LIVE image

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

A weather-resistive barrier combined with insulated vinyl siding had some visible, qualititative results on thermal performance in a new industry study. Image: Vinyl Siding Institute.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:

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.Modest R-value and airtightness improvements were shown in the industry study, though the results in this table reflect conditions only under depressurization. (Click to enlarge.) Source: Vinyl Siding Institute.

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%.


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.


If you enjoyed this article, sign up for BuildingGreen email updates:



1 Installers claim R-10 posted by Lawrence Lile on 02/24/2014 at 10:18 am

A fellow was trying to convince me that this EPS ionsulation behind the vinyl would add R-10 to a building.  I had considered using his company to do some other renovations at one of my buildings, and the subject of siding came up.  I essentially threw him out, and told him not only would I not hire him to side anything, I would not hire him for any other type of construction if that was the kind of exaggerated claim he'd make.  This kind of hucksterism is common in the industry, I believe.  

2 Insulated vinyl siding posted by Kelly davenport on 05/04/2014 at 06:14 pm

We have a 1918 house with both vinyl and aluminum siding. We sustained hail damage in a storm and see this as a good opportunity to remove all the old siding and install the insulated vinyl. The house is drafty and we are hoping by adding the IVS it will help regulate the temp. Is there a difference in quality between manufactures? Noticed on Norandex web site they were " Green Certified"

3 greenwash posted by Tristan Roberts on 05/05/2014 at 09:56 am

Hi Kelly, in my opinion the certification you are referring to is greenwash, in that it gives the impression that the product is higher quality or "greener" in some way, when in fact all that particular certification is saying is that if you use the product as specified, it will contribute points to the NAHB (National Association of Home Builders) green building standard. This is like saying that if you put a lightbulb in the socket and turn it on, it should shine. It is just a product doing what it is defined to do. It does not say anything about quality.

Unfortunately I have no knowledge of quality differences among these products. I would tend to think they are comparable.

Add comment

Login or register to post comments

— Share This Posting!

About the Authors


Recent Discussions

posted by atwilson
on Feb 3, 2016

I believe the Viridian decking we installed is 15/16" thick by 2-1/2" wide; I'm not seeing those dimensions on the Viridian...

posted by Meredith_1
on Feb 3, 2016

Hi there! I've been sourcing reclaimed exterior deck material for a client, and am interested to know:

The thickness of the material you...

posted by tristan
on Jan 29, 2016

Erica, 3-way LED bulbs are sold in stores and online. Just do a search or ask at your local hardware store.

Recent Comments

Saving Wood from the Landfill, Without the Supply Issues

Alex Wilson says, "Meredith, I believe the Viridian decking we installed is 15/16" thick by 2-1/2" wide; I'm not seeing those dimensions on the Viridian website now,..." More...

Meredith Williams says, "Hi there! I've been sourcing reclaimed exterior deck material for a client, and am interested to know: The thickness of the material you used..." More...

Beat the Bulb "Ban": LED Replacement Lamps in a New Light

Tristan Roberts says, "Erica, 3-way LED bulbs are sold in stores and online. Just do a search or ask at your local hardware store." More...

Erica E. Tactical says, "Anyone know a way to replace incandescent 3-way light bulbs with LEDs? My table lamps are all 3-way and I like the ability to have ambient or..." More...

Autoclaved Aerated Concrete (AAC): Will the U.S. Ever Lighten Up?

Simon Hatton says, "Merhis Projects are an Australian Developer and Construction company and have been using their patented Merhis Building System for the last 4 years...." More...