The Most Tightly Kept Secret in Insulation

Most new homes are leaky. In the typical new home, significant volumes of air enter through cracks near the basement rim joists and exit through ceiling holes on the building’s top floor. These air leaks waste tremendous amount of energy.

In recent years, after years of prodding by building scientists, code officials have finally taken a few stabs at addressing air leakage. (For more information on recent code changes related to building airtightness, see New Air Sealing Requirements in the 2009 International Residential Code and An Overview of the 2012 Energy Code.)

Of course, some builders have focused on energy efficiency for years, and many of these builders own a blower door. If you have your own blower door, you have probably learned by trial and error which cracks matter most.

However, the vast majority of contractors build homes without any feedback from a blower door. If these builders want to improve the airtightness of the homes they build, they probably don’t know where to start.

 

 

Get the big holes first

The first step is to make sure that there aren’t any really big holes in your homes. (Joe Lstiburek calls these “the Joe-sized holes”; they’re the holes that are big enough for Joe to crawl through.) You may be thinking, “Can a house really have holes that big?” The answer, sadly, is “Yes, it really can.”

Let’s raise the bar just a little, and make a list of holes that are big enough for a cat to walk through. These include:

  • Holes in the air barrier behind zero-clearance metal fireplaces.
  • Unsealed holes above kitchen soffits.
  • Unsealed holes above dropped ceilings.
  • Attic access hatches or pull-down attic stairs without any weatherstripping.
  • Unsealed utility chases that connect basements with attics.
  • Holes behind bathtubs installed on exterior walls.

Once these holes are patched — in most cases, using OSB, plywood, rigid foam, or ThermoPly — what’s next? If you are a Passivhaus builder aiming to achieve 0.6 ach50, the answer is simple: every conceivable crack in the home’s thermal barrier needs to be sealed. In some cases, Passivhaus builders use a redundant approach — for example, using both caulk and a gasket.