Join the MRA | Member Login
Find a Professional
  • Why Metal Roofing?
    • About MRA
    • Benefits of Metal Roofing
    • Compare Roofing Materials
    • Homeowner Testimonials
    • Inspiring Metal Roofing Projects
  • Homeowner Resources
    • Metal Roofing Buyers Guide
    • Resilient Housing Guide
    • Metal Roof Sustainability Guide
    • Above All Else You Need a New Roof eBook
    • MRA News
    • Member Provided Resources
    • 3rd Party Articles
    • Frequently Asked Questions
    • Preparing for Hurricane Season
  • Gallery
  • Visualizer
  • Ask The Experts
    • Energy Efficiency
    • Existing Metal Roof Questions
    • Installation
    • Is A Metal Roof Right For My House
    • Metal Roofing Benefits
    • Product Selection
    • Other
    • Archive
  • Find a Professional
Join the MRA Member Login
Sign Up Log In Add New Topic

TOPIC: Installation

Plastic under metal roof?

Return to Installation

Return to Forum

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
Guest User

10/5/2016
Share
Or copy this URL: https://www.metalroofing.com/spirit/comment/21556/find/
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. Todd Miller
Todd Miller
Isaiah Industries, Inc.

10/6/2016
Share
Or copy this URL: https://www.metalroofing.com/spirit/comment/21557/find/
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
Guest User

10/6/2016
Share
Or copy this URL: https://www.metalroofing.com/spirit/comment/21558/find/
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. Todd Miller
Todd Miller
Isaiah Industries, Inc.

10/6/2016
Share
Or copy this URL: https://www.metalroofing.com/spirit/comment/21559/find/
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?
Guest User

10/7/2016
Share
Or copy this URL: https://www.metalroofing.com/spirit/comment/21560/find/
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. Todd Miller
Todd Miller
Isaiah Industries, Inc.

10/7/2016
Share
Or copy this URL: https://www.metalroofing.com/spirit/comment/21561/find/

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.

  • About the MRA
    • Member Companies
    • Leadership
    • MRA News
  • For Professionals
    • Join the MRA
    • Industry Resources
    • Upcoming Events
  • Contact
  • Subscribe
©2023 Metal Roofing Alliance. All Rights Reserved. Privacy Policy Terms and Conditions