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how to measure if enough ventilation below a deck?

Posted by jaansu (My Page) on
Mon, Mar 19, 12 at 15:07

I think my 15 year old PT deck failed due to lack of ventilation. Many boards either expanded or were placed so close as to have no space inbetween. Don't boards usually shrink after construction?

I'm about to have an ipe deck put down and I want to be able to determine if humidity will be a problem. The deck is surrounded by ~50% by the house/garage/ family room. I suspect this is the problem - not enough flow through even if the sides were largely open.

Any way to quantitate this or should I plan already on including a fan? What sort of exterior fans are there for this sort of thing? Any other way to handle this?


Follow-Up Postings:

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RE: how to measure if enough ventilation below a deck?

The most quantitative method would be to measure the dew point temperature in the space under the deck, and measure the dew point of the ambient air at the same time. You could get 2 of those remote sensor temp/humidity meter.
(You probably already have one)


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RE: how to measure if enough ventilation below a deck?

That's a great idea - I didn't think of using such devices. But why is dewpoint important? Isn't the humidity itself the best factor to compare?

In any event, how much difference would be acceptable?


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RE: how to measure if enough ventilation below a deck?

"Isn't the humidity itself the best factor to compare?"

Humidity is measured by the dew point. The dew point is a direct measure of humidity. Dew point is the saturation point. Air masses with equal dew points have equal amounts of water vapor.

RELATIVE humidity is the percent of saturation, or dew point relative to the temperature. As the air heats and cools, the relative humidity changes. The dew point remains stable. The space under the deck may heat and cool at a different rate than the ambient air. This temperature difference is not necessarily an indication of bad ventillation. The deck surface may get hot and radiate heat into the space underneath, and even constant airflow will not cool it down to ambient temp. This is OK, as long as the dew point is equal. The air outside could be 80 F with 50% humidity, and the space under the deck may be 100 F with 27% humidity. This is OK, because the dew point is 60 F in both cases.

But let's say the outside air temp is 80 F and the Relative Humidity is 50%, and it's 100 F and 50% relative humidity under the deck. The air under the deck is actually much more saturated. The 80 degree ambient air has a dew point of 60, but the 100 degree air under the deck has a dew point of 78. The air under the deck will cool down at night, lower than 78, past the point of saturation, and dew will precipitate from the air in the space under your deck. As the night air continues to cool, down close to the 60 degree dew point of the ambient air, the space under the deck cools, too. As the air under the deck is "supercooled" past the dew point, water continues to precipitate so the dew point drops with the air temperautre. This is scientifically what is going on with poor ventillation issues.

I have opened sealed steel shipping containers on cool summer morniings to find water run out the door onto my feet. All from condensation, or "dew" as we sometimes call it.

To measure only the relative humidity, may not indicate a problem when indeed one exists. But if the only tool you have is relative humidity gauge, I would take the measurements in the night or early morning, when temps are going to be equal.

If the space under the deck is ever 100% humidity when the ambient air is less than 95% humidity, and the temperatures are equal (within one degree) You have a ventillation issue.


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