Floor Values

Building a Passive House means you do things very strangely sometimes. In traditional “stick-built” construction, each floor gets built out to the structure perimeter, and exterior walls sit atop the floor. But this house has a double shell in which only the inner wall sits on the floor. The outer wall sits on the supporting joists; between the two is an insulation cavity.

We’ll eventually fill the entire insulation cavity with dense-packed cellulose. The insulation was delivered all at once shortly after we lifted the timber frame. We’re talking five hundred forty seven 25-pound bags of basically shredded newspaper, which took us three hours to unload from a 60-foot-long tractor trailer. We piled them high and then covered with tarps to keep rain off.


Ben stacks 25-pound bags of cellulose.

The crew used a renter blower to pack the first hundred or so bags of cellulose between the floor joists. They immediately covered it with Mento, which is not a breath mint but a hardy weather barrier. We rely on the Mento to protect the cellulose from moisture… this stuff does NOT want to get wet.


The floor system is filled with cellulose and sheathed in Mento. (Note the first floor footprint is double the size of the original timber frame.)

The floor is a tongue-and-groove material we installed on top of the Mento. First we installed long spacers to create a service cavity. Kelly led our installation of the floor itself – he works for Montpelier Construction and he’s had lots of experience in this vein. Kelly showed Ben and me how to screw diagonally through the tongue into each spacer and how to use a wedge (such as a chisel or cat’s paw) to force every row tight to the previous. As we pieced each 24-foot row together out of two or three lengths of flooring, we tried to keep the seams away from what will be high-traffic parts of the house.

The flooring varies a little in thickness, so we don’t have a very smooth surface right now. And with all the wall construction yet to come, we’re sure to walk all over it and scuff it up even more. Once things are protected, we’ll give the floor a good sanding to make it beautiful. (Also, it’s possible we will put a finish floor on top of the tongue-and-groove… I never asked Cillian his plans.) But right away, the finished floor enables us to lower the timber frame!


Floor is floored and timber frame is ready to come down. In the back there’s a wall under construction.

Hole Filling

The factory-second oak flooring we laid in Bob’s living room is full of holes. Or at least it was, until Mark and I filled them in with epoxy. The imperfections vary from pinholes to cracks to half-inch-wide knots… which look cool, but they pose practical problems when you’re walking on a floor every day and dragging furniture across it.

Our epoxy is a two-part solution, the resin and the hardener; when they combine they produce a heat reaction and harden uniformly (as opposed to glue which hardens from the outside in due to air exposure). We mixed a little at a time. The two bottles came with instructions to combine in perfectly equal measure, but the stuff clings to the side of any container so you really can’t achieve a precise measurement. Mark got as close as possible by comparing the remaining volume in each bottle.

Mark carefully mixes epoxy.

Mark carefully mixes epoxy.

Getting the epoxy to stay in the holes (without draining out the bottom) proved a major challenge. Our first idea was to stuff the holes tight with cedar shavings from the master closet, but the epoxy soaked right through. I then tried plugging the holes with clear-drying wood glue. That worked a little better, but in many cases the wood glue dripped through before it got tacky, just like the epoxy.

Hole-filling tactics on display. The light-colored stuff is wood putty.

Hole-filling tactics on display. The light-colored stuff is wood putty.

I had the most success spreading some fast-drying wood putty in the open spots and pouring epoxy over that. The epoxy needs 24 hours to become dry to the touch (and 72 hours to set up completely), so for a week I checked the floor every morning and puttied+epoxied all the holes that had drained. As of Friday I’m satisfied that I have plugged them all.

We want a smooth floor with no tripping or splintering hazards. To do that we need to buff out the surface epoxy so only the filled-in holes remain. Soon (perhaps next week) Mark will rent a floor sander and bring everything down to a uniform level.

Living room floor - the latest & greatest.

Living room floor – the latest & greatest.

Float On

Hans and I took a morning to install flooring in the guest bedroom. It’s a floating floor, which means it sits atop the subfloor with no staples, nails, or glue to hold it in place. Hans unrolled some foam underlayment to prevent squeaks and squawks, and then we were off to the races.

Our floorboards are tongue-and-groove wood with what looks like recycled plastic mixed in. They snap together seamlessly in both directions. It took some doing to get started – without fasteners, the first few planks floated all over the floor making it difficult to snap. Eventually Hans grabbed some 3/8-inch shims to space out our assembly from the walls (eventually ¾-inch baseboard will cover the gap), and that helped hold things in place; the increasing weight of the assembly helped, too.

Snapping in the most difficult floorboard.

Snapping in the most difficult floorboard.

The hardest part was fitting around the doorway. A wall slices halfway through this floorboard, and prevented me from sliding it in like I did elsewhere. I had to fit the tongue on an angle, rotate the piece into place, and then install the rest of that row in reverse. I wasted a floorboard trying to cut out both door jambs properly – my first cut left too large a gap, since baseboard won’t extend around the jambs. Once I got it right, the last couple rows of floorboards installed easily.

Hans proceeded to cut and install doorway trim followed by a threshold, which he’ll paint in place. Meanwhile I got on the roof to fit some window trim and replace some siding. I’ll leave that discussion for another day.

Hans faces the guest room doorway.

Hans faces the guest room doorway.