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VINYL SIDING: For a piece of Vinyl Siding of nominal skin thickness of .044” or .046” (these are in fractions in thousandths of an inch), there are no known published ‘R’ values, as they would be miniscule, if measurable at all. It is safe to say this value would be nil.
FIBER CEMENT: Builder's News Magazine states the following: "Fiber cement siding stands up to the competition: An R-value of about 0.15 for 5/16-inch thick siding" ....ref: Builder's News Mag
So, it is fair to say that neither Fiber Cement Siding, nor Vinyl Siding have much to offer in the energy savings arena.
It is in the application of insulation that siding can marry itself to a formidable ‘R’ rating. Rigid insulation gives the siding industry its ability to make increased energy efficiency claims. Insulation can be applied to both Fiber Cement and Vinyl Siding installations and with fantastic results.
Heat resistance: Here is a real world comparison of Vinyl Siding and James Hardie Fiber Cement Siding. It does not take a rocket scientist to figure that vinyl siding which is highly combustible would have little or no heat resistance rating, especially at a little more than the thickness of a matchbook. In contrast, Fiber Cement Siding is 5/16” nominal thickness, or ten times the thickness of a nominal piece of vinyl siding, composed of cement and fibers and very resistant to heat.
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Fiber Cement is Non-Combustible
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Adhered or bonded Vinyl Siding and Insulation retrofit products have hit the market with some acceptance. Certain facts should be exposed about these systems, however. The bonded Vinyl Siding will expand and contract at a far different rate than the rigid insulation to which it is attached. Some manufacturers have answered this dilemma with new insulation products that are less rigid and more apt to follow the fluctuations of the vinyl. Alcoa is one with its "Structure" panel that seems to move with the siding and it is "closed cell," so it won't soak up moisture that gets in behind the siding.
Some of the manufacturers are still marketing their open cell foam products. Open cell foam is visible as little white balls compressed together. "This makes the foam soft or weak, as if it were made of broken balloons or soft toy rubber balls. The insulation value of this foam is related to the insulation value of the calm air inside the matrix of broken cells," as reported by FOAM-TECH. These cells break apart and the insulation looses its cohesion when it has become wet. Given the nature of Vinyl Siding and the fact that it allows moisture to enter behind the siding, what goes on behind the wall that the homeowner cannot see on the surface is a system in flux toward failure. At the very least, the ‘R’ value is usually compromised early on in the system life. In fact, Open Cell Insulation, the kind that is commonly bonded to vinyl siding is known to absorb moisture. Given that Vinyl Siding is known to allow moisture in and labeled a "Supplemental Rain Screen," by the experts, is it not ironic that Open Cell Insulation is used when it is known to absorb moisture and degrade structurally and loose its 'R' value once wet? "Open cell foam should never be used for exterior applications," according to Biobased Insulation.
 Our page, "'J' Channels and the Problems they Cause" explains the moisture intrusion problem in more detail.
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Open Cell Insulation Bonded to Siding |
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Rigid Closed Cell Independently & Mechanically Fastened to Exterior Walls |
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Vinyl Siding must be installed loosely and with variances for thermal movement and these bonded systems leave critical spaces uninsulated at the channel intersections of ‘J’ channels and Corner Posts. Cold spots on the interior of the dwelling will be evident at these junctures during the winter months. The best way to install an exterior insulation is to install a rigid closed-cell insulation board, such as Dow Re-siding Closed Cell Insulation prior to the siding installation.
Fiber Cement, along with solid trim that can be directly sealed with caulking.
Of most importance, if you are planning on adding insulation, is to consider thermal bridges and the Dew Point within the wall assembly. Utilizing professional thermal leak tracing can evaluate hidden heat loss and invisible condensation zones without damaging exterior wall structures.
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