October 8, 2014

Foam Fails Reason #5: Excessive Shrinkage


Fine Homebuilding, Feb/March 2012

With the growing use of foam plastic insulation in building construction, first in commercial roofing and then too often metastasizing over the entire building enclosure, it is being used increasingly as an air barrier component. This is a problem for many reasons – see Foam Fails – and one of them is excessive shrinkage.

Essential to maintaining airtightness is maintaining airtight connections between the building components and materials that make up the continuous air barrier:  plywood, membranes, tapes etc…  Continuity is king. Yet many forces work against the reign of continuity: bad detailing, poor application of materials, plumbers with disregard for airtightness that tear through air barriers, and so on. It can be difficult. To make airtightness more assured we want to select those materials that are the most robust, the most airtight; they may be flexible, or rigid depending on the specific application. In all cases we strive for materials that are dimensionally stable – moving in only predicable and useful ways that support airtightness.  Foam does not do this. Foam shrinks unpredictably and excessively, compromising the air barrier continuity. Foam fails.

Famously, Building Science Corporation’s Joe Lstiburek declared “Foam Shrinks” in a Feb/March 2012 Fine Homebuilding Article. In a lesson to many of his building science classes, Joe tells the now classic story of how the thick foam panels on his “barn” roof shrank. He wasn’t relying on the foam for airtightness, but the thermal performance of the foam was degraded. A couple of months later, in May 2012, Joe clarified his statement saying that it wasn’t an issues of dimensional stability but cycles of contraction and expansion – that all materials, including foam, shrink and expand in different environmental conditions.

The coefficient of thermal expansion/contraction for polyurethane foam is approximately a 1/4″ gap for an 96″ long foam board. A quarter of an inch gap in an airbarrier system is a massive hole, and such gaps/cracks can significantly degrade the thermal insulating value of the system.  (Better tape those seams with something good! Hint: Pro Clima)


Mind the gap! (photo: GreenBuildingAdvisor.com)

Regarding dimensional stability, Dow Styrofoam XPS, states that the maximum shrinkage that can be expected is 2%.  Over a 96″ long board that’s 1.9 inches.  Really.  (Tape might not help in this case; it would seal the join but probably would tear the foam – better use a ProClima membrane in that case).

GreenBuildingAdvisor featured such dramatic shrinkage with the home of the residential programs manager for the Ohio Energy Office, Timothy Lenahan, who stated “…At some of these gaps, the tongue is completely out of the groove, and you can see the fiberglass insulation through the gap.”

Martin Holladay answered on a GreenBuildingAdvisor Q & A: “Foam manufacturers have said that the [sic] continually improve their manufacturing methods to minimize shrinkage problems. Only time will tell whether today’s panels are less likely to shrink than the panels of the past.”    

The only constant appears to be that the chemical companies are continuously reformulating – without any clear declarations of the reformulated results. So while the chemical companies may talk about improved performance, the 2% number is still their printed claim.

Spray foam in particular has been sold as a thermal insulator and air barrier in one. But once installed, spray foam, similar to foam boards, will expand and shrink over time. And both open and closed cell spray foam can and do regularly shrink and pull away from other air barrier components. The air barrier continuity is lost resulting in failure – leaks, drafts, discomfort, energy loss.


Application delivered shrinkage. (photo: GreenBuildingAdvisor.com)

The application of spray foam can needlessly add risk because the dimensional stability can be greatly impacted due to poor on-site manufacturing/installation. This is due to either an improper ratio of chemicals in on-site mixing, improper amount of spray foam applied at a given time, or poorly prepared surfaces (dust, damp) – or some combination of the three. Foam will shrink and pull away from other components and the resulting shrinkage can be dramatic and obvious or more subtle and harder to readily detect. (Tape can’t save you now….)

It has been said repeatedly by the spray foam industry and those repeating the chemical companies that “this is not a problem of spray foam, it’s a problem of the installer’s workmanship!” We ask: So what? The job’s a failure. In an age when we know we should be moving as much construction into factory environments for better quality control, why would anyone purposefully move manufacturing from the factory to the job site?

Who needs to make these problems and this worry? Lose the shrinkage. Lose the foam.

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5 Responses to Foam Fails Reason #5: Excessive Shrinkage

  1. George Hawirko December 1, 2014 at 5:47 am #

    Joseph Lstiburek states at: http://goo.gl/2De6xi where he states

    All foams experience dimensional changes due to their chemistry and manufacturing process. Some are
    “aged” so that they are for all purposes and intents “stable”. EPS is known to shrink immediately on
    manufacture and is typically aged a few days to account for this..

    This website seems to hate the idea of foam but fails to understand that all the problems that are sited are about Polyurethane type foams and XPS STYROfoams, witch are known to shrink and loose R-Value over time.

    Expanded Polystyrene Foam (EPS) does not Shrink, and holds its R-Values forever. More Verified Facts: http://www.epsmolders.org

    EPS Composite Construction encases the EPS in Concrete so it is NEVER Exposed to the Environment, so it is also fire resistant.

  2. Ken December 1, 2014 at 2:52 pm #

    Thank you for the comment. We’ll try to clarify and elaborate on the whole XPS vs EPS issue. Regarding your comment on fire resistance – the concrete is fire resistant, the foam isn’t.

  3. George Hawirko December 3, 2014 at 11:52 pm #

    My point exactly, EPS needs to be covered with Fireproofing, watch this example. http://goo.gl/PcokYZ

  4. John Biasutti August 24, 2015 at 12:48 am #

    One of the problems with extra insulation in houses is that people neglect the importance of humidity control. It is vital that the humidity in the building be maintained below the dew point of the EXTERNAL air.

    Otherwise when small leaks occur there is condensation at the outside of the leak path, leading to corrosion, mold growth, smell and vermin infestation on the outside edge of the insulation.

    When one completely seals a building it is important to then have a heat exchanger and preferably a dehumidifier and maintain a minimum number of air changes. This will ensure that the building is aways drying out, rather than getting damp.

    • Floris August 24, 2015 at 10:28 am #

      Thank you for bringing up this concern.
      Interior humidity is indeed very important for healthy inhabitants. The reason we stress the importance of warm sided air-tightness when adding insulation to our structures has two important aspects.

      First of all, it keep more humidity on the interior of the building in winter, when it is dry outside. This means that, when combined with heat recovery ventilation – you get great indoor air quality and relative humidities (RH) that are between 35 and 45% even without a dehumidifier (you are constantly balancing the indoor RH, with dryer outside air) – these are very good for human health, avoid mold growth on the interior and issues with rodents. Normal/leaky houses in winter are very dry, can be below 20% which is uncomfortably dry and unhealthy.

      Secondly, as you point out – we do need to avoid issues with condensation in highly insulated walls, especially when the interiors are more humid in winter. Since the humidity will want to move outward, we do want to keep this humidity from entering the assemblies. We do this with interior air-tightness and vapor control (INTELLO), so we can have the healthy 35%-40% RH and the highly insulated assembly and not get damp walls that would lead to damages or mold.

      Even when there are small (air)leaks, that you point out might exist (or homeowners might add) – we do want these walls to be save. This can be done by having an exterior that is >5 times as vapor permeable than are interior retarder (see as wel the blogpost “Vented rainscreen>what a high performance building wants“) and having a smart vapor retarder (INTELLO) on the interior. This will allow any vapor that gets into the assembly to dry out fast to the exterior in winter – and to the interior in summer. Providing the walls and roof assemblies with a drying potential that is many times higher than the (un)anticipated moisture stress – allowing you to build highly insulated assemblies, that are safe, durable and healthy.

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