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Community and Q&A

Insulating a “3-Season Room” or Sunroom in the Mid-Atlantic

corsulian | Posted in Energy Efficiency and Durability on

My house was built in 1955 with a north-side carport (driveway extended under, roof extended over – including attic with loose-fill insulation, supported by four load-bearing brick columns presumably with footers below) – in the 70s, it was closed in (2×4 framing, plywood, vinyl siding, and jalousie windows. It was ducted for heat and air conditioning although the ducts remain closed. Since we moved in, we replaced the windows with nicer/newer/normal vinyl, and we replaced our HVAC and had them size it such that it would support the sunroom if it were ever insulated/sealed.

My loose plan has been to replace the two doors which are just simple unsealed storm doors, insulate the open wall cavities (the plywood is only on the exterior side of the framing) with either XPS or Roxul Mineral Wool, remove/replace the vinyl siding after adding house wrap beneath it (since they omitted that step), and I hadn’t thought of the floor beyond putting down dri-core as a subfloor with tile over that.

Are these reasonable plans for making this formerly 1-season room 4-season livable? It’s hard to find direction for this eventual project for a house in the Mid-Atlantic. Especially for the floor which is essentially just the driveway with a few coats of epoxy paint.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, it's rare to have a sunroom on the north side of a house, unless you live in Australia, New Zealand, or South Africa. The north side is the shady side.

    Second, if the floor of this room consists of an asphalt driveway or a concrete driveway that is continuous with an asphalt driveway or a concrete driveway on the exterior of the room, then you really can't use it as a foundation or a floor. You have to demolish the floor and install a real floor.

  2. Expert Member
    Dana Dorsett | | #2

    Alternatively, cutting the asphalt/concrete slab and installing R10-R12 EPS down to a 2' depth below grade along the perimeter would bring it up to IRC code-min for slab-on grade in zones 4 & 5, which would include almost all mid-Atlantic locations. Since it's easier to dig the insulation trench with a 4' wide slab cut, you might as well go for R16- 4" of EPS in two layers, with the seams staggered.

    With a sheet of 6-mil poly between 1-1.5" EPS with 1/2" t & g OSB through screwed to the concrete for a subfloor would work for hardwood or laminate flooring, and is probably cheaper (and certainly more comfortatble than dricore + tile. Since the subfloor is supported fully with the slab & foam everywhere it doesn't need to be as thick as subflooring designed for 16"o.c. joists, and the compressive strength of the foam doesn't much matter, since the weight is distributed across wider surface areas by the foam. (It's common in some areas to install EPS between the bottoms of hot water tanks and the slab which is a much more concentrated weight than the dynamic loading of a Bohemian Polka dance group with a 1/2' subfloor to distribute the weight.) But depending on the flooring type you may have to upgrade to 3/4" goods for fastener retention purposes.

    Since he's peeling the vinyl siding, installing 1" of exterior polyiso (far greener than 1" XPS- usually cheaper, and easier to air-seal to boot), installing carefully fitted R15 batts in the 2x4 cavities would bring the walls fully up to code. Code-min is R13 cavity + R5 continuous- you'd beat that with a bit of margin.

  3. corsulian | | #3

    I see. I appreciate the responses - although both remind me of a nice quote from a friend who is in the trades: "the BEST way to approach any problem in your house is to build a NEW house."

    Is the primary reasoning against a floating floor on the concrete (moisture barrier, wood frame, insulation, plywood) that seasonal heaving may take place?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Is the primary reasoning against a floating floor on the concrete (moisture barrier, wood frame, insulation, plywood) that seasonal heaving may take place?"

    A. No. The primary reason is that when rain and snow fall on the concrete driveway on the exterior side of your walls, the water will wick sideways into your room.

  5. Expert Member
    Dana Dorsett | | #5

    It's (nearly) impossible for a slab under a heated room to frost heave, since that requires that the slab (and the soil directly below the slab) to be cycling substantially below freezing, so that the direction of heat flow is from the ground-upward. Frost heaves propagate in the direction of heat flow.

    With a heated room above, even through R30 floor insulation the likelihood of a frost heave in a mid-Atlantic location the only way the slab to drop below freezing would be if it's leaking massive amounts of air between floor joists or under your foam. With R5-6 foam on the floor, it's never going that low...

    ...except possibly at the wall.

    If it hasn't frost-heaved while unheated, it's not likely to frost heave with the insulated floor. But whatever approach you take to the floor, the 2' depth slab-edge foam (which IS a code requirement unless you create a crawlspace, if that matters to you) pretty much GUARANTEES it won't heave.

    (It helps keep runnoff seepage or wicking from ending up in/under your floor too.)

  6. corsulian | | #6

    Thank you Martin - a break between the floor and the exposed driveway is rather an easier pursuit than excavating a room that has walls and such.

    Thank you Dana - that's somewhat reassuring and gels with the fact that there doesn't seem to have been any movement over the years. I do pay attention to code compliance and recently spent quite a lot on permits for an in-progress basement remodel - I just saw this as essentially an enclosed porch for which I'm seeking the most cost-effective insulation so that I could theoretically sit on a sofa out there in February. It's a long-term residence so I'm not too interested in how the city or any future real estate agent defines the space.

  7. Expert Member
    Dana Dorsett | | #7

    If you're heating it with the central heating equipment it's not just a closed-in porch- it's part of the fully conditioned space. It would also likely be treated as conditioned space from a property tax point of view.

    If you can find a local source for reclaimed roofing foam (there are several in my neighborhood) it's often cheaper R than even low-density batts. Search the local craigslist materials section for rigid insulation, or foam board, see what comes up. For the sub-grade stuff you can't use polyiso (yellowish off-white, sometimes with paper, foil or fiberglass facers) but EPS (the stuff of disposable hot beverage cups & cheap coolers, but higher density than that) , or XPS (usually has coloring- pink, blue, green or grey are the most common) are fine for any below-grade stuff. All are commonly used in commercial building roofing applications, and commonly salvaged during commercial demolition. It's typically 1/4-1/3 the cost of virgin stock goods when bought through salvage operators. Small timers often ask for more, sometimes less. But it's out there eg:

    Depending on just how thick or dense the driveway slab is you might be able to cut the edge trenches with a hammer-drill/roto-hammer on the exterior side of the wall. With Z-flashing separating any exterior framed-wall foam and your slab-edge foam you can end up with fairly low thermal bridging- just the Z-flashing, which also functions as a termite barrier (which is sometimes an issue with slab-edge foam.) Any exposed slab-edge foam can be finished with a cementicious purpose made product such as QuiKrete Foam Coating to protect the foam from UV. If the concrete is too dense you 'd have to cut it out with a diamond saw.

  8. corsulian | | #8

    Alright- I did have a city inspector come by and he said, since the structure already exists and is technically conditioned by having heating/air vents even if they are shut, that we can lay some 2x6 beams across the existing floor, insulate between, then have a standard plywood subfloor on that and then whatever we want. He said, if this was entirely new construction, they'd want us to do underpinnings and generally everything described in this thread but he seemed pleased that we even brought it up since we have a lot of older homes and older residents and they just build whatever they feel like.

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