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Community and Q&A

Insulation on existing wood floors above a crawlspace

reeno23 | Posted in Energy Efficiency and Durability on

Hello. I am turning a home into a duplex in northern PA, zone 5, that is built on a 1′ crawlspace. The crawlspace walls are uneven hand-laid stone. The soil after lots of rain is damp, but no standing water. The joists have no rot and are dry to the touch. It is an old grange, so it is currently an empty shell of a late 1800’s built home (no heating, plumbing, electric, insulation.) The floors are original wood floors in good shape, but I will be covering them with laminate as renters are rough on things.

I have read through every article I can find on how I should handle the crawlspace and floor insulation. I’ve learned turning it into a conditioned crawlspace is the far and away best choice, but the costs and difficulties of tearing up the existing wood floor and subfloor, putting a vapor barrier on the ground, and paying for foam on all the walls is hard to justify.

I have nearly settled on the following:
1) Lay (possibly glue it) 2″ (considering 1″) of XPS, staggering seams, on all of the existing wood floor and tape all seams.
2) Lay 3/4 “t&g and screw down into original plywood on top of joists.
3) Install laminate

Can anyone offer a final opinion on if I may encounter any moisture issues that the house has never experienced in the past 140 years by heating it and putting insulation on top of the existing floors? Are there any other items with this plan that I’m not thinking of? Most discussions on this topic apply to slabs, so I’m mostly questioning moisture.

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    If the building has been heated, it's certainly possible that heat flowing downward through the floor assembly has helped keep the subfloor and joists dryer than they would otherwise have been, and have kept the joists from rotting. Insulating above the subfloor would reduce the heat flow and raise the moisture content of the joists.

    In short, your plan is risky.

  2. GBA Editor
    Martin Holladay | | #2

    With some crawlspaces, it's possible to lower the floor, thereby gaining enough access to install polyethylene on the dirt and spray foam on the walls.

    Otherwise, you might want to consider jacking up the house and raising the foundation walls with new concrete.

  3. reeno23 | | #3


    Thanks for the advice. I have decided to bite the bullet and condittion it. If I tear up the floor, I can simply walk on the ground below between the joists and get the job done.

    With the vapor barrier, I will run it on the floor and up the walls, and try to construction adhesive it to the walls I guess, as the hand laid stone is a very uneven surface. Down the direct center of the crawl space is a poured concrete beam with a mix of wood and concrete piers running down it. Do I run the vapor barrier all the way up these, or just to their bases?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Do I run the vapor barrier all the way up these piers, or just to their bases?"

    A. Up to the bases is good enough. You can also stop the poly at the base of the wall if you want -- you don't have to run the poly up the walls. If you install closed-cell spray foam on the interior side of the stone walls, the closed-cell spray foam will provide insulation; it will be an air barrier; and it will also be a vapor barrier.

  5. reeno23 | | #5

    Not needing it up the walls if it is spray foamed helps the situation a lot. I attached two pictures showing the center beam that goes down the whole center. Some spots are old concrete piers on the poured concrete center, some are newer wood blocks. I guess I should ask how should I wrap this set up with the vapor barrier? Will covering the wood forms for the concrete cause problems when that wood rots? Thank you.

  6. user-6623302 | | #6

    I would want to inspect that grad beam. Take out the wood forming and inspect it before you rely upon the system to support your new work. How about maybe a rat slab. Is each joist supported? I would take out any untreated wood that is that close to the ground.

  7. GBA Editor
    Martin Holladay | | #7

    There are two issues here. One (as Jonathan correctly notes) is structural. You want to be sure that the concrete grade beam is adequate from a structural standpoint, and that it won't heave due to frost action, and you want to be sure that any wooden blocks supporting your joists won't rot. If you have any doubts on these issues, consult an engineer.

    The other issue concerns vapor diffusion. I wouldn't worry too much about achieving a perfect vapor barrier -- minor imperfections in your vapor barrier won't matter much from a performance perspective. Cut around the grade beam if necessary, and overlap the vapor barrier where required. A high quality tape is handy for tricky areas.

  8. user-6623302 | | #8

    Think about insulation on the crawlspace floor to frost protect the foundation and grade beam. Treat in like a shallow, frost protected foundation. How is the rain management system on the outside? Gutters and appropriate drainage can make a big difference inside.

  9. reeno23 | | #9

    Thank you for the insight. I will make sure it is structurally sound and that all wood is treated prior to advancing on any plan that's decided on. I'll also make sure every joist is supported. The home will be sloped on all sides away from the foundations, and gutters will be the same.

    This is a 63' x 20' building. That grade beam runs down the center, and all walls are simply stacked, uneven, hand-laid stone. My current thoughts off of your opinions are:

    Vapor barrier the floor, just to the edge of the beam, and taping it along that edge. Run vapor barrier up the wall and put 1" of foam, or omit vapor barrier on wall and put 2" of foam on. If I put sheets of 2" of rigid foam insulation along all the dirt on the floor on top of the vapor barrier, and tape seams, would that achieve the frost protected shallow insulation goal? I am reading that the rigid is meant to extend off of the bottom of the beam. Do I need to make sure the insulation would match evenly with the bottom of the beam if it happens to be deeper than the dirt? Is this insulation on the floor meant to conserve heat from the furnace supply that would be down there, or is it needed in addition to the heat no matter what for heaving risk to be minimized?

    Sorry for the barrage of question guys. I appreciate your help a lot. This is all an introduction to wrapping my head around basic foundation knowledge. Once I understand it, it won't be asked again.


  10. GBA Editor
    Martin Holladay | | #10

    In your climate zone, I don't think that you'll save enough energy by insulating your crawl space floor to justify the investment in rigid foam.

    As long as you insulate the crawl space walls adequately -- minimum code requirements call for R-15 for basement walls and crawl space walls in your climate zone, so that means at least 2.5 inches of closed-cell spray foam -- your crawl space floor won't freeze, and neither will your grade beam.

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