good insulation/sealing.
Reuben Saltzman
January 15, 2013, 2:29 pm
Hi Chris, I assume that you have a 1-1/2 story home, and that what you’re referring to as the ‘attic crawl space’ might also be referred to as the ‘knee wall attic’. I don’t recommend using a fan in those locations either. Even though a fan may reduce the temperature in this space, there’s a good chance that it’s reducing the temperature because it’s pulling cooled air out of your house, increasing your energy consumption.
MCAS Roofing and Contracting
January 22, 2013, 7:40 pm
Glad to read this post as so many roofers I know insist on electric attic fans and I tend to lean more towards your opinion. It is difficult to find all of the air leaks in your attic and would require a professional roofer, but insulating is something most homeowners can do themselves. It is fairly easy and cheap, and not only will it help prevent ice dams, but will lower heating costs!
Seri
January 27, 2013, 9:54 am
I am very surprised to learn about the ice dam problems that people in Minnesota have faced. I grew up in a very cold area, but have never heard anyone mention anything about this or a similar issue within their attic. I am curious if this is also a problem that is slightly caused by the shape of the roof itself.
gary havens
February 1, 2013, 10:17 pm
How does adding insulation help prevent ice dams? However thick, the insulation doesn’t prevent air flow up and through it. Right? Insulation doesn’t prevent air flow, sealing attic bypasses prevents air flow. Thus, even a completely uninsulated attic, but one with all attic bypasses sealed from upward drafting warm/moist air from the living spaces, with proper passive ventilation within the attic itself … that’s what prevents ice dams, right?
I haven’t read all the links, but another reason not to use attic fans is that they will draft fires upward. I am sure that’s true for what are called “whole house fans.” What say you all?
Reuben Saltzman
February 2, 2013, 7:25 am
Gary – while attic bypasses are a very important part of ice dam prevention, insulation is also critical. Think about is this way: the insulation is a thick wool sweater.
If you go outside on a cold, windy day with just a thick wool sweater on, the wind will cut right through it and you’ll freeze. Put a windbreaker over your sweater, and you’ll be fine. The windbreaker is the equivalent of sealing your attic bypasses. If you wear the windbreaker alone on a cold, windy day… you’ll freeze.
Insulation and air sealing work together.
As for the attic fans, I suppose they could help fires draft upwards, same thing with whole house fans… but so could any exhaust fan. I guess I wouldn’t focus too much on that aspect.