If you've considered adding attic insulation to your home this year, please read this first. Most homeowners go about re-insulating their attics all wrong. What's far more important than simply adding attic insulation is to have air-sealing performed first. This is the labor-intensive part of an attic insulation job that unscrupulous insulation companies may not mention.
Not only is it important to seal attic bypasses before adding insulation, but it's also the law here in Minnesota. Section 1322.0100 Supb. 3 of the 2015 Minnesota Residential Energy Code says that "Attic insulation shall not be installed unless accessible attic bypasses have been sealed. " The Minnesota Department of Commerce even issued a press release in 2014 to help make this information known: "Beware the Insulation Contractor Who Does Not Include Air Sealing"
Attic Bypasses
The Minnesota Energy Code says "An attic bypass is any air passageway between a conditioned space and an unconditioned attic."
So what the heck is that? Just check out the images below. I think these all demonstrate attic bypasses quite nicely.










That last image was from a new construction pre-drywall inspection, and is a pretty common detail on new construction homes.
If a home was built before about 1991, there was probably no attempt to seal attic bypasses. After that, there's a good chance that some of the larger attic bypasses have been sealed, but surely not all of them. To seal up most of these attic bypasses, an insulation company will pull the insulation away and use spray foam insulation to get everything perfectly airtight. Handy homeowners can do most of this work by crawling around in their attic with a can of foam insulation, but let me warn you, this is nasty work. I certainly don't advise it.
For homeowners who insist on performing their own attic air sealing, I recommend downloading this guide from Building Science Corporation: Attic Air Sealing Guide. Also, please read this Building Envelope Guide from the Minnesota Department of Commerce.
Why attic bypasses matter
Attic bypasses are the driving force behind ice dams, as well as frost, moisture, and mold in the attic. If you eliminate all attic bypasses, you'll probably eliminate all of these problems. That's why it's so important to address attic bypasses before doing anything else.