GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Plywood attic floor over insulation: a moisture problem?

shannonunderhill | Posted in Green Building Techniques on

Hello all,

I’ve learned volumes upon volumes about building science reading these forums and related articles. That said, there are always questions. I am working on designing a 1500 square foot, single story net zero house in northern California, about 1.5 miles from the coast. My intention is to build based on the “pretty good house” model. I have what will hopefully be a simple question…

CLIMATE. According to the US climate zone map, I live in climate zone 3C, but it’s actually more like 4C, a pacific northwest climate. California being California, they have their own more specific climate zones and designate us as Climate Zone 1 (out of 16 in the state). See here: We are about 4 hours north of San Francisco and more or less share the same but slightly cooler climate. In general, there are two seasons: wet and dry. For eight months, it’s sunny and dry with lows around 45 and highs around 65-70. For four months, it’s cloudy and rainy with lows around 35-40 and highs around 50-55. It may get close to freezing one or twice a year for a few hours overnight. Although never very cold, we have 4400 heating degree days per year–basically, we are heating a little bit each day, even in the summer.

QUESTION. To save money and reduce the amount of conditioned space, we would like to insulate the attic floor and leave the attic space unconditioned. However, we would like to sheath the attic floor so we can use the space for storage. The roof will be a 10/12 gable with a metal roof. We are attempting to build with minimal or no foam use. For example, we are using Roxul on the interior and exterior of the wall assembly. My first floor ceiling / attic floor assembly would be drywall, 1x strapping, Intello plus, 14″ I-joists 24″ OC with dense pack cellulose, and 3/4″ TnG plywood floor glued and scrailed. All ceiling penetrations will be limited to Airfoil type ceiling boxes and/or carefully taped or foam penetrations. No recessed lights. All mechanicals are on the first floor in a utility room. We are planning to use mini-splits, so no ductwork in the ceiling to worry about. To help isolate the building envelope, the rafters and gable walls would be sitting on top of this plywood floor. In addition, the exterior 3″ of Roxul comfortboard will extend all the way to the rafter tails to help insulate the ceiling rim joist.

In my mind, I am putting the dew point inside the floor assembly with this approach and risking collecting moisture on the underside of my plywood attic floor. I feel like I have to treat this assembly like an unvented flat roof or cathedral ceiling, and do a flash-and-blow or flash-and-batt method instead.

In any case, I’ve researched myself into a corner without the expertise to get out. Is this assembly acceptable? If not, do you have suggestions?

Thanks in advance for any replies.

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
    Martin Holladay | | #1

    Your plywood will be fine, especially if you are paying attention to airtightness when you build your ceiling. (That's clearly your plan.) If any moisture makes its way to the plywood, the damp plywood will dry out on hot, sunny days. (Most attics are dry.)

    Plywood is a "smart" vapor retarder with variable permeance. It's not a vapor barrier.

    As I'm sure you know, building codes require this type of attic to be vented. The attic vents provide a secondary method to help everything stay dry. (The primary method is solar heating, which occurs naturally on sunny days.)

    -- Martin Holladay

  2. shannonunderhill | | #2


    Thanks for the response. That makes sense. I suppose the problem with condensation in roofs is that on top of that "smart" plywood are vapor impermeable layers like EPDM, stick on, or metal roofing. I don't have that problem with my attic floor.

    Last thought. I am thinking I can forget the Intello Plus and just focus on making my drywall the best air barrier I can. This would actually be more in line with local practice of using no vapor retarder under the drywall (though I think folks here are only considering poly sheeting). Reading over Lstiburek's ADA strategies and misc articles from you and others, the method seems cheaper, more easily repairable, and requires fewer specialized materials.

    Thanks again for any final thoughts.

  3. GBA Editor
    Martin Holladay | | #3

    In your climate zone, I think you'll be fine without the Intello.

    -- Martin Holladay

    1. BillTheCarpenter42 | | #4

      Hi Martin,
      I have done the same exact concept. I built my home in climate zone 5A. My 2400 sq ft ranch has 8’ walls and the ceiling are 2x10joists that form the second attic floor of my house. They run all the way out to the rim joist. My exterior zip plywood goes to the top of the rim where it meets my attic deck which is also zip. The two are taped there and my entire attic floor is taped and all penetrations are sealed. I took extreme measures when making sure everything was air tight not just in the ceiling but also in the walls. My ceiling assembly from the outside in is as follows.
      7/16 taped zip
      2x10 ceiling joist
      Cavity filled with r-15, r-23 rockwool to give me r-38 (my town was still under 2015 code when I built in 2022)
      Certainteed membrain
      1/2 sheet rock
      I was not worried when building thinking that if the zip did have any condensation the zip could dry towards that unvented attic with its perm of 2-3. And if things got really soaked the membrain would dry towards the inside. Do u think I made a mistake of using zip and that this could be a big mold maker on the underside of the plywood in the winter? I checked one 16” sheet before taping in October on a very cold day and there was extremely little moisture on the underside of the plywood. I actually had to use a flashlight to see it. Am I better off removing the plywood or leaving and finishing off the attic. I could not finish the attic then due to my budget.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |