Wednesday, December 7, 2011

An Analysis of Radiant Barriers Finds Significant Energy Savings
Steven Winter Associates (SWA) recently evaluated the energy penalty associated with the recessed convectors found in many older masonry buildings. In this construction, the heating source is partially buried in an uninsulated wall. After a collaboration with Jonathan Flothow of The Steam Balancing Company (http://, [they quantified] the benefits of ... [installing] a double bubble foil radiant barrier behind convectors to minimize heat loss to the outside. For the analysis, SWA assumed an average winter temperature of 35° F (NYC average winter) and an average convector surface temperature of 110° F (which accounts for some hours where the convector is not filled with steam).

They looked at a building section that is typical to many older pre-war buildings with a 3 wythe-brick bearing wall and recessed cast iron convectors. Energy modeling and analysis was performed using THERM v6.3. Simulations were run analyzing heat transfer across the wall assembly with and without the radiant barrier.

The model demonstrated savings due to the radiant barrier of approximately 3.4 herms, or 2.25 gals of #2 fuel oil, per typical convector over the course of a 160-day heating season. At current NYC fuel prices, fixing this energy loss equates to an operating cost savings of approximately $9.70/convector for an oil heated building and $4.50/convector for a gas heated building. At an installed cost of approximately $23/convector, these savings would result in simple payback periods of 2 – 5 years. The $23 installed cost is based on a bid by a NYC plumbing contractor coordinating radiant barrier installation work with other plumbing work. If work is performed by in-house maintenance staff, the installed cost is only a few dollars for materials per convector.
Operational variables will, of course, impact real world results. This model is built upon a 24/7 convector run-time during a 160-day heating season. In the real world, occupants may valve-off convectors in an effort to reduce over-heating in their dwelling unit; the extent to which this happens was not quantified. Also, the effect of dust accumulation on the vertical radiant barrier is unclear. Studies of horizontally installed attic radiant barriers performed by the Oak Ridge National Laboratory have determined a decrease in the effectiveness of radiant barriers by approximately 20% over time due to dust build up. With this in mind, it is reasonable to expect a dust-related degradation of radiant barrier effectiveness over the course of several years that may impact realized energy savings. Related to this, a maintenance plan may be necessary to remove dust every few years.

Initial indicators demonstrate that the installation of a post-convector radiant barrier is a cost-effective energy conservation measure, especially in buildings that use expensive heating fuels such as #2 oil. Further investigation is planned for the upcoming heating season in an effort to true-up model data with real world temperature data.

Steven Winter Associates (SWA)
Party Walls
Volume 7, Issue 7; July, 2011; Page 1

No comments:

Post a Comment