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

Carport conversion

user-471779 | Posted in General Questions on

converting a large carport into living space… will be building floor with i joists… there is a concrete floor there… the walls will be 2×6 with r6 zipwall and spray foam inside of that …Zone 3. The “crawl” underneath the subfloor will only be ranging from 9 inches to 13 inches. Should I just put batt insulation in the floor joists? or lay rigid foam on the slab under the joists?

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. Expert Member

    (Edited post)

    Carport conversions are tricky. You are usually starting from a sloped slab, with neither footings, perimeter drains, insulation or a vapour barrier. To this you need to add a perimeter foundation capable of supporting the dead and live loads of the new space, detailed to keep moisture out both from below and the surrounding soil.
    The problem with adding a wood floor over the slab is that most codes set the minimum height of crawlspaces at eighteen inches to two feet, depending on whether there are services present. Ways around his would be to pour another slab, either as a new finished floor, or right under the I joists ( making them in effect expensive spacers). If you opt for this second solution, I'd check with your building inspector to make sure he doesn't still consider a framed floor directly on a slab as an illegal crawlspace.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Malcolm's points.

    Assuming that you have an adequate slab -- and that's a big if -- it would be better to insulate by adding several layers of rigid foam above the slab than by using fiberglass batts.

  3. JC72 | | #3

    Make sure it's permitted otherwise you may have difficulty selling or financing.

  4. user-471779 | | #4

    the carport is 23x40. The floor is sloped. The slab is almost 6" thick.. We added footings under the entire perimeter... and dug and poured a footing down the middle.. For a support wall half way across. Planning on leveling 7 1/2" tall I joists 16 inches on center accross the whole width supported on treated plates around the perimeter. It is permitted. I will check with the inspector if he is ok with the plan.... Lets assume that he is ok... and our new floor will be constructed of the I joists... If the the bottom of the I joists will be ranging from lets say 4" to 2" from the existing slab.... would I use a vapor barrier? and then I was planning on laying down 2 layers of 1 " rigid foam, staggered seams...taped... My framer is thinking I am crazy for insulating the floor... he said just put thick bats in the floor joists and you are done... could you please help me describe the reason why I should do rigid foam instead of the bats? Can I do both? I am going to explore the price difference of pouring the floor instead.. If I poured the floor, I could put down rigid foam on top of the existing slab and pour on top of that?

  5. Expert Member

    That's great - you have all the structural issues under control, and a good thick slab to start from.

    The foam does few things for you. It eliminates the thermal bridging of the I joists, keeps the floor structure within the heated envelope so it will stay warmer and dryer, and potentially solves the problem of the low concealed space bellow the joists - if your inspector agrees.

    Whether you end up going this route or pouring another slab on top of the new foam, a vapour barrier is a must. You don't want moisture to get into the floor as it will have no good way of drying out.

    Good luck!

  6. user-471779 | | #6

    Thank you both!
    Is there a negative about doing the foam and batt insulation?

  7. Expert Member

    I don't think so but I'm going to defer to others on whether the batts are a good idea.

  8. GBA Editor
    Martin Holladay | | #8

    As long as the slab stays dry, and you have a layer of polyethylene and rigid foam between the slab and the floor joists, I don't see any reason why you can't install fiberglass batts between the floor joists -- assuming, of course, that your local code enforcement authority agrees.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |