I have a heat pump in the attic of a rental house that is about 10 years old. My new renter pointed out wet spots on the wooden ceiling of the second floor, and after I checked the roof (o.k.), the HVAC tech went up and told me I had condensation on the outside of the return and supply flex ducts. The previous renter never ran the upstairs AC, and the new renter has had it on non-stop for 2 weeks, but it never had this problem before. I live in the N.C., the land of heat and humidity. I think the attic space has inadequate ventilation, but then again with a tim roof it gets hotter than you can imagine up there, and the ceiling is insulated. Anybody had this happen or have any advice?
thanks,
Bill
Replies
Yep. Everybody who ever ran AC ducts through unconditioned space has had this experience.
Only thing you can do is either make the attic insulated, conditioned space (best), or insulate (a LOT) and seal (REALLY well) all the ducts (not best, but sometimes you gotta do what ya gotta do).
Mike Hennessy
Pittsburgh, PA
Everything fits, until you put glue on it.
I don't know how much ins you have in the attic, but flex duct never has enough
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Insulate your ducts. Also, if your system was not designed for AC, it could be that your ducts are too small. If you have a variable speed blower, you could try lowering the speed.
10 yr old house in NC - good chance they are flex ducts and are R-4 or similar. Current requirement for supply ducts is R-8 in the attic, R-6 for returns. As others said, if your ducts are sweating that bad your problem is that the ducts aren't insulated enough. Also, even though your HVAC guy checked it out, you may want to have a look for yourself. The pan below the unit could be overflowing. It is not uncommon for the pan drain to get plugged. It should also have a float switch, and there likely is an overflow drain coming directly out of the air handler unit.
Why not put a little info in your profile? BTW - I'm in Raleigh.
I have flex duct in my attic. I have a tin roof. I have insulation in the 2nd floor ceiling. I live in NC. I don't have condensation up there.
I do have lots of ventilation: 5Vee on strapping, lot of flow at ridge, and 3 large gable vents.
It's definitely not the best or most efficient situation. Some time I will spray foam the roof and gables... but, it's what I've got.
I used to get dripping ducts in the crawl, before I sorta sealed it and stuck in an intermittently run dehumidifier... I don't have cupping floors, either<G>
I think you need more/better ventilation in your attic.
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Since he said the stains were on a wooden ceiling there are also two other possiblities.
1. the flex duct is leaking at the supply and return air boots causing them to become a condensing surface. Ten years is about long enugh for plastic ties to dry out, get brittle and start breaking.
2. the wooden ceilng has no vb behind it and the excess moisture is coming from inside the house. The area was little used before so it likely didn't sweat enough to cause staining.
We don't use ceiling vapor barriers here in NC. OK - maybe they do in the western mountains but not in at least 80% of the state...
How do you handle vapor transmission through a wooden ceiling?
Same question for air sealing a wooden ceiling?
>> How do you handle vapor transmission through a wooden ceiling << I have not built a house with an interior wood ceiling but I would probably sheetrock and finish it and then apply the wood.
Edit to add: In your question, the word controlling is key. By blocking airflow you block a large amount of the vapor flow too. The idea though is to create a building envelope - wall and ceiling assemblies - that have the ability to dry. That is why we use things like felt paper or house wrap on the exterior - to allow drying. Same thing goes for the interior. Plastic vapor barriers can trap moisture. Then we control moisture vapor through point source ventilation like bath and kitchen fans. For tight houses ventilation is augmented with an HRV, ERV, or some type of ventilation system that introduces fresh air into the forced air HVAC system.
I think what we are experiencing is a paradigm shift. Or, for some of us it is a done deal. Back in the 50s we built loose houses that didn't have moisture problems because they were about as ventilated as a screen door. Then with the 70s energy crunch we said "plastic everything!!!" Vapor barriers everywhere!!!" The idea was to control airflow (heat transfer) that way. Then we learned something when we had mold issues, and the lawyers helped to augment that... The current thinking is to build tight: have impeccable exterior flashing to prevent water infiltration from that source and stop air too, have good interior air barriers, have controlled ventilation, and again have assemblies that have the ability to dry. Sometime in there we figured out that unconditioned spaces had to be ventilated to prevent moisture build-up.
All this applies to the geography in which I build (NC) which happens to be where the OP lives too IIRC. Some applies to anywhere in north America. Your mileage may vary. I am aware that 'up north' the use of plastic vapor barriers in ceilings is still common practice - but this ain't 'up north'.
So back to the initial Q, the absence of an air barrier cold be a contributing factor. Missing plastic - not so much....
Edited 8/27/2009 8:25 am ET by Matt