Specify the Right Amount of Roof Insulation

07/26/2013 | By Christopher Curtland

Let codes, materials, and energy performance guide you

ASHRAE recently updated its roofing insulation standards. New construction and major renovations should comply with these values.
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Having a roof over your head is important, but if its insulation isn’t performing, it could be costing you.

For new construction, architects are responsible for specifying a system that is up to code. Existing buildings are subject to a code compliance update if they undergo major renovation, so if you’re taking on a reroofing project, it’s crucial for you to confirm that the new solution is compliant.

To ensure that your stress levels don’t hit the ceiling and your financial situation doesn’t cave in, take the following steps to specify the correct amount of roof insulation.

1) Follow the Code
The International Building Code (IBC) and International Energy Conservation Code (IECC) are the two primary authorities to follow, and they adopt the ASHRAE 90.1 energy standard. Codes were updated last year and are revised every three years.

Although these bodies are responsible for deciding requirements, your local building department is the only one that counts, explains Darrell Smith, civil engineer and building envelope consultant with Brown Engineering Company, a full-service, multidiscipline engineering firm.

“Typically the local code agencies are about one cycle behind on the model building code, so it depends on whether they’ve passed the 2012 update or are still going off of 2009. Then they’ll write some of their own standards that better fit local situations,” Smith says. “The local officials are the real enforcers in your area.”

Recommended R-values for certain U.S. climate zones increased by five last year (see map for details). It’s important that a staff member or hired contractor is knowledgeable about the code situation, advises Ted Michelsen, president of Michelsen Technologies, a full-service roof consulting firm.

“You have to worry about the actual building code and the energy efficiency requirements to do an acceptable job,” Michelsen says. “Any competent roofing contractor should know those, but you should too.”

2) Select Appropriate Materials
“Insulation must work with the rest of the roof system. If you have the wrong kind, then you can’t properly adhere the membrane, and that’s bad for roof life,” Michelsen says. “Most roof systems don’t play well with others.”

Important characteristics of insulation include high resistance, low conductivity, ruggedness, water and solvent resistance, and high R-value per inch, explains Michelsen.

“There is not a perfect product out there, and cost is the driving factor. Roofs tend to be very large, and no one can afford hundreds of dollars per square foot of roof insulation,” he adds. “You must balance performance with cost competitiveness.”

The most common and current choices are polyisocyanurate board (roughly R-6 per inch), which is compatible with all membranes, and polystyrene foam (approximate R-values range from 3.8-5), which is compatible with single-ply membranes, says Michelsen.

“When reroofing comes around, it’s a concern that an owner might just reroof with what he had before,” Michelsen adds. “What was fine 10 or 20 years ago is probably no longer code compliant.”

Because code likely requires another layer or two of insulation, it’s important to know what your selection offers.

“Very often the roofing contractor selects the system, provides the owner with a bid, and pulls the building permit,” Michelsen says. “Know what’s in that specification and how it stacks up.”

3) Enhance Energy Performance
Seizing energy benefits is tricky, but being proactive will get you ahead of the curve.

“One of the dirty little secrets about roof insulation is that as we increase R-values and the amount of insulation, the amount of heat flow still increases but at a decreasing rate. Going from R-1 to R-2 reduces heat flow by 50%, but from R-19 to R-20 the reduction is maybe only 5%,” explains Michelsen.

“Insulation has a practical limit that you want to get to. After a certain value – which typically the codes dictate – you’re not getting the benefits you think you should by continuing to increase quantities,” he adds.

Reroofing is such a significant concern in terms of budget and capital that you want to make sure you’re doing the project right the first time, says Michelsen.

Whether that entails meeting the latest international and ASHRAE guidelines – or simply keeping local officials at bay by adopting their potentially outdated specifications – is up to you.

“Certainly it’s more beneficial in terms of energy conservation to add insulation up to the point of the most current guidelines, but that’s not always possible due to budgets or structural issues,” Smith says. “When I do a reroof project, I recommend updating to ASHRAE standards, because if the local code hasn’t made the shift yet, it soon will be. Then you’re safeguarding yourself for when it does and capitalizing on the energy efficiency properties in the meantime.”


Chris Curtland christopher.curtland@buildings.com is assistant editor of BUILDINGS.


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