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Difficult conundrum in attic — ventilation, humidity, mold

DaninMass | Posted in Energy Efficiency and Durability on

I’m really hoping someone can help me sort through a difficult conundrum. Let me explain:

Six months ago I moved into a 1920 Spanish colonial with a clay tile roof. There was light mildew in the attic and the home inspector said there was insufficient ventilation — just tubular vents on the gables. But, given the clay tile roof, he couldn’t think of a straightforward way to add ventilation. It’s almost impossible to add a ridge vent. And anyway, the house has no overhang, so no space for soffit venting. I bought the place anyway, thinking the light mildew wasn’t so terrible. But now that winter has set in, I’m noticing major condensation on the sheathing of the roof deck. There’s also spotty mold growth, and in certain sections, it looks more than spotty. I’m trying to determine a course of action, and I’m struggling to think it through.

A few more factors to consider: the attic has cellulose under the floorboards and fiberglass batts around the raised ceiling of the second-floor bathroom. I don’t think there has been any air sealing done.

What is a good plan of attack here? Anything I think of seems to lead to another half dozen necessary steps. For now, it seems like I ought to:
1. Remediate mold
2. Treat the attic as conditioned space by having it spray foamed (since there’s no obvious way to ventilate it with the clay tile roof). 
3. These steps make me worry about resulting humidity in the rest of the house, for which, I assume, I would need to have an HRV installed. 
4. From some folks, I hear the HRV is insufficient and they also require a whole house dehumidifier in addition.

Needless to say, every single one of these steps is a potential budget busting move. There’s probably no way for me to manage two of them, let alone all of them. I’d be grateful to anyone who can weigh in here. For what it’s worth, the house had loose fill blown into the walls in 2009. 

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  1. GBA Editor
    Martin Holladay | | #1

    From the name you've chosen -- "DanInMass" -- I'm guessing that you live in Massachusetts. But perhaps you can confirm your location. That information will help us give you good advice.

    For what it's worth, an experienced roofer can certainly install a ridge vent on a house with clay tile roofing. That said, most clay tile roofs leak a lot of air, so a ridge vent usually isn't necessary. And in any case, your house doesn't need a ridge vent. The problem is elsewhere.

    From your symptoms, it's fairly clear that you have (a) a leaky ceiling, and probably (b) elevated relative humidity in your indoor spaces, perhaps due to (c) a damp basement or crawl space.

    The correct solution is to perform air-sealing work at the ceiling level. This may involve removal of the floor boards in your attic and temporary adjustments to the existing insulation. It's a lot of work, but nothing that will faze an experienced home-performance contractor or weatherization crew.

    Here are links to two relevant articles:

    "Air Sealing an Attic"

    "Blower-Door-Directed Air Sealing"

  2. DaninMass | | #2

    I appreciate this helpful response. Yes, I live in Massachusetts. In a week, I have someone who does infrared scanning and blower door tests coming to help identify air leaks and make recommendations for sealing the attic. I'm a bit nervous, though, that once we seal the attic, I'll just be facing a humidity problem on the second floor.
    The house does have a damp basement, but again, it is a challenging problem. The back of the house abuts a slope — hence the damp basement — and options for re-grading are quite limited. This summer, I dug and installed a drain in the backyard. It helped a lot with water seepage into the basement, but there's probably still a bunch of moisture under the house.
    I'm very grateful for your weighing in.

  3. Jon_R | | #3

    +1 on what Martin said. And cover/coat the basement walls/floor such that moisture flow to the interior is limited. Use hygrometers to verify (attic and interior).

  4. Expert Member
    Peter Engle | | #4

    +1 on all the above. It sounds like you've found your two big things: Air sealing the attic floor and dealing with the moisture getting into the basement.

    Martin's given two good resources for the attic floor, and your blower door guy should also be able to help out.

    Dealing with basement water problems can be tough, but it is very important. There are good articles here and at the Journal of Light Construction website. if your exterior drains have really eliminated the bulk water leakage, you can apply some sort of vapor retarder paint to the interior to help reduce moisture further. Getting a couple of cheap hygrometers will help you to know where you stand and how much more work you've got to do.

  5. GBA Editor
    Martin Holladay | | #5

    For information on improving the situation in your basement, see this article: "Fixing a Wet Basement."

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