Poly on ceiling needed?

[gogolf0401]

So I know this topic has been discussed a few times on here already... but I haven't seen a overwhelming straight answer.

I have a 40x88 pole barn that will be heated to 40 or so (radiant floor) and then bumped up to 60+ when I'm working in there via forced air heater. I already have the walls spray foamed with 2.5" closed cell which is about R16-17. I have will have steel liner on the ceiling and walls. I am planning on cellulose or fiberglass blown in the ceiling. I am wondering if it is necessary to have poly vapor barrier on the ceiling under the steel.

I live in MN where it is common to have several days below -20 or -30 degrees Fahrenheit during winter.

I have heard it both ways:
1. No vapor barrier... you need to let the moisture escape somewhere and it won't be the walls since you have spray foam. If you poly the ceiling you will end up with condensation on the steel.
2. Yes vapor barrier... you want to keep air movement to a minimum and keep moisture from getting into the ceiling insulation.

What have you guys done, what are your results and what do you recommend?

Thanks!

 

[Dick in Wisconsin]

Its my understand that you need a vapor barrier on the warm side of insulation. Otherwise you will get moisture up in the insulation, which will degrade the R-Value of the insulation, which will result in more heat loss, and the moisture in the insulation will lead to mold and mildew problems.

Anything to contrary is opposite what I've been told for 35 years and have seen in practice for those 35 years.

 

[K'ledgeBldr]

If you're doing a steel ceiling- THAT IS your vapor barrier! I can assure you moisture isn't going to penetrate a steel panel.

And just to make sure you have no air/moisture getting to the attic/insulation, make sure all the overlapping seams are sealed with an elastomeric caulk- DONE!

 

[pmiranda]

Steel ceiling has seams and holes that won't stop moisture so you want vapor barrier there, and you have to close up the edge where it meets the closed-cell foam on the walls.
But... why not spray foam the ceiling, too? Although I've heard it's good to have house wrap between the foam and the outside steel so you can swap a damaged panel without disturbing the foam. (Although I wonder if house wrap is strong enough to hold the foam or if it's compatible?)

 

[Dick in Wisconsin]

Steel ceiling has seams and holes that won't stop moisture so you want vapor barrier there, and you have to close up the edge where it meets the closed-cell foam on the walls.

2X. I agree.

 

[gogolf0401]

I haven't seen it myself, but someone said I could have condensation on the ceiling if I install a vapor barrier. He said he knows someone who installed a vapor barrier under sheetrock on the ceiling and had to take it all down because the sheetrock was getting wet. I can only imagine this happening if there is inadequate insulation above the ceiling.

Any other opinions?

 

[Slowgsr]

You need a vapor barrier on the warm side prevents cold air from meeting warm and causing condensating

 

[pmiranda]

Yes, inadequate insulation can cause condensation on the drywall side of the vapor barrier, especially if you try to mud it before insulating in cold weather since the vapor barrier will be cold, and the drywall very damp.

 

[Jess]

You will want to do the vapour barrier and seal it to eliminate air movement from the inside of the shop into the insulation and attic. The moisture will condense in the insulation and reduce its R value and deteriorate the structure. Good ventilation above the insulation will carry off any small amounts that accumulate.