Is Wood Flooring Becoming a Victim of Building Science?

By Bobby Parks

Of course not, but it did make for a catchy title! However, there is some truth in the fact that the science of building better buildings has resulted in the need, and increases awareness, to control moisture throughout the structure. Today’s buildings are better insulated, much tighter, and have more energy efficient equipment. Everything from the Heating, Ventilation and Air Conditioning (HVAC) duct systems to the windows is getting better and better. As someone who has investigated moisture-related issues in thousands of homes in more than 38 different states during the last 20 years, I believe there is a serious lag in the learning curve of many of the trades that are responsible for the proper performance of these buildings and each of the components that make them up. As a result, when there is a bowed or buckled floor, who is the first person the homeowner/contractor calls? The flooring company, of course, despite the fact that there are many trades involved in the successful installation and performance of the wood floors.

Just as a fever is simply a symptom of a deeper-rooted problem within the human body, the flooring issues are often symptoms of a deeper-rooted problem, which is often moisture-related. So, let’s look at some of the trades that impact moisture control and flooring.

Insulation professionals — the moisture from beneath
Today, we find various types of insulations being used, often in a hybrid mixture in the same dwelling. Better and more insulation, combined with advanced air-sealing procedures, means that the homebuilding industry has reduced the heat flow through the building, but not always the moisture flow at the same ratio.

The increased use of foam insulation, especially in the southern climates, has caused us to change how we condition our buildings. We no longer focus as much on the outside getting in. We are directing more of our attention to dealing with what is released inside the home and how are we are getting it out. Showers, baths, cooking, plants, pets, and people are all interior moisture sources that must be considered and dealt with by the HVAC systems.

Of the failures I have investigated, the single biggest moisture-related issues pertaining to wood flooring are often found in buildings with inadequately, or improperly controlled, vented crawl space foundations. Mistakes are common in both new homes and the renovation of an existing structure in how these spaces are controlled. Examples include the builder/remodeler doing nothing, leaving the floor system exposed to elevated moisture conditions; not encapsulating and conditioning the crawlspace; or, most commonly, installing the wrong type of insulation causing even more damage to the structure.

In a vented crawl space home, the No. 1 moisture source is the outside air trying to infiltrate the flooring system. When this happens, the moisture migrates through the subflooring until it hits the wood floor above. At this point, moisture accumulates within the subfloor and into the underside of the wood flooring, and results in a cupped or buckled floor.

So, what is one feasible solution? In my opinion, 2” of closed cell foam on the bottom side of an adequately dried subfloor is one system that will serve two purposes: (1) Not only does it serve as a thermal barrier that will keep those little toes much warmer during the winter, (2) it also protects the flooring system from exterior moisture intrusion. (See Figure 1.)

Figure 1: A vented crawlspace with a foam protected floor compared to an unprotected floor.

Earlier this year, I began a study to illustrate how this works. Graciously, a customer allowed me to turn her problem into an educational opportunity. After several years of repeated failures in her wood flooring, we finally met, and she decided to follow my recommendation of 2” of closed cell foam. To demonstrate how this works, I inserted a wood moisture data logger into one area where it could be covered with 2” of the closed cell foam. In another area (but under the same room above), I intentionally left a “holiday” and inserted a second data logger. I also have a data logger in the room above monitoring the temperature and humidity above the test area. As you can see, in just the first month of summer in Louisiana, the floor with the closed cell foam is consistently maintaining 10-11 percent relative moisture content (which is excellent for this part of
the country).

On the other hand, the section of flooring left exposed has already risen above 27 percent and can be expected to continue to rise, reaching its peak in September before the seasonal change will help to reverse the drying potential. My conclusion: in a vented crawl space, we must incorporate both thermal protection and exterior vapor protection or the moisture can wreak havoc, depending on the following three factors:

  • How permeable is the interior floor system?
  • How humid is the outdoor air? This varies greatly with geography and is probably the most prevalent factor.
  • How cold do the occupants like to keep their home? The colder the home, the worse the problems will be and the sooner it will show itself.
Figure 2: A/C cycled 86 times in a 24 hour period and as a result the indoor humidity averaged almost 65% RH.

HVAC professionals — the moisture from within
And then there is the A/C guy. Many years ago, building code professionals realized the importance of the Manual J load calculations and getting the HVAC systems sized properly. The tighter and better insulated we build our buildings, the more critical it is to get this part right. Of course, the A/C guy often thinks “bigger is better” (guess it’s just a man thing). As a designer of HVAC systems, I constantly deal with the freaked-out builder and/or A/C guy who panics when I design a 3,000 square foot heated home with a 3-ton HVAC unit. Trying to explain the dynamics involved and why it is so important that they size their equipment to properly manage the interior moisture remains a constant challenge.

As time has passed, and thanks to building code officials who, through continuing education, have learned the hazards of oversizing HVAC systems, we are getting better.

Two final thoughts for the flooring companies
“If you’re not testing, you’re guessing.”
Today’s flooring business is not the business of years past. You must evolve with the industry. Good tools and the knowledge of using them will save you many wasted hours and headaches. A good moisture meter and hygrometer are a must. Know your substrates and your interior conditions before you start. Do not depend on the opinions of others and don’t be rushed by the “builder’s deadlines.” You will be the one who ultimately pays the price.

“Educate yourself, your customers, and your builders.”
Understand that we are all ignorant in many ways, and that’s why continuing education is so important. Today’s building practices are changing fast, and it’s up to you to keep up. You want to be the expert; invest in your knowledge, because you should be the person that others call for advice. When you recognize that there is no thermal or vapor protection on the floor structure that you are about to cover with a low perm surface, make sure you explain the consequences to the buyer. When all else fails, sometimes you’re just better off passing on a job.

If you know you’re dealing with a high-efficiency home, there is nothing wrong with talking to the A/C contractor. If he doesn’t conduct Manual J load calculations, if he doesn’t know the Solar Heat Gain Coefficient (SHGC) of the windows, and if he is still basing his A/C on “square feet per ton,” these conversations should be a serious red flag. Do these things to protect yourself. Remember, it’s not always your fault.

Robert (Bobby) Parks is a Building Science Practitioner and Managing Member of Healthy Homes of Louisiana LLC. He can be reached at

5 thoughts

  1. Interested in your take. My friend in NC does HVAC. He insulated his crawlspace and the woodwork dried out and developed cracks. I really feel as if we should have a national mandatory building code standard of 30-50% RH in all homes that have woodwork from real wood at the time of being built. Then we could eliminate about 90% of inspections and BS claims that plague wood flooring contractors.

    1. Even the best of codes are useless if not enforced. We have building codes in place that require that proper calculations are performed, equipment is sized properly and so on… unfortunately the very codes that would ensure proper building operation are overlooked or ignored. I tell builders all the time that just because the BCO doesn’t ask for proof that these codes were followed, it doesn’t release them of the responsibility of following them. It’s not until there is a problem and ligation ensues that many builders realize the importance.

  2. Bobby,

    Good to harp on this issue!
    When AC is running in the house one further adds to moisture condensating and wetting a exposed subfloor with a ventilated (read wet from high RH) crawl space.
    My attitude to this is; “No wonder”!

    It is pertinent to create a moisture barrier to sghield from outside high RH, better yet create a “conditioned crawlspace”.
    The best example is what I have seen in a loghome in the 1990’s, the owner did not want a basement (which is typical in MI) but instead a crawlspace which he wanted to be conditioned, in other words: maintained just like the inside of his home.
    He had the builder pour a concrete crawlspace (like a shallow basement) and tie the space into the HVAC system of the house. This way he had never to worry about frozen pipes either and never a mold problem below the floor either.

    That was the most perfect crawlspace system I have seen!
    A crawlspace with a dirt floor (even when covered with plastic) and ventilated is just asking for problems.
    It is time that the building industry adapt to more functional standards.

  3. What if the foam is applied while the floor system is not dry to 9-10%?
    What if the home in yesrs to come has a water leak? How long does it take to dry that home out?
    If the foam is not done correctly arond the homes plates, it will hold the moisture and rot the outside areas of that home.

  4. This article is generally excellent, though I agree with concerns about whether the home has proper capillary breaks between foundation & framing at the perimeter of the space. My one concern with your photos is the lack of spray foam on the joists themselves. This can lead to the joists wicking moisture and rotting the floor framing. I’ve seen catastrophic failures of this type.

    Joe Lstiburek at Building Science Corporation has an excellent article on the issue, with photos of this particular concern near the top of the article.

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