Heating and cooling account for about 60 to 80 percent of the average Minnesota household’s energy costs. In most cases, increasing the amount of insulation in a home can reduce these costs, but that’s only part of it. Improving a home’s energy efficiency involves an understanding of the “thermal envelope,” the barrier to heat loss and summer heat gain that protects and separates indoor living space from outdoor climate.

Insulation is the heart of the thermal envelope, but it’s part of an entire system including siding, sheathing, sheet rock and other materials. How well insulation works in resisting heat flow depends on where and how it is placed and on what, and how much material is used.

The type and amount of material used also affect how well the insulation works. Insulation material is rated by two measurements:

• Its R-value, or resistance to heat flow. The higher the R-value, the better the insulation reduces heat flow.
• Its density, or the consistent thickness of the insulating material.

Insulating the Millennium Star Home

Structural Insulated Panels
If you refer to Building the Shell on this website, you’ll recall that we used structural insulated panels (SIP) for some of the wall construction and a portion of the roof. SIP panels are comprised of rigid foam insulation sandwiched between two layers of oriented strand board. The oriented strand board provides the structural strength and no wall studs are used in the panels. The SIP’s we used in the walls are 4 x 8-feet, and six inches thick. The panels lock together giving the walls an R-Value of 23. At the highest point in the roof, we used 10-inch foam panels. It’s called a “hot roof” as it’s unventilated. SIP construction is energy efficient, strong, quiet and draft-free.

Wall Insulation
To insulate the stud type of wall construction in the other areas of the home, we used three different types of blown-in insulation. Benefits of blown-in insulation include filling the wall cavity100 percent and the ability to install a higher density material. Insulation density has a big affect on performance. A high-density insulation won’t lose its R-value when temperature drops. For example, a standard 6”, R-19 fiberglass batt that has low density, loses up to 50 percent of its insulating value when the temperature drops below zero degrees. Convective currents occur within the wall cavity because of the low density of the material. The extreme temperature difference between the inside wall surface and the exterior wall surface is what creates a subtle convective loop within the insulation. With blown-in fiber type of insulation, the higher the material density, the better the insulating quality.

In the majority of the walls, a dense pack cellulose was blown-in under pressure where a plastic vapor barrier was attached to the wall prior to insulating. Believe it or not, it’s made of a recycled newspaper product that’s been treated with a fire retardent.

In another wall section we used a blown-in fiberglass material that also completely fills the cavity. In this case, a netting material is stapled to the wall studs, supporting the material as it’s blown into the wall cavity.

Another insulating product used in the Millennium Star home is Icynene spray foam. It’s applied by spraying liquid components into the wall or stud cavity. The foam is inert (containing no formaldehyde) and will not bio degrade, shrink, sag, settle, slip, turn to dust or decline in R-value. Its R-value is 3.66 per inch.

Foam expands to 100 times its size in less than a minute completely filling the space to be insulated. After it sets, in about an hour, any excess material overflowing the cavity, can be easily shaved-off with a knife or saw and then reused in areas of an attic for insulation.

The following chart compares density:
Insulation Density – 6-inch wall cavity

Attic Insulation
In the other attic areas in the house, we used blown-in cellulose 16 inches deep giving the area an R-value of 50. An important energy detail with construction of an attic and roof is to use a “raised heel” or “energy truss.” The raised heel truss allows for full depth attic insulation all the way out to the perimeter of the attic. Careful attention to detail was paid to providing a solid wind block at the outer edge of the attic insulation to prevent air intrusion. In ventilated attics, typical soffit and ridge vents were used for ventilation.

Learn more about energy efficient insulation techniques: