Building efficiently at the start reduces costs later

Building a new home comes with an array of choices — and, in some cases, compromises between couples that result in multiple heating systems — but it’s what’s behind the drywall and the siding that influences future comfort and financial well-being the most, according to Efficiency Vermont.

Builder Brook Longstaff of Morristown invited people into his home on Cote Hill to see firsthand how an energy-efficient home is constructed.

The goal is to make sure that your thermal shell is as complete as possible, because any spot that’s missed could become a problem later on — that includes windows and doors.

The thermal shell, also called the building envelope, consists of four layers: insulation, air control such as drywall, vapor control such as paint on interior walls or a paper facing on insulation, and bulk water control behind siding.

But the process really starts with the concrete slab.

“It’s best to insulate under the slab before pouring the concrete. That’s one thing you can’t fix later to make your house more efficient,” said Longstaff, who poured a 6-inch slab insulated with foam and piped with radiant heat on which to construct his house.

For homes with a basement, at least an inch of foam around a basement wall will prevent moisture from seeping through and lowering the dew point.

When construction starts on a house, a barrier is needed between the concrete and wood to reduce air leaks.

Longstaff typically uses sill seal, a ribbed polyethylene foam, but because it has a tendency to shift around while the wood’s being placed, he recommends also caulking with MasterSeal NP 1— a polyurethane sealant.

“It’s a great air sealer, but can be pricey,” Longstaff said. “It might look like just a tiny crack between the materials, and you think it’s not a big deal, but around the entire building, that’s a lot of surface area potentially leaking air.”

The R-value

The band joist, a board that caps off the floor joists and forms the outer box of the floor assembly, needs to be air-sealed as well to prevent both wasted energy and costly moisture problems.

A lot of people just stuff fiberglass into the cavity, but it should be treated like any outer wall, using a minimum thermal resistance of R20. An insulating material’s resistance to conductive heat flow is rated in terms of its thermal resistance, or R-value. The higher the R-value, the better it will insulate. The R-value depends on the type of insulation, its thickness and its density — in Vermont, it is recommended to insulate up to R40 for the cold winters.

For outer wall insulation, Longstaff is using Roxul, a mineral-based insulation.

It’s a modern replacement for fiberglass and is better for fire suppression, he said.

Inside the walls, the Cote Hill home will have a sheetrock air barrier; on the outside, he’s added a 4-inch-thick foam board on every wall. That board brings the thermal resistance of the building higher than the recommended minimum.

The next thing to consider is windows. Fewer windows mean better heat retention in the winter and cold retention in the summer, because glass has high conductivity.

Heavyweight windows

If you choose to have a lot of windows or large windows, as Longstaff did, it’s best to aim for triple-glass and avoid double-hung windows.

The architect on the project, Harry Hunt, recommended tilt-turn windows. They swing in like a door or tilt the top of the sash down. They are triple-sealed, protected from the elements and offer more ventilation than traditional windows.

The problem is that they are not yet widely available in the United States, and can be expensive.

There are also two things to think about when choosing tilt-turn windows with triple-panes, Longstaff said: They’ll weigh about 10 pounds per square foot, so the frame underneath needs to be strong enough to support them all, and shipping will take a while, because they generally come from Europe.

All windows should be taped around outside and inside to air-seal them, as well.

Windows and walls are the easiest to deal with when it comes to energy efficiency.

Another common problem is the attic. Rather than the usual flat insulation, Longstaff and Hunt used spray foam insulation in between the rafters — offering a tighter fit against studs — as well as 4-inch foam board.

“It’s hard to get enough insulation with just foam board at R5 per inch,” so the spray foam helps to increase the thermal resistance, Longstaff said.

To complete the air-sealed shell, Longstaff and Hunt recommend 3.5-inch poly-iso insulation under the roofing.

What about the heat?

With the thermal shell complete, the heat loads should be low, requiring about 30,000 BTUs of thermal energy per hour. Most heating systems put out 60,000 BTUs.

Longstaff opted for an electric heat pump unit. There aren’t many heat pump units right now that can completely handle the cold Vermont climate. They become less efficient as the temperature drops, and many heat pumps currently on the market shut off at 15 degrees below zero, so it’s best to have a secondary heat source as a backup.

Efficiency Vermont has a list on its website of the best units for Vermont’s climate.

Because Longstaff and his wife couldn’t agree on an alternative heat source, the couple’s home will have a fireplace in the living room and radiant flooring.

“People sometimes have a hard time understanding just how much you can downsize the heating source when you build like this,” Hunt said. “They don’t really need the radiant heat in the slab,” on top of the heat pump and fireplace.

At this point, hot water becomes the largest energy load for the house, on a propane boiler. If the storage tank has a hot water loop with a smaller pipe to reduce the amount of cold water that has to be bled from the lines before the hot comes through, the cost of heating water can be reduced as well.

Hunt and Longstaff agree: Building an energy-efficient home isn’t cheap at the beginning, but it doesn’t take long to recoup the money that would have been spent on heat with a house that leaks a lot of air.