Steel decking sweating

[Steve C]

The sweating, or moisture is condensation. It's caused from the moisture in the air, and the temperature of the metal. All air hold water. It's measured as humidty. When the humidity level is 100%, you have moisture. Below that level, it's still part of the air. Air can only hold so much water. The warmer the air, the more water it can hold.

So at a given temp, the humidity level will be at one point. When the temp increases, the abilit for the air to hold water increase too, and the humidity level decreases. When the temps decrease, the ability for the air to hold water also decreases. When this happens, any areas of that air that decrease to the point of 100% humidity will result in moisture appearing.

Metal is very prone to this because it gets colder faster then most other materials. It's only the air right next to the metal that is affected, but because the metal is cooler then that area of air, the humidity level increases to more then the air can hold. The result is condensation.

A foam barrier works great to stop this, but so does other materaial. Bubble wrap and wood work too. Creating an air pocket behind the foam or wood will actually allow that space to be warmer then the outside air, and you'll have less condensation.

Eddie

Moisture in the air is measured as relative humidity and is calculated as grains of moisture per pound of dry air. This lets you calculate the total weight and therefore volume of water vapor that is available for extraction/ condensation out of any given volume of air. If the area is sealed, once this moisture is extracted condensation can no longer happen and things will stop sweating so a dehumidifier will work well in a closed area that has a good vapor barier envelope.

The problem comes from the fact that most building materials are permeable to water vapor, so even if you dry out the area and it is sealed, it becomes an area of low vapor pressure and the areas of higher vapor pressure will cause a migration of water vapor to the area of low vapor pressure exactly the same way that heat migrates to an area of less heat.

Water vapor will flow right through concrete floors and block walls.
Many people don't bother to put down a vapor barrier.

If you have a dehumidifier that can condese water faster than it can flow through the structure into the area the problem will go away. The other option is to prevent the flow through the walls and floor with a vapor barrier.
Most paint is not a vapor barrier but can buy some that is.

You can use a psycometric chart to determine the properties of the air if you know the relative humidity and the temperature. You can calculate the relative humidity with the use of a dry bulb thermometer and a wet bulb thermometer and a psycometric chart. It will tell you the dew point of the air.

The dew point temperature goes up as the relative humidity goes up. Once it reaches 100% it will start to condense, it is unable to hold any more water vapor, you have fog.

 

[timswi]

Thanks for all of the information guys.

I do understand the thermodynamics of moisture and humidity. I have no external moisture coming in. The real problem here is the temperature differential between the inside and outside of the room causing the deck to sweat. It can rain for a week in the summer, and the deck and the room stay perfectly dry. This develops when it's below 30 outside or there are radical temperature fluctuations.

Has anyone corrected this with rigid foam? I would like to use spray on, but I don't know if it's going to be practical.

If you did use foam board, what thickness/kind? Also, which ones are fire rated?

 

[Steve C]

Thanks for all of the information guys.

I do understand the thermodynamics of moisture and humidity. I have no external moisture coming in. The real problem here is the temperature differential between the inside and outside of the room causing the deck to sweat. It can rain for a week in the summer, and the deck and the room stay perfectly dry. This develops when it's below 30 outside or there are radical temperature fluctuations.

Has anyone corrected this with rigid foam? I would like to use spray on, but I don't know if it's going to be practical.

If you did use foam board, what thickness/kind? Also, which ones are fire rated?

go with the rigid foam. it is cheap and may well do the job. if it doesn't, go from there! If you can get it tight enough to stop the moist air from getting to the cold deck fast enough it will work.

 

[timswi]

go with the rigid foam. it is cheap and may well do the job. if it doesn't, go from there! If you can get it tight enough to stop the moist air from getting to the cold deck fast enough it will work.

That's what I'm rooting for..Just not sure what kind to use.

 

[MikeD74T]

Since it seems to stay above freezing in the basement, have you tried a dehumidifier? MikeD74T

 

[timswi]

Since it seems to stay above freezing in the basement, have you tried a dehumidifier? MikeD74T

I haven't, but I might. In addition to some insulation, that may solve the whole issue.

 

[Steve C]

I haven't, but I might. In addition to some insulation, that may solve the whole issue.

Your porch is 48 foot long by how wide? If it is 8 foot or less you could use almost any kind of insulation, probably 1/2" to 3/4" would do it and would be easy to handle. If you use the kind with the silver reflective surface it will be nice and bright in there also.
If you need wider sheets I know that they are available in Styrofoam, I don't know of other materials in sheets over 8 foot wide, but there probably is some.

 

[timswi]

Your porch is 48 foot long by how wide? If it is 8 foot or less you could use almost any kind of insulation, probably 1/2" to 3/4" would do it and would be easy to handle. If you use the kind with the silver reflective surface it will be nice and bright in there also.
If you need wider sheets I know that they are available in Styrofoam, I don't know of other materials in sheets over 8 foot wide, but there probably is some.

It's about 7 feet wide...Kinda like a bowling alley

 

[Steve C]

It's about 7 feet wide...Kinda like a bowling alley

I would just go and buy the cheapest foam I could find (but that is my unemployed mentality, perhaps you can afford to do better) , glue it in place with as tight a fit as I could (cut it just a little oversize for a press fit) If you cut the end on a slight bevel, it will compress the point of the bevel for a nice seal. If you cut it "square" it will be to dificult to compress the actual edge to get a good seal and fit at the same time. If you bevel both ends it is even easier to get a good fit. I just recently added foam sheet insulation to my old house that was framed with true 2 x 4 1/2 rough cut lumber that was used for the wall studs. I added 3/4" of foam between all the wall studs before putting in the fiberglass using this exact method. Every stud was a different spacing!! Cutting the sheets slightly oversize on a bevel made for a tight press fit. Cut them a little large to start with and trim them down till you find the proper amount of "oversize" that works for you. Get 16 sheets of cheap plastic insulation and some adhesive for the caulking gun. You will be done in about an hour. Use sharp stanley knives to cut the insulation and get a pack of blades. As soon as they start to tear rather than cut, change the blade.

Fast fun and easy......

Avoid the dehumidifier if you can, it imposes a parasitic drain on your electric bill.

 

[dave1949]

If you have a table saw with a decent sized table, you can rip thick foam board nicely. Probably overkill for 1" or less. Of course, it is a messy affair with 'foam dust' floating around. It's the best way to get a clean cut on thick foam that I know of.

If you cut with a razor knife, use a metal straight edge guide to make that much easier.

It does leave a glaze on the side of the saw blade, there are remover solvents for that.

Dave.