Frequently Asked Questions

FAQs » VENTILATION, INSULATION, ETC.

Question: No one mentioned venting. Does this not help emissivity even though the product has absorbed the energy and can be emitted below does this count to these rules?

Answer:

The energy efficiency of a steep roof assembly is impacted by the insulation level, whether the roof system is ventilated and the ventilation rate, the roof location and the direction it faces (or exposure), the thermal mass of the roofing system, and finally the solar reflectance and infrared emittance. All of these features need to be considered when designing an energy efficient roofing system.

Oak Ridge National Laboratory has been monitoring the energy efficiency of a pair of steep slope roofing systems located in Knoxville, TN where the only variable is whether the attic space is ventilated. We find that ventilation has only a second order effect on the energy efficiency of an attic assembly because the bulk of the heat transfer to the insulation surface is through radiation from the roof deck. Reducing the roof deck temperature (through changes in solar reflectance and infrared emittance) is a much more effective means of improving the energy efficiency of attic assemblies.

Question: Has the DOE evaluated ventilation of attics on cooling loads? What is the impact of ventilation?

Answer:

See answer above.

Question: The impact of insulation minimizes the effect of emissivity. All jurisdictions require insulation therefore emissivity should be disregarded because the heat load does not enter the building.

Answer:

As with all other energy efficiency issues, the greatest energy savings benefit is generated by implementing the first energy efficiency measure. Therefore, adding a roofing surface with a high solar reflectivity or a high emissivity has less benefit on a roofing system that is highly insulative. Emissivity should not be completely ignored for highly insulated roofing systems as it still impacts the surface temperature and therefore the roof’s contribution to the urban heat island phenomena.

Question: Does absorption of the roof and heat capacity of the roof not play a role?

Answer:

Thermal mass or heat capacity of a roofing system will have an impact on its energy efficiency by moderating the temperature swings of the roofing system. The thermal benefits of a massive roof vary depending on the location of the roofing system and the amount of solar radiation the roof absorbs. The thermal mass benefits will be maximized in locations where the roof surface temperature cycles above and below the interior temperature. This may be an effective strategy for the Chicago area.

Question: Have you considered the impact of mass and specific heat of alternative materials on potential energy savings?

Answer:

See answer above.