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Community and Q&A

How to insulate under a sunroom porch?

daves88 | Posted in General Questions on

I have a porch that is 8″ off the ground and I want to convert it to an all season room. I started researching the best way to insulate the floor and got pretty discouraged… From what I’ve read, if it isn’t a basement or a cement slab, you can’t really insulate it correctly. I read everything about encapsulating crawlspaces and making the area part of the living space. For this space, 8″ of air over dirt, I can’t see doing that, not to mention, I couldn’t even run HVAC to that space. Every other option I thought of and read about leads to problems and mold. I’m seriously considering having the porch removed and installing a cement slab, but that could get pricey considering it’s supporting a roof.

Before I bail on the project altogether, can someone tell me a good way to do this? Considering I can’t get under the floor and I really want no chance of breeding mold under there.

Thanks in advance for any advice.

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  1. Expert Member


    A few things influence how difficult or easy this may be.

    If you are in a fairly forgiving climate, have adequate head height in the porch, and can accommodate a change in levels at the entrance, you can insulate the floor by covering it with rigid foam and a new subfloor over top. With only 8" of space underneath, this is by far the easiest solution.

    1. daves88 | | #2

      Thanks for the reply. I did see a couple people saying they used this method but wasn’t sure how well this would work. So in this case, would I leave it completely open under the porch or close it in? I do want to keep critters out of there, so at a minimum want to put something in to close it off. I live in central NY and can accommodate raising the floor 2 inches if needed. This seems like a good option, I just want to be sure there wont be mold eventually.


  2. GBA Editor
    Martin Holladay | | #3

    Malcolm's solution is one possible approach. The best way to proceed, as you guessed, is to remove the floor framing and pour a concrete slab. As you also guessed, pouring a slab would be expensive compared to Malcolm's approach.

  3. daves88 | | #4

    I appreciate the input Martin. I guess it's the price you pay when you try converting a space that was never meant to be enclosed.

  4. Expert Member


    A potential problem with my expedient approach is that the floor framing may have benefited from being able to dry upwards. Insulating the floor and enclosing it may make it more susceptible to rot being that close to grade.

    There have been a couple of discussions (and I think a Q&A spotlight) here on GBA about similar situations. The consensus seems to be either a slab, or excavation to create a workable height crawlspace, are the best solutions - but represent considerably more work.

  5. Expert Member
    Dana Dorsett | | #6

    If you can get a ground vapor retarder in there you can probably do a reasonable job of insulating it with a complete fill of blown or poured EPS bead (or EPS shred) such as Styro Pour-Pak, usually sold as core-fill insulation for CMU block. It's not very air-retardent, but it's non-wicking and at 8" thickness would deliver at least R25 or so performance. Air sealing the perimeter would be important to avoid wind-washing effects.

    Most rental blowers are set up for blowing fiber and probably should not be used for blowing using EPS shred, but DIYers have successfully used things like leaf blowers and shop-vac equipment for filling hard-to-pour cavities. (A kiwi named Paul Kennett is an advocate of even using blown EPS beads in framed walls, and has several YouTube videos using oddball equipment as EPS blowers- even hair dryers!)

    There can be a lot of static electricity generated- you may have EPS beads sticking to everything for a day or three, but once installed it doesn't settle. Like all polystyrene insulation it's flammable, so the walls and floor above have to meet spec as a thermal barrier against ignition. (A 3/4" lumber or plywood/OSB subfloor works, as does 1/2" ply/OSB wall sheathing.)

    Paul Kennett has a web page detailing some of his EPS-blower kludge efforts located here:

    (The compressed air venturi type is probably the slickest-quickest/best version he came up.)

    Blown EPS is a fairly common retrofit to all-masonry cavity walls in the UK, but it's never really caught on in the US.

  6. daves88 | | #7

    That's an interesting approach. I was thinking of something like that myself, just filling it with foam but figured it would have to be spray foam which wouldn't be cost effective. I'll look up Paul on YouTube and in the meantime I'm going to get a price on turning this porch into a slab.

    If I go with a slab, can I build it back up to approximately the same height, considering I'm going to have to be about 18" off the ground to the top of the floor? Thinking maybe the concrete slab can be raised a bit and then use sleepers with foam board between to make up the distance. Any thoughts on this approach would be appreciated.


  7. GBA Editor
    Martin Holladay | | #8

    The easiest approach is to purchase blocks of EPS that are thick enough to bring a 4-inch-thick slab to the desired height. However, you can use sand or crushed stone under the EPS if these materials are cheaper.

  8. daves88 | | #9

    If we find this is not cost effective and decide to just use the existing deck, what would the risk be in leaving the floor un-insulated? I know it will be cold in the winter, but would we have to worry about mold? Could we just lay down 3/4" PT plywood, add radiant heat to the floor and cover it with LVT? Just trying to come up with the most affordable, yet solid plan to do this. Last resort will be a screen room instead. This reminds me of a post I read where someone mentioned, if you want to make changes to your house, you are better off building another one.

  9. Expert Member
    MALCOLM TAYLOR | | #10

    Uninsulated houses usually last a long time. Throwing heat at wood framing keeps it dry and discourages rot. The problem is of course that it takes an awful lot of energy to do this - and probably the least energy efficient way to heat the space would be radiant heat in an uninsulated floor. If you decide to go that route, at least consider my earlier suggestion to add fo0am under the new subfloor.

  10. GBA Editor
    Martin Holladay | | #11

    An uninsulated floor -- especially an uninsulated floor with radiant heat -- makes me cringe. If you can't install a continuous layer of rigid foam under the radiant PEX tubing, at least consider installing vertical rigid foam at the perimeter of the sunroom foundation. Ideally, you would dig a trench down, at least 18" or 24" deep, to install the vertical rigid foam, and would extend the foam up to the bottom course of siding. You would need horizontal Z-flashing at the seam between the top of the vertical foam and the bottom course of siding, as well as some type of durable material to protect the above-grade portion of the foundation insulation from abuse.

  11. daves88 | | #12

    Guys, I appreciate the feedback once again. If I go with foam under my sub floor what would be the potential risk in the scenario? With the set up I would have my joists 8 inches off the ground a layer of foam (assuming this would be 2 inches thick) then a layer of 3/4 inch plywood. (Pressure treated?) i would seal around the foam as good as possible to avoid air leaks. I’m assuming the potential risk here would be that if it is not sealed properly, warm air could leak through the floor ultimately causing condensation on the floor joists ?

  12. Expert Member
    Dana Dorsett | | #13

    Warm air doesn't leak downward to the cold crawlspace unless the room above is pressuriized. The bigger risk is in summer, since the insulation keeps the framing cooler. When humid outdoor air reaches the now colder than previously framing it takes on more moisture.

    With the inability to dry toward the conditioned interior, ground moisture is another potential issue, if you can't get mangage to install a vapor barrier over the bare dirt.

    How deep are the joists? I was imagining that it was 8" from the dirt to the subfloor, with perhaps 2x6 joists, but now I'm thinking you have the joist depth to consider as well? With 2x10s or 2x12s and 8" under the bottom edge of the joists you're looking at about 0.7-0.8 cubic feet of volume per square foot of floor.

    In bulk pourable EPS shred suitable for blowing costs something like 60-75 cents per cubic foot. Menards sells 5 cubic foot bags of recycled shred/bead mix for about twice that, but you may be able to beat that price from a local distributor, or buying direct from a local EPS molder. With the space fully filled from the ground to the subfloor with EPS bead or shred all of the wood would stay closer to the room temp (or warmer, if radiant floor), and the mold risks are essentially nil year-round.

    Some DIYers have resorted to using small wood chippers to reduce packing EPS to blowable sized shred, but that seems like more work than it's worth, especially if you have large volume to fill.

  13. daves88 | | #14

    They are 2x10 joists and it is 8” to the bottom of the joist. I’m assuming with this method I would totally seal underneith the porch and add a thick vapor barrier over the dirt?

  14. Expert Member
    MALCOLM TAYLOR | | #15

    Dave, there are two potential issues with sealing the space underneath with a skirt.

    One it that once sealed it better be perfect, as it creates an ideal home and path to your floor for insects and rodents.

    The other depends on how your code interprets this space. Ours recognizes no middle-ground. A building is either open to below, or is considered a crawlspace - with all the attendant requirements of access, depth, ventilation etc.

  15. Expert Member
    Dana Dorsett | | #16

    The ground vapor barrier becomes mandatory once the natural ventilation of the porch to the outdoors goes away.

    Taping the seams of the subfloor for any new flooring would make an excellent top side air barrier, but a sheet of housewrap would be almost as good. Preserving at least some ability for the deck & joists to dry toward the interior would be prudent (ergo no vinyl flooring or sheet polyethylene layers, no foil clad above-the-subfloor type radiant floor system such as Roth Panel, etc.) Plywood subfloor with hardwood flooring would be fine.

  16. daves88 | | #17

    Based on the feedback from you guys, I think the plan is this: Leave it open under the porch (however, I will put some sort of mesh to ward off critters and some decorative wood that allows airflow). Cover the joists with house wrap from the top, then a layer of 2” rigid foam, 3/4” plywood and finally flooring. Does this sound reasonable and hopefully good enough to avoid mold and rot issues?

    Also, when you mention vinyl flooring, are you talking about the wood plank flooring (Pergo type flooring)? We were hoping to put in electric radiant heat under LVT flooring, but if you don’t recommend this, we will have to reconsider.

    Thanks again for the feedback.

  17. GBA Editor
    Martin Holladay | | #18

    I don't think your plan will work. The main reason it won't work is that you can't install a layer of horizontal rigid foam above floor joists, because the floor joists will dig into the compressible foam.

    Here are my suggestions:

    1. Once your old floor is ripped out, install a layer of polyethylene over the exposed dirt. Keep this polyethylene in place with rocks or bricks.

    2. You don't need housewrap above the joists, but you do need to start with a layer of plywood or OSB above the joists.

    3. On top of the plywood or OSB subfloor, you can install a continuous layer of 2-inch-thick rigid foam.

    4. Then you should install a second layer of plywood or OSB, followed by your flooring. If you choose to install electric radiant heat, make sure that your installation (a) follows the installation instructions provided by the electric coil manufacturer, and (b) follows the installation instructions provided by the flooring manufacturer.

    1. daves88 | | #20

      Martin, thanks for clarifying. That plan will raise my floor a bit, but I have the room to do it. I appreciate the advice.


  18. walta100 | | #19

    Please take a look at this porch and try to decide if this porch was actually built to be a room or if this is the result of a series of upgrades.

    The home is built with a back door with step down.
    Owner A lays a few brick as a patio next to the door.
    Owner B puts a roof over the patio.
    Owner C puts a removes the steps and makes it a covered deck.
    Owner D adds a few posts and screens and calls it a screened porch.
    Owner E adds windows between the posts and calls it a 3 season room.
    Owner F adds a duct from the houses furnace and now it’s the family room.

    The problem is owner A did not put a foundation under his brick that will support a family room. Owner B built a low slope roof leaving no room for insulation when he covered his patio. Owner C did not build an air tight insulated floor under his deck. ECT every one did a good job at their little project but there is no way it adds up to well built family room.

    From here I do not know if you have a room or a pile projects that was not built to code or inspected.

    If you really need/want an addition to your home, planing and build real room maybe the best choice.

    1. daves88 | | #21

      Walter, I had the house built 8 years ago. The covered porch was built with the house, but it was built as that, a covered porch. I had planned to screen it in someday, but now want to create a sunroom out of it to get more use during the cold months. I know the best way to do this is to have a foundation built, however it’s not cost effective so I am trying to come up with a good alternative that doesn’t result in problems down the road. Luckily, I am starting with a fairly clean slate as nothing has been done to the porch yet. Thanks for your input.


  19. daves88 | | #22

    I have attached a picture of the space. I probably should’ve included this in my original post.

    1. daves88 | | #26

      Thanks for the comment. There are 3 support posts that we’ll have to contend with. I’ve had 2 concrete guys come by to look at the job and they never came back! I’m thinking the roof support and the proximity to the house deterred them.


  20. user-2642926 | | #23

    Dave, post #15 from Malcolm has a very good point about your local requirements. If that's their definition of a crawl space, you could be facing an inspection problem.

    How big is the area? What's above it that would need to be supported if you poured a slab?

    The slab for my front porch was poured considerably after the footings for the house were poured / fixed, even though a flat roof covers the porch. They basically removed the load from the vertical support post by wedging in a couple angled supports so they could properly pour the post footing and porch, but there was minimal mass to support.

    The attachment isn't a great photo but it demonstrates the finished product, probably making the point more easily than what I wrote above. That vertical post was literally hanging from the roof while the footing for it and the porch slab were poured.

  21. Expert Member
    MALCOLM TAYLOR | | #24

    I envisaged a more finished space you wanted to insulate. Now I've seen the picture, you really aren't losing much by removing the floor and re-building the structure as Martin and others have suggested.

    1. daves88 | | #27

      Malcolm, I should have posted that picture initially. It’s just rebuilding affordably and correctly that seems to be the challenge.

  22. GBA Editor
    Martin Holladay | | #25

    Call your local concrete contractor. It's time to pour a slab.

    I'm not sure of your climate zone, but in most of the U.S., you'll want the slab to be properly insulated. For more information, see "Insulating a slab on grade."

    1. daves88 | | #28


      Is there something about seeing the space that makes you suggest the slab? I kind of like your idea with the plywood/foam/plywood as it’s much more affordable, however I don’t want to invite problems either. Just curious what changed your mind.



  23. GBA Editor
    Martin Holladay | | #29

    Your porch has no walls. That makes installing a slab much easier. The photo shows that you just have three posts, and the loads on these three posts can easily be temporarily supported by installing some diagonal supports, as described by Mike in Comment #23.

    Pouring the slab will be a quick and easy job.

    1. daves88 | | #30

      I live in NY, so i know we would have to get below the frostline (4ft) . I think this makes it a bit more challenging/expensive.

      1. user-2642926 | | #31

        I'm in So Cal so frostlines weren't an issue, but I had to go 12" into native soil so my footings were 32" deep. You don't need to take it to that depth for the entire slab.

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