Patrick’s Barn: Underslab Insulation
How not to estimate a construction project
A few years ago, I gave up telling people how long my building and remodeling projects would take to complete. It was just too demoralizing to think that even my most generous estimates would prove to be wildly unrealistic. This past weekend shows just how wrong I can be.
My plan for the three-day weekend was to level the gravel pad, install the sub-slab insulation, place and tie the rebar, and finally finish up the formwork. I even thought I might do all this work in a single day, maybe a day and a half. Unfortunately, I didn’t even get close to finishing, even while working 20+ hours over the three days.
My wife and I did make good progress, however. We started the weekend by leveling the gravel using a jig inspired by a Fine Woodworking article that showed how to make a Nakashima-style slab table. The woodworker used a big sled-mounted router to level the table top. I adapted the technique to level the machine-placed gravel so the slab would have a uniform four-inch thickness. Thanks to my wife, who helped me throughout this process, it only took a half day, but that was still twice as long as I had originally thought. The rest of the day was spent getting material for the following day’s work.
Sunday was spent placing the insulation and then cutting and fitting the foam around the 17 piers that make up the building’s foundation. Once again, I greatly underestimated how long this would take. I figured on a couple of hours, but it ended up taking almost all day. The next day I worked on placing and tying the rebar as best I could, but I was really beat from the day before and the weekend’s unusually hot and humid weather. When it came time in the early afternoon to go to a friend’s Memorial Day picnic, I was happy to throw in the towel.
You can read more about my barn here.
It should be easy to finish up the rebar and formwork this weekend, though. My plan is to pour the slab on Tuesday—I hope the weather is good the next few days.
Using 2x4s that rest on the foundation piers as guides, I made a screed to level the gravel pad. The gravel provides a sturdy base and capillary break under the future slab, which will be the finished floor in our new building.
My cordless circular saw has a dust port that matches the nozzle on my shop vac. As you can see, the rig did a really good job collecting the dust. After cutting, I disconnected the hose from the saw and sucked up the small amount remaining.
My little boy is generally a good sport about the construction going on around him, and we try to involve him whenever possible. When he's interested, I let him pull the trigger on my cordless drills and impact drivers while I aim and hold the tool. Setting a good example has made me very conscientious about donning my safety gear, and I always make him wear glasses and ear protection whenever he helps.
I thought that placing the insulation under the slab would be as fast as dealing cards, but I forgot about all the cutting and fitting around the many piers that form the building's foundation.
Even though some of my trusted friends told me that rebar was unnecessary in this application, I ignored them, deciding it was cheap insurance. If nothing else, it reduces a slab's tendency to curl, which is common when it's placed over foam.
It's easy to let big home projects become all-consuming, but I try to maintain some balance. Before our son was born, my wife and I would have likely worked until dark and ordered pizza for dinner. But stopping in time for a family cookout is much more important than meeting some arbitrary deadline.