William A. Miller, Ph.D., PE, Oak Ridge National Laboratory and David O. Prevatt, Ph.D., PE, University of Florida
Two years ago, the Florida Building Commission (FBC) approved funding for a research project which would monitor two single-family residential structures constructed with sealed attic systems and document the air tightness of the home, the duct system and the sealed attic. An additional two homes were added to the project (for a total of 4 homes) using funds provided by the Florida Roofing and Sheet Metal Contractors Association (FRSA). The moisture content of the wood roof decks, the indoor ambient, outdoor ambient and attic ambient relative humidity and temperature was to be recorded for a 12-month period. The data from this project will be used to check for moisture intrusion and storage in attics sealed with spray foam insulations.
The goal of the project was to document the risk potential and the effects upon occupant comfort in Florida homes that are sealed using open-cell spray polyurethane foam (ocSPF) or closed-cell spray polyurethane foam (ccSPF). Four residential field demonstrations were setup across the state of Florida to acquire field data. The project focused on Florida homes that have the attic sheathing, gables, eaves and soffits sealed using ocSPF, ccSPF insulations or similar blown fiber sealed system. Attics sealed with blown fiber insulations were also candidate systems for the study. Field data was reduced and used to assess the potential of moisture storage in the foam and sheathing, which can lead to structural damage to the sheathing. The storage of moisture has also been found to affect the indoor relative humidity level causing it to sometimes exceed prescription in ASHRAE Standard 55.
About 77 percent of single-family homes in the southern U.S. are built with slab-on-grade foundations, U.S. Census Bureau (2013). The statistic is higher in Florida, where almost all homes feature slab-on-grade foundations. These homes typically have a HVAC system per floor, and the units conditioning the home are placed in the attic. The convenience of the attic space appeals to builders, who all too often install the HVAC unit and the ducts in the attic to conserve living space while completing the rough-in at a low first cost. However, installing the HVAC and ducts inside an unconditioned and ventilated attic is not the most energy efficient option because of the extreme summer and winter operating temperatures occurring in the attic. Parker, Fairey and Gu (1993) simulated the effects of ducts on space conditioning Florida homes and observed that air leakage and heat transfer to the duct were major contributors
to the peak electrical burden on Florida utility. Walker, I.S. (1998) in his bibliography of duct air leakage reports average measured leakage rates of roughly 7 percent to a maximum of 20 percent of the supply airflow. Air leakage rate is the volume of air movement per unit time across the building envelope including airflow through joints, cracks or a combination of mechanical pressurization and
de-pressurization, natural wind pressures, or air temperature differentials between the interior and exterior of the building envelope.
To improve envelope performance, researchers opted to literally encapsulate the HVAC and ducts by moving the boundary of the insulating planes to the roofline, gables and eaves of the attic. The concept was first introduced by Building Science Corp (Rudd and Lstiburek, 1998). They built and monitored test homes in a hot, dry climate and demonstrated that the prototype homes with unvented attics yielded significant cooling and heating energy savings over a conventional home with ventilated attic. Transforming residential attics into a non-ventilated semi-conditioned attic space has therefore gained approval among builders, Chasar et al. (2010). Boudreaux, Pallin and Jackson (2013) recommend that sealed attics be conditioned to mitigate the moisture buildup issue as
do Roppel, Norris and Lawton (2013). However, field data demonstrating successful implementation in all climates are sparse and there still remains an educational gap in the best practices for builders. The dearth of data in hot, humid climates has caused confusion among builders and code officials because of the confounding variables affecting the hydrothermal (heat and moisture flow) performance of sealed attics.
The 2010 Energy Conservation supplement to the Florida Energy Conservation Code (FECC) provided measures for putting the supply and return ducts inside the building thermal envelope (Section 403.2, 2010 of FECC). However, the code change while intending to minimize risk and improve efficiency, may be counterproductive. In 2014, changes were enacted to section R806 “Roof ventilation”
of the FECC for unvented and sealed attics. The modification to Section 806.5 requires that air impermeable insulation be applied to the underside of the roof sheathing. If instead an air permeable insulation is selected, then the builder must include sheet insulation above the deck for condensation control. Field studies conducted by Oak Ridge National Laboratory (ORNL) in a hot, humid climate
investigated the thermal and hygrothermal performance of ventilated attics and non-ventilated semi-conditioned
attics sealed with open-cell spray polyurethane foam (oc-SPF) and with closed-cell spray polyurethane foam (ccSPF) insulation, (Miller et. al 2016). In the ventilated attics the relative humidity drops as the attic air warms; however, the opposite was observed in the sealed attics. Peaks in measured relative humidity in excess of 70 to 90 percent were found to occur from solar noon until about 8 PM on hot, humid summer days. Moisture pin measurements made in the wood roof sheathing and absolute humidity sensor data from inside the foam and from the attic air suggest that moisture is transferred through the foam and into the sheathing of sealed attics, Lstiburek (2015).
Four residential homes across the state of Florida were selected for conducting a field study to assess the performance of non-ventilated and semi-conditioned attics sealed with open-cell spray foam insulation. The purpose of this research was to evaluate the hydrothermal performance and durability of sealed attic construction where expanded foam insulation is applied directly to the underside of the roof deck.
Field sites are in Florida climate zones CZ-1A and CZ-2A (also ASHRAE climate zones 1A and 2A) where the weather is hot and humid. The homes were instrumented for measuring temperature and relative humidity of the indoor living space, the outdoor air and the attic air. In addition, the temperature, relative humidity and moisture content of the roof sheathing were monitored and
recorded by data acquisition equipment. Air leakage tests on the whole house, on the sealed attic and in the HVAC ducts were conducted on all four homes; results are listed in Table 1. Digital and infrared images were captured to document the thermal performance of the sealed attics. Field tests began June 1, 2016.
FRSA supported the Roofing Technical Advisory Committee (TAC) of the FBC in finding interested homeowners willing to participate in the field study. FRSA also made a $40k donation to the project through the University of Florida Foundation in order to expand the
project to four home demonstrations and to support the acquisition and reduction of field data beyond the project end date of June 30, 2016. Data acquisition will continue for one full year through the hot and humid summer months, as well as Florida’s mild winter season.
Results of blower door testing for the whole house showed House 2 to be the tightest, having an air exchange rate (ACH) of 2.2. Houses 1, 3 and 4 all had leakage rates of about 5 ACH or higher. The attic for the home in West Palm Beach did not appear well sealed and had an ACH of 22, which is high. The attics of homes in Venice and Orlando were each sealed to an ACH of about 5. The leakage rate per square foot of floor space shows House 1 to have the tightest duct system followed by House 2. The majority of leakage from Houses 3 and 4 comes from the conditioned space.
Three of the four homes had about four to five inches of open-cell spray polyurethane foam applied to the underside of the roof decks. The other home had 7-inches applied to the underside of the deck. Digital and infrared images of the roof deck and attic floor infer that the heat flow crossing the attic floor was lowest for the home with 7-inches of spray foam; it having the largest thermal
resistance to heat flow. The approximate R-value in the attic as well as the minimal value required by the active FECC code is also listed in Table 1 below. House 4 is slightly under the code requirement for new construction; however, Houses 1, 2 and 3 were insulated to levels below FECC code for new construction at the time of the spray foam application.
Homeowners commented that they were very pleased with the performance of their homes equipped with sealed attics and did not notice any water problems in or around the attic. Temperature and relative humidity field measures of the attic air and the sheathing revealed good hydrothermal performance in three of the four homes. Moisture content of the sheathing in all homes did not exceed the threshold for mold growth. The residence in Gainesville however, showed the attic relative humidity to increase to saturation
during the late afternoon hours on a hot and humid day. As the attic air temperature increases the relative humidity should drop. The homeowner occupies the home during the winter but vacates the home during the summer and sets the thermostat at 26.7°C (80°F). Field data shows the setting may not enable the air-conditioner to adequately dehumidify the conditioned space. Moist air makes its way through the ceiling and into the sealed attic. The spray foam (being permeable) allows the water vapor to diffuse through the foam toward the sheathing. The movement occurs when the roof sheathing is colder than the attic air, during the late evening as night-sky radiation cools the roof. If the roof sheathing is colder than the dew point of the attic air, then condensation occurs on the underside of the sheathing. House 4 data shows the underside temperature of the sheathing to not drop below the dew point temperature computed at the same location (between the sheathing and open cell spray polyurethane foam insulation). Therefore, it appears that during the summer the moisture flow to and from the sheathing, is by vapor diffusion. It is of keen interest to document these phenomena during the milder winter months which ORNL/UF are interested in better understanding for determining risk assessment and building modifications that would minimize the risk for condensation in the roof deck and thereby improve building durability.
FRM
Parker, D., P. Fairey, and L. Gu. 1993. “Literature Review of the Impact and Need for Attic Ventilation in Florida Homes.” Florida
Solar Energy Center, FSEC-CR-1496-05, 1993.
Walker, I.S. 1998. Technical Background for Default Values used for Forced Air Systems in Proposed ASHRAE standard 152P.
ASHRAE Trans. Vol.104 Part 1. (presented at ASHRAE TC 6.3 Symposium, January 1998. LBNL 40588.
Rudd, A. and Lstiburek, J. “Vented and Sealed Attics in Hot Climates,” ASHRAE Trans. 1998. V. 104, Pt. 2.
Roppel, P., Norris, N. and Lawton, M. 2013. “Highly Insulated, Ventilated, Wood-Framed Attics in Cool Marine Climates. ASHRAE
Thermal Performance of the Exterior Envelopes of Whole Building XI International Conference.
Miller, Railkar, Shiao and Desjarlais. 2016. “Sealed Attics exposed to two Years of Weathering in a Hot and Humid Climate.” in
Thermal Performance of the Exterior Envelopes of Buildings, XII, proceedings of ASHRAE THERM XII, Clearwater, FL., Dec. 2016.
Lstiburek, J.W. 2015. “Venting Vapor,” ASHRAE Journal Aug, 2015, p. 46 – 51.
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