Some of the blog responses actually involve the “expert” pulling out a psychometric chart to prove this point — under design conditions. It reminds me of the old movie joke about the cheating husband that gets caught: “Are you going to believe what you see or what I’m going to tell you?”
We know condensation happens, and the reason why should be obvious — we’re not at design conditions. There are two very common situations where this occurs.
The first is at start up. If you take a space where the HVAC system has not been running, the ambient space around your duct eventually becomes the ambient temperature and humidity outside of the building. Turn the air conditioning on and you quickly get your 55-degree air in the exposed duct, but it takes a while to cool that ambient space around it down to your “design condition” of a conditioned space.
Churches used to be notorious for this because sometimes the sanctuaries would go an entire week with the HVAC turned off to save money then be turned on a couple of hours before services to cool the space down. The second situation is where a window or door is close to the duct and outside air floods into that ambient space around the duct.
The pictures I’ve shown here are of an exposed and uninsulated duct about 8 feet from the front door of a popular barbeque joint in Houston. Every time those double doors open — and they open a lot — our gloriously warm and humid Houston air comes pouring in, helped along by the large kitchen exhaust hoods on the other side of the room. The same thing can happen in retail spaces. Or your modern and expensive loft apartment. That patio door lets in outside air as readily as the front doors on our barbeque restaurant. So rather than arguing that condensation shouldn’t occur in our design conditions, we should be looking at whether our exposed duct is likely to frequently encounter some non-design conditions.
A common place people will go to for guidance are the local building codes — “just do what the code says.” That’s not very helpful in this case. Local building codes have two main objectives: safety and energy efficiency. A little duct condensation doesn’t greatly impact either of those. If your local code follows the UMC, you have an exception to insulating supply air ducts in Section 604.1 for “ducts or plenums located in conditioned spaces where heat gain or heat loss will not increase energy use.”
If your local code follows the IMC, you don’t really get guidance, but you are left on the hook for compliance. Section 603.12 “Condensation” states “provisions shall be made to prevent the formation of condensation on the exterior of any duct.” So, what should you do? Well, the first thing you do is ignore anyone that simply states “you don’t have to insulate the duct if it’s in a conditioned space.”
What you should do is look at your unique design situation and determine if you would expect to see circumstances where the area surrounding your duct would fall far outside of “design conditions.” Because for a significant part of the country, you are on the hook for preventing the formation of condensation on the exterior of the duct. For the rest of the country, you’ll just have to deal with upset customers that assumed you should know that the duct would drip.
Okay, so we’ve looked at the design, and it’s apparent that under some circumstances, the duct will probably produce some condensation. Can you live with it? That’s really a question for the customer, but a good HVAC contractor should give them a little guidance to make an informed decision.
It’s not all about “first cost.” Most of the blog respondents will say they would not recommend omitting the insulation for a “commercial” job. There are lots of reasons for that. Let’s start with liability. A quick internet search will tell you that the average cost of a slip-and-fall claim is around $30,000. As a business, you really aren’t going to risk that for a slightly lower first cost. The second reason is lost business opportunity. You can’t seat customers in a restaurant under a dripping duct, so those tables are not going to produce any revenue for you. The third reason is product/equipment damage. You can quickly lose any perceived first-cost savings if condensation from that duct falls on some expensive dresses or shoes. Lastly, in today’s COVID-19 world, we have to look at how duct condensation affects health. Most of the time when you see regular duct condensation, you’re going to see some mold as well.
Let’s say you are the architect or buyer for a nice loft. You want the look you get with an exposed duct system, but you’re trying to justify that “first cost.” You really need to know what the cost impact is going to be. If I were building a $500K condo, I would not want water dripping on my Persian rug because of a $1,000 shortcut on the construction.
Most building is done with at least a 20-year life cycle, so first cost is not the only consideration. What is your — or your customer’s — tolerance for a little condensation? Last year, I was checking into a resort. As the host was taking me into my room, they noticed a steady drip just inside the door. They immediately offered to move me to a different location or to comp my $300 room for the night. I took the free room and put two bath towels on the floor under the drip. My tolerance was pretty high that night — but only because they made me a great offer.
Most of us don’t want a recurring problem in our home that can be fixed. Buyers remorse will quickly set in if we went too cheap on construction, only to deal with an ongoing irritation later. So I guess this comes down to the question of how much it would have cost to do this right.
Fortunately, I work for a duct manufacturer, so I can quickly get that answer. This is just one example, but I’m going to use the system presented to one of those HVAC blogs. This is a little 3-ton system that consists of:
• A 4’-0” length of 16” diameter spiral duct
• A 16” diameter 90-degree elbow
• A 20’-0” length of 16” diameter spiral duct
• A 16” diameter end cap
• Three installed 16” x 8” rectangular register taps
Our base cost is for the above duct to be single-wall construction with no insulation. I’ll sell you this duct any time for $350.00 plus freight. You’ll likely pay more if you’re going through a supply house or rep, but these are the prices I would sell to your HVAC contractor for, so they should give some perspective.
Rather than tell you “insulating the duct will cost $$$,” here are some real numbers to consider. Yes, it will cost more. But you are getting more.
First of all, if you are going to internally insulate that duct with 1” of fiberglass insulation, the basic outer duct you see will have to be 2” in diameter larger — 18” diameter. That alone will increase your cost by $68.00. I can offer you two common product options that will give you the exposed metal spiral duct look, but with an internal 1” insulation. The first is using a grooved fiberglass board product.
You will have the continuous insulation and a treated fiberglass face, but no metal inner liner. The cost is $1,035.00 plus freight.
The second option is a traditional double-wall spiral duct with a perforated inner liner. That system would cost $1,052.00 plus freight. Slightly more expensive, but the cost of the internal metal liner is offset by using a much less expensive form of fiberglass insulation.
This is just one example and there are some differences in the labor involved with these product options. But for this customer, it really came down to his contractor spending an extra $700.00 on the ductwork for the 1,000 square-foot main room of his nice new loft/condo to not have condensation dripping on his floor when guests went out on his patio in the summer.
If you live in climates where condensation on non-insulated ducts could potentially occur, you owe it to your customers to gather information to give perspective to the additional cost for preventive measures. Having that cheap first cost may get us the job, but the aggravation of a dripping duct is really something our customer should put a price tag on.
