GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Draining/drying capability of insulated vinyl siding

Bryanw511 | Posted in General Questions on

I’m interested in using insulated vinyl siding on a remodel project, more for the rigidity then the thermal value which is negligible. One of the major benefits of standard vinyl siding is the “built in” rainscreen. I’m concerned that insulated vinyl siding will not drain/dry well and the plywood sheathing will remain damp longer. Literature from Certainteed, Alside, and PlyGem all state that the EPS has a vapor permeance of 5 and that it’s breathable. Is this a real concern? Are there any moisture horror stories with insulated vinyl?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Patrick Mccombe | | #1

    I have it on my house. It was installed by the house flipper that rehabbed my house about 10 years ago. I've pulled a piece or two off when installing a vented range hood and when I replaced a faulty hose bib. It was dry back there, but full of bugs--dead ones. I don't know if this is unusual. I'd suggest using standard unbacked vinyl. The EPS insert doesn't really seem to make much difference in rigidity and definitely doesn't look different than standard vinyl.

  2. Expert Member
    Dana Dorsett | | #2

    The Type-I EPS used for the backing might have a vapor permeance of more than 5 perms @ 1" but vinyl doesn't. Nobody cares if the back side of the vinyl can dry out- it can take it. It's the wood sheathing that matters.

    Without any air gap between the siding and WRB there is no capillary break or venting path for drying, so it's not going to meet the "Vented cladding over wood structural panels" exception in the IRC for zones 4 & 5 allowing Class-III vapor retarder on the interior. It doesn't take much of a vent gap to get there though- 1/4" is plenty.

    I can't point to any horror-stories regarding these products, but I haven't looked either.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |