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Community and Q&A

Enclosed, unheated porch insulation

Daniel F. Vellone | Posted in Energy Efficiency and Durability on

I’m in climate zone 6, northern New York. I’ve built a 28 x 10′ porch on the north side of my home with a shed roof that I’ll insulate so the space is somewhat conditioned (walls get insulated as well). It’s unheated and will remain so. I’d like to apply xps to the roof deck, and wonder what r value I should shoot for and if there would be dew point/moisture issues with minimal insulation.

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  1. Jon Harrod | | #1

    Hi Daniel,

    In my experience with these kind of unheated, tempered spaces, a couple inches of rigid foam will be fine, so long as moisture loads are kept low. I'd consider EPS or polyiso as greener alternatives to XPS due to the lower global warming potential of the blowing agents.

    To prevent moisture transport from the conditioned space, make sure that you have a really good air barrier between the conditioned space and porch and that you minimize the amount of time spent with the door between the house and porch open. Also, be careful about the amount of moisture brought in from outside (for example on wet boots and outdoor gear).

    In winter, the porch will tend to stay warmer than the outdoors by at least a few degrees. If the air that passes through the porch comes mostly from outdoors (rather than from the living space), its moisture content will be low. Its relative humidity will drop as it warms; this will work in your favor. If you start to see condensation or frost on the porch windows, crack a window or two to increase flow of outdoor air through the space.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "I'd like to apply XPS to the roof deck, and wonder what R-value I should shoot for and if there would be dew point/moisture issues with minimal insulation."

    A. You can install any R-value of rigid foam you want, without any concern about moisture issues, although a different type of rigid foam (for example, EPS or polyiso) would be preferable to XPS from an environmental perspective. (For more information on this issue, see Choosing Rigid Foam.)

    Moisture accumulation and condensation occur when warm, humid air contact a cold surface. Adding rigid foam to the exterior side of your roof sheathing will lower, not increase, the risk of moisture accumulation or condensation (although the usefulness of the insulation will depend on whether the room is conditioned, either directly or indirectly).

  3. Daniel F. Vellone | | #3

    Thank you for the input. I've become aware of the global warming potential of xps here on gba, and only opted to use it because I've been accessing it for my wrap and strap from a reclaimer.

  4. GBA Editor
    Martin Holladay | | #4

    If you are using reclaimed foam -- that's great. It's been rescued from the dumpster, and you are putting it to good use.

  5. Expert Member
  6. Daniel F. Vellone | | #6

    Dana, you actually gave me the link a year ago to the dealer I got all the xps from. Enough to sheathe my entire house with. I got 2.5" thick sheets of 4x8 xps that had come off a school roof. It was in great condition. They have piles of xps and polyiso in varying thickness all in ready to re-use shape.

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