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Community and Q&A

Building it backwards

Morrill11 | Posted in Green Building Techniques on

This rambling old farmhouse in rural Maine has an attached 3-bay garage that connects to the kitchen. The garage has a two-bedroom, seasonal or guest apartment overhead, so it is a large structure, with 2X4 studs.

As we age we have wanted to find a way to have all of our living needs on the first floor level of this house and have decided the best way to add a master bedroom/bathroom/sitting room suite is to utilize the garage. This will require building a garage onto the side of the existing one, which is an easy and logical way to preserve vehicle storage space without building an entire new structure.

The issue we have is finishing the garage structure for living. Here are the issues:

1. The garage is 2X4 framed with the exterior finished by tongue and groove 1″ profiled boards directly applied to the studs. The first problem will be how to seal the exterior wall cavity for insulation and air/draft sealing.

2. The floor of the garage will need to be raised 8″ in order to mate properly with the existing finished floor height of the adjoining kitchen. The garage floor is concrete, presumed to be merely a poured floor over gravel. The floor/framing assembly will need to be insulated in some manner from thermal bridging and against heat loss. It has been suggested that a first layer of rigid foam be applied prior to framing? comments?

I am thinking now about using mineral wool in the stud cavities, a smart vapor barrier inside of this along with a layer of Insofast. Finally, after sheetrock, a high-perm rated interior paint to finish the wall system. Comments?

The ceiling above will arguably be mineral wool also, although I am wondering if 8″ of fibreglass bats might serve as well given that the second floor apartment will remain in place. Comments?

I have also considered building an additional wall system inside of the 2X4, using 2X3’s which would enable a complete thermal break from the exterior studs. This is quite a lot more work and expense although since we are very much in the theoretical stage of this project, exploring all the options at this point is time well spent. There is only one window in the garage that I do not wish to move, for what that is worth.

Any observations or comments are much appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "The first problem will be how to seal the exterior wall cavity for insulation and air/draft sealing."

    A. The easiest approach is to use the existing layer of 1-inch T&G boards as your sheathing. If you install taped housewrap or a European membrane on the exterior of these boards, followed by either a rainscreen gap plus siding, or (to save money) siding without a rainscreen gap, you will go a long ways toward addressing air leakage through your walls.

    Q. "The floor of the garage will need to be raised 8 inches."

    A. I suggest that you install rigid foam topped by either a 4-inch concrete slab or two layers of 3/4-inch T&G plywood with staggered seams (screwed together). The thickness of your rigid foam will depend on whether you go with concrete or plywood. If you don't want to make the foam that thick, you can substitute a layer of sand or crushed stone for some of the rigid foam.

  2. Morrill11 | | #2

    Good thoughts. My initial reaction is that this does two things that are really unacceptable for this project. First of all, the re-siding the building will completely alter the nature of the building and be very expensive. This would also radically change the appearance of the building also, which is not going to be acceptable.

    The idea of using rigid foam as a base prior to installing a grid has been mentioned before. I wonder what sort of compression the foam would develop once the grid, subfloor and walls are constructed and if this would pose some sort of problem later on.

    I never considered pouring an additional layer of concrete and want to think about that a bit. I guess. The most immediate reaction I had to this was the cost of the concrete and whether that additional pour over old concrete would work well over time. Perhaps the biggest question is how thick a layer of insulation is necessary in order to make a true thermal break from the cold concrete floor?

  3. GBA Editor
    Martin Holladay | | #3

    You have a garage with walls that are covered with boards but no siding. You are worried about air leakage, and I suggested that you add housewrap and siding over the boards installed on your garage wall. You answered that the work would be very expensive.

    Well, if you are trying to convert a garage without any siding to living space, you need to be able to afford the work. The choice is yours. Garage walls are leaky -- especially if the garage has no siding. If you want to turn it into a house, you shouldn't be surprised that the walls need to be detailed like the walls of a house.

    I can assure you that rigid foam has enough compressive strength to be installed under your new floor, whether you choose to cover the rigid foam with a new concrete slab or with a plywood subfloor.

  4. Morrill11 | | #4

    Martin: You misunderstood me. The exterior walls of the garage are properly milled, exterior tongue and groove boards that are profiled to be exterior finish surface. It seems you assume the exterior surface is merely boarding boards, which is not at all the case. The existing siding was installed directly over the studs to provide a true, "finished" appearance once the siding was properly finished. I had considered removing them, resheathing and then reinstalling the siding, but I doubt that is even feasible.

  5. GBA Editor
    Martin Holladay | | #5

    It was very common from 1900 to 1950 to install T&G boards or shiplap boards directly to studs. In those days, this was called sheathing. In most cases (except on some garages), these sheathing boards were covered with a layer of asphalt felt and siding.

    I understand that your garage has nice looking boards. But from a functional (rather than an aesthetic) standpoint, they are still just plain old boards nailed directly to the studs. In my book, they are just sheathing. They don't stop rain and they don't stop the wind.

  6. Expert Member
    Dana Dorsett | | #6

    To get there without adding to your exterior t&g you need to make an air-barrier in each stud bay, but with an air-gap between the air barrier and the t & g. With reasonable roof overhangs (= 1 foot per story of building height) that air gap could be as thin as 1/4", but it's probably easier to do it at 1/2".

    Using 2" wide strips of 1/2" rigid foam tacked to the sheathing/siding on either side of the stud, and at the top & bottom plates to establish the space, cut'n'cobble some 1" rigid unfaced EPS with a loose fit (~1/4"of slop on sides bottom & top) and glue it to the spacers with foam-board construction adhesive, then seal the gap between the foam & stud with can-foam.

    In a 2x4 cavity that now leaves you with ~2" of space for more insulation. Filling it with fiber or foam won't get you to code min by any means, but you COULD add some lateral 2x4s "Mooney Wall" style (see: ) or a non-structural studwall (staggered with the structural studs) to bring the total depth up to 5.5", and with less thermal bridging than simply sistering on some studs. If this is going to be a DIY batt job the better bang/buck is probably to buy a loss-leader "contractor roll" of R19 batts that you split into 3" thick low density batts, to be compressed into the 2" deep space left on the exterior framing, then install R15 rock wool in the new interior side framing cavities.

    If you go with that approach, though I'm loathe to recommend the use of 6-mil poly in most stackups, a layer of poly detailed as an air-barrier (using acoustic sealant caulk to ensure air-tightness wherever it meets the framing) works in a central ME climate, since it puts the vapor barrier at about the mid-point of the center-cavity R, where the average mid-winter temp at the poly should be above the dew point of the interior conditioned space air, and during the cooling season it stay above the dew point of the exterior air even on muggy days. It should be easier to make the poly air-tight than the interior side, and as long as there is at least one air-tight layer between the interior and the t & g sheathing it will be fairly well protected from wintertime moisture drives.

    The stackup as-described has a whole-wall R (with all thermal bridging accounted for) of about R17.5 if you go with the extended studs approach, but nearly R20 if you go Mooney-Wall or staggered stud. IRC 2012 code-min for climate zone 6 is R20 + 5, which comes in at about R18-20 whole-wall (depending on actual framing fraction), so this would be "in the range". Adding only a 2x3 on the interior wouldn't quite get you there, and you be splitting and wasting a lot of rock wool (or compressing R13 fiberglass) to get a reasonable density for the 2x3 cavities.

    At 1" thickness Type-II EPS (1lb per cubic foot density) runs about 5 perms, Type-II runs about 3 perms, which is roughly the range of standard latex interior paint- which should plenty of drying capacity for the exterior half of the stackup. (Half inch OSB or CDX ply is barely more than 1-perm, which is part of the problem.)

    If it isn't already, put a layer of acrylic-latex primer paint on the interior side of the t & g before you insulate, which will even out it's drying rates which keeps it from cupping/warping, and will extend the lifespan of the exterior paint.

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