You can’t talk green building without talking about insulation: it plays a critical function in energy efficient design. Insulation acts like a blanket or wool sweater for a house: it slows down the gradual loss of heat through conduction, thus allowing you to enjoy the warm house you used all those fossil fuels to create for longer, before more fossil fuel input is required to bring up the temperature once again. In the summer, insulation helps keep heat out of your home.

How Much Insulation?

Just like putting on a thicker coat keeps you warmer than a thinner one, more (properly installed) insulation has greater energy efficiency than less insulation—so the answer to the question “how much insulation is enough” is something of a moving target. In the olden days, homes were often built without installing any insulation at all. In the past two decades, however, most state building codes began requiring a certain amount of insulation in the walls, ceiling or attic, and floor. In the past decade, the codes have become stricter and stricter, requiring more and more insulation.

The goal of an exceptionally green house—what we’ve dubbed a “Pretty Good House” in our conversation—should be to exceed code requirements. That’s the “super” in “super-insulation.” The trick is to arrive at an insulation balance somewhere between “more is always better” and common sense, based on the goal of the home: to have much lower energy bills than standard new construction, perhaps as much as 2/3s lower. In Green Building Advisor’s original conversation on the “Pretty Good House” the “R10-R20-R40-R60” rule was posited as best for cold climates. (More R-value means more insulation.)

This rule means going with R10 insulation values underneath slab foundations, (building code often requires none), R20 insulation values in foundation walls (building code requires R10-R15), R40 in exterior walls (building code requires R13 to R20, depending on your climate, with R13 being the maximum R-value you can get with batt of that “pink stuff” installed in a 2×4 wall), and R60 in the attic (building code requires R38 in most “mixed” climates).

In mixed-humid climates, experience has shown me that values can be relaxed some, and I propose that a “10-15-30-50” rule is a good starting point: a minimum value that can be achieved at a relatively good cost payback. If budget and space permits, higher values in exterior walls would still make good sense. Innovations like a double stud wall or a raised heel truss help increase the amount of space one has in which to fit all of this extra insulation.

The Solar Homestead, Appalachian State University’s 2011 entry into the US Solar Decathlon, used super-insulated walls as a strategy to help the home reach “net-zero” performance cost-effectively. (“Net-zero” means consuming very little energy and producing what energy is needed with on-site renewable energy for a “net” energy bill of zero.) Home is built with double 2×4 walls allowing 8″ of space for insulation, double the amount of space allowed for wall insulation in typical construction. While a “Pretty Good Home” is not necessarily aiming to be a “net-zero” home, super-insulation is still a core green design strategy.

Insulation Alone is Not Enough

Most insulation products are only as good as the wall or ceiling in which they are installed—and are designed to be installed uncompressed, evenly, and often in an enclosure that is air-tight. Cramming an R19 batt into space for an R13 batt does not equal R19 walls, and a pile of loose-fill cellulose insulation in your attic does nothing to stop heat loss from any sections that are uncovered. Paying attention to the proper installation for each type of insulation product is absolutely crucial.

Another aspect of the building envelope that is often forgotten in the quest for maximum R-values is the total air-tightness of the home—as warm air leaking out and cold air leaking in are an entirely different source of indoor temperature change that must be corrected with a heating or cooling system. Pretty Good Houses should be exceptionally air-tight, with any and all cracks and penetrations in the exterior of the building sealed. Yes, this kind of detail work takes extra time and attention, but is just as important, if not more so, than the R-values you choose to put into your super-insulation.

Next up in our southeastern climate “Core Green Design” series: “Right-sized” Homes and Outdoor Living Spaces

When building a new home, it pays off to think about energy efficiency, as many construction details done incorrectly or missed altogether can leave a lasting effect on a home’s energy consumption.   Yet armed with knowledge, some easy-to-come-by materials, and a little elbow grease, an existing home can be made more energy efficient.  Indeed, “weatherizing” a home has become a popular way to save money in the face of rising energy costs.  This fall, a handful of Deltec employees are getting together to tackle easy opportunities to weatherize each others’ homes.

Last Sunday we took on the home of our Director of Engineering, where, even having only one weekend morning to work, we found and corrected two all-too-common energy efficiency shortfalls:

1.  Leaky Band Joist

This basement, like many basements, is built of cinder block walls. The wooden components of the house above;  the floor joints and wall framing, sit directly on top of these concrete walls, often with nothing at all installed between wood and concrete.   This makes for a junction that is far from airtight, and in many places we could see light from outside through the gap between wood and cinder block.  Not only was this gap allowing cold air into the basement in the winter, making downstairs TV watching far more chilly than necessary, it was letting in that cold draft right underneath the floor of the house upstairs.  The homeowner had commented that the floors near outside walls sure were cold in winter!

The area where the wooden floor joists of a house rests on the cinder block walls of the foundation is called the “band joist.” In some cases, we could see daylight through the gap between the wood and the cinder block.

The fix was relatively simple: we sealed the gap between wood and concrete around the entire perimeter of the basement with silicone caulk.

2.  Leaking Air Ducts

Duct-work is designed to bring warmed or cooled air from your mechanical unit and into the living spaces of your home.  You would think duct-work would be extremely air-tight, the better to deliver as much warmth or coldness as it can.  In reality, most duct-work is anything but airtight, leaking as much as 25%–one quarter–of the air it carries.  Most of these leaks occur at joints in the sheet metal that make up ducts.

Sheet metal duct work leaks out a lot of the air it carries. That’s because it is made of multiple segments joined together, with nothing to seal the joints.

In this house, we covered these seams with a special kind of fiberglass tape–NOT duct tape, which loses it’s stickiness throughout several seasons of temperature swings–and painted over the tape with a good thick layer of duct mastic.  Duct mastic comes in a tub at a home improvement store and looks and feels like putty–but dries into a tough but flexible air-tight material.

Stay tuned for more home energy efficiency projects we will do for each other.