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Felt vs synthetic

user-959871 | Posted in General Questions on

I built my house using Zip roof sheathing and Zip wall sheathing.
The makers of Zip panels say that an underlayment for the roof and that a house wrap or felt for the walls is not necessary.

The Zip roof sheathing withstood the weather fairly well for the 6 months before I got the metal roofing on.
However, heeding advice from roofers, I did install a synthetic underlayment just before putting on the metal roofing.

On the walls, I was also considering putting on a layer of the same synthetic underlayment before hanging the vinyl siding.
But I have been told that I should use felt instead because the syn. can’t absorb trapped water nor pass through vapor like the felt can.

So my questions are these:

Should I go ahead and put felt over the Zip wall sheathing (maybe at least one layer of 15# felt) or should I just install the vinyl siding directly over the Zip panels ?

Am I ok on the roofing underlayment, or should I have used felt instead of synthetic ?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Should I go ahead and put felt over the Zip wall sheathing (maybe at least one layer of 15# felt) or should I just install the vinyl siding directly over the Zip panels ?"

    A. It's your choice. I think it's easier (and better) to install the felt, because that way you can integrate the window and door flashings with the felt instead of depending on the Zip tape to stay waterproof for decades.

    Q. "Am I OK on the roofing underlayment, or should I have used felt instead of synthetic ?"

    A. You are OK, as long as the underlayment was installed over a vented attic or a vented cathedral ceiling. If you installed the underlayment over an unvented insulated roof, then you violated the underlayment manufacturer's instructions. For more information on synthetic roofing underlayment, see Synthetic Roofing Underlayments.

  2. user-1012653 | | #2

    here is my question about ZIP. It appears to be a good product, and a wash when you add in the cost of felts to osb or plywood. So why are people using both ZIP and felts? Is it because they want the air tightness of ZIP but are still skeptical of the tape for keeping it more water tight? (which also plays into the air tightness....)
    I plan on using both products, but also plan on not putting anything over it as an underlayment.
    just curious....I know people around here that have kept a taped piece in a bucket of water for a month, then took a putty knife to the tape and they couldn't get it to budge from the sheathing surface.

  3. Kopper37 | | #3


    Here's a controversy for you:

    For metal roofs, it's not just about permeance, it's about adsorption.

    Metal roofs have typically been installed over red rosin paper. Follansbee (a long lived company that manufacturers a lot of the traditional terne roofing) requires installers to use both 15# felt, as well as red rosin paper for the underlayment.

    Supposedly, the idea is that metal roofs experience condensation at the interface with the decking. The paper products help wick this moisture away, lengthening the life of the metal. Also, the red rosin paper can also act as a slip sheet, when the metal panels expand and contract.

    Yet a lot of roofers are now installing standing seam and imitation snap-lock roofing over synthetic underlayment.

    Here's a link to an OLD FHB article that talks about standing seam:

    What do you think?

  4. GBA Editor
    Martin Holladay | | #4

    I am aware of the problem you cite. Whenever I install a metal roof -- usually through-fastened panels, not standing seam -- over continuous sheathing like plywood or a solid board deck, I do it this way: First I install a layer of #30 asphalt felt, and then I install 2x4 purlins, parallel to the ridge, 24 inches on center. The metal roofing is attached to the purlins. Any condensation drips to the asphalt felt and evaporates before it causes any problems.

    I can imagine that synthetic roofing underlayment under metal roofing is not ideal, but I have no direct knowledge of problems resulting from its use.

  5. 4y23MuNF7N | | #5

    Is the space created by the purlins ventilated to allow the evaporation to disperse?

    What are the pros and cons of having the 2x4s run from eaves to ridge instead?

  6. GBA Editor
    Martin Holladay | | #6

    A through-fastened roof isn't airtight. Daily changes in temperature cause a lot of air to be pumped into and out of the air spaces under the roofing. I wouldn't worry too much about ventilation.

    You have to install the purlins parallel to the ridge because the purlins have to be 90 degrees to the panel ribs for structural reasons. Your suggested way of installing purlins (parallel to the rakes) would also make it hard to fasten the panels, especially if your purlins drifted away from being perfectly parallel to the roofing panels.

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