Helpful? 0

My intake duct for the HRV seems to be collecting moisture as the insulation is wet

My intake duct for the HRV seems to be collecting moisture as the insulation is wet. There is not enough snow close to the intake so I don't think this is the issue. What else could be causing the issue?

We have been the house for five years and have never seen this problem. We use the HRV mainly in the winter.

Asked by Jeff B
Posted Sat, 02/22/2014 - 18:52
Edited Sun, 02/23/2014 - 06:11

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5 Answers

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1.
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It is possible it has been sucking snow from the falling... or more likely i think that it is condensing moisture due to the cold outside air and the warm inside air. Ideally your insulation should have a vapor barrir on the outside of it with any gaps/seams sealed with a foil or poly tape. this is to prevent moist air from contacting the inner duct.

Do you know what your indoor humidity level is? Possible that your indoor air is more humid this year? Or running HRV longer then normal so it hasn't had a chance to dry?

Answered by Nick T - 6A (MN)
Posted Sun, 02/23/2014 - 11:11

2.
Helpful? 0

The indoor humidity hovers between 35 and 40%. Usually keep the temp in the 18-20 Celsius range. We have run it more with the excess cold weather. Is it best to run it less in order to let the insulation dry out ?

Answered by Jeff B
Posted Sun, 02/23/2014 - 13:52

3.
Helpful? 0

Jeff,
I agree with Nick. The most likely phenomenon to explain your wet insulation is condensation. Your indoor relative humidity levels (35% to 40%) are high enough for interior moisture to condense on cold surfaces. The cold surface in question is the fresh air duct bringing outdoor air to your HRV.

If the insulation on your duct is saturated, that means that the vapor-barrier jacket that should surround your insulation is damaged or imperfect. Without an airtight vapor barrier jacket, warm interior air can contact the cold duct.

Insulated flex duct works, but only if the jacket is intact, the seams are well sealed, and the insulation thickness is adequate. Spray foam on the exterior of rigid galvanized ductwork is even better, but it costs more than flex duct.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/24/2014 - 07:08

4.
Helpful? 0

I'm assuming the condensation is on the outside assembly around the
inner duct, as opposed to inside the duct itself. I had almost
the exact same problem, simply due to leaving a small edge of
the fiberglass wrap exposed to interior air. Even though the
duct in question is only four feet long it was amazing how much
water wicked into the whole system over a couple of months, with
interior humidity below 30%. It's the intake, and therefore the
coldest pipe in the house. The fix was to rework the whole thing
pretty much as shown here and make sure the outer sheath was tied
down firmly over any fiberglass at both ends, making the sacrifice
of an inch or less of exposed inner duct right at the wall and
specifically managing any water that might form on it.

I didn't do something like foam across that gap because I want
this all to be serviceable some time down the road..

_H*

myduct.jpg
Answered by Hobbit _
Posted Mon, 02/24/2014 - 08:52
Edited Mon, 02/24/2014 - 08:56.

5.
Helpful? 0

Thanks - I'll try and find the gaps!
Jeff

Answered by Jeff B
Posted Mon, 02/24/2014 - 09:25

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