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

Post & Beam Sealing

Five20too | Posted in General Questions on

I have been doing a lot of reading on this site and other places around the internet. It feels like my head is about to explode so I’m just going to ask the question. Most homes that I’ve seen talked about that have a crawlspace also have a perimeter wall of cinder blocks or something like that. Mine does not. There are 5 rows of 13 posts per row. I will try and post the pics showing this. To do a liner I would have to run it down 4 different rows and then splice it all together with tape and hope that it totally seals it off. I have also read about people putting rigid board up against the boards from underneath the house and then taping the joints and addidng OSB, thus leaving a deadspace between the floor and the insulation. This might be possible except for the 65 places that the posts rest against the bottom of my floor joists. I used to think about just having some closed cell foam sprayed in but now I don’t know about that either. I was going to wrap up all of my PEX with foam and put a fine mesh screen around the bottom because with all the wood protected I wouldn’t have to worry about rot so much. I live in Northeast Texas and we do have very high humidity here. Drainage around the house is good and I just recently put a swale in my yard to help divert all the rain water out of the yard. I also planned to put down a plastic with wash rock or recycled rubber on top and that would help divert any water away from the house. I recently had my house moved from where it was built 6 years ago and since getting it here I haven’t done anything with it because I simply don’t know what to do. I didn’t have it vented before but I didnt have a barrier down either so there are some spots with mildew that I will need to treat before insulating at all. Can anyone steer me in the right direction? Thanks for any assitance!

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

    Dear Five,
    Start by reading this article: Crawl Spaces vs. Skirts.

    Floors need to be insulated, according to all building codes and common sense, unless you decide to build an unvented conditioned crawl space under your house.

    What you decide to do depends on your budget. The best approach would be to jack up the house a few inches and place it on a new crawl space foundation with concrete-block walls or poured concrete walls.

    If you can't afford that, then closed-cell spray polyurethane foam, protected by OSB, is a pretty good solution.

    If you don't want to use spray foam, a continuous layer of polyiso (rigid foam) insulation, installed on the underside of the joists, would be another possibility -- usually with fiberglass or mineral wool between the joists. This polyiso would need to be installed with attention to airtightness, and protected with a layer of OSB.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Is there anything wrong with using batts?"

    A. Yes. Batts are fairly ineffective, especially as usually installed. However, when coupled with a continuous layer of rigid foam under the joists, installed in an airtight manner and protected by OSB, the batts can be part of a solution.

    Q. "It there anything wrong with just leaving an open air gap altogether with only rigid foam and OSB?"

    A. Again, your suggested approach is less effective than the details I recommend. If there are any air leaks through the rim joist areas -- and there usually are -- your suggestion will result in very poor performance.

  3. Five20too | | #3

    Martin thanks a bunch for that link! There was a lot of good info there as well as the link to another article with some good reading. Sorry it's been so long getting back but things have been crazy! In the article it talks about using fiberglass batts to fill the voids or spray foam. I contacted a contractor about getting spray foam done and he said it costs $2 a square foot for 1" thick. Seems kind of expensive to me, plus I like the idea of being able to remove the batts easily for any issues or remodeling the wife may want to do down the road plus it will hopefully be cheaper. After paying to have the house moved the money just isn't there for spray foam and a cinder block wall. On a side note, is there any recommended product out there to remove the mildew prior to insulating the floor? Thanks for the help with this. I really appreciate it!

  4. Five20too | | #4

    So rigid foam installed and sealed with canned foam the entire perimeter of the rim joist, fiberglass batts covering the remainder of the floor, rigid boards (assuming I can use Polyiso instead) covering the bottom of the house with taped seams, and then osb covering that with caulked seams. Does that sound right?

  5. GBA Editor
    Martin Holladay | | #5

    Your explanation is confusing.

    Here is a link to another article you should read: How to Insulate a Cold Floor.

    I would describe the work this way:

    1. Perform air sealing work at the rim joists with caulk.

    2. Insulate the joist bays with fiberglass batts.

    3. Install a continuous layer of foil-faced polyiso on the underside of the floor joists. The perimeter of the polyiso should be bedded in caulk for an airtight installation. Seal the polyiso seams with high-quality tape.

    4. Install OSB under the polyiso to protect the polyiso.

  6. Five20too | | #6

    I understand now. 2nd to last question. I have seen some videos where the person cuts out a piece of rigid board and, after sealing the rim joist bay, he installs it and seals it with spray foam. The piece is only about 6-8 inches by however wide it is. Basically the area of the rim joist between the floor studs. Is this something I should do also or is the caulking enough with the batts all the way up against the rim joists? I tried to link a video but it denied me saying it was Spam. Ughh.

  7. GBA Editor
    Martin Holladay | | #7

    The type of insulation you describe is necessary for a house with a basement or crawl space foundation. But you don't have a basement or crawl space. Instead, you will be filling the joist bays completely with insulation. You don't need 1 or 2 inches of rigid foam if the entire joist bay is full of insulation.

  8. Five20too | | #8

    Sir you are so much help and I really appreciate it too! My last question would be to ask if there are any chemicals that are recommended by you to help remove the mildew prior to installing the insulation. Doing a quick search I found many manufacturers that claim to do the job but I'm sure there are more preferred ones out there, especially when it comes to spraying it under the house where it may be smelled inside.

  9. GBA Editor
    Martin Holladay | | #9

    Mold can be removed with ordinary detergent and water. Use a bristle brush. After removing the mold, let everything dry out before enclosing the area with insulation.

  10. Five20too | | #10

    I'm gathering all of my materials and I have a quick question. Can you explain why OSB is needed over the foil faced Polyiso that I'm going to install? I've read and re-read everything that you sent me and I really can't seem to figure it out. If the foil acts like the vapor barrier and blocks out everything then wouldn't it be safe to say that the OSB wouldn't really be adding any value to the insulation? I'm not trying to cheap out (added 1k to the project) but if it's really not needed then I'd probably just skip it. Thanks!

  11. wisjim | | #11

    The OSB provides physical protection to the Polyiso. Polyiso is fairly soft and easily damaged, and chewed on by critters, and the foil face may be a vapor or moisture barrier, but it isn't physical protection.

  12. GBA Editor
    Martin Holladay | | #12

    I agree with Jim's answer. If you start visiting crawl spaces -- I've visited dozens -- you'll see how many have damaged floor insulation due to wild animals. How many? Most of them.

    Well-fastened OSB will keep the animals out. Mostly. Porcupines will chew through almost anything.

  13. user-1072251 | | #13

    "Well-fastened OSB will keep the animals out. Mostly. Porcupines will chew through almost anything." which is why you should install a layer of "hardware cloth" which is a galvanized 1/4" screen, and which is hard for animals to chew through. It can go either under or above the OSB; probably better on the outside.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |