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Old 03-07-2010, 05:35 PM   #1
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molding ceiling

built new garage about 6 yrs ago and now noticing mold where rafters and top wall meet.i have infloor heat and the mold seems worse at where the overhead doors are.its only about a 6 inch spot but at every spot where the rafter is secured to the top of the was a very cold winter at times but i do live in saskatchewan.would it be in my best interest to have a dehumidifier but in my garage or do i have another problem.


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Old 03-07-2010, 07:42 PM   #2
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It sounds likean insulation issue. These spots might lack insulation above the ceiling and the warm heated air is condensing in these areas. Check the areas above those locations for missing insulation.


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Old 03-07-2010, 07:46 PM   #3
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Conditions for mold

I'm not a mold expert, but I live in the Pacific Northwest and we have a lot of it here. Mold spores are everywhere. It takes the right consistency of relative humidity (about 70%), temperature and oxygen to start a colony. Check the link for more info:

I'd be looking at my roof first. You say this is new work 6 yr. Is it ventilated? 1 sf of vent per 150 sf of floor space split evenly along the soffit and ridge. Sorry for lacking the metric conversion. if you have a steeply pitched roof 8:12 or greater it would require even more It sounds to me like there is moisture condensing on the trusses and running to the edge, dripping onto the sheetrock and creating the condition for mold.

Ice dam may be another problem. If you are in a place where you get lots of snow buildup on the roof your roof system is vulnerable to ice dams. Code in areas down here that get this condition require a special stick on tar like underlayment that extends about .5 meter up the roof beyond the edge of the heated wall.

I say this because garages usually don't have attic insulation so that means that the heat would go up to the roof and melt snow up there. the water would run down the roof till it froze again at the eve. This creates the ice dam condition.

Another source of moisture in the attic is an improperly ventilated bathroom or dryer that may just terminate in the attic and not get mechanically ventilated to the exterior.

Once you stop the condensation conditions I think you will change the environment for mold growth. I use oil based primer to cover these dead colonies and then cover it with your room color.

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Old 03-08-2010, 06:37 AM   #4
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Molds that thrive on building materials need three things to exist:
An organic food source (drywall paper is perfect), temps from 40-110 degrees F. and moisture greater than 60% RH or.6 water, but less than 100%. At first glance, I would believe that moisture is condensing in the area where the joists meet the walls. It is likely just enough to reach the 60% needed. A leak would probably not be so localized. Add some insulation to keep the area warm and prevent the condensation and/or run a dehimidifier. If the drywall is still solid, clean the mold off with a 10% bleach solution. Dry it well and then prime with a sealer like Zinnser or Kilz. if the drywall is already spongy or soft, then it needs to come out.
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Old 03-08-2010, 07:56 AM   #5
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Don't forget about plain and simple ventilation; an area will turn mouldy if the RH is over 60% at a given temperature, so - sure - one thing you could do is dehumidify the whole garage + contents + car, or better still put up an oscillating fan aimed at the mouldy spot.

Ventilation will make sure the RH doesn't get to 60% - ergo no mould.

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Old 03-08-2010, 01:52 PM   #6
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ridge vent

I had a friend that built a 24 x 30 shop. When I went to finish the drywall off,both soffit ends of the interior walls were dripping wet. He was running a salamandor for heat. The problem began when He blew in 9" of cell in the attic which plugged the screen on the ridge vent. Cleared the vent,all better


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