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Foam first or last in cold enclosed porch floor?

JonesyHome | Posted in Energy Efficiency and Durability on


I have seen this question answered many ways while searching online, so I thought I would pose it here. I know there is another thread on this topic, but it seemed worth posting again.

When insulating an enclosed porch that is not climate controlled, should the layering be: pink foam board affixed to the floor underside, fiberglass batts, plywood for vermin control?

Here’s the application:

Home is built 1923 in the Chicago area. Screened porch with heated room above (sometimes called a “tandem room” here), two walls adjacent to main climate controlled home. Porch has double pane windows and a storm door, but it is and will remain unheated or uncooled. The porch floor is about 4 feet above grade and not enclosed.

My goals for insulating at all is, I feel, pretty realistic: 1) To get an extra few weeks use in the spring and fall. 2) Provide a more stable temperature zone/buffer to moderate or slow loss from and incursion to the main house (if that’s even possible/realistic).

My number one concern here is creating a moisture issue by layering incorrectly.

Thanks in advance!

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  1. GBA Editor
    Martin Holladay | | #1

    If I understand your question correctly, you want to insulate the floor of a porch that is built on a pier foundation -- that is, a porch that allows wind to blow underneath it. Is that correct?

    Assuming I have that right, your plan is a good one:

    1. Fill the joist bays with fiberglass batts.

    2. Install a continuous layer of rigid foam to the underside of the floor joists. The rigid foam should be installed with attention to airtightness (meaning that the perimeter should be sealed with caulk or canned spray foam, and the seams should be sealed with high-quality tape).

    Then you should install a layer of plywood or OSB on the exterior side of the rigid foam to keep out critters and to protect the rigid foam.

    If you follow all those steps, your porch will have an unusually well insulated floor.

    For more information, see this article: How to Insulate a Cold Floor.

  2. Expert Member
    Dana Dorsett | | #2

    Rather than pink foam board (which is blown with climate-damaging HFCs, and loses performance as the HFCs leak out), use polyisocyanurate or EPS which are blown with far more benign pentane.

    Better yet, use reclaimed foam board (any type), which uses no new blowing agents, no new polymers, and costs 1/4-1/3 the cost of virgin stock foam board. A few reasonably local sources are advertising here:

  3. GBA Editor
    Martin Holladay | | #3

    Here is a link to an article with more information on the topic raised by Dana: Choosing Rigid Foam.

  4. JonesyHome | | #4

    Hi Martin and Dana,

    Thanks for the quick reply and the tip about foam.

    Actually, Martin, my thought was to cut the rigid foam and affix it to the underside of the floor deck, which is exposed between the rafters. You're saying, however, to apply the rigid foam across the rafters on the underside, with the fiberglass in contact with the underside of the floor itself.

    Is your method to avoid moisture issues if the foam is affixed to the floor underside? Or, is it due to wind/air movement that is better blocked by having the foam as a barrier to the wind?


    The porch is on a pier foundation with wind able to blow underneath.


  5. GBA Editor
    Martin Holladay | | #5

    The main advantage of the rigid foam layer is that it interrupts thermal bridging through the floor joists. Cutting up the rigid foam into narrow rectangles and inserting the narrow rectangles between the joists significantly lowers the thermal performance of the rigid foam -- and strikes me as rather sad. It's much nicer to see the rigid foam installed in big uncut sheets, rather than cut up into narrow rectangles.

    For more information on thermal bridging, see these articles:

    What is Thermal Bridging?

    Thermal Bridging

  6. JonesyHome | | #6

    Thanks for the articles—very informative and interesting.

    One final scenario, if you don't mind: Let's say I already started gluing panels, ran out of glue, and in between my next trip to the hardware store, I started worrying about trapping moisture and doubting the technique I was following. Would it be advisable, from a moisture perspective, to start over? Keep in mind that I'm not looking to hang out on this porch in January in shorts and a t-shirt—just looking to maintain a more moderated zone. Trying to retrofit a 1923 home without gutting it is all about compromises, after all. Or, put another way, going from zero to something better, but without introducing a future moisture problem, is the goal for now.

    (Not only are narrow panels rather sad, they are a tedious pain... but they're also all cut.)


  7. GBA Editor
    Martin Holladay | | #7

    You can leave any rigid foam that you have already glued right where it is. It won't cause any problems.

    You can switch over to the method I recommended halfway through the job if you want.

  8. JonesyHome | | #8


    Thanks! I have opted against the fiberglass, as I noted some wind blown rain getting in spots that make me nervous. (A recently removed tree blocked that, I believe.) Given my goal of simply moderating rather than actually climate controlling, I'll just stick with my foam board, cover with a fire rated cover, and call it a day.

    Except... I realize that I have film faced Foamular for that first layer I was gluing/have glued. I'm afraid it may trap moisture. I've read a number of things on your site and elsewhere to learn more, but I actually wrote Owens Corning because I couldn't find a definitive statement about the permeability of that film. They do give some perm comparisons and ratings (.02) in a technical bulletin that seem to cover a film, but perhaps not the exact product I have... but I'm not sure. It doesn't seem to state that it is a moisture barrier, so maybe it's okay?

    Clearly, I am a classic do-it-yourselfer in that I know just enough to always have to redo everything I try.

    Thanks again!

  9. GBA Editor
    Martin Holladay | | #9

    There is no good way to insulate the floor assembly of a porch exposed to wind-blown rain. If you've got wind-blown rain, it's better to leave the floor uninsulated.

    If you can remember to close your windows -- solving the wind-blown rain problem -- you can insulate as you plan to do. The foil facing on the foam won't cause any problems (as long as you keep the rain off your porch).

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