Originally posted by @Dave E.:
@Genny Li
Hmm, that was a very aggressive response. Moist air in a crawl space can be very easily solved with some basic ventilation. That is a very different issue than having ductwork with cold air blowing through it. The condensation on the ductwork did not cause the problem. Having a damp crawl space did. Conditioning a crawl space that is not properly insulated will not solve the problem. It may actually make it worse. There are a lot of factors in getting this right, but I agree that the recommendations from the inspection report don’t seem to make sense. I will also admit that we are all making educated guesses as none of us have actually been in the crawl space.
It's aggressive because the guy is an INSPECTOR. I don't expect regular folks to know this, but he is diagnosing problems wrong and giving a "solution" that would make things worse. He should not have a job, period.
He stated flatly that is was, indeed, caused by condensation on the ducts, and he could see it on his report.
Let's go back to the fun thermodynamic steam tables to explain relative humidity....
So, imagine that you are pouring a big cup of icy water. It is the winter time, and it is 15F outside, and you are inside. That cup will get very little to no condensation on it. Why is that? Because the warm air inside is very dry. Except for what you add to the air from cooking, bathing, etc., the air can only hold in it as much water as it can hold at 15F, which is colder than the glass, so without human water additions to the air inside, you actually wouldn't get any condensation on the glass at all. Not even a bead. Because that glass is 32F.
So, imagine that you are doing the same thing, but it's in the middle of summer. At first, you're inside. In air conditioning, the air is squeezed, which makes it hot (P1/T1=P2/T2), and then the heat is taken away by a fluid that is cooler than this super hot air, and then the pressure is released, which by the same law makes it cold. When this happens, the cold air dumps all the moisture it can't hold anymore--it is at 100% humidity at it's really cold temperature. But it's mixed with room air to a slightly warmer temp, so then the humidity drops into a more tolerable range, say 60%. So 60% relative humidity at 74F is a lot more water than 100% RH at 32F. That is, the dewpoint would be reached before you get down to 32F, and the water would begin condensing on your glass.
Now, take that SAME GLASS to a "well ventilated" area--your porch. It's 94 degrees out there, and the RH is again at 60%. Guess what? You glass is now streaming water. Why? Because the amount of water that is held in the 94 degree air is much, much higher than the amount held in the 74 degree air.
When you see that condensation is the issue, with wet, warm outside air hitting cold vents, to recommend "more ventilation" is to recommend "please, let's rot your entire house because I don't know my job."
Oh, and now let's move those same ducts to a ventilated attic!!! MAOR ventilation with the same ducts!!!! What happens? It's raining inside when it's not raining outside. If you're lucky and the attic is brutally hot, the ducts will have a harder time cooling the air down enough, so you'll have less condensation than in your "well ventilated" crawlspace, but that's still not a good bet to take.
It is one thing to see regular joe schmoes not understand this, but it is absolutely infuriating to see that inspector gave literally the opposite of the correct diagnoses.
You can choose to waste obscene amounts of money moving all your ductwork. But what solves it is either 1) the correct conditioning of either the attic or the crawlspace or 2) lots of insulation around the ducts. More ventilation without well insulated ducts just introduces even more water!
Now, let's look at an alternative, where the ductwork isn't the problem (even though the inspector literally said it was). Let's vent a crawlspace better. And let's have no insulation under the floors, because why have no insulation under floors? Now you've made your whole FLOOR an ice-filled glass relative to the outdoors, and you're depending on essentially using a fan that blows enough to dry it out each night when the RH dips to keep from rotting out your subfloors.
Why don't we, instead, understand that the outside needs to stay outside and the inside inside and install a proper thermal and vapor barrier....somewhere? I don't care if you decide to install it against the floor joists and then ventilate the crawlspace or you encapsulate and condition the crawlspace. We need to have the outside be outside and the inside be inside, and none of this "let's play around and make some spaces that are both inside and outside at the same time." Where is your thermal barrier? Put things that are going to be colder than the outside in the summer inside that thermal barrier.
You can pick up a super, duper cheap relative humidity tester and see if there are actually moisture problems (meaning excess moisture being introduced) in your crawlspace pretty trivially. If the difference in humidity percentage is exactly what you'd expect in the difference in temperature, then you don't have a "humidity problem" at all, because you don't have any moisture that is coming in from anywhere but rather have only the moisture that was already in the air before it cooled down. You have instead a thermal control problem.
