Not sure which foam attic insulation to choose… open- or closed-cell? One builder is using both and saving money in the process. To understand the reasoning behind his decision, let’s begin with the benefits of a sealed, foam-insulated attic and move onto the open / closed-cell argument. There are several benefits to a sealed, foam-insulated attic:

* Energy Efficiency: Estimates as high as a 50% energy savings have been noted by incorporating a sealed, foam-insulated attic into a tightly built residential structure. The normal vented attic, with traditional blow-in or batt insulation on the floor, can easily reach temperatures of 140 degrees in the summer and freezing temperatures in the winter.

Because a foam-insulated, sealed attic maintains a temperature of only about 10 degrees difference from the conditioned living space, the entire home maintains a more even temperature. In addition, HVAC equipment and duct systems can be housed in semi-conditioned attic space – a much more energy-efficient scenario. A better insulated house and an HVAC system operating more efficiently are big contributors to energy savings. Further savings are achieved through the use of a smaller sized (tonnage) air handler. Because an efficient HVAC system doesn’t have to work as hard, the size of the air handler MUST be reduced accordingly.

Three inch layer of open-cell foam covering 1 inch of closed-cell

* Added strength: According to testing performed by the University of Florida, a 3 inch layer of closed-cell foam provides nearly a 300% increase in roof uplift resistance compared to traditional framing methods. See Green Builder Magazine, page 47, for more information on the testing methods and results,

* Home Insurance Discount: Your insurance company may offer a discount for the use of closed-cell foam in the attic, particularly if you are located in a hurricane-prone area of the country. According to one of the foam companies in my area (central Florida), it is best to contact the insurance company headquarters, as opposed to your local representative, to inquire about the discount.

* Reduction in Critter Intrusion: Because ridge or off-ridge vents and vented soffits are eliminated in a sealed attic, the opportunity for invasion by unwanted furry guests is significantly reduced. If you live in any of the states that deal with drywood termites, a sealed attic also impedes infestation by these flying pests. See my article entitled, Is Your Brand New Roof an Invitation to Furry Guests, Detailed Solutions Ezine Archives.

The Closed-Cell / Open-Cell Argument: When discussing foam insulation, you will hear the terms “open-cell” or “closed-cell.” Here’s a brief description of the differences:

* Closed-Cell Foam: Because closed-cell foam insulation is rigid and dense (1.7–2.0 pounds per cubic foot), it acts like glue, which adds strength to the structure. Because it is watertight, it provides a vapor barrier in addition to a thermal and air barrier. Some builders prefer this product because a roof leak is encapsulated in the foam. The concept here is that the roof decking will be damaged but the leak won’t find its way into the home. Closed-cell foam is generally more expensive than open-cell.

* Open-Cell Foam: The fans of open-cell insulation prefer this product because the source of a leak can be more easily determined. Because open-cell is not a vapor barrier, water runs through it. Open-cell is softer, less dense (1/2 pound per cubic foot), and more permeable. It also has a greater ability to contract and expand with the decking. You’ll hear arguments for both types. The bottom line is that each has desirable properties and each is used successfully in a variety of applications across the country.

Combining the Best of Both Worlds: One of the homes I’m green certifying is actually incorporating both open- and closed-cell foams into the attic. The builder has achieved the best of both worlds and saved money in the process. First, the attic was sprayed with 1 inch of closed-cell (providing a vapor barrier and added strength to the roof) and then topped with a 3 1/2 inch layer of open-cell foam. This combo was less expensive by about 50 cents per square foot than using a 3 1/2 inch layer of closed-cell alone. This savings is not always the case. Check with your supplier on pricing.

Should open-cell foam have been used by itself, a 5 1/2 inch layer would have been needed to achieve the required air barrier.  Exclusive use of closed-cell foam would have required a thickness of 3 1/2 inches. Icynene was the foam used in this house.The thickness needed to form an air barrier varies between foam insulation products.

As always, The Difference is in The Details.

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