Transfer the layout, and frame the openings a little bigger—just in case
We learned earlier from Ben and his crew how to frame basic exterior walls using 2x4s. It may have seemed like a low bar on the energy efficiency scale, but that is because the house will have 12-inch thick walls, using a double-wall approach.
When your walls are a foot thick, you don’t need to use 2x6s or exterior insulation.
Because. Your. Walls. Are. Twelve-inches. Thick.
(with almost zero thermal bridges)
With the outer walls framed, sheathed, stood, and capped with a roof, the crew can turn to framing the inside walls.
Transfer existing layout to new plates
Begin with plate stock butted to the outside wall.
"Butt the plates tight then I'll measure for a 12-inch wall thickness minus 3-1/2 for the intersecting plate, and mark my plate cut there."
Let’s take another look at what just happened.
Not like that.
He butts the outside sheathing and marks 12 inches. That’s how thick the cavity of the wall is. Next, he subtracts 3.5 inches, for the perpendicular framing.
This is where he will cut the plates off, so the interior walls are not touching the exterior walls.
Ben: "Transfer your layout from the existing layout."
Copy the wall layout on the inner plates including windows and doors.
Break the ends on studs, just like regular wall framing.
These hieroglyphics contain a lot of information. The marks begin at the correct line and cross over the lines to be ignored and indicates what side of the line is waste.
Fill in the rough openings after standing the walls
They frame and stand the walls to be ¾ inch shorter than the height of the ceiling. That way they can fill in above with a piece of 1x stock for friction-assisted wall plumbing.
They transfer layout marks from the plates to the headers and sills and fill in the window and door openings.
To reduce alignment problems, they frame the interior opening to be slightly larger than the exterior one. The header on this door is ¼ inch higher than the header in the exterior wall.
Bridge window openings with subflooring
With the window and door openings framed in, it is time to install window bucks to bridge the opening. Ben uses sheets of ¾ inch subfloor ripped to just under 12 inches, again, to keep away from discrepancies in the framing.
Obviously, the rough openings need to be framed an inch and a half wider than the window schedule calls for if you are going to buck out the openings with ¾ inch subflooring.
Rather than wrestling large single sheets through a portable tablesaw, he’s going to use a track saw to cut two sheets at once. So after flushing up a couple of sheets, he clamps them together with eight-penny clamps.
I can’t really see from here what number he’s marking, but I’ll bet it is 11-3/4.
Combined with the blade thickness of a few cuts, and the size of the gap that's engineered into these sheets, he should end with a whisper of a rip at the end, mostly cutting off the tongue.
Align the edge of the track to the marks, plop the saw in the track and cut.
Looks like it was 11-3/4.
He cuts the sides, tops, and bottoms to length using blocks placed to support the stock or let it drop.
Inside, he places the bottom buck first, but first, he whacks the framing a bit just to remind it who is boss.
The bucks are fastened to the framing of the exterior wall.
They are also aligned with the outer edge of the framing, which means the inside edge is inset ¼ of an inch from the face of the framing.
"The total wall thickness is 12-inches, so I ripped them a quarter inch shy because if we have any bow in our wall, I don't want them protruding past where I'll have to come back and either sand or cut them off.
There is nothing critical about the inside edge, nothing references off of it."
We'll set the bucks, waterproof them, and then later, after a week or two, after everything's had a chance to acclimate, before we go to set the windows, we'll shim the openings so that we know they're dead true, square, plumb.
It's not super critical, but, that's the game plan."
With the bottom and top bucks in place, he slips in the sides.
The critical edge on the window bucks is not the inner one, as Ben noted earlier, nothing references from it, The critical edge is the outer one, snugging up against the wall sheathing.
Cut the openings with a router
Now all that’s left to do it cut out the sheathing. Rather than blind-cutting with a wormdrive, Ben uses a router and flush-cutting bit.
He drills a location hole for the router pokes the bit through the hole, and moves down until he hits the bottom buck. Now he glides the router around the window openings until he gets back where he started.
After cleaning up the edges, he will be ready to apply some liquid flashing to the window openings.
—This is the fifth part of a seven-part series on framing, flashing, and sealing a double-wall house.