inline backdraft damper for kitchen exhaust?

Thermador recommends an additional inline backdraft damper for cold weather installations. From page 4 of their installation manual:
COLD WEATHER installations should have an additional backdraft damper installed to minimize backward cold air flow and a nonmetallic thermal break to minimize conduction of outside temperatures as part of the ductwork. The damper should be on the cold air side of the thermal break. The break should be as close as possible to where the ducting enters the heated portion of the house.
Now from reading this it appears that this is in addition to the damper that will be on the wall cap. I would think that Brooklyn qualifies as a cold weather installation.
Do people here think having two dampers makes sense? It will restrict the air flow some, though the fan currently spec'd is 1,000 CFM, which should be ample.
The wall cap I plan to use is the 10" here: http://www.luxurymetals.com/coppervents.html#anchor_423
A butterfly backdraft damper candidate is here: http://www.luxurymetals.com/butterfly_backdraft_damper.html
Don. www.donwiss.com (e-mail link at home page bottom).
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Don Wiss wrote:

Hi, I'm in Calgary AB, Usually flap in the exterior cap is all common, nothing additional. 1000CFM? If the blower is good one, it could be quite noisy. More added things, more chance of getting trouble, LOL!
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Calgary is a lot colder than here!
It will be a Thermador. I expect I would rarely run it at full speed.
Don. www.donwiss.com (e-mail link at home page bottom).
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On Wednesday, August 13, 2014 12:07:42 PM UTC-4, Don Wiss wrote:

I'd say a key factor is also how windy it is at the location. If it's an area of extreme temps and high winds, then I can see using a double flapper approach. Otherwise, I would be more concerned about the increased flow resistance than I would about the loss from occasional gusts that might push the flap open.
Personally, I've never seen an installation with more than one.
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On 8/13/2014 11:42 AM, Don Wiss wrote:

We have flaps on exhaust vents. Just had experience of birds stuffing our drier vent with nesting materials where one of vent flaps is slightly warped and birds got in. Could have caused a fire with all the stuff we had to clean out. I capped it with a bird nest cage. So even if warm, back draft dampers can be a good idea.
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On Wed, 13 Aug 2014 11:42:02 -0400, I wrote:

I was starting to research Thermador's suggestion of a nonmetallic thermal break. Like what to call it so I can buy such an item. And I found someone has asked the same question I'm asking:
http://www.diychatroom.com/f47/range-hood-two-dampers-cold-climate-90906/
Don. www.donwiss.com (e-mail link at home page bottom). Don. www.donwiss.com (e-mail link at home page bottom).
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Plastic.
Greg
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Well, I can't simply search on plastic. A search term is what I was asking for. Where do I find a 10" round plastic tube?
Don. www.donwiss.com (e-mail link at home page bottom).
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But, if done properly, would you ever actually see the second damper?
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On Thu, 14 Aug 2014 20:46:20 -0700 (PDT), snipped-for-privacy@sbcglobal.net wrote:

No. All ductwork will be inside the cabinets.
I did find a nonmetallic thermal break to put in the ductwork. I can buy 2" of this 10" PVC pipe for a few dollars: http://flexpvc.com/cart/agora.cgi?product=PVC-PIPE-DUCT-Thinwall
Don. www.donwiss.com (e-mail link at home page bottom).
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