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Community and Q&A

Dual problem – Frost in attic / condensation in range hood pipe

Jay Mack | Posted in General Questions on

I will try to keep this as simple and brief as possible.
Central KY
Daytime temps in 20’s – F
Nighttime tems in single digits – F
5:12 Roof asphalt shiingles – osb roof deck
Vented ridgecap (confirmed open and properly installed)
Full vented soffit with baffles to allow insulation
18″ fiberglass blown in attic
all ceiling penetrations are airsealed with foam/caulk
2yr old house 2000square ft
RH = stays around 48%
Use bathroom vents on timer (soffit vented) religously
*Use RANGEHOOD VENT religiously when cooking (broan 634 roof vent)

This morning wife noticed water dripping from the bottom of the range vent. Further examination revealed that water was dripping from bottom of blower housing (about 1cup total at this point) onto the range top. In a panic I went to the attic and noticed that there did not appear to be any water leaking on the OUTSIDE of the metal pipe… assume it is condensing inside the pipe and dripping down. We have not used the fan today, but have used it a lot over the last few days while cooking. I also noticed a lot of frost (thin layer) in an approx 2′ diameter area surrounding the vent pipe in the attic – on the osb. I also noticed frozen ice crystals in the attic covering about 1/2 of the house. The ice crystals were not a “solid frost” layer but a small crystal every 2″ or so.

What have I done wrong here???

The metal vent pipe is placed as follows:
-exits fan through cabinet
-enters attic (sealed around drywall penetration with closed cell foam)
-goes through approx 18″ fiberglass insulation
-exits insulation exposed through airspace in attic above insulation for another 2′
-“dumps” into roofvent (basically just a metal pipe with the roof vent sitting on top of it
**when I had my wife turn the fan on with me in the attic, I could feel some air blowing back down into to the attic. I am assuming the air is being forced up through the pipe, then hitting the damper on the vent and some of it is being deflected back down around where the pipe is just “stuck” into the hole in the roof vent.

How do I “fix” this?

Insulate pipe?
eliminate roof vent?
vent to soffit?
is it possible that I have attic ventilation issues as well, or is the frost coming from the warm moist air that I have been pumping into my attic?

Summary: TWO issues here —
1. water dripping from fan motor
2. light frost in attic

Please let me know what other info is needed. I am almost in panic mode here. I spent 2 years designing the house that I plan to live in the rest of my life…

Thanks so much.

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  1. Jay Mack | | #1

    This is exactly how the vent is attached (round pipe)

  2. Jay Mack | | #2

    here is a crude photo of the attic area.
    Update: The frost area has now disappeared in the attic and the ice crystals are minimum. The temp briefly reached freezing pt today.

  3. GBA Editor
    Martin Holladay | | #3

    Your indoor RH is high. It might be worth investigating why.

    Indoor air is exfiltrating through your range hood exhaust duct when the fan is not in use. The duct is cold. Water condenses on the cold pipe and drips.

    Step one: insulate the duct in the attic.

    Step two: investigate why your indoor RH is high and see if it can be reduced.

    Step three: see if the range hood's backdraft damper is operating properly.

  4. Dan C | | #4

    Hi Jay,
    The best way to insulate that duct would be to disconnect it and install an insulation sleeve over it. Make sure you tape up any joints as well.
    That would be the best option. If your struggling to disconnect the duct, there is insulation you can get for plumbing that comes in a tape form. I would recommend using the sleeve before that though.
    That should stop the water, but you should further investigate martin's points as well

  5. Jay Mack | | #5

    Martin - Thank you so much for your reply.

    The house was completed and moved into August 2015. The RH has always hovered around 50%. I have a hygrometer in basement and main level. I have a dehumidfier in the basement that dumps into the drain (daylight drain/walkout basement). The machine is set on 50% and cycles regularly. During the last few days, the RH has been in mid 40's. I had just assumed that RH was going to stay at this level due to new construction for a while. I guess I was wrong about this being normal.

    Step 1---- What material should I use? Also, should I be concerned that some air blows around the OUTSIDE of the pipe in the attic when the fan is on? As stated in my OP, it seems that some of the air is being deflected back down before it turns 90 degrees to exit the roof jack. The Broan roof vent instructions were followed and there is no mention of any way to secure the pipe to the roof jack. The roof decking has a 9inch square hole cut into it and the 8" pipe simply sticks up through it into the roof jack (which can be used with rectangular or round pipe.

    Step 2----- suggestions on how to do this? We have timers on all of the bathroom vents and run them for atleast 30 min after showering. We also run the range hood vent whenever cooking (maybe every other day)

    Step 3----- I went onto the roof today and had my wife turn the fan on, the damper on the Broan roof vent is working properly and I can hear the primary damper on the Broan vent motor "flop" close when the fan shuts off (system has 2 dampers)

  6. Jay Mack | | #6

    Daniel - Thank you for the link.

    So, a foil exterior, fiberglass insulation is what I need for this application?

    I don't think it will be very easy to disconnect any of the ductwork since it is sealed tightly in the drywall. Not much room to move it up or down if the seal were broke. Would it be feasible to use the sleeve and split it vertically then tape back together? Maybe use two split sleeves and have the splices opposite one another (this method would essentially have "double" insulation).

  7. GBA Editor
    Martin Holladay | | #7

    The product you need is called duct insulation. Some brands use the phrase "duct wrap." Most products consist of fiberglass insulation with a vinyl jacket. You wrap the duct with the insulation and tape the seams of the vinyl jacket. You can buy it at any home center. Here are some examples.

    If you are worried about exhaust air re-entering your attic through cracks near the roof penetration, the obvious solution is to seal the cracks near the roof penetration. If there is an unsealed crack between the duct and the roof sheathing, seal it with caulk, canned spray foam, or high-quality European tape before you wrap the duct with insulation.

    Your hunch that your high indoor humidity levels are due to construction moisture may be correct. Time will tell. As you wait for the RH to drop, operating exhaust fans during cold weather, and operating a dehumidifier, both help. For more information, see Preventing Water Entry Into a Home.

    1. PDXowner | | #10

      Not sure if you're still on here, but I keep seeing recs for rigid fiber board around range hood vent. These come in straight boards, so how would I put them around the round vent. Your fiberglass solution would be easier but I want be using the correct product. Do you have a links or references?

      1. Expert Member
        Zephyr7 | | #11

        The usual product to use is one of the fiberglass duct wraps, or the so called "socks" that are slid over the duct in one piece. I've used this product with range hood vents, and a smaller version on bathroom vents (which also need to be insulated), and I've never had a problem. I've never heard of anyone trying to box around a vent pipe with rigid foam before in an application like this.


  8. Jay Mack | | #8

    Thanks again Martin,

    Do you see any reason why Siga Wigluv would not work in this application? I have a piece of a roll left over from a case I purchased during construction. I have resisted the urge to use this for miscellaneous projects on the farm and have been looking for a legitimate reason to use it.

  9. GBA Editor
    Martin Holladay | | #9

    Siga Wigluv is a high-quality tape. It will work fine for this application.

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