Far easier would be to simply insulate the Dx coil cabinet and other cold exposed surfaces associated with the coil. The insulation needs to also provide a vapor barrier (typically foil faced insulation with seams taped with foil backed tape). Moving air across cold surfaces in a hot attic will significantly reduce the A/C’s efficiency by increasing heat transfer to the cabinet, coil and supply air, and even still may not be sufficient to prevent condensation when dew point is high.

BTW, power attic ventilators are bad news for other reasons. See Allison’s previous article on this topic (and other articles linked at the end): https://bit.ly/3kGtSiN

I haven’t contracted for AC yet but am considering a 1.5 -2 ton unit for a 1067sq’ single level condo. What do you recommend?

And where could I find a sheet metal fabricator to make the.Cylinders in a brushed nickel without all those seams?

Thank you.

This dialog was useful this
This dialog was useful this weekend. I have a personal case study from my own home, built 1979 and I moved in 2014. It’s been a fixer upper on several fronts but I may have discovered something about lack of low eave ventilation contributing to condensation on ductwork. I have a 2 story with the full length of the upper floor supplied by a single air handler (split system). It’s a long run from source to end 70’) with all supply and return on the attic. The exterior has exposed rafters in the front and sealed soffit in the back with 4 high gable louvers for attic ventilation. In the 5 years living here, I knew we had inefficient duct design (high energy bills) but it didn’t cause any visible problems. In repairing some fascia recently, I thought it would be a good time to add soffit venting since IRC and traditional practice states they should be there. I started out cutting in slots for 16’ of 2” aluminum vent from Lowe’s at an area I could reach with plans to extend over next few weekends. It seemed like a good plan.

However, within the past few weeks since cutting in the vents, our ductwork above our upstairs playroom (center of house) started dropping water intermittently. And in addition, the ceiling of the upstairs bathroom became covered with mildew. So this past weekend I crawled into the tight attic (it’s a scissor vaulted ceiling) and that’s when I discovered the condensation on various sections of the ducts that were laid across the ceiling joists. Some had torn insulation, or lacked sealed joints, or straps pinching, all things discussed in this blog. Most had blown cellulose up against the bottom of the duct (nothing buried). And while removing mildewed popcorn ceiling from the bathroom is relatively easy and inexpensive for. DIYer like me, the thought of rewrapping or replacing ductwork in this tight space was way out of my abilities and budget. So I started thinking about what changed recently and all I could come up with was adding the soffit vents. Being in hot humid Memphis in summer, I care to the conclusion that I allowed moist air to enter the lower strata of the attic and have access to the flawed ductwork as well as likely over saturating the attic preventing my upstairs shower steam from leaking as it done previously and lingering below the ceiling instead (we had a 70 cfm fan for a 65 sf bathroom). Whereas previously the attic was slightly positively pressured (duct leakage) and any moist air was limited to the upper strata. Other research led to the reminding me that water vapor is h2o and is lighter than both O2 and N2 (bulk of the atmosphere).

So today I removed the vents and sealed the soffit back up, hoping I’m right. The case study continues.

Don, have you had experience
Don, have you had experience in spray foaming the supply lines from the plenum to the vent penetration through the ceiling, creating completely sealed ductwork? Attic encapsulation is not an option for me, but I was wanting a way to save life of my AC units in 120 degree attic – this was the best option I could find.

Peter, your architect is

Peter, your architect is right.  The amount of attic ventilation is mostly irrelevant to the condensation on the ducts in an unconditioned attic.  In fact, in a humid climate, the more attic ventilation you have, the more moisture is available for condensation on cold ducts.  Here’ an article that explains the problem with humidity:

Two Rules for Preventing Humidity Damage

https://energyvanguard.com/blog/two-rules-preventing-humidity-damage

In talking with an architect, their sense is that no matter how good the ventilation in the attic is, the condensation will be an issue until the ducts have better insulation. This article was helpful to plan that. I feel confident that better insulation and ventilation combined will ameliorate the upsetting amount of condensation that I experienced last year, but have to keep a close eye on it… and get working!

With 2×12 joists, I don’t see
With 2×12 joists, I don’t see how it would be practical to fully bury the ducts, since some are bound to traverse the joists. You’d have to dig out enough of the blown-in to drop the ducts to the top of the joists. At that point, your existing insulation would only come up halfway on larger ducts. You’d need to construct dams out of corrugated cardboard to hold additional insulation above and around the sides of the ducts to neutralize the heat gain. I’ve send that done but it’s a LOT easier said than done!

As noted before, you want to avoid dropping parallel ducts running between the joists onto the ceiling, except as required to connect to ceiling boots.

You might want to consider encapsulating your attic with spray foam applied to the underside of the roof deck.

yes, I have 2×12 ceiling
yes, I have 2×12 ceiling joists with 18 inches of blown fiberglass insulation above the ceiling, and the flex ducts rest on the fiberglass insulation. I live in Pineville, LA. I can feel hot air in the summer when the AC first starts blowing, and the opposite in the winter. I was contemplating to cover the ducts with blown insulation. Suggestion or input welcome.