University of Wisconsin - Extension

MADISON, Wis.--With recent hot weather you may be thinking about ways to keep your house cooler next year. Upgrading your insulation is one way to do that. Insulation will help keep heating bills in check this coming winter as well. Attics should have insulation that provides an insulating value of at least r-38. This amounts to about 12 inches of insulation. Walls should have insulation of at least r-19. The typical products used to provide this insulation are fiberglass batts for unfinished walls and blown-in fiberglass or cellulose for closed in wall cavities. The same products are used in attics as well. Rigid foam sheets are often used under siding to increase the insulation value of walls.

There is another type of insulation that can be useful particularly when walls or ceilings are opened up for remodeling. The first is sprayed-in-place polyurethane insulation. This should not be confused with the urea-formaldehyde insulation that was used for retrofit insulation in the 1970s. Urea-formaldehyde insulation gave sprayed-in-place insulation a bad reputation. It often out-gassed formaldehyde, which caused health problems in some people. Application was also difficult to control, which meant that there could be severe shrinkage problems. No such problems have been reported with polyurethane spray insulation. Installers need to be concerned about gases released during the application process but the product cures quickly and is then non-toxic.

Spray polyurethane is relatively expensive compared to other common types of insulation but it offers a number of advantages for special situations. To begin with, when used at a density of about two pounds per cubic foot it has an r-value of over six per inch. This means that when space is at a premium, such as in an attic with 2 X 6 rafters, you can easily get an r-value of over 30. If you used fiberglass batts, the best you could do is about r-19.

A second advantage is that the foam does not require a vapor barrier in most places and usually does not require a space left between the roof deck and the insulation for ventilation. Fiberglass insulation would require an inch or so between the roof and insulation to allow airflow to carry off any moisture that gets through the ceiling into the space between the roof and the ceiling. Fiberglass and cellulose are porous enough so that air moves through carrying moisture with it. On the cold side of the insulation this moisture may condense and cause moisture to collect on the inside of the roof deck. Since foam is typically sprayed directly against the roof sheathing and is resistant to air flow, condensation on the roof sheathing is unlikely.

It is important to note that some shingle manufacturers require an air space under the roof deck to keep the roof deck and shingles cool. However, the effect of this air space on roof temperature has not been substantiated by recent research.

The same advantages of sprayed foam make it ideal for cathedral ceilings regardless of the size of the rafters. Cathedral ceilings often are plagued with moisture problems particularly if there are penetrations that allow air to leak into the roof cavity.

Another situation where polyurethane spray insulation could be useful is for complicated roof structures with irregularly shaped cavities. Fitting batts or even pieces of rigid foam sheets into such cavities can be a challenge and if not done with extreme care can leave many voids that compromise the insulation. Sprayed foam can readily fill these spaces. The excess on the face of the framing can be removed by using a long saw blade.

Another application is on the inside of crawl space walls. Polyurethane adheres well to concrete, brick and stone as long as it is free of surface dirt. It can warm the crawl space and seal unwanted leaks as well. It also may be easier to install in tight spaces.

While foam doesn't need a vapor barrier, it is combustible and therefore does require a fire rated material over it such as half-inch sheet rock.

The cost, which can be three times other forms of insulation, means that it may not be the product of choice for all applications, but for the hard to insulate situations I have just described it seems like a good option.

For more information about spray foam insulation visit the Energy Efficiency and Renewable Energy Network web site at www.eren.doe.gov/consumerinfo and browse until you find the fact sheet on Foam and Foam Board Insulation. You can also visit the web site of the Insulation Contractors Association at www.insulate.org .

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