Friday, August 3, 2012

Air barriers and thermal barriers are two different things!


This article is from Rick Terry, Quality Assurance Designee at Building Performance Contractor's Association

As an industry, we have made great strides in meeting some of the demands that various energy programs and codes have required relative to materials and processes; however, as a HERS and green rater that works with both manufacturers and builders, I see a constant struggle to accept certain important concepts that enable them to meet requirements.

One of the big ones is the concept of a separate air barrier and thermal barrier that must remain in contact with each other and work together to maximize performance. Air barriers are any materials that prevents air from moving in and out of the building and can be OSB sheeting, rigid foam, housewrap, caulking, approved tapes, etc… they may or may not have additional thermal properties; fiberglass batt insulation is not an air barrier. The responsibility to install and keep intact both the air and thermal barriers are the manufacturer’s and their builders’. 


Thermal barriers are materials that have higher thermal resistance; i.e. the ability to minimize or even eliminate the movement of heat through a material or materials.

The plant needs to adhere to the manufacturer’s installation instructions of the type of insulation being installed in the home. For the majority of homes, this will be fiberglass batts although we are starting to see more options such as spray foam applications, and blown in products like cellulose and fiberglass; but for now, we will focus on fiberglass batts. The standard is simple: Insulation must make contact with all six sides of a cavity without compression and gaps. In the case of ceiling insulation with an open attic, it would be 5 sides.

In addition, the following are key aspects of making that happen:
  1. Paper facing on insulation should be against the conditioned side of the cavity for most of the mid-Atlantic and northern climates of the US. (More southern, humid climates will reverse this.)
  2. Cut batts to proper length and width. Fit should be adequate to fill cavity completely; too long or wide and you can create areas where the insulation pulls away from the wall; too short and you get gaps top, bottom and/or sides allowing spaces for air to move.
  3. For exterior walls with blocking materials, bracing, electrical boxes and plumbing, insulation needs to cut, scored, and fit so that contact is made against all surfaces. (This can also be accomplished with a combination of rigid foam in between the blocking and fiberglass batt w/o facing, over top.)
  4. For horizontal obstructions like electrical wires, the batt should be split to go around the obstructions and still contact all surfaces plus fully fill the cavity.
  5. DO NOT POKE INSULATION INTO PLACE WITH A STICK! This is one of the most common practices we see even in plants that have been doing it right for years, as soon as there is a personnel change, they seem to go back to the stick. It creates compression areas and/or voids in the insulation that have potential for future problems like air and moisture movement in the walls.

These key aspects take time…more time than most manufacturers and builders want to commit but it needs to become a priority as codes and programs require higher standards. These standards are typically verified through performance tests like the blower door leakage test, duct leakage test and infrared imaging. Use your HERS/Green raters to develop a standard and/or checklist for greater performance and to review best practices with your team.

Rick will have his article on AIR BARRIERS coming soon.

1 comment:

haddad said...

Great article Rick. One comment on fiberglass batts - some factories are still using batts with tabs. These tabs create a gap and compression at each stud.

Another issue is the thermal barrier in the ceiling floor cavity in two story homes.