Welcome to Structure Tech

Insulation vs. Air Leakage (Attic Bypasses)

By In Attic Bypass, attic insulation On January 17, 2009

With winter officially here, I’m on a big insulation kick.  This is the time of year when I can really tell which houses have attic problems, oftentimes just from looking at the house on the outside!  Today I’ll talk about a few things I look for when inspecting an attic when it comes to air leakage, insulation, and ventilation.

When I inspect houses in the winter, the first thing I look at is the roof.  I do this out of habit, and I can usually identify a house with attic problems without even going in the house.  The most obvious things to look for on the outside are ice dams, icicles, and patches of unevenly melted snow or frost.  These are all signs of heat loss from the house to the attic space.


If I find a Minnesota house with obvious problems on the outside, I look for two conditions in the attic which cause heat loss; lack of insulation and attic bypasses.  Most people readily understand what lack of insulation is, but attic bypasses are generally less understood, and much more time consuming to correct.  An attic bypass is a passageway for heated air to rise in to the attic – insulation won’t fix this.  To understand how attic bypasses work and why insulation doesn’t help, think about wearing a knitted sweater on a cold windy day; the wind will cut right through the sweater, but if you wore a thin windbreaker over the sweater you would be much warmer.  The same principal applies to attics – warm household air will pass right through a foot of fiberglass insulation, but if the air is stopped by a physical barrier (such as the drywall at your ceiling), it won’t pass through the insulation.  According to the Minnesota Department of Commerce, attic bypasses can reduce the overall effectiveness of insulation by as much as 70 percent!  This is why it’s critical to fix attic bypasses before adding more insulation to an attic space.

When I evaluate the insulation in attics, I focus on areas with missing or minimal amounts of insulation.  Missing_and_Insuffient_Insulation Any gaps can drastically reduce the overall effectiveness in the attic.  Attics in new homes should have insulation that provides an insulating value of at least R-38, which equals out to about a foot of fiberglass rolls, or a foot of loose-fill cellulose, or about eighteen inches of loose-fill fiberglass.  If I can see the bottom chords of trusses in an attic, this is an obvious sign that the attic needs more insulation.  While I don’t have any hard and fast rules for my recommendations, I generally say that an insulation level over R-38 is good, between R-19 and R-38 is marginal, and less than R-19 needs correction.

Attic bypasses are much more difficult to identify because they are usually covered by insulation.  While national home inspection standards don’t require inspectors to move insulation, I always make a point of doing this anyways because this is typically the only way to identify them, and they’re important to know about! While bypasses are major sources of heat loss, they can also allow moist household air in to the attic space, which will often condense on the roof boards, creating a frost covered attic space.

Frost_in_Attic Frost_in_Attic_on_Plumbing_Vent Frost_on_Nail_Heads_in_Attic Black_Stains_Around_Nail_Heads_in_Attic

In the summer there is never frost in the attic, but a dead giveaway that frost accumulates in the winter is small black stains around the roofing nails. The frost in attics will form the heaviest around nail heads because they’re the coldest components.  Possibly the worst bypass that I frequently find is a bath fan exhausting in to the attic – these pump warm moist air in to the attic at a ridiculous rate.  Another common place to find bypasses is around the furnace or water heater vent.  I frequently find gaps around the vents that are several feet wide, and these areas are always covered with insulation.  The Minnesota Department of Commerce puts out an excellent brochure on finding bypasses in the attic and how to fix them, which you can download here.

The last thing I look for in an attic space is ventilation.  Attic spaces are almost always unconditioned spaces, so they need to be ventilated to the exterior.  Attic ventilation helps to keep the roof cooler throughout the year, which will help to minimize ice dams in the winter and help prolong the life of the roof in the summer.  While the traditional reason for ventilating an attic space was to prevent condensation in the attic, I’ve read a number of studies lately that say that this may not be as important as we once thought.  I always verify that proper ventilation is in place, and I make suggestions on how to improve ventilation when necessary.

Reuben Saltzman, Structure Tech Home Inspections – EmailMinneapolis Home Inspections

About the Author


Reuben is a second generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben has worked for Structure Tech since it was purchased by Neil in 1997, and is now co-owner and President of the company.


  • Insulation For Attics 5 YEARS AGO

    Hi, another helpful article for those searching for information on the subject, hopefully I will be able to put it to good use.

  • LCD Screen Cleaner : 6 YEARS AGO

    ceiling fans are great for cooling a warm room with large area. i prefer them over multiple desk fans,'"

  • Brady Bryant 7 YEARS AGO

    in our place, the weather could turn out to be uncomfortably humid that is why we bought an air conditioner at home to reduce h.''