Building Codes and Building Science are Beginning to Better Align

This post first appeared on the Green Building Advisor website.

I’ve heard Mike Guertin mention that every home built is a building science experiment.  It can take some time for problems in buildings to be known.  For instance, a bad detail on a window installation, like shown in this photo, could take more than 20 years to present as a problem.  If the problem is being repeated, building science might identify the issue and propose a change.  Eventually, the change may end up in the building codes, but this whole process is slow.

That being said, there are many building science principles that we have figured out, with several already added to the codes.  Some have been known about for decades but are still slow to be adopted.  Let’s discuss a few of each.

  1. Airtight structures.

The work on finding and sealing air leaks began in the 1970’s with the airtightness requirements officially hitting the residential building codes in the US in 2012, some still argue about these changes.  There are a few areas around the United States that don’t require blower door testing, or only require a visual inspection of the air barrier.  I regularly have discussions on the topic that “buildings need to breathe”.  (My argument is by how much, give me a number.  Quantifying how much air moves through a home is a good start for an educating discussion.)   So, why is air sealing important?  Take your pick, durability, operational costs, comfort, indoor air quality.  Let’s just focus on durability.  Air movement is a double-edged sword.  It can be a drying mechanism, but it can just as easily be a wetting problem.  Air contains some amount of moisture, which can be changed to liquid form when it contacts a cool surface (occurring during the winter in heating dominated climates and, in the summer, when air conditioning is in use).  When this happens inside a wall, basement/crawlspace, or in an attic, things can become wet.  If we limit the amount of air that is moving through these assemblies, the air cannot carry the water vapor that causes trouble, reducing the risk.  Simplified explanation, but hopefully by now many of us have the understanding that uncontrolled air moving through assemblies is bad.

The good news is that I’ve seen an air leakage rate reduction in most new homes built in my market since the blower door test was implemented.  When testing began, just under 3 ACH50 was the norm.  Now, the number has reduced to just under 2 ACH50, in less than 10 years.

  1. Continuous insulation.

This one is a bit prickly.  CI was put in the energy code, but, in my opinion, insulation installed outboard our wall assemblies is much less about energy savings and much more about the durability of the structure.  We are trying to warm any moisture sensitive components in wall assemblies, keeping them above any risk of a condensation problem.  Warm equals dry, cold equals wet.  If the CI is installed correctly, I think we will be adding decades to the durability of our homes.  The key is correctly installed with enough R-value based on the climate you live in.

It looks like my state (Minnesota) will be adopting the 2021 energy code (as written) with its requirements for CI.  We are also a poly state, meaning most contractors are accustomed to installing polyethylene sheeting on the interior of our wall assemblies (still allowed by the codes).  I’m not a fan of poly, but I’m even less excited about combining poly and CI.  Educating our builders (and inspectors) is going to be crucial.

  1. Indoor air quality.

ASHRAE 62.2 has been a driver in the new construction industry for a couple decades now.  Ventilation rates aren’t always agreed upon, but at least we’ve had codes requiring ventilation.  What I am excited about is the requirements of balanced ventilation coming into the codes.  Beginning with the 2021 IRC, all new homes built in climate zones 7 and 8 are required to have a heat or energy recovery ventilation system.  I suspect this will move to warmer climates in future codes.

I know this is a little morbid, the deaths by risk factor in 2019 (pre-covid) have air pollution ranked 9th, outdoor air particulate matter pollution ranked 11th, and indoor air pollution also making the list for the United States.  We currently have the technology to improve indoor air quality.  The technology is expanded even more by using automated sensors measuring air quality and controlling ventilation rates.  Of course, ventilation works best if the source of “fresh” air is actually fresh and if the pollution isn’t already in the house itself.  Keeping the home dry and free from mold and reducing the risk of radon poisoning is also important.

Speaking of radon, are you aware of how and when we learned that radon was a problem in homes?  There was testing being conducted by the EPA and DOE in the mid to late 1970’s, researching the risks of radon in homes, but it wasn’t until 1984 when a Pennsylvania resident working at the newly constructed Limerick Nuclear Power Plant set off the radioactive monitors as he entered the facility.  The problem was the plant was not yet active.  The source of the radioactivity was traced back to the worker’s home.  A level of 2600 pCi/l was detected in the basement.  (4 pCi/l is the current actionable level.)  Here’s an interesting paper on the history of radon written by Robert K. Lewis, History of Radon.pdf (

  1. Duct tightness testing.

There has been a lot of work done trying to understand air leakage in ductwork by these two fellows, among others.  This is John Tooley (left) and Gary Nelson (right).  Their work in the 1980’s and 90’s helped develop some of the codes related to duct tightness and how it relates to building pressures when using forced air heating and cooling systems.  All this work eventually led to the codes requiring ductwork sealing.

The first codes mandating duct tightness testing was required when ductwork left the conditioned space of the home.  Newer codes (2021 IRC) require that all ducts be tested and meet a certain tightness metric, whether the ducts leave the conditioned space or not.  What’s the reason?  Unsealed ducts can still create pressure issues if the ducts are leaking between floors or inside walls or other duct chases.  Plus, if air is leaking into or out of the ductwork, that air is not getting to where it was supposed to go.  I’ve seen this time and again, that room furthest from the heating system has an issue maintaining comfort.  Or maybe the return air pressure is greater than the supply in a room, causing the space to become negatively pressurized.  This pulls outside air into the room, the air might be hot and humid, or cold.  Either result in comfort complaints.

  1. Vapor retarders.

The codes for this topic still need some work, but building science has vapor retarders pretty well figured out.  Do we need to consider vapor when designing assemblies?  Absolutely!  But what I worry about is how vapor will leave the assemblies and less about how it’s getting in.  If I’ve addressed bulk water management, air sealing, and thermal control for moisture sensitive surfaces, water moving into assemblies by way of vapor diffusion is much less of a concern.

Codes still allow a class I vapor retarder (polyethylene sheeting, glass, and metal are all examples of a class I), to be installed in climates marine 4 through 8.  This means you could be in Northern Missouri and install poly on the interior side of your walls.  Not what I would recommend, but it is allowed.  Put enough continuous insulation on the home and you can move to a class III vapor retarder, painted drywall.  Make the assembly airtight and do a good job at bulk water management, most vapor problems are solved.

Building science and building codes are beginning to better align.  I listed just five, there are many more.  (A really good one recently appeared on GBA, Recurring Mold in the Attic – GreenBuildingAdvisor).  What is your opinion on where we should concentrate efforts on getting the building codes and building science in better agreement?

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