Southeast Coastal region wet AC insulation. 2

[NativeGator]

I have a 2000 sq ft house less than a year old on a river in the South Eastern costal region of the U.S. it has insulated Metal AC ductwork in the crawlspace.
The insulation surrounding the ductwork is wet and dripping water. The inside of the ductwork appears to be dry.
Can some one enlighten me and suggest a possible solution to my dilemma before I start having to harvest mushrooms in the crawlspace.

Background:
The 2000 sq ft house is located 100' feet off a river and the crawl space is on the order of 5+’ tall. I have 6 mil black plastic over the earthen floor in the crawl space which is attached to the crawl space concrete stem wall. The main floor of the house has 10+” fiber glass batts separating the main floor from the crawlspace. (they are dry).
I have over 26 air vents around the crawl space perimeter to allow for good cross ventilation. I have no signs of mold or mildew on the floor joist or subflooring above the 10” batts.
The crawl space temperature stays between 72 and 76 degrees over any 24 hour period during the summer [summer temperature here ranges from low 80’s to mid 90’s],(I have not been in the house over the winter yet).
Both conditioned and unconditioned air ducts are in the crawl space.
The duct work is insulated with ~ 1" thick fiberglass that has bright outside foil. The duct work is suspended below the 10" batts and on the order of 3' above the ground.
The duct insulation does not show signs of mold or mildew. (yet)but it is wet!
The house is kept between 74-78F during the day and 70-73F at night.
The inside humidly stays between 48 – 57% during any 24 hour period.

If you need more Info I will try and supply it.

Thanks

 

[AbbyNormal]

1) You need to stop ventilating your crawlspace.You will have high ambient dew points in the SE

2) You need to make sure that the metal ducts are not leaking.

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[NativeGator]

Abby, if you do not ventilate the crawlspace how do you keep mold from growing since you now have a closed air space without airflow?
My HVAC company, builder, and county all said I needed the cross ventilation to mitigate any mold problems that could arise in the crawl space. Am I in a dammed if you do, dammed if you don't situation?
It seems that the metal duct work itself is sweating on the outside and then saturating the insulation.
Do I need to have thicker fiberglass insulation or a different type of insulation to move the dew point away from the metal duct work surface or am I all wet (no pun intended)with my assumptions?

 

[AbbyNormal]

seal the crawl, insulate the skirting, blow 25 CFM down there or perhaps the duct leakage you already have will take care of it.

The big factor for mold that we have some control over is moisture. Those vents do nothing but humidify that crawlspace all spring summer and fall.

The surface of duct insulation, and even very thick insulation is always going to be so many degrees colder than the surrounding air.

When those ducts are in an attic, probably the attic air is superheated, could be 130F. The surface of the insulation may be 120F, 125F etc. Ambient air in the coastal south will have dewpoints in the 70s maybe even hit 80 once in a while, so 120F insualtion will not sweat.

Now take the scenario of a crawlspace,odds are it will be cooler than the ambient air by a bit and with aminet dewpoints in the 70s the crawl air is almost 100% RH.

Now you have ducts surrounded by 78F air with a 75F dewpoint. Even with a lot of insulation, the surface temp of that insulation will be say 70F and it is going to drip, just like you see now.

Seal it off, and if you are too worried about moving a small bit of conditioned air down there, then add a dehumidifier.



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[NativeGator]

I guess by trying to improve the efficency of the system, a different problem was generated, sweating and soaked insulation.
I wonder why the HVAC company who designed the system, and charged me big bucks to install it, didn't tell me about the possible moisture problems I would have. It seem that the ME/PE who designed and signed off on the work should have enlighten me,or am I wrong?
Would going to the black flex pipe have helped?

 

[imok2]

Take a look at this web site

http://crawlspaceinfo.com/Crawl_Space_Insulation.htm?gclid=CIC5ic7o9pwCFSNQagodS1AIbg

 

[AbbyNormal]

http://www.joelstiburek.com/topten/south.htm

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[NativeGator]

Thanks, Abby good artice.
The HVAC company is going to come out to the house (they have not set a time yet) and let me know how THEY are going to fix the problem. Lets see what they say.
I will keep you all posted.

 

[MintJulep]

Get yourself a copy of this month's AHSRAE Journal. Interesting article on moisture problems typical of SE construction.

 

[NativeGator]

I have a dumb/uninformed question.
If I have two cooled surfaces, one say 20 degrees below he dew point and one 5 degrees below the dew point,which one will have the moisture condense on it first. The coldest or will they both have moisture condensed on them? Which one will condense the most moisture out of the air if they both have the same surface area?

 

[AbbyNormal]

they should both condense, just one more profusely than the other

once they are in there for a prolonged time, the surfaces can both look quit wet, but I would say there will be a bigger puddle under the colder surface

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

I saw an additional response to this thread which seems to have disappeared.

It referred to a study involving sealing off crawlspaces.It looked at insulating the skirting and insulating the floor, and of these two options I would go with the insulation at the skirting myself as the preferred way.

I believe the OP has the insulated floor already, I do not think it would hurt to add it at the skirting as well.

It would not take much air to keep the crawlspace dry. Perhaps existing supply duct leakage would do that, or a deliberate small amount of air like 25 CFM. In my locale, the only time you would see a crawlspace would be for a modular building like a temporary class room.

I deal primarily with attics and lobby to have them sealed. In a humid climate it is the way to go and if you are in the hurricane belt, it eliminates the weakest link in your defence against wind driven. If you are experiencing hurricane force winds, attic vents will let your insulation get soaked and bring your ceilings down while your roof is still intact. That is a fact jack.

So the concept of venting a crawlspace was a way of addressing mositure coming up from the ground. You vent it to dry it out. But in many locations the vent air is too humid, so it is better just to keep the mositure out, poly on the ground and no ventilation openings in the crawl.



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[rogzog]

Just thought, does the insulation on the duct have vapor barrier? Is the vapor barrier sealed at all laps and joints? If not, the hummid warm outside air will condense on the cold duct and soak it.

rogzog

 

[Compositepro]

In reality a vapor barrier on the ducting will simply prevent the insulation from drying-out once it gets wet.

The link to the article about insulating the outside wall and conditioning the crawl space was excellent. It was posted in another thread , though, and some comments may have appeared to be inappropriate in that context. I would like to see that link posted here in this thread because it was very appropriate and I learned something valuable.

 

[ChasBean1]

I'm chiming in late on this one, but before changing anything make sure you don't have to conform to International Mech. Code. The 2006 code (Sec. 406.1) says all attics and crawl spaces shall be naturally ventilated. If for a crawl space mechanical exhaust is used, it needs to be no less than 0.02 cfm/ft2. If automatic controls are used, the fan should cycle on any time the %RH is above 60.

That's code, not opining whether its right or wrong - I can definitely see the benefit of sealing and dehumidifying... To stay 'legal' however I'd suggest stripping all your supply duct insulation, finding/sealing all air leaks with some of that ductmate gray goop, and re-insulating it with maybe thicker insulation, with extra attention to taping/vapor barriers.

 

[Compositepro]

The article in in link that went missing was published and well researched and and presented the data to say that the code needs to be changed.

 

[AbbyNormal]

You can end up with a vented crawlspace full of air at almost 100% RH in the southern locations. It is obviously shaded from the sun and the ambient air gets cooled by the earth and the house above.

You get the right weather as in ambient dewpoints in the high 70s and you can have as much insulation as you want, with a vapour barrier but the ducts will be doomed to sweat.

The surface of the insulation will always be a little cooler than the surrounding air when you have cold air running in the ducts.

Metal ducts leak and tend to inflate the insulation. Get thermal short circuitting, the vapour barrier gets cooled.

Running duct work in unconditioned spaces is always an accident waiting to happen. Simple solution is to close off the vents and even condition the space with a small amount of air.

When the surrounding air is cool with a high RH the insulation is going to sweat.

Example

metal duct in a crawlspace is insulated with perfectly applied R8 duct wrap. Crawlspace air is 77F and has a 76 dewpoint. 55F air is flowing through the duct.

Consider the bottom of the insulated duct, heat flow is up, so perhaps an air film of R0.65

Heat transfer per sqaure foot estimate as (77-55)/(8+.65)= 2.54 Btu/hr

So use heat flux through the air film to get temperature differential across air film

DT=2.54x.65 = 1.65F

So the insulation vapour barrier surface temperature is 77 - 1.65 = 75.4 and the dewpoint in the crawl is 76. Its going to sweat.

How thick does the insulation have to be to get it warmer than 76? R8 is already a couple inches of duct wrap.

Maybe work on keeping a lower dew point down there if you are running the ducts down there.

Keeping the humid air out is the first step to lowering the dewpoint in the crawl.



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

Codes like the IMC try real hard to be 'one size fits all' but that is impossible.

Your local climate tells you what you have to do.

I built a fourplex in the tropics and I put my money where my mouth was. I sealed the damn attics.

No when you seal them, you are supposed to condition them. Well I cheated, did not condition the attic void, and I kept an eye on them to see how they were working.

Conclusion

Attic space, averaged in the top right hand corner of ASHRAE's comfort zone.

My attic, August 2007, the big dip in RH was me pre-chilling the place as Hurricane Dean Approached, the spike back up was the 8 hour power failure during storm. The drop in temp and RH is due to increased heat transfer through sheet rock ceiling to conditioned space below. Insualtion is on top of roof deck not at ceiling plane.

It averaged at 80.5F and 47% RH

I have since stored some valuable items up there and as insurance I added about 30 CFM of air, temperature averages 79, RH in the 40s. The other three units do not even have any deliberate conditioned air supplied to them. Been 3 years now, no problems.

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install