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Porch Ceiling Mildew

ElegantSolution | Posted in General Discussion on May 10, 2009 03:37am

I made the mistake, seven years ago when I built my screen porch, NOT to finish the pine rafters and car siding I used for the ceiling of my screen porch. I can’t figure out why it is getting covered in a dark green mildew. The 12’ x 20’ space is fully screened, high up onto its gable roofline.

I am scrubbing the mildew off with a scouring pad in a heavy bleach solution, and will follow with a spray of the same stuff. However, I don’t know what to do after that? Leave it and spray each year with chlorine bleach? Varnish with spar to keep the natural wood look (will mildew return UNDER the varnish?)? Paint with a solid body stain with mildew killer added?

Or, can I just cover the whole thing with a new ceiling, such as metal siding? In that case, the rafter bays will be enclosed and not ventilated. Is there a problem with that?

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  1. User avater
    Dam_inspector | May 10, 2009 04:41pm | #1

    An application of Disodium octaborate tetrahydrate would be a good idea. Bora-care, Borada, termite pruf etc. It kills and prevents fungus and wood boring insects.

    1. ElegantSolution | May 10, 2009 09:50pm | #2

      Thanks for the tip. Do you think that if I treat the wood like this, I can simply keep it unfinished?

      1. User avater
        Dam_inspector | May 11, 2009 01:59am | #3

        I would think so.

  2. User avater
    Dinosaur | May 11, 2009 02:26am | #4

    I can’t figure out why it is getting covered in a dark green mildew. The 12’ x 20’ space is fully screened, high up onto its gable roofline.

    You haven't filled in your profile, so I don't know what region you're in and that could make a difference in the answer. But at a guess, I'd say there is not enough ventilation up in that gable to allow warm, humid air to get pulled out of there. Remember, there's no direct sunlight on that ceiling to help dry it out, so you're completely dependent on air cirulation to do that.

    There's got to be both an 'in' and an 'out' for effective ventilation to take place, and you mentioned that your rafter bays aren't ventilated. Just because the bays are open on the bottom doesn't mean hot humid air won't get trapped up there between the rafters under your ceiling boards.

    Two suggestions: 

    1. You could install a ridge vent on the roof to allow flow-through ventilation in those rafter bays. Hotter air will flow up and out the ridge vent, drawing cooler air from down low in to replace it. This process will help keep the ceiling boards and rafters from getting damp enough to support mould growth.

    2. If the ridge vent is not an option, you could install a large 'Casablanca'-type ceiling fan in the peak and set it to draw air upwards. That will 'wash' the ceiling with cooler, drier air drawn up from below and should force the hot, humid trapped between the rafters air to blow down and out of there through your screens.

    Most important is that you understand you need to address the cause of the problem, not just treat the symptoms. Scrubbing off the mould with Javex and hoping it won't come back is like taking Benedryl for a skin rash without knowing what's causing it.

    Dinosaur

    How now, Mighty Sauron, that thou art not brought
    low by this? For thine evil pales before that which
    foolish men call Justice....

    1. ElegantSolution | May 11, 2009 03:34am | #5

      Dinosaur,Good points, well taken. I live in southern Wisconsin, near the top of a hilly ridge. I would think the open screen porch would get enough ventilation, but obviously it is not enough. We do get some humid weather in the summer.I have a ceiling fan installed already, but rarely use it because it never seems to get too hot or stuffy. I kind of hate to keep it on all the time just to keep mildew at bay. The ridge venting won't work because I am built on a heavy ridge beam. And, I am worried about covering everything up with a new ceiling skin (agreed?).I am hoping that a good cleaning and a paint job will do the trick. I think you are disagreeing with that, right?

      1. User avater
        Dinosaur | May 11, 2009 04:49am | #6

         hoping that a good cleaning and a paint job will do the trick. I think you are disagreeing with that, right?

        I'm never really satisfied just treating symptoms instead of curing the disease. In your case, I would be skeptical that a one-time cleaning with fungicide followed by a paint job will permanently solve your problem. I have seen what is probably the same type of gray-green mould grow on poly vapour barrier or vinyl siding in areas where it's damp enough.

        Since you have the ceiling fan already in place, the easiest thing to do after you finish your cleaning would be to turn it on low at the beginning of the warm season, and leave it on until the snow starts to fly. If you have a recurrance of the mould problem in spite of that that'll tell you you need even more aeration. You may want to kick the speed of the fan up higher, or consider something else.

        And, I am worried about covering everything up with a new ceiling skin (agreed?).

        Very much so. Given the problem you have now, I do not think that would not be a good idea at all.

        Dinosaur

        How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

        Edited 5/10/2009 9:51 pm ET by Dinosaur

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