I was under the impression that rigid sheet metal duct work was superior to flex duct in terms of indoor air quality. But I could be wrong about that. Flex duct is reputed to be more leaky. It also has more nooks and crannies for dust and mold to accumulate.

Consider my build, however. The house is being constructed of steel SIPs with a conditioned, non-vented attic. So all of the duct work is contained inside of conditioned space (no crawl space and nothing underground). So I would think flex duct's pressure loss wouldn't be as much of an issue, and the likelihood of mold growth is pretty minimal.

Is the increase in cost for rigid duct work worth it? Am I missing other elements to consider regarding indoor air quality and selection of duct material?

HVAC installers have told me that flex duct runs should be kept to a minimum since it offers more resistance to air flow.

Is there enough moisture in ducts to support mold growth? Perhaps during spring/fall when they aren't being used.

Mold should not be a problem in ducts unless you have unusual conditions that provide enough moisture. When designing a house the manuals J and I think its D should be used to provide proper air distrubition.

The keys to good indoor air quality are to have a tight house and sealed ducts. Flex duct should have a nylon strap properly applied and the joints sealed with tape. For sheet metal all joints should be taped or have mastic. The return side typicall uses the insides of wall cavities and panning below the floor joists. This lets a lot of air to be pulled in from areas that dont get cleaned after construction. These are also difficult to seal as there are so many hidden seams inside the walls.

I asked Joe L about flex duct and he did not seem to concerned with its use. I think it shoul be properly sized for length and installed so there are no kinks or restrictions. What I see with flex duts is that many time the installer uses a 25' length is used for a 5-10 run, the excess may be kinked or routed like a slinky.

There are only 2 areas in a HVAC systems that get wet.  There is condensation on the A coil and drip pane. A humidifer also has water.  A UVC light can be used to improve air quaity.

Am I correct that the UV light is aimed at controlling mold spores in the air? Dust and allergens however would need to be trapped with an electronic filter?

air quality is affected by three things. Particulates, think dust, pet dander etc, which a filter removes some of but the smaller stuff gets by. Gases/vapor such as voc's. cleaning agents etc are removed by a charcoal filter, not used much in HVAC. Living organisms such as viruses and bacteria are killed by the UVC light. UVC lights do not produce any harmful byproducts.You should ne have direct exposure to UVC light, if installed in the duct work you will not be exposed. Basically the organism gets a sunburn. You have heard of UVB sunburn hazard UVC is just another part of the UV spectrum. UVC lights will kill anthrax if you are worried about a terrorist attack.



http://www.americanairandwater.com/al/al_prod.htm

There is one other method to clean the air and that is Ozone. Ozone should not be used in an occupied building. Consumer reports said of Ozone air cleaner is that at low level they do not clean and at high levels are harmful. Ozone should be avoided particularly if you have asthma or other breathing problems.Ozone has its place in restoration. Electrostatic air cleaners produce ozone as a by product.

HVAC systems are not air purifiers there is a limit to the filtering. More filtering =more restriction =less efficiency to the tune of 30% or more and greater strain/wear on equipment. Use mid level filter for air flow and filtering. I like the Tri-dek 4 panel filters for air flow and filtering. http://www.tridim.com/Products/Brochures/100-3.pdf

Room air purifiers can supplement the furnace filter. We like Austin Air. HEPA filer is designed to last 5 years. Charcoal filter has 7 or 15 lbs of charcoal versus an ounce or 2 in a home center model.

You have to understand that there are over 100,000 types of mold and most are not a problem. You can not get away from mold. Excessive amounts of certain molds my be a problem for some people. It is very hard to prove mold is a health issue im most cases. Mold is a sign of a moisture problem, mold is decay. It needs moisture generally above 60% and a food source- almost anything organic, paper, wood, dust, soap scum etc. Control moisture and you control mold.

Alton;

resistance to air flow is also determined by the length and configurationof the duct, flex dut allows shorter runs with smooth turns.

Jelly;

one of the great benefits to running ducts in conditioned space is any leakage stays in where it belongs, hard duct or not mastic all joints

I had new AC units installed in my Houston townhome earlier this year. The old ductwork was trash, so I asked for it to be replaced along with the addition of some retrun air "jump ducts." When I talked about hard duct he said "huh?" At least in Houston, if you want hard duct, you need to find someone who does commercial work. The price was a LOT higher and would have taken another week or so, which didn't fit with limited time we were there. It was not worth it to me.

The main problem with flex is friction, which means more static pressure and requires more power to move the air, especially if it has the duct has turns in it. In commercial work we limit flex to 6 or 8 feet.

If you can get a good job that is sized properly without a lot of bends, flex is OK. A long run of flex is probably less prone to leaking than hard duct in sections.   Flex is much harder to clean (maybe inpossible), but when is the last time you cleaned your ducts?

The IAQ problem with mold is typically in the condensate pans which should always be well drained. We use UV lights in all our air handling units.

Bruce