1. Gaps around boxes should be sealed
Light Fixtures & Junction Boxes
The average 4” electrical junction box, where the opening has been cut with a roto-zip tool, has an opening the equivalent of 1-1/2 square inches.
Of course these gaps are greatly mitigated by whatever fixtures are installed, as well as by the type of insulation installed, but with receptacles and switches, air will still move by electrical boxes.
This air movement can be quite pronounced with fiberglass insulation.
Gaps around boxes should be sealed

2. Exhaust fan housings
Exhaust fan housings:
Exhaust fan housings are perhaps the worst because the covers do nothing to restrict air movement through the gaps.
Gotta love the duct tape.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

3. Recessed lights (can lights)
Can you see the darkened area where the arrow is pointing?
This is the telltale sign of air being filtered as it flows through the insulation from a can light---through the 14 inches of white fluffy.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

4. Picture courtesy of Charles Buell---Charles Buell Inspections, Inc.
This is the same area “excavated” a bit and exposing the top of the can light.
Even more of the dirty insulation is now visible.
Picture courtesy of Charles Buell---Charles Buell Inspections, Inc.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

5. How Can-light by-pass works
Because the edges of the round can are too often not adjusted to make contact with the gaskets on the trim---air can move into the attic around the lights even though this is an ICAT can.
Does everyone know what ICAT stands for?
Insulation Contact Air Tight
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

6. Can-lights with holes can be a significant bypass—and typically are not rated to be in contact with insulation.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

7. Some can-lights are worse than others
Can you see the CFL through one of the holes?
There were two of these can-lights above a shower stall and each can had five of these holes /2 square inches x 2.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

8. Crawl Space access hatches
Ghosting indicative of air coming and going through the carpet at a crawl space hatch. This is a sign of missing or inadequate weather-stripping. Older carpet will show this condition as dirt is filtered out of the air.
Keep in mind this connection is almost always under negative pressure.
Every time you turn on an exhaust fan, or open a window or door, air can be pulled into the house from the crawl space--especially with exhaust fans, windows and doors opened at upper levels.
Most people do not like the idea of air being replaced in the home via the crawl space—filtered or otherwise.
Perhaps the biggest source of negative pressure is stack effect--which we will talk more about it a bit. This is especially true the taller the house is. It can pull crawl space air into your home 24/7/365.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

9. If the carpet is brand new, thermal imaging can show the “ghosting” in a different way.
At some point in the future, you can count on the carpet looking like the first picture, unless the hatch is properly weather-stripped.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

10. Staples?
Even the staples would keep the weather-stripping from making full contact.
Obviously the paper still being in place is going to prevent a good seal.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

11. Through Carpet
Through Carpet:
But it is not just carpeting at the crawl space access that we see air by-pass. Any place air can move through, or under sill plates or around stairs, the carpet can show similar types of staining.
It is also happening where there is no carpet---the carpet just makes it visible.
This will generally be more pronounced on exterior walls on any floor level but can even happen on interior walls as well---all there has to be is a path for air from the exterior to be drawn to that location.
Inadequate sealing of rim joists will send air to the interior partition areas---if there is a breach---like a plumbing or wiring hole.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

12. Under baseboards
Under baseboards:
Along baseboard and hardwood floors you can often physically feel air moving into the home (dry finger or wet finger test)
With IR it can be very apparent. Turning on lots of exhaust fans will often show very pronounced movement.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

13. This was new construction and should have been sealed better.
What effect on a blower door test is this likely to have?
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

14. Windows that do not latch properly
Thermal imaging can find leaky windows easily in the winter.
Note how the cold air drops down and makes the wall under the window cooler?
Windows that do not latch properly
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

15. Poor air sealing around gas fireplaces
Inadequate air sealing around fireplaces is very common.
This is especially true of ones that hang over the foundation.
It often results in poor blower door results on new construction.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

16. Open Dryer Vents
There is no secondary flap in the dryer itself, so if these flaps are blocked open or missing, this will be a significant location where fresh air comes into the home.
Who has opened the dryer in the winter and felt how cold it was inside the dryer?
Good chance there was something wrong with the damper---although some amount of air will still leak around most flaps.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

17. Missing bottom boards
Missing bottom boards can allow tremendous amounts of air infiltration. It can also allow vermin entry to the home.
Missing bottom boards
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

18. Leaks around door & window weather-stripping
Another place that air by-pass can be noted is at doors where weather-stripping does not fit tightly.
Of course when you see it at the garage/house door it can be even more problematic.
Leaks around door & window weather-stripping
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

19. Knee Walls Knee-wall attics
Knee Wall attics can be very difficult areas to insulate and air seal.
They are typically poorly insulated and air comes and goes under the wall

20. Knee Walls We need to block and air seal under the knee wall
And then adequately insulate
This is difficult in older construction and compromises will be made.
Not only will the vertical wall need to be insulated but air sealed/encapsulated on all six sides.
Obviously the slope above the knee wall is going to be a problem and solutions will depend on lots of factors.
Types of insulation
How much space there is
Adding insulation to interior
Hot roof method
Etc.