Thermal Performance of Unvented Attics in Hot-Dry Climates: Results From Building America

Hendron, Robert; Farrar-Nagy, Sara; Anderson, R.; Reeves, Paul; Hancock, E.

doi:10.1115/isec2003-44070

Cited by 5 publications

(8 citation statements)

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“…Actual measurements of the energy performance of sealed and insulated attics have been rarely reported in the literature, and they have almost exclusively included homes in hot-dry climate regions (Hendron, Anderson, Reeves, & Hancock, 2002;Hendron, Farrar-Nagy, Anderson, Reeves, & Hancock, 2003;Parker et al, 2002;Rudd & Lstiburek, 1996). In general, when measurements have been made, they are for only a very small number of homes, for limited time periods ranging from two days to one month.…”

Section: Measured Energy Performancementioning

confidence: 99%

A Literature Review of Sealed and Insulated Attics—Thermal, Moisture and Energy Performance

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Walker

Levinson

2016

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In this literature review and analysis, we focus on the thermal, moisture and energy performance of sealed and insulated attics in California climates.Thermal. Sealed and insulated attics are expected to maintain attic air temperatures that are similar to those in the house within +/-10°F. Thermal stress on the assembly, namely high shingle and sheathing temperatures, are of minimal concern. In the past, many sealed and insulated attics were constructed with insufficient insulation levels (~R-20) and with too much air leakage to outside, leading to poor thermal performance. To ensure high performance, sealed and insulated attics in new California homes should be insulated at levels at least equivalent to the flat ceiling requirements in the code, and attic envelopes and ducts should be airtight. We expect that duct systems in well-constructed sealed and insulated attics should have less than 2% HVAC system leakage to outside.Moisture. Moisture risk in sealed and insulated California attics will increase with colder climate regions and more humid outside air in marine zones. Risk is considered low in the hot-dry, highly populated regions of the state, where most new home construction occurs. Indoor humidity levels should be controlled by following code requirements for continuous whole-house ventilation and local exhaust. Pending development of further guidance, we recommend that the air impermeable insulation requirements of the International Residential Code (2012) be used, as they vary with IECC climate region and roof finish.Energy. Sealed and insulated attics provide energy benefits only if HVAC equipment is located in the attic volume, and the benefits depend strongly on the insulation and airtightness of the attic and ducts. Existing homes with leaky, uninsulated ducts in the attic should have major savings. When compared with modern, airtight duct systems in a vented attic, sealed and insulated attics in California may still provide substantial benefit. Energy performance is expected to be roughly equivalent between sealed and insulated attics and prescriptive advanced roof/attic options in Title 24 2016. System performance can also be expected to improve, such as pull down time, performance at peak load, etc. We expect benefits to be reduced for all advanced roof/attic approaches, relative to a traditional vented attic, as duct system leakage is reduced close to 0. The most recent assessments, comparing advanced roof/attic assemblies to code compliant vented attics suggest average 13% TDV energy savings, with substantial variation by climate zone (more savings in more extreme climates). Similar 6-11% reductions in seasonally adjusted HVAC duct thermal losses have been measured in a small subset of such California homes using the ducts in conditioned space approach.Given the limited nature of energy and moisture monitoring in sealed and insulated attic homes, there is crucial need for long-term data and advanced modeling of these approaches in the California new and existing home contexts.

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Section: Measured Energy Performancementioning

confidence: 99%

A Literature Review of Sealed and Insulated Attics—Thermal, Moisture and Energy Performance

Less

Walker

Levinson

2016

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“…Numerous studies have evaluated the energy savings associated with in-creasing the energy efficiency of the United States residential building stock [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Many studies have focused on various aspects of residential buildings such as heating, ventilation, and air-conditioning (HVAC) equipment, façade, air tightness, insulation levels, duct leakage, and construction type.…”

Section: Introductionmentioning

confidence: 99%

Using BEopt (EnergyPlus) with energy audits and surveys to predict actual residential energy usage

Rhodes

Gorman

Upshaw

et al. 2015

Energy and Buildings

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“…Also, since an unvented attic is better sealed from the outside than a vented attic, the whole house infiltration is usually decreased which further improves the space conditioning energy consumption (Hendron et al, 2003). The sealed attic design has been measured to save 8% in cooling energy in hot-dry climate zones (Hendron et al, 2003). Because of these benefits, unvented attics were incorporated into the International Residential Code (IRC) sections R806.5 and N1102 in 2006R806.5 and N1102 in (IRC, 2015.…”

Section: Introductionmentioning

confidence: 99%

“…Since the temperature differential between the duct air and attic air is decreased, less energy is lost from the ducts. Also, since an unvented attic is better sealed from the outside than a vented attic, the whole house infiltration is usually decreased which further improves the space conditioning energy consumption (Hendron et al, 2003). The sealed attic design has been measured to save 8% in cooling energy in hot-dry climate zones (Hendron et al, 2003).…”

Section: Introductionmentioning

confidence: 99%

Investigation of the proposed solar-driven moisture phenomenon in asphalt shingle roofs

Boudreaux

Pallin

Jackson

2016

Journal of Building Physics

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Unvented attics are an energy-efficiency measure to reduce the thermal load of the conditioned space and decrease the space conditioning energy consumption by about 10%. This retrofit is usually done by spraying polyurethane foam underneath the roof sheathing, and on the gables and soffits of an attic to provide an air barrier and a thermal control layer. Unvented attics perform well from this perspective, but from a moisture perspective sometimes homes with unvented attics have high interior humidity or moisture damage to the roof. As homes become more air tight and energy efficient, a better understanding of the hygrothermal dynamics of homes with energy-efficient envelopes becomes more important. One proposed reason for high unvented attic humidity has been that moisture can come through the asphalt shingle roof system and increase the moisture content of the roof sheathing and attic air. This has been called ''solar-driven moisture.'' Oak Ridge National Laboratory investigated this proposed phenomenon by examining the physical properties of a roof and the physics required for the phenomenon. Results showed that there are not favorable conditions for solardriven moisture to occur. Oak Ridge National Laboratory also conducted an experimental study in a home with an unvented attic and compared the humidity below the roof sheathing before and after a vapor impermeable underlayment was installed. There was no statistically significant difference in absolute humidity before and after the impermeable underlayment was installed. The outcomes of the theoretical and experimental studies suggest that solar-driven moisture does not occur in any significant amount.

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Thermal Performance of Unvented Attics in Hot-Dry Climates: Results From Building America

Cited by 5 publications

References 0 publications

A Literature Review of Sealed and Insulated Attics—Thermal, Moisture and Energy Performance

A Literature Review of Sealed and Insulated Attics—Thermal, Moisture and Energy Performance

Using BEopt (EnergyPlus) with energy audits and surveys to predict actual residential energy usage

Investigation of the proposed solar-driven moisture phenomenon in asphalt shingle roofs

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