Insulated Vinyl Siding: Worth the Extra Cost?

Two studies indicate some benefits to using insulated vinyl siding, but more data is needed to win over this skeptic.

Setting aside the overall environmental profile of the oft-demonized PVC (check our coverage in this month’s EBN feature “The PVC Debate: A Fresh Look”), I’ve been getting a lot of questions about insulated vinyl siding—the vinyl siding with form-fitted expanded polystyrene (EPS) insulation permanently built into the back side of the double-four courses of vinyl siding.

Thanks to claims being made by the Vinyl Siding Institute and specific manufacturers, I’ve been hearing questions like these:

• Does the EPS act as a significant thermal break, since it is continuous on the exterior of the building’s frame and sheathing?
• Does insulated vinyl siding make a building more airtight?
• What impact does the EPS have on the moisture performance of conventional, wood-framed exterior walls?

Intuition vs. numbers

It’s pretty easy as a building-science wonk to dismiss this product: the added R-value of EPS is R-2 to R-2.7.

Also,the long strips of EPS are only step-jointed, and therefore not airtight. And eliminating or significantly reducing the free-draining and ventilating space—which are touted as so important to the moisture performance of conventional vinyl siding—is certainly a significant moisture-management change.

Fortunately, we don’t need to rely entirely on speculation. Two recent field studies include evaluation of the hygrothermal performance of insulated vinyl siding:

• “Insulated Siding Energy Performance Study” (A June 2013 field study conducted by Newport Ventures for the Vinyl Siding Institute)
• “Effect of Cladding Systems on Moisture Performance of Wood-Framed Walls in a Mixed-Humid Climate” (A 2010 ASHRAE paper by Craig Drumheller, NAHB Research Center)

Increased R-value

The industry study focused on thermal performance: increased exterior wall R-value, decreased thermal bridging, and increased airtightness. The study evaluated five different products from a range of manufacturers installed on five single-family detached homes, in five cities spanning three climate zones.

Standardized testing showed a range of increased R-value from 2.0 to 2.7; modest reductions in thermal bridging (as qualitatively assessed from thermal imaging); average increase in building airtightness of 11%; and utility bill savings ranging from 1% in Indiana to 11% in Colorado.

Conclusion? Insulated vinyl siding provided measurable but quite modest improvements in the thermal performance of these retrofitted existing homes.

Next time around, I’d like the Vinyl Siding Institute to measure airtightness under both pressurization and depressurization. (I checked with Newport Ventures, and airtightness testing was only under depressurization.) It would be interesting to see if pushing the insulated vinyl siding out and off the wall assembly (during pressurization) would generate different airtightness results than pulling it in (during depressurization). Both pressurization and depressurization are relevant to air leakage in real-world buildings.

Improved drying capacity

The NAHB Research Center study was a 22-month field investigation in Maryland (mixed-humid climate) comparing structural sheathing moisture content of multiple conventional wall assemblies that had wall claddings ranging from brick to stucco to conventional and insulated vinyl siding. The study included water injection testing (in August) at or near the structural sheathing layer. The water injection was designed to simulate leaking that might accompany a multi-day storm.

Both the insulated and conventional vinyl siding showed among the best drying capacities. The insulated vinyl siding performed the best (lowest structural sheathing moisture content) when there was no wetting event, and the conventional vinyl siding performed the best after the water injection testing. The slightly better drying capacity of the insulated vinyl siding was attributed to the warming of the insulated wall cavity by the siding insulation.

Conclusion? The introduction of the form-fitting insulation to the vinyl siding did not significantly reduce the drying potential of the vinyl siding in this field study.

Reasons are elusive

I have to admit to being quite surprised at the superior drying ability of the insulated vinyl siding; I would think that filling most of the free-draining space of conventional vinyl siding with form-fitted EPS insulation would significantly reduce both free drainage and convective drying between the vinyl siding and the rest of the wall.

It’s quite possible that the specific conditions of this field test—including the mild climate—explain some portion of the results; it would be great to test this supposition with field-testing in harsher climates and different wall configurations, varying more than just the exterior claddings.

What do customers think?

Enough building science; what is the demand for this unique product in the marketplace?

I asked Brian Knowles, project consultant with a Vermont company that installs quite a bit of vinyl siding, Jancewicz & Son, what he thought of the insulated vinyl siding products.

“The studies confirmed what my general sense of the insulated vinyl siding was,” says Brian. “The modest improvement in thermal performance is less of a selling point than the improved stiffness and sense of robustness that the insulated vinyl siding provides.”

Based on Brian’s estimates, the cost premium is significant: the insulated version of a colonial white siding, compared with the conventional product from the same manufacturer “carries a 50% cost increase” just for siding materials, he said. “The trim and finishing components for insulated siding are also at a premium and should be considered as well,” Brian adds. “However, the increases are much smaller,” up to 20%.

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Editor’s Note

While the issues Pete has focused on here are about the hygrothermal performance of insulated vinyl siding, he and the rest of our GreenSpec editorial team took a more holistic view when deciding whether to list insulated vinyl siding.

We don’t list vinyl siding due to life-cycle concerns, so if we add the performance benefits of the EPS, are those issues overcome?

We’ll look for more data on an ongoing basis, but we are not currently listing this type of cladding. Our team is simply too concerned about the many health and environmental problems associated with PVC.

EPS also has its own problems—such as its use of flame retardants—and we’re not seeing enough evidence that insulated vinyl siding provides benefits that clearly overcome these problems. And speaking of performance benefits, if people are replacing a home’s cladding and choosing an insulated product, we think it makes more sense to consider a deep energy retrofit that would provide much greater R-value and airtightness than this product can offer.

–TR