Hi All, I am in need of some guidance! If you are interested in Building Envelope issues, mold, and air circulation, please read on. 🙂
We built a home with a covered deck 10 years ago and 2 years ago added a carport. The homeowner noticed mold on his carport painted plywood ceiling about a year after we finished. 🙁 I went and looked at it and analized all the attic and found mold on both sides of the plywood at the carport and mold on the back side of the plywood at the covered deck. At the carport, the mold is sheet specific as some sheets have it and some don’t.
To help you better understand the house and my progression of understanding:
- The house and covered deck and carport all have an open attic with hip style roofing, spaced soffit vents, and mushroom roof vents. We have calculated it and we have more than adequate venting.
- The ceilings of these outdoor covered areas are not insulated
- We are in the mid-Willamette valley in Oregon, Temperatures range from about 20-100F, but are probably most often in the 50-60F range, with a lot of rain/drizzle, and moist air this house is not in the shade or near any trees, rather on an open hillside surrounded by fields.
- I have had the plywood compnay out and they wnat to place blame on the conditions as they claim, mold is everywhere, it just takes the right conditions to allow it to grow
- I have had a home inspector out and he claims it has to do with the dewpoint being on the plywood and the difference in temperature between the attic and ouside air. At different times of day, and as temperature moves, the dewpoint moves from one side of the plywood to the other, hence the mold on both sides
Finally, I have my ideas on how to resolve it, but I am looking for more advice or experience to back me up. Any thoughts, or recommended consultants to direct me to would be greatly appreciated!
If you have more detailed questions, please ask! any help is greatly appreciated.
Kent
Replies
What the inspector said was true -- in your area the dewpoint is always real close to the temperature. Lots of things can happen.
For instance, since you lack trees the roof of the carport will radiate heat into the sky at night and cool down rapidly (especially if it's a dark color). This will drop the temperature in the attic, causing dew to form on the carport ceiling (both sides) just as it forms on the grass overnight. Venting may actually be counter-productive since it allows the attic to stay cooler during the day.
A modicum of insulation over these areas might help -- just 4" unfaced fiberglass, maybe. I'd also paint both sides of the plywood with mildew-resistant paint.
If it's only a normal suface moisture issue and the wood is dry, simply have your painter use a mold inhibiting additive (available in any decent paint shop) to the paint. Done! Not much of an issue really.
Mold
We worked on an extensive remodel last year, the new roof sheathng was installed in July and no ceilings or insulation installed through November. This is about the time the mold started to grow, but like you said, only on some of the sheets. I checked the sheets with the mold and they appeared to be of the same outer veneer, maybe poplar or cottonwood?? The sheets with fir/hemlock did not grow mold. Ventilation could not have been the problem as the attic space was wide open.
Final action was to spray the moldy sheets with a bleach solution, the mold stopped growing. Any thoughts??
gbaune, the owner seemed to think it was due to the specific sheets of plywood as well. I think that is a contributing factor, but I also think that it is more likely an atmosphere thing, as it happened all over the second story deck cover soffit, this soffit was 10 years old, and different plywood.
I guess my concern is more for the long term, how can I kill the exisitng mold as well as stop any future mold growth?
Thanks Dan for your reply! My question about insulation is, will mildew form within the insulation now that you are just moving the dew point to within it? What are your thoughts about air movement, and installing a fan that would draw air through the attic when a certain humidity is reached?
The purpose of the insulation would be to help the plywood retain heat and not get so cold due to the black body radiation from the roof. You don't have a moisture problem, you have a cold problem. There might be temporary condensation in the fiberglass, but it would be of no consequence, since fiberglass does not absorb moisture.
Surface mold on an unheated suface isn't a problem with how it's built - it's a problem with a surface that supports mold. The SURFACE supports the mold currently. Use a better product and it will go away.
Mold condensation dew point wood solution
Mould spores are all over the world, but you don't see mold
everywhere, as mold needs food and water to grow.
Mold feeds on wood and paper, it gets its water out of the
air or the wood or paper it grows on.
It also grows on dirty walls etc.
You get condensation on surfaces, when the object is below
dew point.
Condensation forms inside untreated wood! The molecules of
water vapor are so small, they can enter the surface of the
wood (and some other things)
You see condensation on windows, during cold weather, when
the warm wet air inside your home condenses on the window
surface that is below dew point.
You see condensation outside on your truck, greenhouse etc
as, condensation, dew, frost, ice - depending on the time
of year.
The air we breath out is 100% full of water vapor.
Condensation does not form on surfaces/or in objects that
are warm - that is to say, above dew point.You can pick up
sheets of paper that are damp and limp with condensation.
Condensation can form on many things, in most cases it
merely dries out, evaporates and disappears into the air.
Like when you get a can of beer out of the fridge, the warm
wet air rushes to it, in no time it is covered in condensation.
Then the can warms, the condensation dries and disappears.
That's how water vapor, condensation works.
To stop mold growing on any surface, wood -whatever, you
need to deprive it of either food or water.
You cannot deprive mold of water, as the water vapor is in
the air. You must deprive it of food!
You can do this by painting the surfaces with gloss paint
or varnish - then like your beer can, the condensation will
form and then dry/evaporate and disappear into the sky.
Kent
This may or may not be a possible answer to your problem, but it might be something to read through.
This brand/type of plywood could have a propensity for mold growth.............
http://forums.finehomebuilding.com/breaktime/general-discussion/anyone-use-sandeply