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Community and Q&A

Insulation/Vapor Barrier For Sunroom with PT Decking

mikefrig | Posted in General Questions on

Hi, we are converting our screened porch into a 3 season room. The roof was insulated when it was built years ago in case we decided to utilize the space more. The room sits 2 1/2 – 3′ off of the ground. There is gravel under the decking that sits on top of plastic sheeting. The sunroom will get sun the majority of the day. We will be installing a propane space heater for the handful of winter days we want to heat the room (holidays when kids are home, etc). My plan was to insulate under the PT decking between the joists with rigid insulation. I was going to lay luan plywood as my subfloor over the stained PT decking, and then install peel and stick outdoor carpet tiles on the luan. Questions are; 1. Is that insulation sufficient? 2. Should I install a vapor barrier, and if so where? Does the luan need to be treated or stained? The room will remain dry as the screen system has vinyl windows we’ll close in inclement weather. Any help will be greatly appreciated!

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  1. Expert Member


    I would try and mimic as closely as possible floors on piers designed from the start to be insulated. Those are typically from bottom to top:
    - A layer of CDX plywood to protect the insulation.
    - Rigid insulation under the floor joists to minimize thermal bridging.
    - Batt insulation in the joist cavities.
    - A sub-floor (for this I would use CDX again)

    No need to an additional vapour-retarder

    If you want to leave the existing decking, make sure to cover the ends of the boards to keep the joist cavities protected.

  2. mikefrig | | #2

    Thanks for the info! It is more involved than I anticipated. I was figuring since the room won't be used all that much in cold weather, we could get away with a lighter approach. Is what you're suggesting more for heat retention/keeping out the cold or to manage moisture?

    1. Expert Member
      MALCOLM TAYLOR | | #5


      Thee are a couple of reasons I suggested what I did.

      - Once you add insulation you need to protect it, so the underside plywood is necessary. All I've added that might seem optional is batt insulation in the floor cavities, which is cheap.

      - Covered porches often change over time. You now want to heat it occasionally. The next step is often incorporating it into the conditioned area of the house. To me, rather than have to backtrack and add layers as the room evolves, it makes sense to go the extra step and have the floor ready for any future use.

  3. Expert Member
    BILL WICHERS | | #3

    Since you're planning on putting on putting luan and "peel and stick carpet" (you probably mean "carpet squares") here, you presumably don't care if the PT decking gets covered up. If you can tolerate the floor getting higher, you could put insulation right over the PT decking, then something a little thicker than luan over the insulation (1/2" plywood at least, or full 3/4" like you'd used for a "regular" subfloor), then carpet over that. This puts all of your insulation above the floor, and will let you put it down as full sheets for an easier installation. I'd tape the sames and seal the edges which will let the rigid foam act as an air barrier for you, since the PT decking is probably fairly open I would guess.

    You'll want to use rigid insulation rated for compressive strength here. One of the higher (denser) grades of EPS is probably your cheapest option.


  4. mikefrig | | #4

    Unfortunately, I do not have that much room above the decking. 1/4" subfloor on top of the decking plus the carpet tiles is all I have room for.

  5. mikefrig | | #6

    Malcolm, I appreciate the insight - It makes total sense. We had the ceiling insulated at the time of construction "just in case" we decided to convert the porch. Years later I'm glad we did.

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