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moisture in basement ceiling from fresh air vents


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Hello,

I had my home built in 04 with an unfinished basement.

Minnesota winters get cold, and every winter it seems I get a great amount of moisture (ok...water) dripping off of the 4" insulated fresh air vent tube.

The 1st winter we noticed this and had the contractor take a look, he replaced about the 1st 7' of insulated tubing. The next winter it did it again...builder went bankrupted. anyway this past fall I decided to insulate the celing for noise and hoping it would take care of my problem with the moisture. Nope..i just removed the sheetrock to check on it and it's wet up there again. i put up 5/8" (Noise) r19 (noise & insulating value) and those darn fresh air vents are soaking wet inside it's insulation. course it leaks right out of it because they replaced 7' of it and only taped it.

The two fresh air tubes run along the west wall to the north wall where they are vented. In that area of the ceiling, like the next rafter over to the east is the heat vent for that room in the basement, so there is definately heat with in the general area of the insulated fresh air vents. I did put r19 all around my heat vent just to help keep the ducting somewhat insulated from the cool air up there.

The rest of the basement is not yet insulated in the ceiling, althought all the walls are insulated and sheetrocked. I pulled the sheetrock on the wall just under this area and the walls are dry, just the darn fresh air vent is cold and wet.

anyway..how can I stop the moisture from building up?

I'd like to finish this basement but I'm afraid of all of this moisture.

I have a choice furnace, direct vent system to the west side of the house (about a 10' run), the two fresh air vents go out the north side of the house (about a 35' run). The two fresh air vents..one is dropped next to the furnaced 7" off the ground and the second one goes directly

into the fresh air side of the furnace.

I do run a humidifier in the basement next to the fresh air intake by

the furnace, and have a humidistat upstairs and downstairs, I keep the humidity about 32 to 38 percent. Last week I shut it off for three days

(temps outside where like -20 below) and it was so dry the sinuses hurt.

a few pictures...

in/out for furnace

fresh air vent into furnace fresh air side

fresh air vent 7" off the floor

in/out with exhaust heading towards outside

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my last house we had the heat exchanger system and had no problems.

do these insulated fresh air vents (even in a fully finished basement) always get moisture in them?

but just don't leak normally?

Anyone have some direction for me on this.

Thanks for you time,

Randy

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Hi,

I'm wondering why you have a fresh air vent into that area when you have a Category IV appliance that gets all of its combustion air from outside. What's up with that? Is there also a Category I gas water heater in there that needs combustion air?

Those white PVC pipes are not "fresh air vents." One is the exhaust pipe for the furnace and the other is the fresh air intake. They meet at a combo vent before it passes through the wall to the outside. If the joints of that exhaust vent are leaking you need to get an HVAC guy out there stat to cement them because condensation dripping out of that pipe means gas is escaping as well.

ONE TEAM - ONE FIGHT!!!

Mike

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Mike, I don't think it's the PVC exhaust/combustion air vents he is talking about. It seems he has some type of fresh air exchange system with one of the insulated ducts running from the exterior to the return plenum below the furnace. The other seems to be attached to the supply plenum??? The systems we typically see have a single, larger duct to the return with a motorized damper on a timer. If his system is constantly open (no damper) to exchange air I could see the intake, including the insulation, getting cold enough to cause consensation on the exterior of that duct. Although I would have thought that complete insulation would have prevented it.

But, frankly, it's not a set-up I'm familiar with and I'm loathe to comment on the root cause of the moisture. He may be better off getting an independant HVAC tech out there for a look-see.

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I have to agree, you do have a 'choice' system and you shouldn't have moisture problems.

Let's take this one step at a time.

You have two corrugated pipes that should be insulated and wrapped in black plastic (vapor retarder). One pipe is attached hopefully to the return plenum called a 'fresh air' duct. This duct provides fresh air for the occupants. Judging by its size you must be referring to the other pipe, the 4inch one.

This duct provides combustion air for possibly your two water heaters, boiler or another gas burning appliance, it's called a 'combustion air duct'.

Here is what you need to check and then get back to us for more problem solving.

1. Determine exactly where the leak is coming from.

2. Check both ducts to ensure the black plastic is completely sealed, especially at the joints. Any warm moist air will condense on the cold pipe and cause a drip.

3. Make sure the pipe is continuously insulated and the straps are not too tight.

4. Locate the intake hoods and determine if they are exposed to wind driven snow. How far above the ground are they? Submit a photo. If the moisture is not coming from condensing warm inside air, then it must be coming from outside.

Additionally, where is your humidifier located, directly on top of the furnace or on the other plenum? This is unrelated to your problem... just curious.

Finally, measure the combustion air pipe, it might be 5 inches and not 4. Visit a big box store of your choice and get a combustion air termination fitting. The ‘boot’ contains the cold outside air and prevents it from dumping or flowing freely. It's inexpensive, around $35.00 and you can do it yourself.

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Whenever you have a surface that has a mean temperature of 5 degrees situated in a warm, moist basement the result will be condensation.

A heat recovery ventilator would solve the issue.

Dew point can occur at any temperature dependent on RH

Not sure how an HRV would solve a leaking fresh air duct.

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thanks for all of the responses! much appreciated.

I had the hvac company come out that did the original install, not much from them except that I need more insulation...hummm, that's why they replaced both of those vent tubes years ago. O'well there is clearly an air leak! But they said they would put in an HRV for about $2k.

The problem is in the Northwest corner and right at the fresh air vents where they enter the home. The plastic covering does have several holes in them so that doesn't help either. I hold my lighter next to the tubes right where they enter the home and when I kick on the furnace the lighter flame starts to dance around. There has to be a leak there.

I'm begining to think perhaps it's the fresh air vent frame (hope that's the term)on the outside, I would think that should be calked. I'll check this weekend when we temps get to the low 30's.

Question?

is there a calking that I can use in those low temps and work?

I'll need to replace the insulated tubing as well.

The tube that drops to the floor (water heater vent) is a 4" tube (R-4.2 F214)

The second vent tube is what the Hvac people said was an 11" insulated tube that they put in on their second trip years ago. I can

feel what feels like a 3' tube or pipe inside from where it meets the house

and headed towards the plenum.

Only one of the insulated vents are wraped in black plastic, the other is the 4" one that is sliver in color, but would appear to be a vapor retarder.

both vents on the outside of the house could be exposed to some wind driven snow, they are about 1' 6" off the ground.

I believe you are correct that the air is coming from the outside.

My humidifier sat next to the fresh air vent that drops off 7" off the floor.

I've shut it off now because the hvac tech says that my house is too moist. His meter says 42% today and mine said 38% (30deg outside temp).That's a bit on the high side...it's warmed up.

I haven't been able to measure the combution air pipe yet, I'll do that this weekend when I have new tubing and large ty wraps to seal it back

to the pipe. I would think that the tubing should be calked as well to the wood just inside the house.

Here are the vents outside..

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a picture of both tubes about 2' into house

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thanks for the help.

Randy

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thanks for all of the responses! much appreciated.

I had the hvac company come out that did the original install, not much from them except that I need more insulation...hummm, that's why they replaced both of those vent tubes years ago. O'well there is clearly an air leak! But they said they would put in an HRV for about $2k.

The problem is in the Northwest corner and right at the fresh air vents where they enter the home. The plastic covering does have several holes in them so that doesn't help either. I hold my lighter next to the tubes right where they enter the home and when I kick on the furnace the lighter flame starts to dance around. There has to be a leak there.

I'm begining to think perhaps it's the fresh air vent frame (hope that's the term)on the outside, I would think that should be calked. I'll check this weekend when we temps get to the low 30's.

Question?

is there a calking that I can use in those low temps and work?

I'll need to replace the insulated tubing as well.

The tube that drops to the floor (water heater vent) is a 4" tube (R-4.2 F214)

The second vent tube is what the Hvac people said was an 11" insulated tube that they put in on their second trip years ago. I can

feel what feels like a 3' tube or pipe inside from where it meets the house

and headed towards the plenum.

Only one of the insulated vents are wraped in black plastic, the other is the 4" one that is sliver in color, but would appear to be a vapor retarder.

both vents on the outside of the house could be exposed to some wind driven snow, they are about 1' 6" off the ground.

I believe you are correct that the air is coming from the outside.

My humidifier sat next to the fresh air vent that drops off 7" off the floor.

I've shut it off now because the hvac tech says that my house is too moist. His meter says 42% today and mine said 38% (30deg outside temp).That's a bit on the high side...it's warmed up.

I haven't been able to measure the combution air pipe yet, I'll do that this weekend when I have new tubing and large ty wraps to seal it back

to the pipe. I would think that the tubing should be calked as well to the wood just inside the house.

Here are the vents outside..

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a picture of both tubes about 2' into house

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thanks for the help.

Randy

Hi Randy,

Let me say the last thing you need at the moment is an HRV.

In simple terms an HRV is a complicated system added to your FRESH air duct. In light of your past experience with your HVAC contractor I'm guessing they wouldn't have the expertise to install it right in the first place. Save your money and further aggravation, besides your problem is NOT related to improving fresh air efficiency.

But, I have a concern with this photo

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There is clearly condensation on the flex vapor barrier which would suggest the moisture is coming from the inside of the house and not from inside the pipe leaking out. What has me concerned is the presence of the additional insulation, the white fiberglass insulation, I suspect you peeled back to expose the duct.

By adding insulation over your vapor barrier, you've effectively tampered with the location of the dew point.

A vapor barrier MUST be on the warm winter side of the envelope.

Here is what's happening.

Mother Nature Rule No. 14. - Warm travels to cold.

As warm moist air moves across the insulation toward the flex, its temperature drops. And as the warm air cools, it also looses its ability to hold water vapor. Somewhere in the gradient (insulation) the vapor turns to water or condenses, that point is called the dew point.

The function of the vapor barrier is to prevent warm moist air from reaching the dew point. We don't want warm moist air at the dew point.

By adding insulation you've effectively placed the vapor barrier beyond or downstream of the dew point, if that makes sense. In other words the vapor barrier is now on the cold side of the envelope.

I would suggest you expose both ducts by removing the additional insulation and make the necessary repairs to the plastic and duct insulation, the silver pipe.

The exterior wall connection is equally important and I'd bring the plastic as close to the wood framing (rim joist) and foam them in place to seal any air leakage.

With regard to your comments above:

You DO NOT need more insulation, you need LESS

You DO NOT need an HRV

You DO NOT need new ducts, the one you have are just fine.

You DO NOT need caulking at the exterior

You DO NOT need to service your exterior hoods, they're ok.

You DO NOT need to shut off your humidifier, it has nothing to do with your problem. 40% RH is normal.

Let us know how you make out.

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Uh Rob, those temps are in fahrenheit[:D]. In a building tight enough to need fresh air intakes, 40% RH at 30F outside will be enough to cause that condensation. In my experience the RH should be around 25% assuming indoor temps around 68F, if the noses aren't happy you could bump the RH to 30% but expect to see small amounts of condensation on cool surfaces.

Tom

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Uh Rob, those temps are in fahrenheit[:D]. In a building tight enough to need fresh air intakes, 40% RH at 30F outside will be enough to cause that condensation. In my experience the RH should be around 25% assuming indoor temps around 68F, if the noses aren't happy you could bump the RH to 30% but expect to see small amounts of condensation on cool surfaces.

Tom

Yes you will and that's the reason we want to remove insulation from the VR to keep the pipe warm.

Condensation will occur on windows but I've never seen it on FA/CA Flex.

Nearly every home in the Calgary and surrounding areas I inspect have this system without condensation.

Remember, if the pipe, the VR, is exposed to indoor air it will remain warm enough to prevent condensation.

As we speak, my humidistat is set at 40% and my office reads 54%, window are clear.

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i was able to get to my project on Saturday.

Ladybugs in the general area of the fresh are vents....yep, sure enough the bottom side of the large 6" vent was not caulked good enough. I don't think they could get a caulk gun under there too seal it. I used the great stuff before I put the new insulated piping back on.

Even though it's only been 12 hours since sealing it back up I don't see any frost next to the vents and no condesation anywheres so far, although it's a warm 27 deg outside.

Thanks for the ideas and comments.

Randy

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Our area gets cold and they tap into the lines to direct the condendation to a condensate pump and this problem is solved. This system works fine when properly set up with a condensate pump.

If your condensation is on the outside of the fresh air intake tube then it must be very, very cold where you are, not just very cold.[:-bigeyes

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  • 7 months later...

Hi,

To prevent possibility of moisture condensation, you need constant air movement between the bottom and top of the roof, through the space between rafters / above the insulation layer installed on top of the finished ceiling. if properly installed between the rafters, on top of the ceiling material, there should be approximately 2" of space left above it. This space allows air to flow between the bottom and top of the roof. Ideal installation would include vent chutes / baffles, secured to roof decking, which ensures proper insulation spacing and undisturbed air flow.

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