Stopping Condensation in Shop(Help!)

[EdenRose]

I have my boat shop set up in an unheated barn in the Pacific NorthWet and am having terrible condensation problems overnight. I have a metal roof and uninsulated slab. The roof is uninsulated. In the mornings the place is soaking wet, countertops, saws and the floor.
(there are no leaks).
What would help this? Insulation, ventilation?
My saw tables are getting rusty and it is not good for epoxy and paint work.

[Alan D. Hyde]

Think of what happens in hot weather: a cold glass sweats on the outside where it's exposed to warm air.

The same thing is happening in your shop, but in reverse: the cold metal is sweating when exposed to the warm interior air.

The practical ways to deal with this problem are to make the "outside" air warmer (indirectly, by installing insulation and a vapor barrier), to make the inside air drier (a de-humidifier), and to store tools under cover to minimize condensation on them.

From what you've said, I assume that the building may be heated (by some means, maybe even just the sun) during the day, and then is cooling off at night.

If it's economically feasible, you may also wish to keep the building at (say) 50 degrees or better overnight.

Alan

[ 10-09-2002, 02:28 PM: Message edited by: Alan D. Hyde ]

[Wayne Jeffers]

I agree with what Alan said.

If your building layout and budget will allow, the best solution would be to install an insulated ceiling with vapor barrier well below your metal roof and provide for generous ventilation in the space between the ceiling and roof. A suitably sized attic fan would probably be a good thing.

Note that it's not very practical to install insulation directly against the metal roofing because it will still sweat and get your insulation wet. You need a healthy gap with air movement between your insulation and your roof to prevent this.

In heating season, adding only a little heat will go a long way to dry the interior air, once you take care of the condensation.

Wayne

[Ian McColgin]

Since it's the upper left edge and the land of temperate living, I'd not spend the $$ for insulation.

How about suspending a frame a few feet below the roof with clear plastic. Easy enough to lay out such that vapor will vent up and fluid roll down. Folk do that all the time around here with all shrink wrap boat temples. Get some ventilation between the plastic and the roof and all will be well.

G'luck

[thechemist]

At night, on clear, cloudless nights, planetary surface-objects lose heat by radiation to the night sky, which is at the temperature of deep space, over four hundred degrees below zero Fahrenheit.

Eventually the temperature drops below the Dew Point of the air, and Condensation Happens.

Polyethylene, it turns out, is opaque to the long-wavelength infrared radiation emitted by objects at planetary-surface temperatures, and so acts as an infrared-radiation-mirror, much as do overhead clouds.

A few layers of poly sheet, spaced, or bubble-pack, or even better a "space blanket" of layers of aluminized plastic film, all these things covering the object of your warm attention will reduce its heat loss overnight.

[Leon Steyns]

Another way to capture moisture in confined spaces: get some old nylon stockings, fill up to one-third with salt, suspend from the ceiling in a corner and put a bucket underneath.

The salt will absorb the moisture from the air, the surplus water will drop down into the bucket. May sound ridiculous, but actually works very well.

Greets, Leon Steyns.

[ken mcclure]

Doesn't sound ridiculous at all. There's a company who is, or at least was, making a dehumidifier that carried a charge of calcium chloride through which a fan would pass air.

The moisture from the air would collect in the CaCl and drop into a catch bin. You could then empty the bin periodically or, for a more permanent installation, run a hose from the bin to a drain or to the outside.

[PaulC]

Does your shop use catalytic or unvented propane or natural gas heaters? These will generate a lot of moisture.

[WFK]

A metal roof will condense even if its open on all four sides. How big a roof are we talking here? The ONLY way to properly fix this problem is to remove the roof, lay down whats called a condensation blanket, designed specificaly for this purpous, directly over your purlins, or rafters, which ever it may be, then fasten your roof right over the top of this. Sorry, but it really is the only answer.
Bill

[EdenRose]

Thank you for all the help so far.
To answer a few questions....
There are 2x4 horizontal rafters below the roof
that I could either put plastic, or plastic backed
insulation on. Do I need some way to drain water
that condenses on top of the layer?

I would then need ventilation
between the new ceiling and metal roof.
Would gable end passive ventilation, passive roof vents or wind/convection driventurbine vents do the job?
I would like to avoid noise and electic draw of a fan.
Heat is from a wood stove, and used only when I am working in very cold conditions.

[wolfietuk]

Dropping a ceiling is a good idea. and gable vents work well for ventilation without penetrating the roof. In this area (South Carolina) I would recomend a cheap gable fan from Lowes (40$) to help with heat in the summer. Check with a good local builder to hear what he says for your area. I wouldnt go as far as ripping off your roof. With an insulated ceiling you will get some moisture on the top of the insulation. This should evaporate during the day. keep an eye on it for a while to make sure. I would use ply or sheetrock for the ceiling. The poly will dry and crack after a while. You do not need to tape the seems but it will better seal your workspace and make heating and lighting easier. Rock is pretty cheap.

Rick

[EdenRose]

Rick,

Heat is not a problem in the summer, but a fan would be the way to make sure I was getting the ventilation.
The only problem with sheetrock is the ceiling is high, like at least 15' and working with sheetrock that high is not fun. I have seen warehouses that use the white plastic covered insulation as the ceiling. It should make for a bright shop. Would the seams be taped or nailed with battens?

[Ian McColgin]

Don't work too hard at this. If it's just a normal sized roof over a normal boat you probably have less than 20' - 30' width.

Have slightly limber slats a bit over half the span. Tack clear plastic to a suitable number of slats for each side - 6' to 10' intevals should be fine. If the boat is in the way, lean this on each side of the boat. Then work your way down, working from the deck, joining the two sets of slats. A couple of short semi-verticl members will do it, since you want a verticle gap in the final job.

The final goal is to spring up in place a bunch of inverted U's with the plastic overlapping at the apex, a bit of a verticle gap there to let the air go up.

Leave the plastic a little way away from the walls. You may want to roll the plastic at the bottom edges such that it makes a bit of a gutter to direct fluid to some conveneint point.

The moisture will still condense on the tin roof, dip down to the plastic, and now roll harmlessly away.

As I hinted before, this also works inside shrinkwrap style structures.

Cheap and easy.