This animation shows one way to create a thermal boundary within an attic surrounding a new living space—the hard way. You can also move the entire thermal boundary into the rafters, placing the knee walls within the thermal envelope. We'll cover that method next.
OFFICIAL TRANSCRIPT:
I just want to say that adding a riving knife to a tablesaw is a good way to prevent a kickback accident. And keeping your sandwich in the truck is a good way to prevent Ruff from co-opting it.
But that’s not what we’re talking about in this video. We’re talking about saving energy when adding living space in an attic.
Normally, insulation is on the attic floor, held in place with ventilation baffles that allow air to come in at the eaves so it can exit through a ridge vent.
If you are doing remodeling work, you’ll have to scoop the insulation out of the way so you can get some work done. But since we are building this house right now, the insulators are still a month away, giving us time to seal up the attic properly.
Define the thermal boundary of the attic living space
The tops of the walls are huge air leaks running the length of the wall. Wood framing shrinks back from the drywall and from framing connections. Unless you seal those gaps, air will whistle through them all day, every day.
The other thing we will do is install a series of rigid insulation blocks along the edge of the living space we are creating to define the thermal boundary.
We’re also going to replace those mini baffles with full-length baffles because the rafter cavities will get insulated in a little while. When we do that, we need to seal the baffles to the framing with canned foam and seal the framing gaps with sealant. In every rafter bay.
Now, we can plumb up from the foam blocks and add more foam blocks where the knee walls will be. Those blocks also need to be sealed to the baffles and framing.
Frame the attic living space
It’s a great idea to insulate below the floor for acoustical isolation. Now we can glue and screw the subfloor to the framing.
Frame the knee walls.
Insulate the knee walls
Seal the edge of the subfloor to the foam blocks and add a layer of foam to the outside of the knee wall. Tape the seam.
We can insulate the wall cavities now and add drywall to keep the insulation from falling out.
Seal the rigid foam board to the rafter blocks with canned foam, and you’re ready to set the insulators loose, filling the floor outside the living space, and rafter cavities above the living space.
This is one way to create a continuous thermal boundary to keep that living space livable.
But good luck with that sandwich struggle.
