Building Science-Building Shell Layers-Air Control Layer

Last week we talked about the outer most layer in a building, the rain control layer.  This week, we will continue with the building shell layers discussion.  The air control layer is next on the list.  The four layers, excluding the structural layer, are:

1. The rain control layer or WRB
2. The air control layer
3. The vapor control layer
4. The thermal control layer

Traditionally, the air control layer or air barrier in my northern Minnesota climate has been combined with the vapor barrier.  Polyethylene sheeting with it’s very low perm rating of .04-.06 perms and poly’s ability to block air movement works very well when properly installed and is not damaged throughout the life span of the home.  We will hit the vapor control layer next week.  These layers can be, and sometimes, should be separate.

The purpose of the air barrier is to limit the amount of air movement through a building’s shell.  Current building code requires us to test all new homes.  A tightness level of 3 air changes per hour or less while a blower door is operating at 50 pascals must be achieved.  Most new homes I test, which use either poly or spray foam as the air barrier, average 2 – 2.5 ACH50.

Why do we care if some air naturally moves through the building’s assemblies?  As it turns out, building scientists have known for decades this air movement is the second highest source of moisture in wall and roof assemblies.  Just as bulk water leaks can damage our homes from the outside, air leaks can also cause damage, in a northern climate, this damage is from the inside.  I see this every spring when I’m called to several homes and businesses in my area where it has begun to rain inside the structure.  Frost that has accumulated over the winter in roofs and walls has melted, causing damage.

Cold air infiltration through poorly air sealed roof assembly while blower door testing.

Stack effect and ventilation equipment are the forces that move air inside a building and the wind is the force outside.  With stack effect, during the winter months, air is exhausted out the upper levels of the home, creating a negative pressure in the lower levels.  Outside air is pulled into the home in the lower levels.  During the winter, this air is cold.  The opposite can happen during the cooling season, though the forces are typically much less during the summer.

Improperly designed heating and cooling equipment can also cause positive and negative pressures within the home, effecting costs and comfort.  Wind will create a positive pressure within a home on the side of the home that the wind is blowing against, a negative pressure will be created on the opposite side of the home, pulling air out of the home.

These air leaks can cause increased energy costs.  I’ve conducted testing on several homes where extremely high heating bills were caused by air leaks (and low insulation levels).  Air leaks can also cause discomfort within the home.  I often hear complaints about drafty areas in the home.  As an example, I was in a home where the homeowners stored cases of fresh fruit along the outside walls of their home because the temperature in this area stayed between 35 and 40 degrees.  The bottom plate was not sealed correctly and the stack effect was moving cold air under the plate.  Indoor air quality can also be affected. Moving air across insulation, construction adhesives, or across carpets can cause allergy issues in sensitive people.  Negative pressures in the lower levels of a home, which is common with the stack effect, can pull soil gas, such as radon, through the slab.

Thermal Imaging of cold air infiltration under poorly sealed bottom plate.

There is a saying some builders try to achieve, “build it tight and ventilate right.”  Getting a house “tight” requires an understanding of where houses leak.  Changes in building materials, such as concrete to wood, seams and connections in building materials, corners, windows and doors are all common areas of air leakage.  A good air barrier will enclose all sides of a building, a great air barrier will have several layers through the assemblies. House wrap, if properly installed and sealed can help.  Taping and caulking wall sheeting, sealing framing connection on window and door openings, using closed cell spray foam on the hard to seal areas, such as a rim joist and using a combination of sill seal and acoustical caulking on bottom plates will tighten an envelope.  Using poly that is well sealed, or my preference, installing air tight drywall will further reduce air leaks.  Several layers working as an air barrier.

One area of air sealing that, in my opinion, is not addressed properly is coordination with sub-contractors.  Most plumbers and electricians do not understand blower door testing and air sealing methods required in modern home construction.  Basic building science and building energy codes should be taught to all the trades!  General contractors and job supervisors should have policies in effect so everyone understands who’s job it is to seal the penetrations created during the construction process.

Damaged air barrier. The polyethylene sheeting used both as the air barrier and vapor retarder was cut by an electrician to modify the electrical system.

If a home is constructed “tight”, moisture levels will increase and indoor air quality will decline.  This is where the “ventilate right” statement comes into play.  Minnesota code requires balanced ventilation on all new homes, usually achieved by an air exchanger.  A heat recovery ventilator, or HRV will allow for control of the air exchanges within the home.  Air exchangers will be discussed in a future blog.

Most new homes being constructed in my area are custom, I do not know of any production home builders working in north central Minnesota.  Custom homes have many challenges in achieving low air tightness levels.  These challenges also exist in the other building shell layers.  Sharing knowledge, techniques and getting to know materials that might not be common in our area is one of the focuses of this blog.

Next week is the vapor control layer.

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