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Posts tagged: R Value

The recently published NRCA position on R Value of roofing polyisocyanurate (ISO) Roof Insulation is a good opportunity to re-evaluate your roof insulation choices and roof system design criteria.

By , June 17, 2010 3:00 pm

Thermal performance, energy savings and durability are at the core of sustainable design.  Roofing insulation plays a critical role in building energy use and should receive closer scrutiny not only based on true thermal performance but also practical experience and considerations.   Typically, ISO is simply the default, based on years of practice, copying old roofing specifications or following the past direction taken by peers.  There are other drivers of this habit including the fact that roofing manufacturers also produce the ISO and promote it, and the perception that ISO provides the highest thermal performance per inch thickness.  Possibly there are even thoughts that ISO must be used to obtain the total system warranty from the manufacturer.

While ISO continues to be specified, there is a key fact about its use that we should pay greater attention to – when the roof approaches the end of its warranted life it typically requires complete removal, disposal and replacement.  The insulation is typically quite wet and cannot be salvaged.

A change in perspective may be valuable in the quest to achieve practical, sustainable goals for your clients.  There is a product and design method that enables the insulation to be re-used and provide long-term stable thermal performance.  Dow Deckmate Plus Extruded Polystyrene Insulation.  A simple, cost effective solution that may mean simply swapping it for the ISO that has been used in the past.

THERMAL PERFORMANCE:

In November 1987 the NRCA and Midwest Roofing Contractors Association issued a joint bulletin regarding “in-service R value” for polyisocyanurate insulation.  Their recommendation at that time was an R value of 5.6 per inch thickness.  The NRCA reaffirmed this position on ISO R value in May 1994.

ISO manufacturers disregarded that recommendation, and continued to advertise R-7.2 per inch for design use.

In January 2002, after a decade of pressure from the NRCA, the ISO manufacturers agreed to a new method of testing and describing thermal performance known as the LTTR method.  This method dropped the R value of roofing ISO to R-6 per inch thickness, and is currently the referenced standard in guide specifications and manufacturers literature.

The NRCA recently completed R value testing research similar to the original program reported in 1987, as a follow up to the validity of LTTR.  The NRCA published “Tech Today” in the May 2010 Professional Roofing Magazine, reaffirming their position developed in 1987:

“The NRCA maintains its longstanding recommendation that designers determine polyisocyanurate board insulations total in-service thermal resistance based on an R-value of 5.6 per inch.”

They also add this important comment:

“However, based on NRCA’s testing, it may be prudent for designers to use an even lower R value when designing for cold conditions . . . ”

The published thermal performance of Dow Deckmate Plus extruded polystyrene (EXPS) has always been R-5 per inch.  When tested in accordance with LTTR methods, the R value actually moves to R-5.3 per inch.  The final comment in the NRCA article is interesting.  It recognizes the fact that ISO R value actually follows a temperature curve that peaks when tested at 75 degrees F and decreases as the temperature moves lower AND higher than that point.  Of the samples analyzed by the NRCA the average R value of ISO products were only R-4.74 at 25 deg F, R-5.39 at 40 deg F and R-5.05 at 110 deg F.

Dow extruded exhibits the following R value properties at various temperatures:  R-5.6 at 25 deg F, R-5.4 at 40 deg F, R-5.0 at 75 deg F.  When considering actual rooftop temperature conditions, Deckmate Plus outperforms ISO thermal performance during periods of peak cooling and heating loads.

MOISTURE PERFORMANCE:

The NRCA data does not address moisture issues in roofing application and the impact of moisture introduced into the assembly has on long term thermal performance.  However, practical experience indicates that moisture is insidious and will be present within the roofing assembly whether introduced by moisture laden airflow from the interior of the structure into the assembly, mechanical damage to the membrane over time, workmanship issues and tie-offs during construction or long-term deterioration of the seams and roof assembly.  Moisture intrusion will occur.  Eventually that moisture deteriorates the ISO, making it unsuitable as a substrate for the roof assembly.  Until it reaches that point, the continual accumulation of moisture in the assembly and trapped by the deck below and the membrane above, will gradually degrade the R value of the assembly.  Once the roofing system has reached the end of its warranted life, 17 years on average (NRCA data), it is typically entirely disposed and replaced.  Although not the fault of ISO, its lack of moisture resistance is certainly a contributing factor.  Not a sustainable solution.   See SPRI Moisture Bulletin for detailed information.

Consider the use of Dow extruded polystyrene insulation as the answer to this design concern.

Duke Medicine has found some great advantages to the use of Dow extruded polystyrene in their roof assemblies.  Tim Pennigar, Project Manager, Structural Systems, Duke University Health System, published a technical piece entitled, “Duke Medicine:  High Performance Buildings for Extraordinary Health Care” in 2007.  This was aimed at designers and builders of future Duke medicine buildings and was based on accumulated years of experience.  When discussing roofing in this article, Mr. Pennigar’s experience created a clear focus:

“Roofing System Design

Few building components possess the power of the roofing system to impact facility operations.  Done well, the roofing system is a reliable ally, protecting both our property and our mission.  Done poorly, the roofing system is a deep and continual draw on our institution’s resources – resources that find their best use in support of outstanding patient care or cancer research, not in the funding of a premature roof failure.  Duke Medicine has established a firm rationale for roofing design that ensures the highest performance and durability for our medical buildings.  In general, we promote the following principles for membrane roofing design:

  1. Favor insulations or insulating assemblies that are highly resistant to water and physical damage
  2. Favor roof assemblies that position the roof membrane directly over a permanent or semi-permanent substrate
  3. Favor roof designs that prohibit or highly discourage the entrapment of water within the roof assembly
  4. Favor membrane and insulation designs capable of in-place reuse or recycle in future roof iterations

Over time these guiding principles have produced a dramatic improvement in roofing performance on our campus.  We particularly ask our designers to emphasize adaptive reuse of materials.  Adaptive reuse will sharply lower future costs and will produce minimal impact on the environment and hospital operations, opportunities certain to be welcomed by future generations.”

The insulation that Duke chooses is Extruded Polystyrene.  Highly moisture resistant.  Reusable.

Reusable. Only extruded polystyrene can be used to design a roof assembly where the insulation can be reused after the membrane fails.  In fact, Duke had “a 2007 roof replacement that mustered a materials salvage rate of 90%, diverting 718 tons of solid waste stream.  Salvaged materials from this effort, including 296,000 board feet of XPS insulation, have been reused in new roofing construction on two of our buildings.”

Top Ten Design Mistakes:  Roofing professionals share the top ten design mistakes they see in the field.

DESIGN CONSIDERATIONS FOR EXTRUDED IN CONVENTIONALLY BUILT ROOFS:

  • The building code allows direct application of extruded polystyrene to steel decks.  Only FM and rated assemblies require the use of 5/8″ Type X gypsum on the deck first.
  • The NRCA recommends that ALL conventional roof assemblies have a cover board.  Roof consultants suggest that one of the top ten design mistakes is not following this long standing NRCA recommendation.  The use of extruded polystyrene requires the use of a cover board.
  • Tapered extruded available down to 1/16″ per foot slope.
  • Can be specified as a part of NDL and up 30 year waranted assemblies for major manufacturers.
  • May be used with a variety of membrane types and application techniques.
  • Can be specified with 30-year re-use warranty.
  • The only roof insulation product carrying a thermal warranty – whether the roof leaks or not.

THOMCO can guide you through the system design, detail and specification process when considering Styrofoam for your roofing application.  Click here for technical articles referenced above.  We look forward to assisting your toward your sustainable goals and toward practical solutions to your roofing problems.  Click on Need Assistance to contact us with your questions, or give us a call.