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Old 10-24-2021, 01:26 PM   #1
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Preventing condensation on transitions

I've got a 14" roof raise, and I'm struggling with condensation collecting on the transitions and running down and dripping on the floor. I've insulated with XPS foam board (as with the rest of the bus) and I'm getting water dripping around it.

I tried sealing the edges of the foam board with expanding foam, and maybe didn't get every crack, but it didn't work and the issue with that seems to be expanding foam not wanting to bond to the metal and just dripping off in great globs before it can cure.

It's looking like my best best is to take down the foam board and use spray foam instead so the metal of the transition is totally sealed off from the warm interior air, but I'm wondering if there's any alternative, some product that would seal and adhere, or something i could line the metal with before adding the foam board.

any suggestions appreciated!

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Old 10-24-2021, 01:49 PM   #2
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Ventilation is the primary prevention of condensation.

Getting a perfectly sealed envelope of the living space is difficult but something to shoot for.

Then well controlled ventilation, high CFM when needed.

And also high BTU heat sources to help get everything dried out when needed.

Some people use actual dehumidifiers but not so practical away from shore power.

Avoiding high humidity climate regions helps.
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Old 10-24-2021, 03:01 PM   #3
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Quote:
Originally Posted by Erin F View Post
I've got a 14" roof raise, and I'm struggling with condensation collecting on the transitions and running down and dripping on the floor. I've insulated with XPS foam board (as with the rest of the bus) and I'm getting water dripping around it.

I tried sealing the edges of the foam board with expanding foam, and maybe didn't get every crack, but it didn't work and the issue with that seems to be expanding foam not wanting to bond to the metal and just dripping off in great globs before it can cure.

It's looking like my best best is to take down the foam board and use spray foam instead so the metal of the transition is totally sealed off from the warm interior air, but I'm wondering if there's any alternative, some product that would seal and adhere, or something i could line the metal with before adding the foam board.

any suggestions appreciated!
Are you certain it is not water leaking in? (just checking)

Any pictures?

What are the weather conditions that are causing the condensation? Average outside temp when this is happening, average inside temp?

Do you already have heat inside the bus? What kind? You may already know that propane heaters like Mr. Buddy create huge condensation problems even with minimal temperature differences inside and out.
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Old 10-24-2021, 03:05 PM   #4
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Yes, only use heaters that vent combustion to the outside
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Old 10-25-2021, 10:51 AM   #5
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Originally Posted by Rucker View Post
Are you certain it is not water leaking in? (just checking)

Any pictures?

What are the weather conditions that are causing the condensation? Average outside temp when this is happening, average inside temp?

Do you already have heat inside the bus? What kind? You may already know that propane heaters like Mr. Buddy create huge condensation problems even with minimal temperature differences inside and out.

I don't have photos with me right now, but the transitions are 20 gauge sheet metal with five lengths of square tubing as support from the high part to the low part. I'm getting water dripping down those tubes.

I'm nearly certain there aren't any actual leaks, but there were some i had to fix, so it's possible I missed something small, though this problem is happening where there is the exposed metal on an angle, which seems a prime spot for condensation to collect and run down.

I'm in the pacific northwest (Vancouver Island, BC) so there will be almost non-stop rain for the next six months, ha. Average outdoor temp has been between 8 and 12 (celsius), and indoor is generally around 12 overnight and up to 20 during the day.

I have a small wood stove (no propane for exactly the reason you mention), which works pretty well to dry the place out, but doesn't help for the water collecting overnight.

I know the exposed metal needs to be covered and I've been trying to figure out what the best way to do that is.
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Old 10-25-2021, 11:27 AM   #6
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Expanding foam seems to stick to everything, so if it isn't sticking, what exactly are you applying it to that causes it not to stick. Is the surface clean of grease and moisture?

You could try applying butyl sheets to the metal, and then applying your foam board to that. Use the sheets to seal off all the gaps between the board, and then apply your foam to the sheets and between the boards.

But if you're getting that much drippage from condensation, I have to question where the moisture is coming from, or is the exterior just that cold? Like others have said, "ventless" units create a lot of moisture, and should be avoided in sealed up situations.
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Old 10-25-2021, 01:19 PM   #7
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I'm assuming that you've insulated between the square tubes in your transition, but the tubes are still exposed to the inside. In this case the condensation you're describing is normal and to be expected. The bus fills with warm, moist air during the day and then overnight the roof cools (and the tubes along with it, since steel is an excellent conductor of heat), which causes the moisture inside to condense on the tubes.

Your best solution is probably to seal the tubes behind an additional layer of XPS (1/2" or 3/4") and a layer of paneling or wood (whatever you're using for you're ceiling). This will prevent cold external elements (like the tubes) from coming in direct contact with the warm internal air.
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Old 10-25-2021, 01:40 PM   #8
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Originally Posted by musigenesis View Post
I'm assuming that you've insulated between the square tubes in your transition, but the tubes are still exposed to the inside. In this case the condensation you're describing is normal and to be expected. The bus fills with warm, moist air during the day and then overnight the roof cools (and the tubes along with it, since steel is an excellent conductor of heat), which causes the moisture inside to condense on the tubes.

Your best solution is probably to seal the tubes behind an additional layer of XPS (1/2" or 3/4") and a layer of paneling or wood (whatever you're using for you're ceiling). This will prevent cold external elements (like the tubes) from coming in direct contact with the warm internal air.
I agree on this assessment-it is most likely condensation.

I'm wondering if anyone in the forum has foamed the inside of the channels before-that is my first thought about reducing air-to-metal contact.

Insulation board versus foaming: the challenge is to eliminate the air gap against the metal completely.

I would probably try to apply a vapor barrier over a large section of the interior (straight up plastic secured by tape) to see if condensation is forming on the inside of the plastic barrier to find where you might have that dew point challenge. That would also help locate any leaks if they exist.

I would not just insulate and hope for the best. In other conversations and posts I think we all agreed that BC is one place where the temperature differentials were almost perfect for the rain forest effect!

Also, if you have a diesel heater and you experimented with vapor barriers you might see some reduction of condensation depending on how warm and circulated the interior air is.
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Old 10-25-2021, 02:13 PM   #9
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Eliminating the air gap inside the channel doesn't eliminate the thermal bridging that occurs.
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Old 10-25-2021, 11:16 PM   #10
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Originally Posted by Booyah45828 View Post
Eliminating the air gap inside the channel doesn't eliminate the thermal bridging that occurs.
This is true but if you were referring to my recommendation to foam the inside of the channel, I was focusing on removing the source of air (and therefore the likelihood of condensation).

Where the outer part of the channel is in close proximity to the air inside the bus, yeah, with thermal bridging there may be condensation. That's why I suggested a vapor barrier, even temporary, to see where the drips were forming.
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Old 10-26-2021, 08:57 AM   #11
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This is true but if you were referring to my recommendation to foam the inside of the channel, I was focusing on removing the source of air (and therefore the likelihood of condensation).

Where the outer part of the channel is in close proximity to the air inside the bus, yeah, with thermal bridging there may be condensation. That's why I suggested a vapor barrier, even temporary, to see where the drips were forming.
Ah, I assumed you were recommending foaming the inside to prevent thermal bridging. Foaming in there to remove air volume and humidity access would be a good idea.

But I think the frame still needs insulated over as the thermal bridging from the outer skin will still create condensate on it.

I'm also thinking they're trying to use a ventless heater, and have created a "greenhouse" effect in the bus with all the humidity.
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Old 10-30-2021, 06:47 PM   #12
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How many bodies (human & animal) living in the bus? That can determine how much moisture is going into the air. A dehumidifier might be in your future. Spray foam on bare metal should create a thermal barrier between the metal skin and the interior. If the insulation is too thin, you'd see condensation on the foam interior. Add a fan to distribute the heat evenly, a wood stove can create local heat but allow areas of cold that create condensation in your humid environment.
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