Insulation for Aluminum ducting in Attic?

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Posted by: from Coquitlam
1/5/2012 at 11:58:51 PM

I am installing two bathroom fans with aluminum ducting througgh the attic and to the roof. I was told that to avoid condensation and eventual mold that the ducitng in the attic needs to be insulated. The problem I have is where can I find this insulation to buy in Vancouver? ANy thoughts, even on other options/alternatives. Thanks.

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Dave from 360renos in Navan
Date/Time1/7/2012 at 9:51:39 PM

Good Evening Mark,

Yes, that is correct that your ducting that runs through a cold zone (attic) need to be insulated.

Check most home improvement stores in your area. You should be able to find a wrap that goes on your duct like a sock. It will be like a tube of insulation with a black/grey plastic covering on it. Make sure you buy the correct diameter.



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Mark in Coquitlam
Date/Time1/8/2012 at 12:32:09 PM

Thank you Dave for your response.

You describe something that I have been searching for but with no success. I have tried Home Depot, Rona and recently even Andrew Sheret -an HVAC Specialist. The closest product that I have found is an insulated bubble wrap product produced by Reflectix that provides R4.2.

Seems strange that in the rainy Pacific Northwest where mildew is prevalent that they would not sell correct ducting insulation,

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Mark in Coquitlam
Date/Time1/8/2012 at 3:54:45 PM

Update: This morning, I went again to Home Depot. Another guy that seemed fairly experienced told me for a bathroom fan to use the pre-insulated ducting in a box. I opened the box and the ducting seems to be very lightweight and overly flexible. The HD guy told me this would be much easier than solid aluminum ducting. I was sold on this today until I asked him what new house construction uses and he replied with aluminum ducting and then they place an insulated sock over top of the ducting. The sock idea is what I am trying to locate.

I ended up leaving HD again empty handed and back researching. He mentioned that there must be a duct wholesaler that sells the sock type insulation. Any ideas?

Another interesting point this HD guy told me was that you need to cover the bathroom fan unit with vapour barrier and use tuck tape to secure it to the vapour barrier on the attic floor? Any thoughts on this?

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Dave from 360renos in Navan
Date/Time1/11/2012 at 7:53:23 AM

Good Morning Mark,

Yes you need vapour barrier over your fan. You do not want to have any air leakage around the fan opening. Warm air in your attic will cause mold growth and condensation problems.

Clear away the insulation from around your fan. Install and staple a 6ml vapour barrier to the joists and wood fan support around the fan. The sheath tape (red tuck tape) all of the perimeter of the vapour barrier 100%. The pile the insulation back over the top of the fan.

I use the flexible pre-insulated ducting when doing fan retro fits and new installations here in Ottawa. Not sure if you your Building Code is different in B.C.

Are you going through the roof or side wall of your home?



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Mark in Coquitlam
Date/Time1/12/2012 at 12:09:45 AM

Thanks Dave,

I am going through to the roof. Does the flexible pre-insulated ducting provide adequate R value? Also, I am curious on your thoughts with comparing the pros/cons on aluminum ducting versus the flexible pre-insulated ducting.

I believe I have the right picture of a vapour barrier over the fan unit. With the unit resting on the attic floor vapour barrier and dry wall which is essentially the ceiling of my main floor are you saying to place a sheet of vapour barrier over top of the unit and seal it in and around the 2X4 support structure and to the attic floor vapour barrier?

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Dave from 360renos in Navan
Date/Time1/12/2012 at 2:50:02 PM

Hi Mark,

Yes, you are correct with your description of how to complete the vapour barrier over the fan. But when you say the fan is resting on the drywall I presume you will attach it to the joists?

The pros to flexible plastic duct is much easier to work with if bends and turns are necessary. No cons that I'm aware of with the aluminum other than it can get bent out of shape if it is squeezed to much.. I've used aluminum but prefer plastic for ceiling fans for ease of installation. Tape your duct ends well at the fan and where you exit the roof, then ensure the insulation sock is snug to the fan port and roof vent and tape again.

Good Luck!


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Mark in Coquitlam
Date/Time1/17/2012 at 2:12:39 PM


Thanks for htis reassuarance. I have given up trying to locate an insulated sock and have now purchased a pre-insulated vinyl ducting. I returned the large collection of aluminum ducting that I have purchased. The pre-insulated ducting process seems so much easier than rigid ducting although I was willing to install but just couldn't delay my project anylonger no finding an insulated sock.


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Frank in Oakville
Date/Time4/8/2012 at 11:04:31 PM

Rona, check in the section where they have duck pipe, e.t.c

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Ken in Delta
Date/Time11/23/2012 at 2:30:23 PM

We have installed bathroom ceiling fans in our main and ensuite batrooms (Nutone & Panasonic). The fan exhaust duct assembly has a 4 inch opening. We found that a plumbing supply outlet in North Surrey (Emco) sells insulated 4 inch ducting in 25 foot lengths which we used successfully.

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