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Community and Q&A

How to insulate a screened porch floor

CTarch | Posted in Energy Efficiency and Durability on

I am looking to insulate the floor of a 3-season porch that is outfitted with removable sashes that will be removed for the entire summer season, thus exposing the floor to some blown-in rain. The sashes will be reinstalled in the fall so that the porch can be used in the fall and spring and will be opened up to the main house to benefit from the heating system in the house.

Is it safe to insulate beneath the porch floor, knowing that the wood floor (tongue and groove garapa) may get wet when the window sashes are not installed? I was thinking about using closed cell spray foam, but not sure if that will trap moisture at the underside of the flooring or if the moisture will eventually evaporate back up through the wood. I also thought about providing an air space below the flooring, with closed cell foam sprayed onto pressure-treated plywood run beneath the floor joists, but there really isn’t any conditioned area to vent the airspace to, and venting it to the exterior would defeat the purpose of insulating.

Any ideas would be appreciated!

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  1. GBA Editor
    Martin Holladay | | #1

    You have two choices.

    1. Extend the roof overhangs so that no wind-blown rain ever reaches the porch floor.

    2. Enclose the crawl space under the porch with an airtight, insulated skirt. If you take this approach, you'll need to install a layer of 6-mil polyethylene on the dirt floor of the crawl space.

  2. CTarch | | #2

    Thanks for the suggestions. Unfortunately, the overhangs will not keep out all rain and the porch is supported by wood posts on piers, so no crawlspace either. Should we just give up on any plans to insulate the floor? It might mean that the room just becomes a 2 1/2 season porch, instead of a 3-season one!

  3. GBA Editor
    Martin Holladay | | #3

    I assumed that the porch is on piers. The skirt I am talking about would need to be built. The usual recommendation is to prepare a shallow trench, perhaps 4 inches deep, for the base of the skirt. The skirt is built with pressure treated lumber -- typically using 2x4 framing. Include a continuous layer of rigid foam, as thick as possible, on the exterior side of the 2x4 wall. Then install pressure-treated plywood on the exterior side of the rigid foam, with flashing as necessary to bridge the seam between the bottom of your siding and the top of the pressure-treated plywood.

    All of this work needs to be performed with attention to airtightness. Include an insulated access door with hinges, weatherstripping, and a latch.

  4. brendanalbano | | #4


    If you decide not to insulate, consider using some of that money to invest in some really delightful heirloom quality insulated slippers, cozy blankets, and an attractive little box to keep them in next to the porch! That might extend your porch from 2 1/2 seasons to 3 seasons just as effectively as insulation would.

  5. dickrussell | | #5

    Consider also changing out the windows, with reframing of the wall as required to accommodate the new ones. We have a 10x14 side porch with an exterior full-glass door on each end and four six-foot wide sliding windows with removable screens covering the rest of the three sides facing the outdoors. The window frames are vinyl. I got the four of them for $150 each at a big box store six years ago, and they still look great. Just as you'd like to do, we sometimes leave the interior door open and the let the house heat the room in the shoulder seasons. There is insulation in the attic space over the porch, but nothing yet under the floor, something we may change in time.

  6. CTarch | | #6

    Thanks to all for the useful advice and clarification. Another question - should the dirt floor of the crawlspace beneath the porch also get a layer of rigid foam? Also, is there any risk of getting mold growth on the undersides of the potentially rained upon wood flooring boards if that crawlspace is airtight?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Should the dirt floor of the crawl space beneath the porch also get a layer of rigid foam?"

    A. In most climates, adding horizontal rigid foam above the dirt floor is not needed. (If you have some extra rigid foam, add it to the walls of the skirt to make the rigid foam thicker -- or bury some of the rigid foam at the perimeter of the porch. The buried foam should be installed vertically, to a depth of 2 or 3 feet.)

    Q. "Is there any risk of getting mold growth on the undersides of the potentially rained-upon wood flooring boards if that crawl space is airtight?"

    A. The risk depends on the volume of water (wind-blown rain) that reaches the porch floor. If the rain is not too heavy, the boards (and the crawl space below) should dry outward on warm, sunny, windy days. But any system can get overloaded with too much moisture. If you are on the seacoast, and you get regular gales, all bets are off.

  8. CTarch | | #8

    New concern on the insulated apron framing:
    How to avoid movement of the apron due to frost heaves in a 4" deep trench
    The porch is located in climate zone 5
    Should I go back to the original idea of insulating below the floor joists and venting the joist space?
    Thanks for any new thoughts!

  9. GBA Editor
    Martin Holladay | | #9

    The type of skirt I described is routinely built without frost heaving problems. If your soil is unusually damp, digging a deep trench and filling the trench with crushed stone is the standard approach to frost-heave worries.

    Well-drained (dry) soil is less subject to frost heaving than saturated soil.

    -- Martin Holladay

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