Building Science-What does an air leak look like?

The past few weeks I’ve been discussing air leaks and the need for fresh air in our homes.  (It’s best if your fresh air doesn’t come from uncontrolled air leaks.)  This week we will see what an air leak looks like and what can be an outcome of these leaks.

Air moving through building assemblies can be very difficult to pinpoint.  A hole may be in one location but present itself in another.  The building will often give you clues of where to start looking.

I was recently at a job doing some work unrelated to energy auditing.  (I was at the home building a custom shower, the ultimate test of water management.)  A contractor was removing drywall on an exterior wall so that some electrical work could be performed during a kitchen renovation.  The home was built in the late 1970’s, standard 2 x 4 construction, fiberglass insulation with a 4 mill polyethylene vapor barrier.  The exterior of the home has wood lap siding over a fiber board sheathing.  I know the fiber product as Builtrite but there are other trade names for this type of sheathing.  I took a couple pictures that first day after the drywall was removed.

Staining on fiberglass insulation.

The dark colored staining on the fiberglass is from years of air leaking through the fiberglass.  As the air moves through, dust and particulates suspended in the air are moved through and filtered by the insulation, causing the dark spots.

Moisture behind the insulation.

This picture was taken after some insulation had been removed from the wall. (The next photo, which was taken the next day, shows the source of the air leakage and the moisture problem.) What was happening is warm, conditioned air was leaking around the kitchen exhaust vent. As the air came in contact with the sheathing, which was much colder, the moisture in the air was condensed, forming frost behind the insulation. The photo shows how the moisture followed the electrical cable. The air was exiting the wall cavity through the hole for the wire which leads into the attic. The frost followed the cable all the way to the attic.

I took this picture a day after the first.  Moisture is gone.

Notice the dirty insulation under the vent.  This was one of the areas of air leakage around the vent.

Framing and insulation below the kitchen exhaust vent.

Notice the water staining on the stud and bottom plate.  As the frost melts in the spring, the water moves down the wall and onto the plate and stud causing the water staining.

This event has been happening for around 40 years.  Why is there no rot or mold?  There is polyethylene sheeting on the wall which limits the drying potential to the inside of the home.  The poly wasn’t well sealed so there was probably some drying, but the majority of the moisture was moving outward during the spring and early summer.  Remember there is fiberboard sheathing on the exterior with wood lap siding.  The fiberboard and lap siding both have a perm rating of around 20, which is vapor open.  Moisture can move through both materials.  In contrast the poly is around .06 perm, a class I vapor barrier.  The moisture was able to dry completely before the next wetting event by moving through the fiberboard and wood siding to the outside, no rot.  Had this been OSB with a low permeance siding, there probably would have been rot and decay.

The picture above was taken on the south side of a different home on a bright, sunny day when the temperatures were in the teens.  Water was dripping out the vinyl siding.  As warm air (which can hold much more moisture than cold air) is moving through this wall assembly, it has made it all the way to the vinyl siding before condensing.  Frost and ice has accumulated between the siding and what ever is behind the siding.  Even though the air temperatures are well below freezing, the sun has warmed the siding enough to cause the frost to melt.  We do not know exactly where the air leak is, but we do know it is somewhere close.  It could be leaking around the window or it could be above the window, possibly in the attic.

This next photo shows the results from an air leak in an ceiling at another location.

Notice the icicles hanging from the soffit vent.  Air leaks leading into the vaulted ceiling of this home has caused moisture to accumulate during the winter, which is melting during the spring and running out the soffit vent.  There was some water damage to the ceiling on the inside.

Another home with air leaks leading into the attic space.  The brighter white spots under the roof deck are where the frost first forms, the nail points from the shingle nails that have penetrated the roof.  The frost on the boards forms later in the frosting event.  This is a common sight in a cold climate.

The final two pictures are of air leakage in two different ceilings, both while my blower door was operating.  I thought it would be fun to share a little thermography insight.  Take a look at the two pictures.

Both photos show the tell tale signs of rays or fingers projecting outward from the source of the leak, but why are the rays in the first picture dark blue and purple and the second orange and yellow?  It has to do with the time of the year the photos were taken.  The first was a winter time test with cold outdoor temperatures.  The second was a summertime test with warm outdoor temperatures.  Both photos are good examples of what an air leak looks like.

That’s it for this week.  Do you have a topic for me to discuss?  Leave me a comment here or direct message me on Instagram.

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