# Attic Smell from A/C - I'm at wit's end!



## didn'tdoit

I guess I do not understand...musty and dusty...Is it wet up there? Have you checked the condensate line from the AC/? Is it broken, does it have a pump on it? Is it working? You should be able to find the condensate drain line and water should be dripping from it while the AC is running. check this out and post back.


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## brownerd

is the smell coming from all the vents the same or from 1 more than the other??? If it is from all the vents the same then you might want to check the ducting going to the return. If the smell is stronger from 1 vent then check the ducting going to that vent. When you had insulation blow in they may have accidentally broke/put a hole in a duct (but you checked that). Good luck. You may need to have a HVAC tech come out a pressure test the ducts and seal them from inside if needed.


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## cidsamuth

*Seal from inside?*

How do you seal the ducts from the inside?

The only logical conclusion to this is, despite my efforts at foil taping every duct, boot, and joint, the hot attic air is still getting in.

So, how do you seal from the inside, as the lost post stated?


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## brownerd

cidsamuth said:


> How do you seal the ducts from the inside?
> 
> The only logical conclusion to this is, despite my efforts at foil taping every duct, boot, and joint, the hot attic air is still getting in.
> 
> So, how do you seal from the inside, as the lost post stated?


Well, from what I can tell it's a little luck and a lot of magic! :wink: 
Apparently they use some sort of aerosol that sinds the leaks and fills them with a glue of sorts. From what I hear it is an involved process that requires the duct to be sealed off while they are spraying the sealant (you don't want it coming out the vent). Here in California it is required that duct sealing be done on all new HVAC installations.


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## Pete Gotchall

Cid,
In the summer, I notice the "hot attic" smell when I come in @ 5PM and the set-back thermostat turns on the A/C. 
I have sealed and fixed all of the original mess left by the "experts".
It may just be the hot smell you notice.
Pete
By the way, the Great Stuff foam works great for sealing duct work to plenums if you have the flexible duct.


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## coolmen

add a attic fan to circulate the air in the attic on high temps,does the drain to the evaporater have a trap installed and where does it drain to?the filter location pulling in attic air?all duct work, plenums well insulated


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## cidsamuth

Pete Gotchall said:


> Cid,
> In the summer, I notice the "hot attic" smell when I come in @ 5PM and the set-back thermostat turns on the A/C.
> I have sealed and fixed all of the original mess left by the "experts".
> It may just be the hot smell you notice.
> Pete
> By the way, the Great Stuff foam works great for sealing duct work to plenums if you have the flexible duct.


Yea, the "experts" really do lousy jobs.

I found insulation on the coil (after an expert said the coil was clean). As stated, I had to seal all joints after an expert installed the system.

The smell is still there, even though the coil is clean. I am having the ducts professionally cleaned Tuesday, so maybe that will fix things.


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## 747

best way is to have a insulating contractor come in and spray the ducts in the attic with close cell poly foam insulation. It has to be closed cell not open cell. That will seal them tight and make unit more efficent also. This is always done in new construction when homeowner chooses to go with poly spray foam insulation. The only difference is the use a open cell for walls and ceilings and close cell for ducts. Or is it the other way around. I think i have it correct.


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## didn'tdoit

747 said:


> best way is to have a insulating contractor come in and spray the ducts in the attic with close cell poly foam insulation. It has to be closed cell not open cell. That will seal them tight and make unit more efficent also. This is always done in new construction when homeowner chooses to go with poly spray foam insulation. The only difference is the use a open cell for walls and ceilings and close cell for ducts. Or is it the other way around. I think i have it correct.


 Can this be sprayed inside of the ducts? Who does this? Do you know of any national contractors?


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## brownerd

Pete Gotchall said:


> Cid,
> In the summer, I notice the "hot attic" smell when I come in @ 5PM and the set-back thermostat turns on the A/C.
> I have sealed and fixed all of the original mess left by the "experts".
> It may just be the hot smell you notice.
> Pete
> By the way, the Great Stuff foam works great for sealing duct work to plenums if you have the flexible duct.


Do you have a whole house fan? I have noticed that smell in my house too. My HVAC return is near the louvers for the whole house fan - I suspect air is being drawn from the attic via the fan louvers. Something to think about if you have a fan.


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## blackandgold

*Musty Attic Smell from Upstairs AC*



cidsamuth said:


> I am getting that musty, dusty attic or insulation smell from by A/C vents, and it is more pronounced when very hot outside.
> 
> To paint a proper picture . . . the house is only 2 years old and has dual-zone cooling. The upstairs furnace is in a small part of the attic where the insulation is rolled. The rest of the attic has blown insulation. Because the "blown" side represents the largest part of the attic, most of the return and supply ducts run to it from the "rolled" side.
> 
> Last year, to help with costs and efficiency, we had additional insulation blown in to the "blown" side. Since then, the A/C has smelled like the attic, though we notice nothing from the heat in the winter.
> 
> I recently when up there and taped up all the joints, nipples, and boots so minimize cold air blowing into the attic (or being sucked from the attic). The contractor had failed to do this. I checked the ducts, and none have fallen loose or broken (all have R6 insulation around them). I had the original A/C contractor back, and he claims no insulation is collecting on the coil (I don't know how to check this myself).
> 
> We have ridge and soffit vents, though I noticed that insulation has made its way into (and blocked) several of the soffit vents.
> 
> I am not saying the A/C ducts are now air tight, but I certainly did as good a job as most contractors would do. I don't think I can put any more foil tape on the ducts! Any ideas? Could the lack of ventilation in the attic be the culprit? Is there something else to check on the furnace?


I know that it has been a year since your air conditioning problem but I wondered what the resolution was? We are having the same attic smell from our upstairs AC that you reported and the professionals say that there is nothing wrong yet the smell is intense as the outside temperature increases. Thanks for your time.


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## mrtech

What type ducting do you have? If it's fiberboard duct, a musty smell will develope if the fiberglass gets wet for a period of time. I've seen this when the board was installed after getting wet during the rain and being hauled to the job site in a truck. The glues in the duct don't set well with watersoaking.
You could also, perhaps have a spit off from the indoor coil, depositing moisture on the fiberboard. This will cause a smell especially after sitting for a while and molds will also develope on the surface.
Cut into the supply (and some cases the return) duct at the air handler or furnace to inspect the duct.
Duct that has gotten wet enough to produce the smell, then dried quickly will show no signs of getting wet, but will stink anyway.
Let the unit sit in the off position for 30 minutes, then just as you turn the fan on, smell the air from one of the outlets the farthest away from the indoor unit. If you smell the stink imediately- it is the ducting, if the smell takes seconds to reach your nose, the stink is coming from the indoor unit or right near it or the return.
If you have metal ducting, the insulation in the air handler or furnace may have gotten wet and smelly and need replacing.
The drain should be trapped and draining properly and not allowed to dry if it is going into any house plumbing.


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## mikemy6

clear the soffits add insulation baffles then you can push the insul. back into place


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## Brent65

*best guess*

You are getting suction out of your attic from one of two places or both. They are the attic access door if it is inside your home or from can lighting fixtures, when your a/c kicks on the return air vent is sucking this heated air in thru these areas .


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## Craig93

*Dust in my House and it has an 'attic smell' when A/C runs*

I've read these replies and they all sound good. 

BUT, I had same problem and I think I located it after 2years also.

My heat/dust/smell was coming from the attic access pull-down stairs. I left a light on in my attic and at night noticed a faint bit of light coming through the door edge. 

I sealed it and thought I was the next guest on This Old House....but it still happened again.

I turned that light on again at night....Bammm!!! Dang warm air and dust from attic coming through the main piano-style hinge on the ATTIC DOOR. I have a second air return located on the wall/floor under that door that was pulling that dust and attic air right down through the hinge.

I opened attic access pull-down ladder, sealed it with duct tape and I swear I can tell a difference 5 hours later. 

Hope that helps anyone, if it doesn't then there are other good ideas.


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## missalbany92

*AC Duct Smells, too*

I joined specifically to search on this very topic. I, too, am at my wit's end over a smell coming from our AC registers. The smell is only noticable when the air is running, not the heat. The smell is a musty, moldy smell and is most noticable when it gets hot outside and I live in Georgia so it is hot a lot! The weird thing is that is is only present in a few of our vents, not all. I did have one really helpful Heat/Air person come out who noticed we had an air flow problem. We have a RUUD split system with the air unit in the attic. Turns out our plenum (sp?) box was built too large for the unit which didn't allow enought pressure to force air through the ducts. Also, several of our ducts were crimped at the register. We have flex ducts. These were replaced with elbows and the duct line leading off the supply side was replaced (which is where two of the ducts were stinking.) Following the replacement of the plenum box, we noticed the smell in two of the registers was gone and is still gone. However, the smell remains in one other register in our kitchen. What I do not understand is the significance of replacing the plenum and changing out the duct line leading from it and why this alleviated the smell. I think the other register might be from the return side. I'm thinking of having the entire line replaced in hopes it might get rid of the smell in the kitchen register. It's really perplexing and sounds crazy but it is real. Our house is only 6 years old and we just started noticing this smell two years ago. It is also more noticable when the air is not running and seems to disappear when the air kicks on. Any help or thoughts appreciated!


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## coolmen

If the supply side of the furnace/airhandler in the attic have majer leaks this would cause positive preasure in the attic and negative preasure in the rooms. All air would push/suck into the living space causeing smell/dust. do you notice dirt or streaks of dust @ the registers? if you do take off the register and seal between the sheet rock and the boot to eliminate leaks. As for the attic ducts main, remove all insulation from the main and mastic tape all seams.


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## desertguyphx

*Attic Air Smell*

This problem of attic air smell began when I had a new roof installed. I discovered when the roofers removed the shingles around my skylights, it disturbed the flashing seal on the skylight curb allowing attic air to come into my living area. Resealed around skylight curb and all is good again.


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## Theissfamily

Has anyone on this thread completely gotten rid of their smell? We bought our house in Destin 3 years ago and have pumped thousands into trying to find the same smell over our kitchen. We're replacing one of our AC units because it was pushing most of the air but it was the smaller unit and the duct work in the attic wasn't properly routed or installed causing condensation to drop out of the vents and soaking the insulation anywhere near it. It wasn't visible on the white ceiling so we didn't catch it. I thought it was the hood vent because it smells in our walls and cabinets too but only near our kitchen area. Apparently that's where the duct work screw up is centered. Guy cut a hole in the ducts to show my husband the jizz in there. 3 years, duct cleaning, mold remediation, lots of Scentsy, lots of guys in and out of our house and no answers until recently when I threatened my husband with burning it to the ground. This guy is hopefully our savior but we'll still have to suck out the old insulation and replace right? Who does that without ripping out all the dry wall? Walls are hollow from attic access over that area. Vacuum cleaner to suck it out? Will it work? We're literally going broke trying to get rid of that damned smell!


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