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Converting old outbuilding to heated workshop

A. Bradford | Posted in Energy Efficiency and Durability on

I have a wooden garage, built in 1929 in coastal Maine, in remarkably good shape. It’s got a post and pier foundation and had the front ripped off to convert to a 2 car garage in the 90’s. I plan to reconvert it to it’s original intent as a workspace. Like most of these old outbuildings in our neighborhood, the lawn has grown up to the base of the siding (or maybe they were intended that way) and there is a major negative grade inside, kind of like an earthen crawlspace foundation. When I started digging around and found this out, I got very worried about rot. Turns out after digging far enough to really see under that the structure is in amazing condition underneath! The only rot we have is where a concrete slab was poured at the front to accommodate the drive up to the garage door. The slab was poured against structural posts and those rotted. I assume this situation has worked out ok because any moisture that accumulates underneath can breath through the perimeter and up through the uninsulated and unheated space.
My concern is this. I intend to insulate the interior space. This won’t be a rough workshop. I make musical instruments and need fairly consistent conditioning. I also cannot afford to lift the building and pour a new foundation. If I insulate while leaving the underside to breath, could I cause a moisture trap inside the building? Or is it better to excavate the sides for better airflow underneath, then air seal and insulate the floor?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    A. Bradford,
    Your description is unclear.

    1. Does the building have a wood floor or a dirt floor?

    2. If the building has a wood floor, I assume that there is a dirt-floored crawl space under the wood floor. Is that correct?

    3. What type of floor is there on the part of the building that was converted to a garage?

    4. Are the piers of the pier foundation made of concrete or wood? If wood, what species?

    5. You mentioned that the grade is up to the siding. What is behind this high-grade situation? Boards? Concrete? A continuous dirt floor joining the indoors to the outdoors?

    6. How low does the wall sheathing go? How does the wall sheathing made a transition to the foundation or to the grade?

  2. A. Bradford | | #2

    I was worried my description was insufficient. I would take pictures, but the snow is still heaped up around the whole thing, despite it being the first day of spring. Maybe I should've waited to post when I can take pictures...

    1. The building has a pine floor. Below this is a pine subfloor. Below this are 2x6 joists. Below this is a beam perimeter plus a central beam.

    2. There is a dirt crawl space below the floor. I am not sure there's enough space for a human under there. Maybe an extremely skinny and adventurous one.

    3. The whole building is still wood floor. The front side which was opened to accommodate the garage door has a slab ramp. Whoever did it poured it over the rim joist but no further. Vehicles have parked in there for a few decades, with no sign of deformation of the structure. This leads me to believe the posts, piers, and structural beams are well settled and stable. This is part of why I would like to leave the foundation the way it is.

    4. From what I can see, there are concrete pads that come up to a little above the grade of the dirt crawl space, with wood posts above that, about 4x4 equivalent. There are lots of them, maybe 16. The space is 20'x24', by the way. None of these concrete pads are any higher than exterior grade. In fact, most are slightly below it. I have this vision of the whole building being jacked up and the piers getting an extension to relieve the sunk-in sensation.

    5. The siding is clapboard all the way to the base, with pine board sheathing behind. In some cases, like the back of the building, the siding is up to a foot below grade. I've dug away some of what I can there, and the siding is remarkably low in rot. The point where the lawn meets the building, the interior crawl space drops below this. So if the building was originally intended to be build on or above grade, the decades have built up a little height around it, so the underside of the building is now the lowest point. The crawlspace goes from less than a foot below grade in the front, to maybe up to 2 feet below grade in the back.

    6. Wall sheathing and clapboard end together at the equivalent of the rim joist, or the base beam. The ground is a few inches below that at the edge.

    So if I weren't trying to do something to the space, then I wouldn't even know any of this information. I found out by digging. From the outside, it all looks lovely and continuous. Green grass goes right up to clapboard siding and it looks like a lovely old New England barn scene. I've already committed myself seeing this project out, as I ambitiously began by removing most of the concrete ramp leading to the garage door.

  3. GBA Editor
    Martin Holladay | | #3

    A. Bradford,
    If you want to insulate this building, the right way to proceed is to jack it up one or two feet, to get the joists above grade. But you tell us that you can't afford to do that. (By the way, the work may not be as complicated as you think. You'd have to start by excavating under the building and lowering the crawl space floor.)

    If you don't want to jack up the building, you should (if possible) lower the grade around the building, and adjust the grade so that it slopes away from the building in all directions.

    Ideally, you would gain access to the crawl space and lower the dirt floor enough to work down there.

    If none of this is possible, and you still want to invest money in the building, you have to be aware of the risks -- namely, that the lowest wooden components of the building will rot.

    Without any excavation or jacking, you would need to install floor insulation above the existing pine flooring -- for example, by installing 4 inches of rigid foam on top of the pine flooring, followed by 3/4-inch plywood, secured to the pine floor with long screws.

  4. A. Bradford | | #4

    I should have clarified. I can't afford to convert the foundation to a slab. I am pretty sure of that, at least. My main asset is sweat equity. We call it yankee ingenuity up here. I just don't want to be the yankee that leaves it worse than when I started.
    So it's fairly uncomplicated to lift and raise the pier system? Would you accomplish this by excavating the crawl space to further reveal the low concrete piers, then lift the building and support the additional height with wood posts? Or is it possible or preferred to add height to the old concrete another way? Internet searches on this have been fruitless. If I am bothering to lift the building, I want to do it in a way that lasts. I have the added complication that the exterior grade at the rear of the building gets much too high to ever be correct, as it is the neighbor's yard. I will have to approach the rear more creatively, with an eventual retaining wall. All in due time.
    Although I understand it's not ideal and more risky, have there been cases where it works to place rigid above the floor joists? Regardless of whether this is installed above or below the joists, what is the best way to deal with the primitive crawl space below? I've read about creating conditioned crawl spaces, unvented crawl spaces, etc. Some put vapor barriers on the ground. Others spray plastic foam all over the place. Is there any advantage to just leaving it be, and making sure enough area is graded down to let air through? The crawl space, it seems, will always be the lowest point, and therefore susceptible to moisture accumulation. Does excavating it further mean I need to treat the perimeter with some kind of barrier?

  5. GBA Editor
    Martin Holladay | | #5

    A. Bradford,
    To jack up the building, you need to have several screw jacks, some sturdy beams -- 6x6s or 8x8s -- and some cribbing (4x4s or 6x6s). But if you aren't sure where to start, it might be safer to hire someone to do the work for you.

    There is no advantage to leaving the crawl space the way it is except for saving money.

    It's possible (since you are in Maine) that your soil is sandy and very well drained. If so, you can do nothing about the crawl space if you want -- as long as you are willing to accept the associated risks.

  6. Rick Van Handel | | #6

    I like Martin's idea to jack it up. As he said, the process is simple. Is there a company near you that moves houses? If so, they would be a prime candidate since they 'll already have the jacks and crib work. If not, try to rent some jacks from a rental house. Odds are your building isn't attached to the foundation. If u have plenty of wooden piers you may not even need cribbing. What size is your building? Install your beams to span as many floor joists as possible and then start to evenly jack up the building. As you raise, start placing SOLID concrete blocks at your existing concrete piers. A standard big box store should have 4" and 8" thick block. Thinner blocks are probably available from a block supplier or special order (they're used for blocking mobile homes). Worst case scenario you could use treated wood shims. Anyways , keep jacking and adding the block and shims. When your building is at the height u need, slowly lower it back down . If u need additional height u need to lower your jacks, add blocking under them and keep jacking.

  7. KEVIN ZORSKI | | #7

    Bradford - If the perimeter beams and central beam are substantial enough, as you jack up you could place wooden blocks or cribbing as Martin is suggesting, but you may be able to get away with this cribbing every 8 or 10 feet on the perimeter,and maybe 1 or 2 on the central beam. You could raise each piece of the building 6 inches or so at a time. It takes time, but that is what you have. So you may end up with 12 cribs, made of 6 by 6's, and you go around with your jack(s) raising 6" at each spot. But when you arrive at your final desired height, I would pour concrete into sonotubes cut to the desired height or slightly less than each set of the cribbing. This way you have one post at each point that is solid, substantial, and less likely to shift when you add weight into the building and it takes on wind and snow loads. Then you roll your custom height concrete piers into position, lower the building onto them and shim to level. If you can get old pieces of slate roofing, they make excellent shims, if water is not allowed to collect between them too much. You could also use pressure treated 6 by 6's or 8 by 8's as your new piers, with cedar shims to level. You want the structure to protect the piers - not allowing water to pool between the piers and the perimeter beams. Instead of cribbing, you also temporarily slowly build up with concrete blocks, as Rick suggests.I recently moved a 16 by 16 shed and raised and leveled it. I used just a 2 ton hydraulic auto bottle jack. Once you have 18"to24", you can insulate, covering the bottom of the insulation with something vermin proof ( cement board or heavy screening) as the mice and other small critters will want a winter refuge in your warm insulation. Best of luck to you. You can do this !

  8. A. Bradford | | #8

    Thanks for the support. I an finding a way to get the building jacked and extend the piers with poured concrete. Sounds like the best way to insulate this type of building is to create a continuous layer of foam underneath, covered by cement board, once I have gained sufficient working room. Looking forward to this! Thanks again to all at GBA. This is a fabulous resource.

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