TASK 2.5.6 FIELD EXPOSURE TESTING AT ORNL

Abstract

A combined experimental and analytical project was conducted to document the thermal performance of cool roofs and to investigate other roof and attic strategies that will complement cool roofs. Cool color roofs are a first line of defense against heat transfer penetrating the conditioned space. However, radiant barriers, above-sheathing ventilation , low-emittance (low-e) surfaces in the inclined air space, insulation, and conventional and advanced thermal mass (i.e., phase change materials [PCMs]) are all possible candidates that can enhance cool roof and attic performance. A steep-slope roof and attic test stand, the Envelope Systems Research Apparatus (ESRA), was used to field test and document the heat transfer crossing the roof decks of cool color roofs separately or in combination with other energy saving strategies. The attics are adjacent to one another, and their shed roofs face directly south. The footprint for each attic is about 16 ft (4.9-m) long by 5 ft (1.5-m) wide. The roofs are inclined at 18.4. Soffitt and ridge venting are provided with vent openings of about 1:300. Salient features of the ESRA and additional details of the roof and attic assemblies are provided by Miller (2006). A commercially available asphalt shingle with solar reflectance of 0.093 and thermal emittance of 0.89 (SR093E89) was the control for comparing the thermal performance of the prototype roof systems. The Tile Roofing Institute (TRI) field tested clay and concrete tile on the ESRA to assess the effects of cool color roofs, thermal mass, and placement of batten and double-batten systems under the tile. Results implied that the further the cool color tile was elevated off the deck the better in terms of reduced deck heat flows. A medium profile tile with conventional color pigments and on double battens showed deck heat transfer very similar to that of a similar but cool color tile attached directly to the deck. The ventilation effect under the conventional color tile appears to compensate for the effect of the infrared reflective pigments on the adjacent tile roof. However a flat tile roof showed similar performance whether nailed directly to the deck or offset on double battens. Results imply that the thermal mass of the flat-tile is the stronger effect and that profile tiles can better exploit the thermally induced convection occurring under the offset of the tile roof. Miller et al. (2005b) measured at solar noon buoyancy-induced airflows and found that the high profile tile had the highest airflows. Stone-coated metal roofing is not as massive as tile roofs; however, deck heat flows were significantly reduced by elevating the metal roofs on double battens and the heat transfer was comparable to that of tile roofs. The results herein suggest that thermally induced buoyancy and/or the effective thermal resistance of the air space significantly effects on the amount of heat penetrating into the attic. The best roof and attic assembly tested on the ESRA was an infrared reflective clay tile spray adhered to the deck with 1¼-in (0.032 m) of EPS foam [RUS-6.25 (RSI-1.1)]. Miller and Kosny (2008) had also tested a painted metal prototype that had similar heat transfer performance as the clay tile system. The prototype metal had a 4-in. (0.102-m) air space and was also fitted with PCM. Two different fabrics, impregnated with 10 lb (4.5-kg) of PCMs, were placed on the sheathing and covered by one of the two low-e surfaces. Overall, the peak day heat transfer for the clay tile assembly or the painted metal assembly with PCM was reduced by about 90% compared with that measured for the control shingle roof. Asphalt shingle roofs are by far the least expensive roof system so shingles with and without cool pigmented colors were tested on the ESRA. A prototype roof and attic assembly having asphalt shingle roof was shown to drop the peak day deck heat flow 75% of a control shingle assembly and its attic air temperatures did not exceed the outdoor air temperatures for field testing during August in East Tennessee. The assembly contains a corrugated and foil-faced EPS insulation that combines the effects of conventional insulation, radiation control and natural convection to help limit the heat transfer through the roof deck. Another main objective for this task is to benchmark the roofing materials tested at ORNL against ORNL’s AtticSim and/or other candidate attic computer tools. Task 2.5.4 takes the benchmarked tool and couples it to a whole building code for use as a web-based roof calculator. The design tool ASTM C 1340 (2004) was reformulated for above-sheathing ventilation and validated against the field data (Miller et al. 2007). Simulations for pre- and post 1980 roof and attic constructions were made for the hot, dry southeastern region of California, in El Centro and for the more moderate climate in Sacramento. An attic of 1261 ft2 (117.2 m2) footprint with a roof pitch of 18 was modeled that had air-conditioning ducts in the attic. Simulations indicated that a cool color shingle retrofit on an existing roof deck drops roof and attic cooling energy 6% of that used by the base roof (SR05E75); savings for the cool color shingles are about $4 per year per square, while the cost premium for cool color shingles is about 40% (Mullenax, 2010) above standard materials. Therefore, the cool color shingle can pay for itself in roughly 10 years. Results show that retrofit work conducted on the roof (without encroaching on the dwelling) yields savings of about $13 per year per square for the best roof assemblies tested at ORNL. Further improvements require working within the attic. Adding a radiant barrier showed the most significant drop in building heat transfer. A building owner might ask whether it more economical to retrofit the radiant barrier or add more insulation to the attic floor already covered with RUS-19 (RSI-3.3) levels of insulation. If the building owner opted to not retrofit a radiant barrier but added RUS-19 (RSI-3.3) to the existing level of insulation, the cooling energy savings would increase about 39% of the control roof and attic as compared to 63% reduction of the control stated earlier for the radiant barrier. Including both radiant barrier and additional insulation to bring the ceiling insulation in compliance with Title 24 [RUS-38 (RSI-6.7)] drops cooling energy 65% and heating energy 30% for the very reflective clay tile roof assembly, (see Table 3 in report). Finally, if the owner opts to also correct duct leakage and increase duct insulation to Title 24 code then the cooling energy drops 76% and the heating energy drops 52% of the control. Clearly, ducts in an attic cause major energy losses.