air infiltration

Dangers of Too Much Air Sealing

Every home has to breathe. Air has to enter and air has to leave. Otherwise, pollutants can stay in the home and affect your health. Moisture can build up and damage the home's structure and/or create mold. We are certain that you want to be healthy and have a solid home structure to have a good resale value. 

We talk so much about air sealing because it is so important to limit the amount of air changes occurring in a home. There are a lot of companies out there who will spray foam your wall cavities, attic spaces, basement spaces - or will even rent you the equipment to do it yourself to save a few bucks. The thing they do not do is actually test your home for air infiltration. Effectively they seal up your home, stop air from entering, but have no idea how much should be entering and much actually is entering. Sounds a little unsafe to me. 

Every home has a standard level of how many air changes it needs per hour called the Building Airflow Standard. This is easily calculated after finding the volume of your home. Next, you need a blower door test to confirm how leaky or tight your home is. You may seal up to 70% of the BAS levels before needing mechanical ventilation to bring more air into the home. 

At Sellair, we want to air seal properly. We want to make sure your home is left with proper ventilation so that you can feel confident knowing that your home is not only more comfortable and energy efficient, but that your home is also attaining the proper amount of ventilation because it was air sealed the correct amount. 

The right thing to do is to get an Energy Audit before air sealing your home. Then after air sealing is completed, run the blower door again to confirm your results. If you air seal too much, bring in mechanical ventilation to the home. This will give you adequate amounts of airflow and filter pollutants in the home. 

While this may be a lot of information, we are here to educate you. Please contact us if you have any questions or concerns regarding this process. 


What is a knee wall?

A knee wall is probably the LEAST COMFORTABLE and most ENERGY INEFFICIENT part of your home. 

A knee wall is an area you've seen hundreds of times and may have no idea what it is till now. 

A knee wall is also an area that do-it-yourselfers normally 'fix' wrongly. 

Physically, a knee wall is an area of a home that would normally be a full attic space. Instead of a full attic, a room is built in that space. The room is typically conditioned (heated and/or cooled) and is drywalled off. 

Now what does a knee wall look like? A few key indicators that you have a knee wall:

  • Low sloping ceiling with attic or roof space above that leads down to a wall. The blue wall in the picture below is the knee wall. 
  • An attic access that is perpendicular to the floor

From the inside of the attic, a knee wall looks like the image to the left. The wall on the left with yellow insulation is the same wall as the blue wall in the image above. As you can see in the image below, the joists run perpendicular to the wall, which means the joists run below the floor of the living space. As insulation does not stop air flow, cold air in the winter can pass under your floor as well as hot air in the summer. As a side note to this image, the insulation on the knee wall floor was installed upside down. About half of the Do-It-Yourselfers install insulation incorrectly.

From the outside, what does a knee look like? Below is a picture of the same house. The orange represents the walls that form the conditioned space in the bedroom. This is the livable space shown in the first image of this blog post. 

Then you can see the ventilation where air enters into the knee wall space. Unwanted air will travel under the floor, causing discomfort and a severe lack of energy efficiency. 

Now, what are the solutions?

We want to air seal any penetrations or top plates, as we do in all attic spaces. But then for the knee wall, it's important to air seal the wall with Tyvek or foamboard. This will help reduce the wind damage on the insulation to keep it in good shape as well as increase its effectiveness. Then, we bay block the cavities to create a continuous thermal boundary, and block air from passing under the floor. 

Knee walls are tight, complicated areas. Our recommendation is to get an energy audit to first determine your home's air leakage. The recommended solutions all stop air from entering or leaving the house. Not knowing your Building Airflow Standard nor your blower door number can be severely dangerous as you air seal. Air seal too much, and you can create moisture and health problems - bad for both the inhabitants and the home. 

Our recommendation is to get an Energy Audit and have those knee walls air sealed and insulated properly.

The Blower Door

The blower door is an essential tool to determining how to fix your home's comfort and energy efficiency problems. It is very difficult to determine the leaky and troublesome spots of your home  without the blower door - as well as impossible to find the actual air infiltration level.

Every home has to breathe. Air has to enter the home, and air has to leave. Sufficient air flow in a home helps clear the home of pollutants, carbon monoxide, and moisture (a.k.a. mold's best friend). Too much air flow makes the home drafty, uncomfortable, and inefficient.

By taking the volume of the home, and a few other factors, we can scientifically determine how much air should enter and leave your home - also called the Building Airflow Standard or B.A.S. By using the blower door, we can determine the actual infiltration of air into your home. 

Since we now know the home's actual air infiltration and the B.A.S, we can figure out how much the home may be air sealed. By minimizing the air infiltration, the home is now more comfortable, energy efficient, and safe.