The story and a half arrangement has been around for centuries. It may have originated as a sleeping loft under a thatched roof. The design appeal is simple. Space under the roof is used for living instead of storage. The shapes of these living spaces often take on interesting angles as they follow the rooflines. A story and a half design normally uses fewer materials than a similar two story plan.

The story and a half design also offers some unique energy saving opportunities and challenges. The details below show four examples.

Coved Ceilings

First, here’s the challenge. Inevitably, somewhere upstairs the ceiling finish will follow the roofline, leaving too little space for adequate insulation. High-density fiberglass batts fit between the rafters. Sheets of rigid foam insulation can be attached to the underside of the rafters to bring the total insulation up to at least R–38. Use a few long nails with washers to hold the insulation temporarily. Strips of six-inch aluminum flashing in the angled joints of the cove provide more secure backing for drywall screws. They also help support the foam until the drywall goes up. Be sure to tell the drywall hangers to bring long screws. If rigid insulation isn’t your first choice, you could attach furring strips to the rafters and create a cavity deep enough for batt insulation. In both cases, remember to leave an air space beneath the roof sheathing for ventilation as required by code.

Kneewalls

Super-insulated kneewalls are the biggest energy saving opportunity in the story and a half design. With almost unlimited space you could install R-38 or higher. Use lumber scraps or wood lath to build a support that extends to the desired depth. Thirteen inches between the support and the drywall is enough to hold two layers of standard density R-19 batts. That’s an R-38 wall. Blow-in-blanket contractors can hang their netting off a similar support structure to install BIBS in the kneewalls.

Flat Ceilings

It’s not difficult to achieve an R-49 ceiling in all the flat ceilings with batts or loose fill material.

Eaves

Here’s a detail that’s unusual for most story and a half designs. Install a full platform floor over the entire second story. It means extra rim joist material and several additional sheets of plywood. There are two energy saving results. First, the rim joist raises the rafters off the plate, making room for up to R-49 over the outside wall. From below, install R-30 high-density batts between the joists. R-19 standard-density batts lay on top to the floor sheathing. Placing insulation on the floor eliminates this as storage space, unless an additional platform is built.

The second advantage of the fully sheathed second story is air sealing. Plywood blocks air movement between the attic space and the floor cavity that runs through the house. Without the plywood, air could easily escape from every plumbing and electrical penetration between the first and second floors. If your energy saving plan calls for advanced air sealing the plywood is a key part of the continuous air barrier. An alternative would be to install solid blocking between the joists and caulk each block. 

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