New ductwork sweating.....

[sktn77a]

We just had the ductwork in our house replaced. Downstairs (crawlspace) is sheet metal trunk with R8 external wrap and R8 flexduct branch lines. Both the trunk and branch lines are sweating significantly (water dripping off onto the plastic sheet covering the dirt in the crawlspace). I recall our old internally insulated trunkline (R?) sweating like this but not the flexduct (R6). We live in central North Carolina (Raleigh, Durham, Chapel Hill) and it's just got hot and humid this week.

Is this normal? Or is the trunk insulation badly done (the R8 Flex duct looks real flimsy and cheap). Upstair in the attic (same R8 externally wrapped trunk and R8 Flex branch) is sweating a little but not as much.

Thanks.

Keith

[beenthere]

If the air flow is to low, it will sweat.

[toddb]

We just had the ductwork in our house replaced. Downstairs (crawlspace) is sheet metal trunk with R8 external wrap and R8 flexduct branch lines. Both the trunk and branch lines are sweating significantly (water dripping off onto the plastic sheet covering the dirt in the crawlspace). I recall our old internally insulated trunkline (R?) sweating like this but not the flexduct (R6). We live in central North Carolina (Raleigh, Durham, Chapel Hill) and it's just got hot and humid this week.

Is this normal? Or is the trunk insulation badly done (the R8 Flex duct looks real flimsy and cheap). Upstair in the attic (same R8 externally wrapped trunk and R8 Flex branch) is sweating a little but not as much.

Thanks.

Keith

wow not typical for insulated flex to sweat like that as long as its ul listed flex it all looks flimsy but if its listed it is what it says it is. how hot is it in the space where the duct i sweating. must be quite a large temp difference between duct temp and surrounding air temp.

[Gunnery Sergeant Hartman]

It's sweating because the outside temp of the duct is dropping below the dew point.

Close up your crawlspace and put a duhumidifier in the middle of it with the drain piped to the outside.

I get a dozen or so calls about this every year. Ventilated crawlspaces had to be the worst idea ever.

[sktn77a]

Beenthere: It's a 2 ton condenser with the specified 800 CFM airflow.

Toddb: No, I don't recall the old R6 sweating (even though the internally insulated trunk did). I was a much sturdier flex than the new R8.

Gunny: I thought the moisture was coming up from the soil. Do you think closing the vents will improve things?

Keith

[toddb]

Beenthere: It's a 2 ton condenser with the specified 800 CFM airflow.

Toddb: No, I don't recall the old R6 sweating (even though the internally insulated trunk did). I was a much sturdier flex than the new R8.

Gunny: I thought the moisture was coming up from the soil. Do you think closing the vents will improve things?

Keith

i dont know what flex they used so i wont comment on that but gunnery sgt picked up where i left off the temps need to be regulated closer u could seal off the space and de-hemidify as he points out or go the opposite way and insulate and condition the space. the flimsy flex appearance may also hold the answer if it looks that way because its improperly installed and is sagging instead of pulled tight this would explai appearance and lack of airflow

[Gunnery Sergeant Hartman]

Gunny: I thought the moisture was coming up from the soil. Do you think closing the vents will improve things?

Keith

I did a "poor man's" version of a sealed crawlspace on my house where I layed 6 mil plastic on the ground, cut 2" thick blocks of styrofoam DOW board insulation and blocked off my crawlspace vents, and put a dehumidifier in the middle of the crawl and piped the drain outside.
Sweating problem solved.
With the vents open you're having hot humid air come into the crawl and hitting the colder (below dewpoint) ductwork causing the sweating.
The moisture will also affect yor joists and subflooring over time.

[sktn77a]

Beenthere: It's a 2 ton condenser with the specified 800 CFM airflow.

Actually, the 2 ton is the 2nd floor condenser. I checked the furnace settings on that yesterday and it was set for 400/800 CFM, so I re-set it for 600/800 CFM, per the installation instructions for the condenser. Downstairs (the one with the sweating problem) we have a 3 ton. I checked the dipswitch setting on the furnace this morning and the first stage setting was also wrong. It was set to 600/1200, so I set it for 900/1200 and we'll see if that helps.

Thanks all!

Keith

[HVAC/Stud]

You sure its R-8? I've seen duct insulation compressed and not providing the correct R value.

It's not bubble wrap is it? They sell it in SC and if it is not used with spacers correctly it will only be R- 1.1.

The outside of the insulation is below the dew point, you need to correct the insulation or lower the dew point of the space.


STUD

[sktn77a]

Well, it is R8+ (3") fiberglass wrap on the trunk and R8 Flex (says so on the outer material) for the branch, but the flex so cheap that it may have compacted in utero(?)

[HVAC/Stud]

If the duct insulation is correct, then you need to do something about the dew point,

BTW what is your supply temp?


STUD

[sktn77a]

Supply temp (at the register) with the new flow rate is 60degrees when the room temperature is 76.

[Shophound]

Gunny is correct. Ventilating crawl spaces in the hot, humid south is a DUMB idea.

Click on this link for good crawl space info:

http://www.buildingscience.com/docum...rm=crawl+space

This link has very insightful info:

http://www.buildingscience.com/docum...erm=crawlspace