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Excessive attic moisture

We renovated my 1923 Bungalow home in 2004 by adding a new second floor. During the renovation we installed 2 zone air conditioning. On the second floor we have been experiencing condensation like dripping from the diffuser in our master bedroom closet. All diffusers/vents were closed for the winter. The dripping in the closet seems to correct itself by mid-December.

This winter I found excessive moisture within the Air Handler in the attic. The moisture is found everywhere in the air handler – wiring and fan chambers. This winter season has been so bad that the water alarm for the drip pan under the air handler went off. I have also noticed drops of moisture on the roofing nails in the attic.

I’ve done everything I can think of to fix the problem, including opening as many covers, plugs and drainage outlets in the air handler to get more ventilation in it. But i’m worried because I now see mold growing on my 4 year old air handler’s fiberglass. I've also painstakenly covered each diffuser with window shrink wrap, to see if this is where moisture was originating.

Our new roof has deep soffits. These soffits are 14 inches deep of free ventilation – my contractor used continuous perforated vinyl soffit material.

Using the 1/150 formula I calculated how much ventilation I actually needed for an attic with 1050 square feet; I came up with 7 square feet (my home is 25 x 42). My ridge vent extends only 14 feet and is ventilated using GAF COBRA Ridge Vent. If I’ve calculated correctly, my roof has an unbalanced ventilation by a factor of 9 to 1, and is over ventilated.

I'm wondering if balancing the Soffit ventilation area to match that of the Ridge (14’ x 14.1 net free = 197.4 square feet), is the answer.

I would appreciate some advice as to if I am understanding this problem correctly and what may be my alternatives. Additionally, I am concerned my air handler will need extensive cleaning to correct the mold problem. My 5 year old has asthma and this problem has given me great concern.

Re: Excessive attic moisture

I'm wondering if balancing the Soffit ventilation area to match that of the Ridge (14’ x 14.1 net free = 197.4 square feet), is the answer.

Huh? You have closer to 2.5 sq ft of ridge venting, me thinks. If 14' of ridge was opened to 10' wide......it would only amount to 140 sq. ft.

If you're instead suggesting that you add yet more soffit venting......if anything you need more ridge venting to balance what you already have in the soffits.

Added a complete second floor to the house? It would be kinda normal to see *some* excess moisture up there for a year or so as the materials dried down.

Any bathrooms added up there? Kitchen? If so, where do the exhaust fans for these rooms dump their load?

PS- Cobra-vent is cheap, but surely not my first choice.

Re: Excessive attic moisture


Thank you for your reply. You were closer on the ridge vent calculation; 14.1 square inches/lineal foot (not feet as I calculated).

No kitchens on the 2nd floor, or vents emptying into the attic. We have one functioning (& one roughed in) bathroom on the 2nd floor. The fan in to the functioning bath exhausts to the soffit (using a soffit vent).

Another issue I did not discuss: Although this season has been wetter than most, we occasionally get water in our basement – other seasons have been dry and we still find moisture in the air handler. As I said – this winter has been excessive.

I am curious as to the common recommendation that the Ridge and Soffit ventilation should be balanced. Will having too much ventilation at the soffit increase the possibility of moisture build up?

You also have me wondering if I should look into replacing the current ridge vent with a better alternative – do you have suggestions? Would additional ventilation up on the ridge line help?

Again, thank you for your insight.

Re: Excessive attic moisture

The fan in to the functioning bath exhausts to the soffit (using a soffit vent).

That could be a/the problem right there. Soffit ventng is not recommended for houses that have soffit intake vents. Reason being that the warm moist air being ejected is likely to be sucked up into the soffit vents....dumping it right back where you don't want it - in the attic space. An exhaust vent in the gable end or even thru the roof would be a better alternative.

I prefer Omni-Roll or Shingle Vent II to roll-vents for the ridge on most applications.



A 50-50 balance of soffit and ridge vent square footage is the goal to shoot for. The more evenly these vents are distributed around the perimeter of the house, the better. However, having extra venting on either side of the in & out equation isn't apt to hurt anything.........it's just that there won't be any real benefit either. The lesser of the two will control the final flow rate.


Are there any unsealed penetrations from the second floor into the attic space or even from the first floor or basement? (Such as around pipes, ducts, flues, chimney chases, etc. )

The moisture is definitely coming from somewhere and I guess my first suspicion would be that bath exhaust fan in the soffit. This could/would contribute to attic moisture during both wet weather periods and dry periods.

Re: Excessive attic moisture

Hello Goldhiller,

I found a large area of wet flooring near a bedroom flex duct connection. I cut that out and found it dry underneath. This duct connection was found to be loose and when placing my hand in the boot - I found it warm. I've resealed this duct connection and improved upon the insulation around this boot.

Before closing up the air handler I sucked up as much of the condensation and sitting water from handler, trunk and ducts. Next I removed the plastic from the diffusers, opened them and the air return, then I turned on the air handler fan for 6 hours to circulate warm air from my home into the air handler and ducts. I will continue to do this for a couple of days in an attempt to dry out the system.

I'm going to do as you recommend and tighten-up all penetrations from the second floor with expanding and bat insulation. I'm going to keep all covers in the air handler sealed to see if I was creating a venturi effect and drawing warm air into the system. I'm also going to run the fan at least every-other day once things are dried out.

Long term - looks like I'll need to call a better HVAC tech to investigate and recommend a solution.

I thank you for your advice and if you have other suggestions - I'm all ears.

Re: Excessive attic moisture

Bill you missed one of the major points of Goldie's post. The bathroom vent fan blows hot moist air out a soffit vent, that warm air rises and goes right back into the soffit vent into the attic. It's almost as bad as discharging directly into the attic. Even on very hot days the natural flow up the soffit vents will carry the moisture into the attic.

Re: Excessive attic moisture


Good catch; I failed to mention that I planned on moving the bathroom vents (there are two - one for future use) to the roof after the winter ends.

I hope these solutions will keep my attic and A/C system dry.


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