Powered Attic Ventilation Is Unwise, Unnecessary, and Dangerous

A Dallas homeowner called me at my Austin, TX, office one day with an unusual story. His family had purchased an existing home in a classy, close in Dallas neighborhood (that will go unnamed to protect the innocent) that everyone in Texas knows. They had gutted the house down to the studs and rebuilt it to the best quality standards, with the best materials and with one of the Dallas areas best builders and contractors. The problem was that since they had moved in, the house had been a nightmare! It started to grow mold almost immediately. Not just a little, but a lot--on everything. It was miserably uncomfortable and sticky with high humidity levels. The electric bills were far higher than they had expected. Given the complete lack of comfort, that was really unbearable.

Now, I don't do mold remediation, but I do the critical step that must precede a remediation if you intend for it to be successful. I diagnose and identify the source(s) of the moisture and write a scope of work to make the house dry and ensure that it stays dry.

They had hired an engineer to solve the issues, and he found that their a/c's were oversized. The house was 3,800 square feet, two stories, and the builder had installed 9.0 tons of cooling capacity on the house. Now when they had asked the a/c contractor about the sizing he responded, "We always make sure we put enough a/c on the house to handle the hottest days. Like most a/c contractors, we usually install one ton of cooling for every 400 or 500 square feet of house area. Since you have ten foot ceilings, we went with 9.0 tons which was one ton for each 422 square feet of conditioned space. I can guarantee that you have enough air conditioning." They were assured that this is a "standard sizing ratio in Texas."

Well, he was right about that being how a/c's have been sized in Texas (for that matter in most of the U.S.) for the last forty or fifty years. Back in old days, before we had computers to do the calculations, it took a whole day to perform a heat gain/heat loss sizing calculation. The old uninsulated, leaky houses with single pane windows really needed one ton of cooling capacity for each 400 or 500 square feet of space. This became a "rule of thumb" used throughout the industry. Being an ACCA (Air Conditioning Contractors of America) Manual J (a/c sizing calculation methodology) trainer, I've done workshops all over from Texas to Wisconsin, and I've found that this ratio of square feet to tons of cooling is applied pretty consistently wherever I go. The problem is that our homes today are far more energy efficient, better insulated, and with better windows, too. Using the old "rule of thumb" today results in massively oversized units.

The mechanical engineer did the ACCA Manual J heat gain/heat loss sizing calculations and his calculations showed that the house actually needed only 5.5 tons of cooling! The house had 3.5 tons of excess cooling capacity or about 3 percent oversizing. That will often be sufficient to create short cycles and result in poor dehumidification.

Ok, ok, I can hear you now. "What's wrong with having an a/c be too big?" Isn't bigger, better? No. Bigger isn't better in air conditioner sizing. First, an oversized air conditioner runs in short cycles and that means it doesn't remove indoor humidity well. It never runs long enough to dry the air inside the house, so the house stays cool, but it also stays damp. You get that cool, clammy cave effect. The right sized unit will deliver greater comfort, for more hours of the summer, and for less dollars upfront and fewer dollars in electric bills each and every month. This is especially true in humid summer climates. But, that's an in-depth discussion for another article. (Look around in my blog to find it)

"By all the saints (or something to that effect), we've found the problem," the engineer announced. They sued the builder and the builder lost so he replaced the a/c units with new, properly sized units. Hurray. We won. Our house will be liveable," cried the owners! But, alas, after another summer of mold growing on everything, lousy comfort, and bad odors, the owners went looking for a real solution. A friend of mine at the greater Dallas home builders association referred them to me.

I flew up to Dallas and went to the house. The first thing my initial walk through revealed is that they weren't kidding about the mold. It was on the walls, on the clothes in their closets, on the pot holders and dishes in their kitchen drawers and cabinets--on everything. It was going around every light switch and electrical outlet in the house and their beautiful solid oak flooring and stairs were all so warped that you had to pay attention when you walked or they would trip you!

I had noted when I walked up to the house that it appeared to be built over a crawlspace. I asked and the owner confirmed that it was. Knowing that crawlspaces can be a massive source of moisture for a home, I asked to be shown the access opening. It was located in the floor in a closet under the stairway to the second floor. Just as we got to the scuttle opening, the owner stopped and looked at me with a strange expression. I didn't know what to make of that look but then he said, "Before we open this up, I want you to run your hand around the gap at the edge and feel the wind blowing up from down there!" I did as he asked and sure enough, there was what I would guess to be a 20 mile an hour "wind" blowing up from the crawlspace! "Now, the wind outside is dead calm today, so you explain that to me. It does that all of the time," he said as if announcing that the house was haunted.

Intrigued by the ghost story aspect of this information, I put it in my file as an anomalous but intriguing finding. I then opened the hatch and went down into the crawlspace. It was a mud pit! I sunk in to my wrists. There was the usual code required ventilation, but the relative humidity was 94 percent and the wood framing members read as saturated (over 30 percent moisture content by weight) when tested with my Delmhorst two pin wood moisture meter. Enough of this. I knew what I needed to know about this crawlspace. It was a massive source of moisture. I got out and removed my clean room suit.

Building science research by Dr. Bill Rose, P.E., AIA at the University of Illinois, Urbana-Champaign, has proved that exposed soil in crawlspaces and basements is a massive, even dominant source of moisture for a home. His studies showed that even powder dry soil evaporated 12.2 gallons of water per day per 1,000 square feet. The average family of four makes only 5-7 gallons of water per day inside of the house by bathing, breathing and cooking. So a home with 2,000 square feet of exposed soil in the crawlspace will be dominated from a moisture source perspective with at least 25 gallons of water a day coming into the house from below.

I had the beginnings of a theory in my mind. I was sure that the crawlspace was the source of the massive moisture affecting this house, but I needed a transport mechanism. Remembering the "wind" from the crawlspace, I decided to measure the pressure that the house was under. Many factors can cause a pressure in a home. Among them are air leaks in the a/c and heating ducts that lead to imbalanced air flows and closing interior doors and isolating rooms can cause negative pressures.

I turned off all of the a/c equipment, all exhaust fans, and opened all interior doors. They didn't close their doors as a routine, so that took that possibility off the table. The digital micro-manometer showed that the house was under a whopping -7.8 Pascals of suction pressure! What the....? It takes a lot of air flow to induce that much negative pressure across the entire area of the envelope of a large two story home.

I now knew the scale of my transport mechanism, but what was causing it? That much negative pressure easily explained why the air was virtually whistling up out of the sodden wet sauna of a crawlspace into the house and saturating everything on the inside, but I had turned everything off. What could be producing that much airflow? Time to go on a hunting trip.

I took my smoke puffer and looked for air movement. Everywhere I looked that connected the inside of the house to the outside like plumbing penetrations behind the sink, electrical boxes, windows, I found the outside air was being sucked in as if the house were a vacuum cleaner. Then I puffed some smoke near a recessed can light in the ceiling of the master bathroom on the second story. For once, it didn't blow back at me; instead, it was instantly and rapidly sucked into the attic! I tried a few more recessed fixtures and this same thing happened. Now I knew what was going on.

I asked confidently where the attic pull-down stairs were located. When I pulled them down, I heard the sound I had expected. Hummmmmmmmmm! The sound of a power attic vent fan. In fact, two of them. Each rated at 1,250 cubic feet per minute. When I examined the attic, I noted that there were no soffit vents in the house at the eaves, and that the gable end vents were unusually small. Now I had all of the pieces. (Have you figured it out yet?)

I decided to do a demonstration to show my homeowners what was happening in their house. I took several pieces of toilet paper, each about four or five sheets long. I placed one over the face of each recessed can light in the master bathroom ceiling. As I had hoped, the suction from the attic held them in place. I then went downstairs and told the owners I had identified the source of their problems and if they would come upstairs, I would show them what was happening.

Well, I wish I had taken a photo of their faces when they walked across the master bedroom and saw all of that toilet paper hanging from their bathroom ceiling like some kind of Halloween prank! It was priceless. I puffed the smoke and showed them the suction the attic was placing the house under. I explained to them that their attic ventilation fans were sucking a total of 2,500 cubic feet of air per minute out of the attic and how because of the tightly built attic most of that was coming from inside of their house through recessed light fixtures and up hollow interior walls! Witness the magical hanging toilet paper I said. I told them that in building science, we always say that "a house is a system." When you suck air out of the top of a house, it depressurizes the whole house, and an equal amount of air is drawn into the house from outside to replace the exhaust flow.

I then dropped my big bombshell on them. That's what is causing the mystery "wind" to blow up from the crawlspace. They looked a bit dubious, but I persisted. I asked the man of the house to go down to the crawlspace access scuttle and tell me when the wind stopped. He agreed. I went into the attic and disconnected the hot lead from both fans (replaced the wire nut of course) thus shutting the fans off. He started yelling downstairs that it had stopped! The toilet paper banners also fell to the floor at the feet of the lady of the house. They were convinced.

Yes, the house is a system and when you do something to the attic, the problems can go all the way down to the crawlspace or basement! The whole house responds to any and all changes or forces placed on the system. The last nail in the coffin was that so much of the make up for the attic exhaust air was coming from a crawlspace that due to poor construction techniques stayed at nearly 100% relative humidity all of the time. There was no way that even properly sized air conditioners could deal with the massive amounts of moisture introduced into the house by that much air flow.

The solutions:

  • Remove all powered attic ventilation and install a balanced passive ridge vent and soffit vent system -- this removes the transport mechanism for the moisture.
  • Cover the soil in the crawlspace with ten mil thick polyethylene, overlapped by two feet and sealed at the seams, piers and walls -- this stops the moisture from coming up out of the soil thus eliminating the source of moisture at its source.
  • I considered making the crawlspace a sealed, unvented crawlspace, but due to the way it was built, that wasn't possible in this case -- but sealed, unvented crawlspaces are a topic for another war story. (Look at my blog for this topic.)

Problem solved. Mold can't grow in a dry environment. I remember getting an email from the owner two days after the solutions were put in place. It read, "Indoor relative humidity is now 48 percent! That's fantastic! I've been recording these readings for two years and that's the first time it's been under 55 percent and it's usually over 65 percent. Thank you so much!"

...Just another day in the life of a forensic building science geek.

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