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Condensation from kitchen vent duct

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  • Dec 9th, 2018 9:41 pm
[OP]
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Jul 7, 2017
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Condensation from kitchen vent duct

I installed a kitchen hood last year and had to vent through the roof - there is no option for through-wall venting and suboptimal venting thrugh the soffit was the other choice.

No leaks from the roof but when I went to inspect the ducting today, I noticed that there's moisture in the ceiling drywall under the vapour barrier. Not too much so I don't think it is due to rain but would be consistent with condensation from the very recent "cold" spell - hard frost and overnight temperature dropping below freezing. I don't want any ongoing problems with wet drywall.

When I installed the ducting, I had foil taped the duct joints hoping any water would drip down into the kitchen rather than anywhere else. I think I also foil taped the elbow swivels. Vapour barrier on the attic side and drywall on the room side were taped to the duct to prevent condensation from air leakage (was up in the attic checking ceiling light boxes and bath vents for air leaks prior to topping up insulation). The duct was wrapped in foiled bubble wrap.

What can I do to seal this? Spray insulation foam around the ducting where it comes up from the ceiling and makes the first ~60 degree turn?

Kitchen hood backdraft flaps close (just checked) but I am sure they're not air tight.
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13 replies
Deal Fanatic
Feb 9, 2006
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Brampton
thriftshopper wrote:
Dec 7th, 2018 9:01 pm
I installed a kitchen hood last year and had to vent through the roof - there is no option for through-wall venting and suboptimal venting thrugh the soffit was the other choice.

No leaks from the roof but when I went to inspect the ducting today, I noticed that there's moisture in the ceiling drywall under the vapour barrier. Not too much so I don't think it is due to rain but would be consistent with condensation from the very recent "cold" spell - hard frost and overnight temperature dropping below freezing. I don't want any ongoing problems with wet drywall.

When I installed the ducting, I had foil taped the duct joints hoping any water would drip down into the kitchen rather than anywhere else. I think I also foil taped the elbow swivels. Vapour barrier on the attic side and drywall on the room side were taped to the duct to prevent condensation from air leakage (was up in the attic checking ceiling light boxes and bath vents for air leaks prior to topping up insulation). The duct was wrapped in foiled bubble wrap.

What can I do to seal this? Spray insulation foam around the ducting where it comes up from the ceiling and makes the first ~60 degree turn?

Kitchen hood backdraft flaps close (just checked) but I am sure they're not air tight.
If you can stop air movement you should be ok.

Unfortunately as you learned the flaps on the hood are trash as they are on most of them.

What you can do is add a small EPDM gasket all the way around the backdraft section of piping in the vent Hood. So that when the flap is in the closed position, the flap rests ever so gently on the EPDM gasket.

High end bathroom exhausts like Panasonic's have this as a design feature, where cheap companies like Broan will send a piece of foam out when you complain about flapping noises from the wind. I know not the same type of fan but it's the same idea.

You also got flaps on the exterior vents right?
[OP]
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Thanks. Where do I get such thin EPDM gaskets? Hood is actually a Vent-a-Hood (being in Texas probably doesn't help with design and most people probably vent through the wall which would negate this problem). Flaps are (a soft) plastic which IIRC aren't flat and air tight either.

I guess I could get something to plug the fan intake when it's cold.
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Feb 9, 2006
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Brampton
thriftshopper wrote:
Dec 7th, 2018 9:19 pm
Thanks. Where do I get such thin EPDM gaskets? Hood is actually a Vent-a-Hood (being in Texas probably doesn't help with design and most people probably vent through the wall which would negate this problem). Flaps are (a soft) plastic which IIRC aren't flat and air tight either.

I guess I could get something to plug the fan intake when it's cold.
Vent a good actually has a good back draft prevention design. What I think is happening in your case is the stack effect acting on those backdraft flaps.

Home Depot has the gasket material tho I'm not sure it'll help in your case.

You got any pictures?
From what you describe you have air movement/warmer air escaping
Deal Addict
May 23, 2009
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Mississauga
I agree that it could be air movement related. When I installed my range hood I also had condensation issues on extremely cold days and could sometimes feel draft coming in. Back story is I upgraded from 4" to 8" duct and the plastic back draft flap on the new exterior wall cap wasn't sufficient in stopping the air movement. I ended up installing a spring loaded butterfly flap like it one linked below which slides into the duct.

It has worked great for the last 4 years but downside is I think the spring is now getting greasy and needs replacement.

https://www.amazon.ca/Continental-Fan-M ... aft+Damper
[OP]
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Jul 7, 2017
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I'll go take a photo tomorrow.

I don't think it is a backdraft problem. Don't hear any flapping in winds. The roof vent (also Vent a Hood) has a magnetic catch and shuts well (not 100% seal but it's air leakage rather than huge amounts of air from wind) and the hood back draft flaps work as intended.

I think it could either 1) be warm air rising up, condensing in the duct and dripping back down, or 2) I was boiling water last night for pasta and had only one fan on half speed which may have resulted in water condensing in the tube (was below 0C at that time last night).

I've used Continental Fan dampers on 5 other places in my house. I was hesitant to use it in a kitchen duct due to grease and the fact that I have 2 other flaps already and the hood may not work as well with that much pressure (only spring dampers are available in 8", no gravity dampers in 8") . Not to mention it is a PITA to change the damper. They're also only rated to 180F.

Might get a pair of 4.5" plugs to stick into the fan opening when not in use (still have the downdraft).
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Dec 19, 2009
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Proper duct insulation would probably be better than foiled bubble wrap. The condensation is probably forming on the outside of the duct because of it.
[OP]
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pootza wrote:
Dec 8th, 2018 1:21 am
Proper duct insulation would probably be better than foiled bubble wrap. The condensation is probably forming on the outside of the duct because of it.
Could be that too. What duct insulation should one use?
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thriftshopper wrote:
Dec 8th, 2018 1:29 am
Could be that too. What duct insulation should one use?
Probably something like this ... slide the insulation out of the duct and slide it overtop your duct.
Not sure what the class number is but would be the same as ducting used for HRV's.

Image
[OP]
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Jul 7, 2017
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Went up to the attic this evening (been above freezing for 36+ hrs now).

Image

Good news is the damp/moist spot has not grown so I think it was from using the single vent fan at low speed to vent a pot of boiling water.

I think I'll add more blow-in insulation for now and redo the duct insulation next year. Fortunately it s close to the attic hatch.
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Apr 19, 2018
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You might find the blow in insulation gets wet fairly quickly if the condensation issue isn't solved. The photo makes it look like condensation is leaking down the inside of the elbow and spreading outwards by capillary action first through the elbow swivels (might not be tight enough to prevent water leakage) through the drywall paper and under the tape on the drywall. The bubble wrap isn't doing it for insulation. You might find it easier and just as effective to use an insulated pipe wrap rather than the pull-through insulation. Just pull off the bubble wrap and wind the pipe wrap with the duct in place using aluminum tape to seal up the joints in the pipe wrap..
[OP]
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pootza wrote:
Dec 8th, 2018 7:15 pm

but the duct wrap you posted would be fine and should be able to get in 4ft widths instead of 1ft widths.
What is the advantage of 4' width over 1' width? Do I not use a spiral wrap?
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