HVAC duct against joist

[garrett1812]

Buying a newly built home, and one of the few items the inspector found was surface mold on a joist. This joist has an HVAC duct very close to it, and in some places touching. All the other ducts are centered between joists. The builder cleaned off the mold, but did not move the duct. Is this something that may have happened once while building and will probably not happen again, or should I push to have this duct relocated?

 

[thooks]

What kind of duct is it? Supply or return should be insulated. Exhaust does not matter

 

[garrett1812]

Pretty sure its a supply, but did not look close enough.

 

[AndyCBR]

I suspect the mold is a result of the duct sweating in the non conditioned (or semi conditioned) space it is running in.

The supply duct needs to be insulated.

 

[garrett1812]

What about the ducts which are centered between joists? Will they also be a problem later on and thus should also be insulated?

 

[garrett1812]

Also, how can it be insulated when its tight against the joist? It still would need to be moved I believe.

 

[AndyCBR]

Unless you can change the RH of the space they are running in they will sweat. I assume the basement is unconditioned where the duct is running?

If this is a brand new house and everything has just been started up there still may be some residual moisture working it's way out of the space.

As far as insulating if there is not room take the duct down and insulate and reinstall. Another option is to fill the joist cavity with insulation and box it in under the joist.

 

[garrett1812]

Yes, new construction, and basement is unconditioned.

 

[AndyCBR]

I'm no basement expert but a lot of people run dehumidifiers in those spaces that stay cool but muggy.

In my mind it should be insulated regardless as you are losing supply temp through the ductwork. I've never seen supply or return ductwork uninsulated.

 

[zmaxmotorsports]

I suspect the mold is a result of the duct sweating in the non conditioned (or semi conditioned) space it is running in.

The supply duct needs to be insulated.

Insulating 6" single wall duct in an unfinished basement? Ive never seen it required around here anyway.
The duct work and the install job and the nasty looking pex are just 2 of the reasons I have always stayed away from new construction,looks like **** to me but builders want cheap/fast.
I subscribe to neither.

 

[AndyCBR]

We don't do many basements around these parts.

But the fact remains his basement is an unconditioned space. Why someone wouldn't insulate supply duct in an unconditioned space escapes me. Whether it is a crawl space, under a pier and beam house, a basement, or an attic, your supply duct should be insulated.

Granted in lower humidity, heating only conditions you could get away without it (you'd duct losses would just be heating the basement).

But as the OP is experiencing, cold supply duct in an unconditioned space sweats...

Additionally most hard duct is sealed with mastic.



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[reader2580]

The only insulated duct work I see in new construction locally is flex duct. My previous house was built in 2001 and no insulated duct work. I thought the HVAC did an incredible job in that house with the duct work compared to what I see in a lot of new houses.

The basements of new house in my area are generally considered conditioned spaces. Even unfinished they usually have some heat registers in the basement. An uninsulated duct will help heat/cool the area it travels through. If the basement of a new house is damp then the builder didn't waterproof and drain tile the house properly.

I don't have an issue with PEX for plumbing. We had a thread here recently with the PEX versus copper debate. It was stated that most plumbers will default to PEX these days unless the customer requests copper or they are repairing a copper pipe. I have toured new million dollar houses with PEX plumbing.

 

[larry4406]

In his picture, the band board is insulated, so the basement would be "inside the thermal envelope" of the house. As the 1st deck is not insulated, this is not a crawl space.

In the unfinished basements my company provides in VA and MD, we still provide a few supply registers to the basement (2-3) and we provide a cold air return off the furnace return (our furnaces are located in the basement). This tempers the basement and prevents the musty high humidity conditions that may occur. We counsel our customers on the importance of tempering the basement (many think they are saving money by closing them off then they complain of musty basement).

Duct work inside the thermal envelope is not insulated as any heat loss from the duct directly contributes to the conditioned space of the building.

I suspect his duct is sweating due to high humidity in the basement due to lack of tempering the basement air. Either cut in some registers and a return, or install a dehumidifier. I often see duct work sweating on startup of the equipment which dissipates.

Remove the moisture source (duct sweat) and the mold will cease.

 

[Ron Lombardo]

Insulate it whether supply or return and exhaust should be insulated within 10' of the
exterior discharge. Those pex lines should be spread apart at lease 2" and I would insulate them also ... something is causing condensation to create mold.

 

[zmaxmotorsports]

We don't do many basements around these parts.

But the fact remains his basement is an unconditioned space. Why someone wouldn't insulate supply duct in an unconditioned space escapes me. Whether it is a crawl space, under a pier and beam house, a basement, or an attic, your supply duct should be insulated.

Granted in lower humidity, heating only conditions you could get away without it (you'd duct losses would just be heating the basement).

But as the OP is experiencing, cold supply duct in an unconditioned space sweats...

Additionally most hard duct is sealed with mastic.



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Yeah they sealed the joints with that nasty *** mastic,you can see it in his 1st picture.

 

[zmaxmotorsports]

In his picture, the band board is insulated, so the basement would be "inside the thermal envelope" of the house. As the 1st deck is not insulated, this is not a crawl space.

In the unfinished basements my company provides in VA and MD, we still provide a few supply registers to the basement (2-3) and we provide a cold air return off the furnace return (our furnaces are located in the basement). This tempers the basement and prevents the musty high humidity conditions that may occur. We counsel our customers on the importance of tempering the basement (many think they are saving money by closing them off then they complain of musty basement).

Duct work inside the thermal envelope is not insulated as any heat loss from the duct directly contributes to the conditioned space of the building.

I suspect his duct is sweating due to high humidity in the basement due to lack of tempering the basement air. Either cut in some registers and a return, or install a dehumidifier. I often see duct work sweating on startup of the equipment which dissipates.

Remove the moisture source (duct sweat) and the mold will cease.

 

[garrett1812]

Okay, I did misspeak earlier. There are a few registers in the basement, so based on what I am hearing this would be considered conditioned space.

 

[AndyCBR]

Okay, I did misspeak earlier. There are a few registers in the basement, so based on what I am hearing this would be considered conditioned space.

Is there a return also?

Supply registers with no return won't let the humid air ever see the coils. You just get a mix of humid basement air and supply air.




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[garrett1812]

I am not sure. Will try to see next time I am at house.

 

[NUTTSGT]

Also, how can it be insulated when its tight against the joist? It still would need to be moved I believe.

If I were to insulate the ductwork, I'd probably use something like double bubble and foil tape.

 

[cowboyjosh]

I can see the reason the duct work hugs the joist, is because the hot PEX line wouldn't allow the HVAC duct to be centered like the rest. I don't know why they ran the PEX the way they did based on the one picture.

No the duct does not need to be insulated, the basement is conditioned space. As far as return air in the basement at least in the areas where I build, there are various code issues with return air in an un-finished basement, that might be the case depending on your AHJ as well.

I see you are in Indiana, I am in Colorado where we cannot even find a dehumidifier in a store (we don't need it, were dry here) but in Indiana its a must. I know of at least 3 houses that had fires in Ohio from cheap Home Depot / Lowes / Menards dehumidifiers over the past few years, so I might suggest you talk to an HVAC company and have a whole house dehumidifier installed as opposed to the cheap, free standing units; because depending on the size of your basement you might need more then 1 free standing, and for a few hundred more you could get one installed with a rock solid warranty and one that is more efficient.

Congrats on the new home.

 

[kbs2244]

Get a box fan.
Hang it from a joist as far from the sump hole as possible.
Aim it at the sump, put it on low and forget it is there.
Then get a dehumidifier and put it so it can drain into the sump.

Mold grows in damp, still air.
You need to move and dry the air down there.

 

[zmaxmotorsports]

Oopsies,had my head up my *** thinking about whole house humidifiers instead of dehumidifiers!
As far as the lack of returns in an unfinished bay basement without a separate utility room its normally done to avoid pulling flue gasses out of the chimney/flue pipe .

 

[garrett1812]

The duct is centered between joists further away in the room. At the end of the wall it goes up into the main floor via a rectangular register, which is right up against the joist. So maybe its to avoid the PEX, but also because of the register position. I was hoping a 90 degree could fit in there to move it over a bit.

Good thoughts on the whole hose dehumidifier. I will have to look into this. I have not had a chance to monitor RH levels yet, but anticipated needing at least a portable dehumidifier in the basement.

 

[Average_Joe]

Running a dehumidifier seems a bit extreme. I don't ever see residential exposed un-insulated ductwork here in SC, but I know ductwork between floors is always insulated.

I would get a few local hvac contractors to look at it and maybe give you a written inspection letter on the whole system. If they recommend it, I would push the builder to have all ductwork insulated.

Also, if the basement is conditioned, the existing system should remove the excess humidity.

 

[LS6 Tommy]

Just a few thoughts.

Cooling supply ducts are ALWAYS be insulated, whether they're in conditioned space or not. They will sweat.

If the system has gas/oil heat, you can't have a return in the same room the unit draws it's air.

Not to be insulting, but long runs or lots of round duct/flex are usually a good sign it was a relatively low buck job. I'd scrutinize the whole install.



Tommy

 

[volleyball]

It could have been from the pipe getting cold during construction and dew forming on it against the bare wood causing mold.
But as LS6 said, the supplies should all be sealed at joints and insulated. That filed faced double bubble wrap isn't thick though it likes 1/2" gap to work. There is plenty of room in that joist space to fix it. And have the builder do them all. The returns can be left bare, just sealed.