Billy's Blatherings

Building Practices to Mull Over for New Homes

All too often HVAC ducts are in an attic, and the ducts are poorly insulated and are not well sealed so they leak conditioned air into the attic. Simulation results indicate that homeowners typically pay an added $100 to $300 more per year because of leaky and poorly insulated air conditioning ducts operating in an unconditioned attic. Energy costs are also increased if the attic floor leaks air to or from the home. Duct location and sealing the attic floor are of paramount importance and should take precedence over all other energy efficient roof system and attic strategies. The simulation results of Figure 6 illustrates why these practices should be a priority component of a building program. The dark blue bars represent a dark heat absorbing roof and attic that contains poorly insulated and leaky ducts and a leaky attic floor. The orange bars show energy use where the practitioner repaired the leaks in the attic and sealed and rewrapped the ducts in RUS-8 (RSI- 1.4) insulation. The light green bars show the benefit of moving the ducts into the conditioned space. The light blue bars are for the new-design ventilated and insulated roof deck with the attic floor repaired for leakage and the ducts moved to the conditioned space.

Figure 6.  Comparison of energy effects of leaky ducts in attic space, sealing attic floors, insulating attic floors, and eliminating energy losses from HVAC ducts in unconditioned attics.

The heat gains and losses from leaky ducts and a leaky attic floor are double if not triple the heat gains and losses crossing the attic floor.  Adding insulation to the attic helps but the heat transfer tends to level off for floor insulations exceeding RUS-30 (RSI-5.3) because losses from the ducts predominate.  Adding insulation does reduce the energy bill for all assemblies represented in Figure 6, but if all on does is add insulation then the energy consumed due to the roof system and attic is not minimized as clearly seen in Figure 6.  Sealing all duct joints with mastic and wrapping the ducts with RUS-8 (RSI-1.4) insulation drops the energy losses for ducts in the attic by roughly 40 percent (Fig. 6 dark blue bars compared to orange bars). However, more savings can be achieved if the ductwork is simply kept out of the attic. An attic with RUS-60 (RSI-10.6) floor insulation but with leaky ducts and a leaky floor (Fig. 6 dark blue bar) has 30 percent greater heat energy losses than an attic with just RUS-5 (RSI-0.9) floor insulation but with no ducts and no air leakage across the attic floor (Fig. 6 green bars). In many homes, the ductwork increases air-conditioner energy use by roughly 18 percent for moderately leaky ducts in a well-insulated attic [14] and [15].

Pre 1990 homes were hopefully built to the presiding ASHRAE Standard 90-80 “Energy Efficient Design of Low-Rise Residential Buildings” [18]. Therefore the payback for adding insulation above the 1980 code level set at RUS-20 (RSI-3.52) was computed for an attic assembly with sealed attic floor and inspected ductwork3, Figure 7. Adding RUS-19 (RSI-3.3) of insulation pays for itself in about 35 years when added to an existing RUS-20 (RSI-3.52) batt. Increasing the ceiling insulation to RUS-60 (RSI-10.6) yields a 34- year payback if the basis of savings starts at RUS-20 (RSI-3.52) batt.

Building America (BA) has a residential benchmark [16] that calls for no ducts in the attic, sealing the attic floor and RUS-50 (RSI-8.8) insulation placed on the attic floor. The seasonal roof heat transfer was computed for the BA benchmark and for the new roof and attic design (Figure 7). Increasing the level of insulation on the attic floor from IECC [9] code level of RUS-30 (RSI-5.3) in Austin, TX to the BA benchmark of RUS-50 (RSI-8.8) lowers the ceiling heat transfer by 41.5 percent of that computed for the code level of insulation (view red squares in Figure 7). At RUS-50 (RSI-8.8) there is only 2,900 kBtu per year crossing the attic floor; however, the new attic design with RUS-30 (RSI-5.3) shows heat flow of about 3,000 kBtu per year. Therefore, the ventilated and insulated roof system performs as well as the BA benchmark while using only IECC code level of insulation (i.e., RUS-30 for insulated and ventilated roof deck; RUS-50 for BA benchmark).

Figure 7.  Building America Benchmark at RUS-50 (RSI-8.8) is compared to the new roof system design having an insulated and ventilated roof deck.

3 Simulated data is also represented by the green bars in Figure 6.

Winter Field Tests of the Prototype Roof and Attics

During winter nights, field data revealed that night sky temperatures were much lower than the surface temperatures of the test roof systems, a situation that drives radiation heat loss to the sky. Good roof design should ideally limit heat gains during the summer while also limiting heat losses in the winter, which is why insulation works better than cool roof systems in cold

Computer Simulations

ASTM C1340-04, “Standard Practice for Estimation of Heat Gain of Loss through Ceilings under Attics Containing Radiant Barriers by Use of a Computer Program,” [12] was benchmarked against field data for the insulated and ventilated shingle roof system (Figure 4 and 5).  Simulations were made for the hot climates of Miami; Austin, TX; Atlanta and the cold climate of Baltimore with and without air-conditioning ducts in the attic. An attic of 1539 square feet (143 m2) with a roof slope of 18 degrees was modeled with and without a cool color shingle roof; the cool color shingle’s solar reflectance was 0.25. The supply duct surface area was set at 304 square feet (28.7 m2). The return duct assumed 176 square feet (16.4 m2) of surface area exposed in the unconditioned attic. Energy Plus [13] estimated the hourly run times for an air-conditioner certified with a Seasonal Energy Efficiency Ratio (SEER) of 13 and for an 85 percent efficient gas furnace that heated the home. The hourly indoor air temperature for the house and the run time for the HVAC were estimated by Energy Plus and read by AtticSim to better estimate the roof and attic load as coupled to the building envelope.