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Community and Q&A

Thermal breaks when enclosing porch

nycjstar | Posted in Green Building Techniques on

I have a stone porch in front of the house (6’x25′) in lower New York (climate zone 4), that is surrounded on three sides by the house and garage (fiberglass batt insulation c.2005 construction). Only the front is exposed and open. The floor is stone, presumably on a concrete slab. The siding is cedar shakes, with a 6″ protruding stone chair-rail around 3′ off the ground in it, and stone facing below that. The roof line continues down over this area (with a nice bead-board ceiling with high-hats in it). I’d like the room to be enegy efficient as I’d like to use the minimum in HVAC for the room.

The lip of stone porch floor sticks out around a foot beyond the natural formation of the new wall filing in the space between the supporting columns, which are also in line with the garage and house. So I think I have to insulate on top of the stone floor: building a new floor up around 4-5″ (there is room), as I couldn’t have a thermal break if I insulated outside the porch (because the door would be set 1′ inside it … thus exposed stone with no thermal break). ALSO, the stone chair rail and stone facade, which comes around the front of the house and garage, and then into this porch area will conduct cold too.

So a couple questions:

1) Is there that much heat transmission through the exposed stone floor that I need to insulate it, and if yes, then I see my only option as a built up insulated floor with 2×4 sleepers, 2″ XPS, plywood, and then tile or something.

2) I assume I’ll remove a 4″ width strip of cedar shakes, and build the new wall flush up against the tyvek underneath it maybe with a strip of silicon caulk between the new 2×4 and the tyvek. But the lower half of the wall has the stone chair rail and stone facing. Should I cut a 4″ channel into those too? Is the thermal bridging that big a deal?

3) and finally for the roof/attic, I was planning on somehow getting a flexible cover up through the high-hat light holes to protect the lighting, and then filling the whole cavity with cellulose. My thoughts are: that’d be easier than spraying foam, as it would distribute itself better … and I want the insulation against the ceiling, rather than the roof, as I wouldn’t want the heat to be above the ceiling.

4) in terms of actually heating it, I was thinking of just an electric heater – maybe blowing from under a bench, but this is south facing, and I’m hoping will have excellent insulation properties (including lots of fixed glass), so hoping for some significant solar gain (maybe even incorporating a Trombe wall?)

Any advice and thoughts greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    J. Star,
    Your post is confusing, but I think I finally get it.

    You have an existing open porch that you intend to enclose by building insulated walls that "fill in the space between the columns." Once you are done enclosing this porch, it will become a conditioned room, fully insulated. Is that what's going on?

  2. nycjstar | | #2

    Correct. Only one wall need be built (the south facing), I believe, as I'd use the outside walls of the house and garage for the other 3 walls (unless there is a downside to using an insulated wall in what will become the interior of this space).

  3. nycjstar | | #3

    To be clear, the outside walls of the house and garage ARE the current walls of the porch. (3 sides only).

  4. GBA Editor
    Martin Holladay | | #4

    Normally, any conditioned room needs insulation on all sides. An uninsulated slab is not a good idea for a conditioned room. But it's your house, and how far to go with the insulation depends on how you intend to use the room. Will it be heated all winter long? Or only used occasionally?

    If the slab protrudes beyond the walls, you're right that the best way to insulate the floor is by installing a continuous layer of rigid foam above the slab, followed by a plywood or OSB subfloor.

    For the ceiling, the best solution is to remove all of the recessed lights and throw them in the dumpster. Choose a different type of lighting, and install an airtight ceiling. Once you do that, you can blow deep insulation above the ceiling.

  5. nycjstar | | #5

    found a pic here ...

  6. nycjstar | | #6

    Looks like the pic didn't come through. What about that thermal bridge of the stone coming around the corners and back into the porch ... do I need to cut that off and seal to whatever is behind it? (if anything). Or can I build up against it, with that thermal bridge carryiing through the wall?
    As per the ceiling, it's a nice beadboard ceiling that I'd like to avoid disturbing if possible.

  7. nycjstar | | #7


  8. GBA Editor
    Martin Holladay | | #8

    Q. "What about that thermal bridge of the stone coming around the corners and back into the porch?"

    A. Well, do you want to do this project the right way or the easy way? It's your house. The right way is to rent a gasoline-powered masonry saw -- looks like a monster chainsaw -- and cut through the stone cladding where the new wall intersects the house. A fun job!

  9. nycjstar | | #9

    OK understood. How big a deal to you think it is? I.E. if I don't cut out the stone rail, would it be equivalent to using, say R4 windows instead of R5? Or presumably much less of a difference, since it's only a little bit of horizontal exposure/bridging ... What would you suggest? (Considering you never know what other problems it could open up, expense, etc.)

  10. GBA Editor
    Martin Holladay | | #10

    The photo shows what looks like a nice house in a nice neighborhood. If you do a sloppy job of enclosing your porch, the house will look like a trailer, and the value of the house will drop.

    Do it right, or hire a contractor who owns the necessary tools.

  11. nycjstar | | #11

    Thanks for your advice. I would think most homeowners posting on this site are learning more about how to do their projects the right way than the contractors would know themselves ... and while I won't do the work myself, I want to be able to spec out exactly how it should be done to the contractor - so that it gets done right (or at least rightish). (would you think XPS, EPS, or Poly-ISO for the floor insulation? - this must be an age-old question by now! Cost is not an issue since it's only 150sf)

  12. GBA Editor
    Martin Holladay | | #12

    Green builders don't use XPS, which is manufactured with a blowing agent that has a high global warming potential. More information here: Choosing Rigid Foam.

    Since a slab might be damp, EPS would be the best choice here. Polyiso shouldn't be used in locations where it might get damp.

  13. user-2310254 | | #13


    Have you checked your local zoning? You may need an approval to enclosure your front porch.

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