Metal Roof Condensation Prevention Question

[dieselscout80]

We have a metal building with no insulation so it has condensation issues.

Would a external roof treatment like BLACK JACK 5-Gallon Elastomeric Reflective Roof Coating be enough to prevent condensation from forming inside the building?

Shop BLACK JACK 5-Gallon Elastomeric Reflective Roof Coating (1-Year Limited Warranty) at Lowes.com

This would be much cheaper than spray foaming the inside especially since we want to build a real garage at some point.

Shop Dow FROTH-PAK Foam Insulation Kit at Lowes.com

 

[EddieWalker]

I doubt it. Condensation forms when the outside temperature lowers the inside temperature to the point of 100 percent humidty on the surface of the metal. In humid climates, this happens almost every morning from Spring to Fall. You need enough of a barrier to prevent those temperature changes from happening.

Insulation is the easiest way to do this, and spray foam is probably the best. You want closed cell foam,which is about twice as much as open cell foam. Open cell foam absorbs and holds moisture, so over time, there will be a small amount of condensation that forms between the bottom of the metal and the top of the foam. This moisture will build up inside the open cell foam and eventually lead to mold and decay of the foam. It will also cause the metal to rust.

 

[newbury]

We have a metal building with no insulation so it has condensation issues.

Would a external roof treatment like BLACK JACK 5-Gallon Elastomeric Reflective Roof Coating be enough to prevent condensation from forming inside the building?

Shop BLACK JACK 5-Gallon Elastomeric Reflective Roof Coating (1-Year Limited Warranty) at Lowes.com

This would be much cheaper than spray foaming the inside especially since we want to build a real garage at some point.

Shop Dow FROTH-PAK Foam Insulation Kit at Lowes.com

Top "coating" won't work.
What size metal building? How soon do you plan on building "a real garage" ?
When I had a new metal roof put on my 40x60 workshop they put in 1" roll insulation on the underside. With the old roof it would start raining inside due to condensation.
If the underside of your roof is easily accessible it seems like you might could tack up roll insulation tightly then reuse it on your garage.

 

[Snobdds]

To fix it right...

1. Remove and save metal roof.
2. Install OSB decking.
3. Install vapor barrier on top of the OSB.
4. Install purlins on top of vapor barrier.
5. Attach metal roofing to the purlins on top of the vapor barrier.

This creates an air space between the metal roof and vapor barrier.

 

[tommu56]

1/4" bubble bubble insulation installed below roof and above purlins on my cabin and garage prevented it

tom

 

[LD1]

Eddie hit the science spot on.

Cold metal, with warm humid air inside is what the cause is. The cold metal cools the air right near the metal below the dew point and the water drops out of the air. Same way a cold beverage sweats.

Only way to prevent is to:
1. Keep the metal warm. Which is not an easy thing.
2. Keep the air right next to the metal cool enough.

To achieve that, as others mentioned, vapor barrier works well. Another solution, which may be easier given the long term plans, is to go ahead and insulate. You can insulate the bottom chords of the trusses, and with proper ventilation, keeps the attic, and air near the metal, at the same temp as the ambient air. And if the metal and air surrounding it is at the same temp and outside ambient, the only way it will be wet is if it's raining

 

[brown40]

+1 on the foil backed radiant barrier. I used double bubble on the walls and a cell product called prodex on the roof and have not had condensation issues. On a couple of occasions the dew point was low enough that the concrete floor was wet while the ceiling and wall remained dry. Easy handling and installation and the product is unable to hold moisture. I believe the variety of products come precut to size for installation between joists/rafters--I used the straight 4' and 6' widths prior to sheathing and roofing. Makes for a very bright interior without paint.

 

[EddieWalker]

If you go with bubbles, or a vapor barrier, or any other water proof liner, remember that does not stop the condensation from forming. It just stops the moisture from dripping into the shop. The water is still there, but now it will run down the top of the barrier and out the sides of the roof. If you have gutters, you will never know this is happening. Usually you wont notice it even if you don't have gutters because the water from under the roof is minimal that goes down the side of the building. You might notice green or black mold growing at the top of the wall if it's on the shady side of the building. But usually by the time the sun is up, it will evaporate.

How is the airflow on the roof? Do you have a continuous ridge vent along the full length of the roof? Air coming in from the eves and traveling up along the bottom of the roof, and out through the vent is very effective at minimizing how much water is formed during condensation.

 

[dieselscout80]

This is a metal carport type building it does not have eves or a ridge vent.

Do you think the spray form with cause the metal to rust?

 

[dstig1]

OK, I'm going to get all engineer-y on you here.... The key measure is the dewpoint of the air. The inside surface of the metal/roof must be kept above the dewpoint to avoid condensation. If the interior surface temp of the metal drops below the dewpoint, dew forms and you get the rain you so love... So there are 2 ways to do this: keep the interior surface of the metal warm or reduce the dewpoint of the air that contacts the metal surface so the dewpoint is below the temp of the metal. The first one is pretty obvious to most people but the second one is often harder to get your head around. What it comes down to is if you can reduce the amount of moisture in the air that contacts the metal, then you can solve it that way. Ways to do that? Well air conditioning for one The other big item is to block the vapor from getting to the metal. This goes to Eddie's point of using closed cell foam as it is a vapor barrier. So it does double duty. It insulates the surface so that the surface temp is warmer, and it blocks the vapor from the inside so even though the metal will still be cold from the night air, it will not see enough moisture at the metal surface due to the CC foam to form dew. This is why you need to use CC foam as open cell is vapor permable and will create a bigger problem in the long run.

Make sense?