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Old 09-20-2011, 08:03 AM   #1
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Cold Air Return Ducts & Aluminum Foil


I've been trying to seal in odors that originate in my cold air return ducts. First, I used Zinsser B-I-N shellac, which eliminated the musty/moldy odors, but off-gassed its own toxic odor all last winter. This summer, I painted over the Zinsser with Safecoat Transitional Primer (three coats), which eliminated the toxic odor from the Zinsser product but has an odor of its own, which has been off-gassing all summer. As a last resort, I plan to apply aluminum foil over the Safecoat and hope that this blocks all the odors.

My question is what should I use to attach the foil to the three sides of the duct (the fourth side consists of sheet aluminum that I attach in sections with wood screws and Liquid Nails)? A hand stapler comes to mind, but I'm concerned about the foil sagging from the top surface of the ducts (really the underside of the subfloor). An adhesive would remedy this and also be easier to apply, but it would have to be low odor and capable of bonding aluminum foil to a painted surface (the Safecoat primer). Does anyone know of such a product, or have another suggestion? Your suggestions would be greatly appreciated.

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Old 09-20-2011, 08:40 AM   #2
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Cold Air Return Ducts & Aluminum Foil


I think it would be easier, and maybe even less expensive to replace the return air duct. It sounds like you have access to the duct if you are able to line it inside.

My main concern with using aluminum foil is that if any comes loose it will cause problems with air flow and may even damage your equipment, especially if you have a fuel burning furnace. Another concern is that the glue you will use to apply the foil may cause worse air quality problems than you already have.

Is this RA duct in a joist space? If it is you can simply use metal or foil-sided duct panning.

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Old 09-20-2011, 10:49 AM   #3
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Cold Air Return Ducts & Aluminum Foil


Thanks for the fast reply! Yes, the cold air return is between floor joists - they're what I painted with the Zinsser product and then Safecoat's primer (the sides of two adjacent joists plus the "duct roof" which is really the underside of the subfloor for the floor above). Wouldn't the aluminum foil be cheaper and easier to install than foil sided duct panning? And you're right about the foil coming loose, which is why I asked for advice. It looks like I need to use lots of staples, or find a low-odor adhesive, but wouldn't the aluminum foil itself block any odor from the adhesive?
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Old 09-20-2011, 11:02 AM   #4
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Quote:
Originally Posted by kim23 View Post
Wouldn't the aluminum foil be cheaper and easier to install than foil sided duct panning?
If a piece of the foil came loose and blocked the air flow at the RA filter you could damage your furnace by causing it to overheat. This could ruin the blower motor or even crack the heat exchanger. Both would be expensive to repair and the latter could even be deadly.

Do a search for "ThermOpan". You could even have RA duct fabricated at a sheet metal shop.

Yes, it's going to cost more than a box of aluminum foil, but it will work much better.
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Old 09-20-2011, 11:16 AM   #5
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What should I do about the cross bracing between the joists?
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Old 09-20-2011, 11:21 AM   #6
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Cold Air Return Ducts & Aluminum Foil


To be honest I've never seen anyone try to cover the wood in their RA joist space. All we ever do is pan the bottom of the joists and the space then becomes the RA duct. (BTW, this is only for return air, it's not safe or legal to use wood as a supply duct for heated air.)

I've never had a situation that the wood smelled bad enough that it needed to be covered. Do you know what caused the odor problem to begin with?
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Old 09-21-2011, 04:12 PM   #7
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Cold Air Return Ducts & Aluminum Foil


Photo 7,8: http://www.buildingscience.com/docum...39-five-things

And: http://www.buildingscience.com/docum...ong-from-start

Gary
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Old 09-23-2011, 11:31 AM   #8
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Cold Air Return Ducts & Aluminum Foil


In reply to fabrk8r - Thanks for your reply - the original smell seems to have been mold and years of accumulated pet hair in the returns. I found all of this stuff when I bought the house, and two rounds of professional duct cleaning did nothing to remove the smell (using the BIG truck-mounted vacs and 8" hoses). Hence, as a last resort I tore off the duct panning, threw it out, sealed with the Zinsser B-I-N, which is supposed to block even smoke odor, and covered the ducts with new panning. Unfortunately, the Zinsser product off-gassed, despite my leaving the panning off for the time period the manufacturer recommended to accommodate this (two weeks, as I recall). This led to my use of the Safecoat product, which is non-toxic (unlike the Zinsser coating), but which ALSO has an odor that seems to be off-gassing after four months of continuous air movement (and, I followed this manufacturer's recommendation for wait time to accommodate off-gassing before reinstalling the duct panning).

Thanks to GBR for the links - I'd seen one of these previously, and the other is interesting, but doesn't quite match my situation (the joists appear to be new - a situation not as challenging as my old, damp, musty smelling joists, or the painted surfaces as they are now). I guess that I'll try stapling the Reynold's aluminum foil directly to the painted joists and underside of the sub-floor. But my concern here is that moisture may creep in between the aluminum foil and the painted surfaces, which is why I was asking about no-odor adhesives that bind aluminum foil. Given the air movement through the ducts, would this be a problem?
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Old 09-23-2011, 11:48 AM   #9
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Cold Air Return Ducts & Aluminum Foil


Okay, I'm far from an expert here, but I'm wondering why you can't just 8 X10 rectangular duct from the cold air truck of the furnace to the grille? I know you'll have to MacGyver the very end with Thermopan or sheet metal + tape + luck, but that'd leave you with a system that gets all of the wood out of the mix.

As for the cross bracing, my understanding (and again I'm not an expert) is that it's good stuff but not critical structural per se, so you can move the occasional bit or replace it elsewhere. I've seen plumbers and HVAC guys do that.
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Old 09-23-2011, 12:09 PM   #10
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Cold Air Return Ducts & Aluminum Foil


Thanks. I thought about installing 8x10 metal ducts between the joists, but figured that aluminum foil would accomplish the same thing (a metal surface that blocks ALL odor) and be easier to install as well as cheaper. It would also allow me to either wrap the cross braces by hand (with the foil) or just leave them asis (since they don't have much surface area that's off-gassing).

Do you think there would be a potential problem with moisture getting behind the foil - if it was only stapled in place? This wouldn't be a problem here in Wisconsin in winter (with dry inside air), but it might be a problem in our humid summer months.
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Old 09-23-2011, 01:00 PM   #11
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Cold Air Return Ducts & Aluminum Foil


[QUOTE=kim23;734515]Thanks. I thought about installing 8x10 metal ducts between the joists, but figured that aluminum foil would accomplish the same thing (a metal surface that blocks ALL odor) and be easier to install as well as cheaper. It would also allow me to either wrap the cross braces by hand (with the foil) or just leave them asis (since they don't have much surface area that's off-gassing).

Do you think there would be a potential problem with moisture getting behind the foil - if it was only stapled in place? This wouldn't be a problem here in Wisconsin in winter (with dry inside air), but it might be a problem in our humid summer months.[/QUOTY

When you think about all of the steps that are required and that you've done so far, to try to keep things cold air return the same shape, the foil plus everything else is actually a lot more work. Rectangular ducting snaps together and can be strapped up with metal screws remarkably quickly.

And yeah, I'd worry about completely covering wood with metal foil. the moisture issues as you've identified them would worry me. Whereas if you use sealed duct for the cold air return, you can have that section of floor closed back up again without having to do any more worrying about it.

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