I have a small room off of my kitchen that appears to have previously been a back porch with cement blocks as its foundation. I already re-insulated the walls and ceiling, but there's still obvious cold coming up from the floor. I want to get this area insulated to at least reduce this occurrence, but sadly there's no crawlspace. We plan to pull up the oxboard flooring to replace it anyway, so I will have a window of time I can get underneath and add insulation.

I've read various articles about insulating raised 3 season rooms, or rooms that have crawlspace access, and there are different answers as far as vapor barrier, venting, etc. So my questions are: 1. considering this area will be sealed up again, do I need a vapor barrier, if so just on the floor, up the walls or even on the underside of the floor joists? 2. with no apparent ventilation will moisture become a problem under there or in the floor? 3. What is the best way to insulate/vapor barrier that won't require access to the space again? (because I can only imagine taped plastic would eventually drop off of concrete walls etc.)

Any help or advice would be most appreciated

  • Is there "no crawlspace" or "no crawlspace access"? You said both. The "oxboard" you refer to - is it ThermoPly? laid flat? under the flooring? What are the specs?
    – herb guy
    Mar 3, 2018 at 4:11
  • I'm unsure, but judging by the way the cold flows in, I'm assuming there's space under there but no access. I think its just standard oxboard (cheap) laid flat, the only flooring was peel and stick tile that has been removed.
    – adam
    Mar 7, 2018 at 23:36
  • As soon as you pull up that oxboard take some photos and add them to your original question and you're sure to get good advice from fast 'n cheap to NASA specs. If there is space down there but no access from the exterior you can cut a hatch or trapdoor from the inside.
    – herb guy
    Mar 8, 2018 at 3:32
  • I sealed off a room using plastic sheeting hung over the floor beams providing enough of a belly to hang the batts in place, it was a plant room I think similar to what you are describing, not enough clearance to get underneath. The plastic sealed the air space and the batts created the insulation needed this was over 20 years ago with no problems since.
    – Ed Beal
    Nov 26, 2018 at 19:57

1 Answer 1


I'm going to assume you're in a mainly-heating climate, and the room in question has a masonry subfloor and masonry walls.

First, you'll want to make sure there isn't a moisture wicking issue in your floor or walls. The simple test for this is to tape a square of clear plastic sheeting onto the floor and see if has condensation (More info: I have a basement floor with lots of moisture, can I paint or seal it to make it waterproof?). If you do have a moisture issue you need to use a masonry waterproofer before you add insulation.

Because you don't have full exterior access, you'll need to insulate the room from the inside. Vapor barrier should be on the inside in a cold climate, as you'll have more moisture in the house than outside. Most likely the easiest way to achieve this in a small space without any existing studs is to use foam boards (XPS or EPS). Many of these function as a vapor barrier as well when seams are glued and/or taped, which removes the worry about attempting to fasten a plastic sheet to concrete with tape. Properly installed foam board is a very good insulator for its thickness and fairly easy to DIY.

From there you'll have to install wall and flooring materials. You can glue on some wall board products and use a floating floor, or install some strapping or wooden structure on the inside of the foam boards to attach wallboard and/or subfloor. If you don't want to use foam board for some reason, building interior structure and insulating between the studs would be another reasonable approach. You may also look at basement wall systems that combine insulation and structure.

The premium option would be to build some structure first, and then have someone coat the floor and walls with spray foam. This has similar insulating properties to foam board but molds perfectly to your building. If you go that direction, ask the insulation contractor about the flooring options, as you'll likely want to use foam board there anyway to maintain level.

In any case, make sure you understand the installation requirements of your selected insulation, structure, and floor/wall materials when you design this project. In particular, make sure you're working within their specified load ratings and using manufacturer-recommended fasteners and adhesives.

  • I'm guessing I didn't go into enough detail. I live in St. Louis Missouri, we get a fair amount of both cold and hot weather. The room is already framed in, with 2x4 and insulated walls/ceiling. The floor of the room has oxboard or similar on it currently. I'm not sure if there's framing below the oxboard or not. If I'm understanding you correctly, this info could make a large difference in your suggestion.
    – adam
    Jan 25, 2018 at 23:41
  • @adam If the oxboard flooring has been there for a long period of time, the good news is there is no moisture penetration from the soil beneath the floor, which, I guess, is a concrete slab poured directly on the grade. Do you have enough headroom to accept a false/raised floor? It could be the solution you need.
    – r13
    Apr 18, 2021 at 22:27

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