Humidity Explained

Condensation on wooden windows is more problematic

Humidity. Moisture. Water vapor. Evaporation. Condensation. Mold. Rot.

These are all words that go together in people’s minds when the topic of humidity comes up. But what is it and why is it so important?

I’m going to try to explain this as simply as possible, so for the scientists and engineers reading – please cut me a little slack. I’m going for clarity over precision. However, if you catch the inevitable factual errors, please point them out so I can correct them.

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Why Doesn’t My Mini-split Heat Pump / Air Conditioner Work?

I recently had an interesting question – a reader asked what could cause a Fujitsu mini-split air conditioner to cause the air to become *more* humid. In fact, they noted that the air became highly moisture laden and the house was just yucky humid.

I really scratched my head on this one because, from a physics standpoint, under “normal” conditions, this is impossible with a mini-split. Why? Because a mini-split system has an air handler unit in the house with the only connection to the outside (and outdoor humidity) is through a small hole in the wall where the electrical and refrigerant lines run. And yet it happened.

The questioner noted that multiple units were involved and that various parts of the electronics had been changed, and yet the problem persisted. He noted that he’d heard of a number of other people with the same problem. I admit, I was baffled!

Then it came to me. In fact, I had worked with an associate, helping them to track down this exact problem. While I can’t state with 100% certainty that the problems were the same, the symptoms are the same. In addition, I realized that my own home’s systems exhibited the same issues, but I automatically made the adjustments to make the systems work properly!

Here’s what’s going on…

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April showers bring May flowers, August storms bring…roof leaks!

Summer rains bring flooding

There’s nothing like a gentle Springtime rain. But Summer often brings us torrential downpours, and along with them, roof leaks, incredible moisture and mold!

A few days ago, we had record rainfalls in Eastern Pennsylvania – five inches of rain in some areas. This fortunately came after an extended dry spell, so the rivers didn’t flood this time. We got really lucky.

These heavy rains often bring high winds, fallen branches and roof damage. Sometimes, they’ll just lead to enough leaking for you to notice. Maybe there’s a discoloration on your ceiling or window jamb. Whatever the sign, please pay attention. Failure to deal with a “small” leak now can lead to thousands or tens of thousands of dollars of damage to your house later. Worse, the accompanying mold can pose a severe health/allergy risk.

When dealing with leaks, you need to take several steps in order to minimize the chance of more serious damage to your home:

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High Electric Bills, Wasted Energy & Central Humidifiers

It’s winter, and for many people, that means dry skin, cracked lips and nosebleeds, so I’m often asked about whole-house humidifiers – humidification systems that connect to your central heating system to distribute moisture throughout the house.

As noted in another post, I’m not a big fan of these units. Any time you concentrate humidity, you run the risk of growing mold. A little leak in your duct system and you could be squirting moisture into walls, ceiling cavities or other areas where you might not know there’s a problem until the entire thing rots out.

It is vastly preferable to use standalone units that use cold water, a small fan and a pad to soak the water. These units are very energy efficient. But beware, there are energy hogs among the small units too. some of them have electric heating elements to evaporate the water and use ten times the electricity as the simpler models! 

In spite of my personal aversion to these units, I recently worked with a friend to track down the source of her high electric bills. She’s an engineer too, so she was reading manuals and trying to uncover the problem. 

As it turned out, she told me “Ted – I have a whole house humidifier, could that be an issue? It runs on hot water.” I’ve heard of this before. Units that squirt hot water into the air stream of the heating system. The trouble is, her unit worked very inefficiently, running the hot water the entire time the heating system is on!

Let’s look at this. This system was basically leaving the faucet on for 8-16 hours per day, using 50-100 gallons of hot water just to put a few gallons of moisture into the air. What a horrible waste!

When you run the numbers, you find that, at the low end, this was costing her $30/month to run. However, with high electric rates in the northeast U.S. and cold winter days, this number is about $100/month! 

Fortunately, there are other ways to solve the problem. If you must have a whole-house humidifier, look for one that doesn’t use hot water and doesn’t run the water the entire time your heating system is on. You’ll have to do your homework, and maybe argue with your heating contractor who will just want to install whatever unit they have sitting on their truck. But it’s your utility bill, so don’t settle.

Guidelines for choosing a whole house humidifier:

  • It should run on cold water, not hot water
  • It should only run if the humidity is below the set point
  • It should not run water continuously when the heating system operates

In addition, whole house humidifiers can be breeding grounds for mold, depending upon the design, so make sure you service it regularly. That means draining the unit and opening it up to clean it out as per the manufacturer’s instructions. Off-season (typically Spring, Summer and Fall), they should be disabled, otherwise they work against your air-conditioning system causing it to work extra hard.

So if you have high utility bills this winter, check to see if you’ve got a whole house humidifier. It could be pouring your utility $$$ down the drain!

If you find Ted’s Tips to be useful, please support my work and visit my Amazon store. In it, I’m collecting the products mentioned in these articles so you don’t have to hunt all around the web to find them.

How Can Your Bath Fan Cost You $20,000?

Bath fan venting ino the atticAn improperly vented bath fan can lead to tens of thousands of dollars of damage to your home and even create a health risk. How? *Moisture!*

The main purpose of a bath fan is to remove moisture from the bathroom after you take a shower. The reason you want to do that is that all that excess moisture can lead to mold and potentially cause your paint to peel and may even rot out your walls. So why do people dump all that water into the attic? Basically, because they’re too stupid and lazy to vent it properly – straight up and out through the roof.

How bad can it really be? Bad! I have had many clients with moldy attics due to this improper fan venting. But one in particular stands out in my mind. It was a beautiful custom home – no expense was spared. I was called in due to mold in the attic. Usually, this means a little patch of black on the roof plywood. But when I opened the door to this attic, I was greeted by a roof covered with fuzzy mold. Everything was damp. This was a problem!

After searching, I found the culprit – the builder had routed the bath fan duct under the fiberglass insulation over towards the soffit (the overhang where the roof meets the house). Often this are has a little bit of ventilation, so many lazy builders run bath fans to this area. However, in this case, there wasn’t even any ventilation slots in the soffit, so the bath fan was just dumping all the moisture straight into the fiberglass insulation.

The homeowners had to spend thousands on mold remediation. They then decided to re-insulate the entire area using spray foam and had to pay to get added attic ventilation to avoid this problem in the future. All because the builder was too cheap and lazy to add a $25 roof vent cap and run the bath fan to it.

Broan roof vent cap

So if you ever consider venting a bath fan into the attic, remember this story. Always vent bath fans straight up and through the roof. Use insulated ductwork from the fan to minimize the chance of condensation in the duct. And use a high quality roof cap, like this one from Broan.

Why not vent out the soffit?

I’ve had some builders argue with me, saying that it’s ok to vent out the soffit. They claim it’s safer because you don’t want to put more holes in the roof because of the risk of leaks.

First off, a high quality, self flashing roof cap like the Broan shown here is very easy to install water-tight. I did two myself and I’m an engineer, not a builder. So scratch that argument – it’s bogus.

Next, think for a moment. What does warm, moist air do? It floats up! Duh. So if you try to vent a bath fan out the soffit, that warm moist air is just going to rise back up through the soffit and back into the attic. It’s hardly better than venting straight into the attic. So under no circumstances should you accept soffit venting of a bath fan. Save $50 on installation today, pay thousands for mold remediation and a new roof tomorrow!

The only acceptable alternative to roof venting is sidewall venting. You might do this when you have a slate roof. In this case, you can route the vent to the nearest exterior sidewall. it’s not perfect, but it will do if it’s not too far away.