Crawlspace HVAC issue/question

[TGTIW]

There seems to be a wealth of information and resources among the members here, so I'm hoping someone can give me some advice regarding the HVAC ducting I have in my crawlspace. The ducting is roughly 20 years old and rather worn, I'm getting a good amount of condensation forming on it. I realize this also indicated that my crawlspace retains a good amount of humidity, but one problem at a time. I got a rather outrageous quote to replace and reinsulate the ducting, and I'm considering just doing it myself. Off of the main supply line branches off a number of ducts running to the floor registers. Each branch has a rigid duct, with flexible duct connecting it to the register and supply line. Would there be and issue with just replacing the entire run with a flexible line? If it's properly supported and doesn't have any severe bends, would the airflow be restricted at all?

[ltusler]

I'd guess you could replace it all with insulated flexible. As long as you keep the sizing correct for the right flow.

[TGTIW]

I would keep the diameters of the lines the same as the existing ducts, I just think that using a flexible line would make the process easier, as well as have less joints.

[futuresoptions]

Silly question, but what kind of duct do you currently have? Insulated/covered steel? Fiberglass Board?

[Zeke]

Another silly question, how does ducting get "worn?" But, yes, as long as the sizing is close to the amount of area used for the existing, the flow will remain the same. You can wrap some of it with the rolled fiberglass lengths to save tearing it apart.

If it's poorly insulated or just rigid bare duct, do something. The flexible stuff is nice with a sanitary liner and an exterior sheath over the fiberglass insulation. That old fiber board interior/metal exterior is nasty.

[TGTIW]

I'm not sure what the current duct is constructed from, my guess is insulated steel.
As for the wear and tear, pretty much every place the ducting is hung from a strap the insulation is torn.

[futuresoptions]

Is it round or square (rectangular) duct?

Oh! and how long of a run is it? Any ducts branching off the main?

[TGTIW]

Round.

[john70t]

The cheap way:
-Put down some plastic to prevent ground moisture upwards migration(and so you you can slide around), tape all the seams with awesome metal tape, insulate the runs and tape seams.
-Spray all mold/mildew with bleach solution.
-Wear a mask, long sleeves, nitrile gloves, and glasses. Blow off outside and take shower after.

Get more airflow through the crawlspace somehow. Maybe a solar powered fan.
Take a look at the exterior grading and fix any surfaces running towards the house.

I'm assuming you're in the south(high humidity), with a dirt surface underneath.

[red-beard]

The insulated flex duct is a lot easier to install. but...

It comes in 2 forms. Foil on the outside only and foil on the inside and outside. Obviously, inside/outside is better. In both cases, the ducts are better traps for dust than hard duct. The unlined duct can also pickup moisture, which can lead to mold.

These ducts are common in newer construction because the labor to install is much less.

Insulated hard duct is better since it doesn't collect tons of dust and is easier to clean. It is much more expensive and much more labor intensive to install.

[futuresoptions]

I would pull off all the old insulation. Inspect and repair any pin hole rust if any, purchase newer/higher rated insulation and recover your duct. I would then purchase a gallon or so of mastic and put a thin layer over all of the new insulation and tape. This dries hard but flexible like silicone... It will prevent the new insulation from getting damaged from moisture and should last for many years to come. I would think while uncomfortable, would be an easy job to do and you could save yourself some bucks...

[johnsjmc]

The type of duct in a crawl space depends a lot on your location. In Canada we always see metal duct (which never wears out here)but in Florida insulated flex duct is much more common.
I just had the heat pump replaced in a Tampa winter home and was told the metal duct rusts away very quickly because of the high local humidity so it,s rarely used there.
I was also told by more than one hvac contracter that ductwork in a crawlspace wears out in about 5-10 yrs. avg life for a heat pump there is 7 yrs . The crawlspace is open to the outdoors with the house raised on piers, and fruit rats and other vermin will eat the ductwork,plus exposure to wind driven rain
I had quotes to replace ductwork for 7 rooms $800-1500. I ended up having the existing repaired for $300, with $150 came from the local electric co.(Progress energy)
I found detailed info on flex duct recommendedon on the net,but didn,t bookmark it.

[TGTIW]

Thanks for all the info, as it is, I have a vapor barrier on the ground, all the downspouts exit well over 10ft from the house, and I've graded all the surrounding land. I'm in Tennessee and we've been having an unusually humid summer. I think I will re-wrap the existing solid duct and replace the flexible lines. As far as crawl spaces go, it's pretty roomy so it shouldn't be too unpleasant. I was quoted $2600 to re-wrap the existing ducts and $3200 to replace them.

[Tim L]

Get more airflow through the crawlspace somehow. Maybe a solar powered fan.
Take a look at the exterior grading and fix any surfaces running towards the house.

You don't want to ventilate your crawl space. letting worm moist air into your crawl space is a bad thing. It's going to condense on any thing cool.

[TGTIW]

That seems to be the big disagreement these days. Vented vs. nonvented. Obviously, on humid days you would just be pulling humid air in, but I don't understand why so many contractors still believe the best way to dry out the space is by pulling the outside air in. Completely sealing the space right now and dehumidifying isn't really an option due to $$, and the house is 22yrs old and I don't have any mold issues at the moment so I think I'll be good for a bit if I can reinsulate and fix my ducts.

[sprio]

I concur with futuresoptions on this one - I'm in Tennessee, as well, with a vented crawl space, and a 30 year old house. I'm in the process of slowly sealing all of the seams with tape and mastic, and wrapping the ducts with insulation. Not the most fun job in the world, but, like adding extra insulation to the attic floors, will pay off in a more comfortable home, with the satisfaction of having done the job myself (so I know it's right), for less money.

[johnsjmc]

My take on sealing or venting a crawlspace depends on if you want max efficiency during a heating season (sealed) or a cooling season (vented or wide open) Home Depot and Lowes sell all the stuff needed to seal up ,replace or repair and rehang
My Tampa house had a distribution box made from ductboard which was all split open. The individual runs were then 6" flex wjth lots of droopy spots. half my air was heating and cooling the outdoors. . I first considered doing it myself but got a really good deal from a contractor
I found the best deal from a guy on craigslist . I insisted they take out a permit. No problem except a little slow to come back for something small they missed.

[john70t]

Originally Posted by Tim L

You don't want to ventilate your crawl space. letting worm moist air into your crawl space is a bad thing. It's going to condense on any thing cool.

That's a good point, thx.
A crawlspace is above the frost line though, and the surface soil temp differences may only occur when it's hot outside.
You probably are right.

I know that is very true about basements from experience. A friend had water literally dripping from the joists and walls in his cold basement which was open to the hallway/garage upstairs.
-He put batt insulation up under the floorboards and it was rotten within a year.
-In those situations, I had heard it was best to insulate the walls and rim joist and circulate air to make it part of the "warm zone".

[Geoz1]

Im am a union sheet metal worker, I have a little expierence regarding your issue. Flexible duct is fine when the system is designed for it. It will kill your air flow (CFM) when not sized for it. Your best bet is to re insulate the old duct if it is not rusted out.

[scottmandue]

As has already been said... stick with the existing duct sizes... and when I was a HVAC installer here in SoCal around 20 years ago we did just about everything with 100% flex duct.

Edit, some duct board, some rectangular sheet metal on commercial, some spiral steel and some aluminum, some fiberglass pipe (FS), but I would say 80-90% home instillation was flex duct.