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Need advice on closing in porch

Linda Wolf | Posted in General Questions on

My house is in the SE Texas Panhandle, in zone 3B, but only 15 miles from Oklahoma (zone 3A) and also only 15 miles from the majority of the panhandle (zone 4B). So we have a very mixed climate that in my lifetime I have seen range from -15 degrees to +115 degrees F. Some years it is quite dry and some years quite rainy and humid.

A few years ago we built a lean-to covered porch along the full north side of our house, approx. 10’x32’. We poured a 4” concrete slab with 8”x8” footings, but no insulation, vapor barrier or anything else underneath. The porch roof is white steel, over plywood decking, completely unfinished underneath. Due to an overhang over one small portion of the porch, there was room for only 2×4 “rafters” under the decking, not 2x6s, which we would have preferred.

Now I would like to enclose the porch, dividing it into 2 rooms, a guest bedroom and a mud room (and also to serve as a buffer for the main part of the house from the north wind), but I have questions about how to best insulate the area, given my restrictions.

The mudroom portion of the porch I am okay with just leaving bare concrete (although I may decide on covering it with porcelain tile). I was thinking about putting a good pad and carpet in the guest bedroom, as the only way I could think of to help warm the floor up a bit, but I didn’t know if I was just asking for mold to build up under the carpet. Should I go with some good porcelain tile and throw rugs, instead?

Since I have only 4” of insulation space in the ceiling, what will give me the best bang for my buck? One thought that I had was that after insulating, instead of (or perhaps in addition to) using sheetrock was to install metal ceiling tiles, the shiniest that I could find, to possibly work as a radiant barrier … would something like this reflect the heat back down into the room, rather than allow it to leave through the roof? Would I need to seal all the seams between the tiles to keep heat from escaping, and if so, what is the best product to use?

To minimize thermal bridging, there will be no openings in the ceiling/roof or outside walls at all (for light fixtures, fans, electrical outlets, light switches etc.), and I plan to buy the best insulated exterior door I can find. There will be only one small triple-pane window in each room.

I would appreciate any and all advice/ideas.

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Linda,
    To start, you need to insulate the perimeter of the concrete slab with vertical rigid foam insulation. EPS would be the best choice from an environmental perspective, but XPS would also work. The insulation should be at least 2 feet wide, and should extend from the top of the slab downward. Of course you will need to dig a perimeter trench to install this insulation.

    You will also need to install Z-flashing at the top of the insulation, integrated with the porch siding. The above-grade portion of the rigid insulation will need to be protected from abuse and UV light with metal flashing, pressure-treated plywood, cement board, or stucco.

    It's also possible to install a horizontal layer of rigid foam above the slab, followed by plywood subflooring secured to the slab with TapCon screws.

    If at all possible, consider temporarily removing the steel roofing so that you can install a layer of rigid foam above the roof sheathing, followed by 1x4 strapping. Then you can re-install the roof sheathing.

    If you can't do that, you should fill the 3.5-inch-deep rafters with as much closed-cell spray foam as you can fit in there.

    Metal ceiling tiles aren't going to help; just use ordinary drywall.

  2. Linda Wolf | | #2

    Martin, could you please clarify what you said about insulating the slab? When you say it should be 2 ft. wide, do you mean it should extend 2 feet out from the edge of the slab, or that it should go down 2 ft. into the ground?

    How important is adding this insulation in my climate? I ask this because before I decided to enclose the porch to add a bedroom, I toyed with the idea of adding on a master bedroom that was passive solar, highly insulated, etc., and of the 3 concrete contractors that are in my area, none of them had ever heard of insulating a slab (which I was asking them to do). Since I am attempting to over-insulate elsewhere, and the room will see little use in the cold months, I am mainly concerned about moisture infiltration. Do you think this woud be a problem in my project?

    Thanks,
    Linda

  3. GBA Editor
    Martin Holladay | | #3

    Linda,
    Q. "When you say it should be 2 ft. wide, do you mean it should extend 2 feet out from the edge of the slab, or that it should go down 2 ft. into the ground?"

    A. This is vertical insulation, not horizontal insulation. The top of the insulation is even with the top of the slab. It extends downward from there into the ground (vertically). The insulation should surround the perimeter of your slab on all exposed sides.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "How important is adding this insulation in my climate?"

    A. Since you told us, "We have a very mixed climate that in my lifetime I have seen range from -15 degrees to +115 degrees F," I would say, "very important."

  5. Linda Wolf | | #5

    And how do we protect the insulation from damage, other than covering it with dirt?

  6. GBA Editor
    Martin Holladay | | #6

    Linda,
    Q. "How do we protect the insulation from damage, other than covering it with dirt?"

    A. If you read my first answer (Comment #1 on this page), you'll see I answered that question when I wrote, "You will also need to install Z-flashing at the top of the insulation, integrated with the porch siding. The above-grade portion of the rigid insulation will need to be protected from abuse and UV light with metal flashing, pressure-treated plywood, cement board, or stucco."

    For more information on this issue, see How to Insulate a Basement Wall -- scroll down to the section of the article that begins with the heading, "If I insulate on the outside, how should I protect the above-grade foam?"

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