A Homeowner asks about Metal Roofing Underlayment & Condensation

March 1, 2023 | Filed under: Roofing Help

Ron M. of  North Carolina writes: 

Before I get an estimate from a contractor, I want to learn more about roofing underlayment and condensation. I am considering replacing the 23-year-old screw-down metal roof of my home.

Here’s some information about the construction of my house and the current roof:

  • The roof has a 45-degree slope, a porch, and no attic.
  • There are 3 dormers.
  • The screw-down metal roof has asphalt paper underneath.
  • The next level is OSB (Oriented Strand Board).
  • Under the framing and insulation of the interior sections are drywall and scrim-coated ceilings.
  • I have soffit vents on the exterior, but I’m not sure how well they work with the batt insulation between the rafters.

I want to replace the asphalt paper underlay with a better product, as I’ve heard that a metal roof will outlast this type of base. I’m worried about condensation with different types of underlays. 

Please give me some advice before I ask for quotes.


Todd Miller’s Response:

Ron, have you ever checked to see if you have had high moisture levels or condensation? You are at risk, and I would like to help you determine if there are existing problems. 

Here’s a webinar that may help you understand the dynamics at play with your roof.


Ron…

An interesting video. Thanks for the reply. 

I have not checked condensation levels. Without removing the drywall, the interior side of the roof is inaccessible.

I have noticed a single water stain line (½” wide by 4” long) that I would bet is at a drywall joint within a few feet of the ceiling’s peak. It’s barely noticeable. I used my camera’s zoom lens and didn’t see a problem on the exterior roof at or above this point. 

We recently experienced extreme cold here in western North Carolina, accompanied by 25 mph winds that drove temperatures below zero. In this weather, condensation may have formed high up, even under the ridge cap, and dripped down through the insulation to the drywall and then to the first joint. 

Whatever I do with a new roof, I don’t want to make any condensation worse.


Todd…

I am troubled by what may be happening with your roof, yet I realize that tests and checks will be invasive. 

One way to check from the outside would be some fastener pullout resistance tests. These tests are done up near the ridge because that is usually the area most affected by condensation.  This test could be done by a handy DIY’er. An easy way to do this would be with a farm scale, and the fastener is driven into the decking through a steel strap attached to the farm scale.  Then you pull up and see what pressure it takes to dislodge the fastener.

My concern is that you don’t have an effective vapor barrier between the living space and the area beneath the roof. Once moisture gets into that area, it will condense on cold surfaces. Any metal roof will create a cooler roof deck, increasing the chance of condensation. This situation is always worse at the roof’s peak because warm air rises and carries moisture with it. 

My thought is that, ideally, you will:

  1. Remove the old screw-down metal roof
  2. Check the decking condition and replace sections as needed
  3. Install a vented cold roof.  This is basically a vented roof installed over the existing roof, including a new layer of decking.  
  4. Typically, 2 x 4’s are installed as “sleepers” over the existing deck following the rafters. Then a new layer of decking is installed.  The resulting chamber is then vented with intake vents at the eave and exhaust vents at the ridge.
  5. Follow this with another layer of decking
  6. Install the new metal roof

I understand that is a lot to do. If the current decking turns out not to be in bad shape, I suggest Installing a breathable roofing underlayment over the deck and then an entangled mesh such as Dry-Tech to create a thermal break between the new roof and the decking. 

I hope this gives you enough information to confidently call a professional metal roofing contractor for a worry-free roof to protect your beautiful home.


More questions about roofing underlayment and condensation? Readers, I’m happy to answer your detailed and specific questions on this subject or anything about roofing. Your roof is the first line of defense from water and wind damage. It can also play an important role in preventing fire in these very dry times.

todd Miller

has spent his entire career in the metal building products manufacturing industry. He is president of Isaiah Industries, an organization recognized as one of the world’s leading metal roofing manufacturers. Todd is currently Vice President of the MRA (Metal Roofing Association) and a Past Chair of MCA (Metal Construction Association). Through his website, he strives to raise the bar on standards and practices to provide property owners with the best possible products for successful roofing projects.

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