Proper attic ventilation
We have recently developed an issue in our vented attic where we have condensation and mold beneath the insulation, on top of the ceiling drywall. It’s soaking wet. There’s 3000 sq ft of attic space and it’s kind of chopped up from additions over the years. We have powered attic fans in each section to pull air from the soffit vents. I would like to know what I can do to stop this from happening. Do I possibly need more ventilation?
I feel lost. I’ve had two mold remediation companies out and one had no ideas about the attic and the other thought I needed to add more fans and pump air from the living space in there. From what I’ve read that seems like a terrible idea. One of the guys had a moisture meter and it read 80% relative humidity in the attic and it’s only 47% outside. Please help! I need some ideas. A real solution.
Thank You,
Ryan
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Replies
Hi Ryan, where are you located?
Central Alabama
If you have a leaky ceiling, attic fans are just as likely to pull interior air through your ceiling as exterior air through your eave vents.
The first thing is to make sure your ceiling is thoroughly sealed. Start with any obvious penetrations such as can lights and go from there. The next thing would be to make sure your eave vents aren't blocked. The attic insulation itself can block them if you don't have vent baffles. After that make sure you've got good venting at or near the ridge. Make sure your bathroom fans and stove hood are actively vented to the outside, and make sure to use them. Do you have HVAC ducts in your attic? Make sure they aren't leaking.
If everything is in order you shouldn't need the attic fan, which could actually be contributing to the problem. So, do all the above, then turn off the attic fan.
You shouldn't need power vent fans in a properly vented attic. You state you have soffit vents, do you also have a ridge vent? If you don't, you should probably add one. The normal vent path is in the soffit vents and then out the ridge vent at the top. This will draw off moist air, since moist air normally rises.
The fans will be depressurizing the attic and likely drawing MORE indoor air up from any penetrations (such as holes for wires in the top plates of the walls, recessed can lights, etc.). Normally you do *NOT* want indoor air getting up into the attic.
Where are these wet spots located relative to the rest of the structure? Their location can provide a lot of insight as to what the problem might be. As an example, if the wet spots are near a bathroom, I'd suspect moisture from the bathroom is leaking up into the attic. Steamy showers add a *lot* of moisture to the air, and if that gets up into the attic, you can have problems. One thing to check there also is to ensure your bathroom exhaust fans are vented properly to the outdoors, away from any soffit vents, and aren't just blowing into the attic.
If things are *really* wet, I'd also check for roof leaks. It's entirely possible for rain to be getting in somewhere, then running down to a wet spot on the attic floor. Usual suspects hear are flashing and valleys. You can usually see dark and/or moldy spots along the roof framing to find leaks if the leaks are bad enough.
Bill
The house is basically dived into two different sides. Each side is 1500 sq ft, but the roof on each side is different. One side could have a ridge vent but it would only be about 25-30 feet long. Im not sure if that would be enough or not. If it is enough I could get rid of the fan and get a ridge vent.
The other side doesn’t even have a ridge. It just comes to a point. Would it better to just have a some type of passive vents or stick with the fans?
As far the condensation goes, it all appears to have started around areas where cold air was being pulled up from the living space.
I checked and all the bathroom and kitchen exhaust pipes are properly vented through the attic and the soffits are not being blocked.
I also checked for any roof leaks but did t see any signs of a leaking. The moisture seems to be trapped between the insulation and drywall in the ceiling.
I can’t help but wonder if making the attic a conditioned space would be the right move. Just seems so expensive.
EDIT: There is ductwork in the attic. It’s covered with 2-3” of closed cell spray foam. Also, the showers aren’t running. We’ve been working slowly on this house for last 5 years and still haven’t moved in.
If you've found cold air being pulled up from the living space where the moisture is, then you've found your problem. fix the air leaks and your problem will likely be solved. Cold air against the drywall can chill it enough to condense moisture out of the very hot and humid attic air. This often happens around duct connections to the registers where leaking air and/or contact between the cold registers and attic air causes condensation.
Your RH readings only measure a part of the problem. You need to measure both temperature and RH to really know the conditions of the air. It would also be helpful to know this information at several locations and levels in the attic and outside. Supply duct air temperature and RH would also be helpful. With this information you can begin to understand where and why you are hitting the dewpoint and causing condensation.