Metal Ceiling Insulation

[Bruce Elasik]

The CEILING in my free standing garage/shop is white metal roofing screwed up to the rafters . I just acquired enough FREE insulation to insulate the attic. The insulation I have is faced with white plastic. My question is........ Should the plastic go UP---or Down ?--- The attic has two eave vents and there are also soffet vents. The humidity here is below 25% in the summer and quite cold with moderate snow (humid) in the winter. My ceiling is exactly like the one in the attached picture.(not my shop----found the pic on line from which i got the idea to use metal roofing for my ceiling) .
http://myweb.cableone.net/bge1/ceiling/ceiling.jpg

[Larry Edgerton]

I don't like plastic on a ceiling because of condensation problems. If it was mine and I had free insulation, I would put the plastic up and slice it with a knife so there was no chance of condensation buildup.

Years ago the electric companys told their customers with electric heat to insulate their floors and staple plastic on the bottom of the joist. I replaced way too many floors that people fell through because they are so rotted. When codes called for vapor barriers, I refused to do it, and now it is not code any more here, because of mold issues. I hate plastic in a house. I'd slice it.

[Dave Beauchesne]

Bruce:

Insulation vapor barrier should go on the '' warm ' side - i.e. facing the inside of the shop - that being said, if the integrity of the vapor
barrier is not sound, you will get condensation on the metal roof on cold days when you are in the shop breathing ( we hope ! ),
or green wood off gassing water vapor etc. I say insulate the roof however you wish, but ensure you have a proper vapor barrier
installed that complies with local code(s) - do not rely on the plastic on the insulation to be your vapor barrier.

BTW, I am not a builder per se, but a HVAC guy who has installed many walk in freezers / coolers. In their case, the vapor barrier
goes on the outside ( remember, warm side ) of the structure, and really nasty things happen if the integrity of the vapor barrier is not
kept to a high standard.

Dave Beauchesne

[Bruce Elasik]

Thanks for the replies . --Larry---I do recall one year when the power company supplied plastic faced fiberglass jackets for everyone's hot water heater. They rusted them out in a short amount of time and even caused some pipe freezing problems for structures that relied on the hot water heater to keep exposed pipes from freezing in an enclosed space. I just may pull all the plastic off the insulation or make regular trips up in the attic to see what is happening in the colder humid months.
Now im sure the plastic should not go down and touch the topside of the steel ceiling---


Dave, i think i may have not been clear that im trying to insulate between the rafters above a metal ceiling-- I'm concerned about the condensation above the ceiling in the attic space---
thanks for the replies---

[Jim Laumann]

Bruce

I have a ceiling as you describe - pole building steel. I have a vapor barrier - it was attached to the rafters first, followed by the steel. I have 14-16" of blown fiberglass on top the plastic. No sweating or condensation issues.

I would do a vapor barrier - but putting one in after the steel is installed won't be easy. Depending on the number of sf and lengths of the steel sheets, it might be easier to take the steel down, put in a vapor barrier, then re-install your steel.

Jim

[Bruce Elasik]

Jim
Im afraid its a little too late for installing a VB onthe bottom of the rafters before the steel is installed--removing the steel is out of the question at this point--
how would this be different than installing the plastic faced insulation with the plastic side down on the steel ?

[Jim Laumann]

Bruce

The differance will / would be the fact that the vapor barrier would be "seamless" - to the extent of the width of your poly film. In my shop, they used 15' wide poly, so I ended up w/ 3 seams (they overlapped the rolls/sheets poly about 2'), while w/ the poly film on the batts, you have a seam on both sides of every batt.

The only other option I can see would to try to lay the poly down over the steel - working from the attic side, but w/ the webbs of the trusses in the way, it will be a challenge. If you could start on the side of a truss - go down to the steel, across the steel to the next truss, then up the side of the truss.

See my crude ascii art below....the | represent the trusses, the * is the poly, the - is the steel.


| * * |
| ******************************* |
---------------|-----------------------------------------|-------------------

Good luck

Jim


2nd edit....the forum software is stripping my leading blanks, as a result, the | & * are shifting left. Sorry....

[Bruce Elasik]

Thats an option for sure---I just may attempt to remove screws -one rafter at a time and slip the poly under each rafter---replace the screws in that line and move on to the next one--tedious -but its not as bad as removing the steel altogether. I should be able to do it in 10 ft wide sections overlapping the seams.
I appreciate the input.