Plastic under metal roof?

Guest User
10/5/2016
Hi, I'm helping build a cabin in north Minnesota. Here is the roof plan so far. We have 2x6 joists 2' apart running vertically along the single pitch roof. On top of that are 2x4 purlins 2' apart running 90 degrees of the 2x6s. We want to put fiberglass insulation between the 2x6s, and have an air gap between the 2x4s. I am worried about condensation dripping off the steel roof onto the insulation. Would a plastic sheet on top of the 2x4s help in this case? After reading thru this forum, it seems plywood decking would have been a better choice. But, winter is coming and we can't afford the decking this year. I believe the interior moisture will be dealt with well by vents and windows. The roof pitch is low, about 2:12. I appreciate any advice you can offer on how to avoid condensation problems. Thank you
Todd Miller
Isaiah Industries, Inc.
10/6/2016
I would suggest a vapor barrier, such as a layer of polyethylene ... but the purpose is to keep moisture from inside the structure from reaching the cool back side of the metal roof and condensing. Many folks assume that in a case like this, condensation occurs from outside moisture. Rarely if ever would that be the case on the back side of the metal. So, adding a vapor barrier, preferably on the house side (bottom side) of the fiberglass insulation, would prevent warm moist air from inside the cabin from reaching the roof and condensing. Yes, I'd prefer to decking. An additional consideration -- can you add baffles and actually vent vertically on top of the insulation -- bring fresh air in at the bottom and out at the top? The 2:12 pitch is a little problematic in that regard but it would be nice, from the standpoint of moisture control, energy efficiency, and ice dams, to have some actual ventilation with fish air and exhaust beneath the metal roof. All things considered, I'd sure like to see decking with underlayment between the decking and the metal roof.
Guest User
10/6/2016
Wow, thanks for the quick and detailed response. How would you suggest making the baffles? Is the goal to hold the insulation down enough to let air underneath the purlins? I will skip the poly directly underneath the metal. Does paper faced batt insulation act as a vapor Barrier, or should I also use plastic underneath the insulation. Thank you
Todd Miller
Isaiah Industries, Inc.
10/6/2016
There are commercially available baffles -- usually made of foam ... available at the big boxes. Yes, you'd want to allow air to move under the horizontal battens and upward between the vertical joists. My company has a product in stock called ThermaDeck that may interest you. It would act as a baffle and it also has a radiant barrier and foam. For info, please email me at [email protected] Some backers on batt insulation are vapor barriers and some are not. If you do try to rely on the insulation backer as a vapor barrier, I would tape the seams.
Guest User
10/7/2016
Thanks so much for your advice. I feel allot more confident about the project now. One last question, should I tape/caulk the baffles, or is it better to not seal them tight?
Todd Miller
Isaiah Industries, Inc.
10/7/2016
I would not seal them tight ... so that in addition to circulating outside air through the baffles, moisture can also get into them and be exhausted out .... you will also leave a gap at the top for moisture and warm air from the attic to get out at the exhaust vents.

If you would like to reply to this thread, please log in. If you do not have an Ask the Experts forum user account, create one here.

Find a Contractor

Get Started Today

Take the first step to increasing the value of your home with a great looking, durable, fire resistant and energy efficient metal roof. Browse our list of qualified MRA Member Roofing Contractors in your area for a free consultation and estimate.