Editor’s note: This is one in a series of blogs detailing the construction of a net-zero energy house in Point Roberts, Washington, by an owner/builder with relatively little building experience. You’ll find Matt Bath’s full blog, Saving Sustainably, here. If you want to follow project costs, you can keep an eye on a budget worksheet here.
After a couple weeks of frustration, I was rewarded with my second successful inspection today, giving me a green light to pour the concrete slab that will complete the foundation. It is hard to believe that at the next inspection, the house will be completely framed!
The frustrations I experienced over the last two weeks resulted from once again underestimating the fantastic power of concrete. I should have removed the stakes and forms for the stem walls as soon as 24 hours had passed, but I was nervous and decided to wait an extra day. When I finally got the nerve to pull the stakes they didn’t budge a millimeter. I had purchased a fancy stake puller called a JackJaw that my mentor had used to remove his form stakes, but I had decided to go for the $225 unit instead of the $450 one. The result was the rapid destruction of the tool.
Thankfully, JackJaw has outstanding customer service and offered to accept the unit back at a full price credit towards the more expensive one. After a week of waiting, it finally arrived, and though it was many times more powerful it was still a battle to get the stakes out. They had been setting in a layer of concrete about 16 inches thick in footings beneath the stem walls for more than a week.
With advice from customer service, I used a sledge to pound each stake in a few inches and then used the JackJaw to pull it up until it wouldn’t move any farther, then repeated the process. It was an agonizingly slow process, and I still have four stakes in the ground as I’m typing this, but that’s about 92 stakes less than I had in the ground a week ago.
With the stakes out, you might think it would be quite easy to pull off the form boards. The smooth plywood forms were still greasy from the last time they had been used so they didn’t adhere to the concrete. All the screws and stakes holding them together had been removed. Unfortunately, because of the way the concrete had curled up and around the bottom of the form boards, removing them was just as excruciatingly time-consuming as pulling out the stakes. Like running a marathon, it didn’t seem like a lot of fun while I was in the middle of it, but looking back on it the experience was much more rewarding this way and I surely have some additional muscle tone to show for it.
Adding mineral wool insulation
With the forms removed, I placed 2-inch thick Comfortboard 80 on the interior vertical face of the stem walls. Many people are puzzled by a layer of insulation sandwiched inside two pours of concrete, but it will serve two important purposes. It will protect the slab from frost heaves during the winter, and work in unison with the walls and roof to reduce the heating and cooling loads required to maintain a comfortable temperature in the house all year long.
Mineral wool insulation doesn’t have the disadvantages that some types of rigid foams have. It really is an incredible product. You can soak it down with a hose all you want, peel back the outer layer, and the inside will be bone dry.
I cut a 45-degree angle on the top edge of the insulation. This will allow for maximum insulation while also giving the slab a firm connection to the concrete.
More insulation will follow, but in the meantime, I capped off the entire DWV (drain, waste, vent) system and filled it with water. I added a 10-foot section of ABS pipe to one of the vents so I could show the required “10 foot head of pressure” for the inspector. The force of gravity on the column of water held in that pipe applies a force to the entire system; the code recognizes this test to show there are no leaks in the system.
A gravel capillary break
Capillary forces are very powerful. Have you ever been to a redwood forest and wondered how water gets from the roots to the leaves at the very top of the tree? The answer is capillary force, and surprisingly, it works even more efficiently in concrete than it does in trees! Scientists believe concrete has such powerful capillary force that it theoretically has the ability to drive water 6 miles upward against the force of gravity. Wood maxes out at about 400 feet, which is why you don’t see any trees taller than that.
In an effort to curb these powerful forces, building codes require that a layer of gravel and a vapor barrier be placed underneath a concrete slab. The gravel allows any standing water to drain away, and the vapor barrier takes care of any water vapor.
I added a gravel layer inside my stem walls (see Image #2 below). The long 2x12s you see in the photo act as barriers to prevent the gravel from falling into the “shovel footing” at the center of the slab. That is necessary to support the bearing wall that will soon be framed directly above it. Once all the gravel has been added and compacted, the boards will be removed and a 12-inch-wide and 8-inch-deep ditch will be left behind. It is much easier to create the ditch this way rather than by shoveling the gravel out.
When we pour the concrete for the slab, the concrete will flow into this ditch and the slab will be 8 inches thicker along that line, giving added support to the bearing wall. The entire roof of the house is supported on only two exterior walls, so none of the interior second-story walls have any weight to carry. I could have supported the weight of the floor joists between the first and second stories in the same way, but I would have had to use special engineered I-joists. Instead, it was much easier to use two lengths of 2×12 joists and have them meet on top of one of the first story interior walls. This wall is the bearing wall.
More insulation caps the gravel
After all of the gravel had been added, I leveled it out and installed the horizontal layer of mineral wool, as you can see in Image #3 below. The 2-inch-thick insulation extends inward from the stem walls a total of 2 feet. (The center of the slab is not insulated.) Keep in mind that the more insulation added now, the lower the heating bill will be in the future. Investing an extra $300 now will quickly pay off in a year or two, and then I will reap the benefits every year after that for the life of the house.
Once the insulation was added, I ran a plate compactor around everywhere to ensure the gravel was well compacted. Then I pulled out the long 2x12s to create the shovel footing.
The last steps before the pour were to add the vapor barrier and the rebar. The 15-mil vapor barrier comes in a large roll so it was simply a matter of rolling it out and cutting it to fit. Wherever a pipe penetrated the plastic I used vapor barrier Tuck Tape to seal the hole. I tucked all the edges of the barrier in between the two layers of insulation.
I lined up the rebar in a neat, 4-foot grid and set it on 2-inch-high chairs so it would end up right in the middle of the 4-inch slab (see photo at left).
And now the slab
A freshly poured slab is an incredibly beautiful thing. When the concrete first comes out of a truck it looks like a messy slop of oatmeal with blueberries in it (the blueberries being the gravel). The crew used a long, straight 2×4 as a screed and ran it back and forth to level the slop.
After about 20 to 30 minutes, the slop has started to gel a bit, and the gravel sinks down just a little. At this time, a long pole with a metal “float” attached is run across the surface (see Image #4 below). The float pushes the gravel further down and brings this very creamy part of the concrete up to the top. It is at this stage when the concrete really starts to take on a new personality, like an awkward teenager emerging from acne-filled puberty.
Another 30 to 45 minutes go by, and the concrete really starts to harden, but when you run a float over the top you can still get a tiny layer of cream to appear. You continue working this cream back and forth over the top, smoothing and smoothing until it, too, begins to harden. At this time, a strong force over a small area of concrete will create an indentation, but by kneeling on pieces of plywood, the pressure is applied over a larger surface area and the concrete doesn’t move. Using these “kneeboards,” you can get over to the center of the slab and work that as well without marring the surface.
I must admit that I had another learning experience during this pour as well: We ended up running out of concrete with a tiny corner of the slab left to go. I made a quick trip to the hardware store and snagged 13 bags of concrete. We mixed it up and finished the job. All said and done, you can’t tell the difference, and hopefully there will be no negative side effects down the road.
After the concrete hardened, I went out and sprayed it down with a fine mist every 15 minutes or so. While excess water added to the concrete before it has set will reduce its strength, when water is added after setting it slows the curing process and prevents cracking that can occur when the concrete sets too quickly.