Building Envelope

The building envelope is the most important element of energy efficiency in residential buildings, especially in the context of the IECC, which excludes equipment efficiencies from its scope.1 The 2012 IECC considerably improved the efficiency of building envelope components over the 2009 IECC. Efforts to increase the efficiency of building envelope components beyond the 2012 IECC levels were limited, and the 2013 code development cycle saw the incorporation of a number of code changes intended to simplify the code language and clarify the requirements set in the 2012 IECC. Among all the building envelope-related code changes approved for inclusion in the 2015 IECC, the one code change that impacts energy efficiency in a quantifiable way is CE66-13, Part II.

4.2.1.1 CE66-13, Part II: Definition of a New “Tropical” Climate Zone

Part I of this proposal targets the commercial provisions of the IECC, which are not the focus of this determination analysis. Part II of the proposal applies to the residential provisions of the IECC. This approved code change added a new “tropical” climate zone to the IECC along with a set of requirements that if met would imply compliance with requirements set in Chapter 4 of the 2015 IECC Residential Provisions. The tropical climate zone includes Hawaii, Puerto Rico, Guam, American Samoa, U.S. Virgin Islands, Commonwealth of Northern Mariana Islands, and islands in the area between the Tropic of Cancer and the Tropic of Capricorn. The requirements to be met in order to demonstrate compliance with Chapter 4 (Residential Energy Efficiency) are as follows:

• Not more than one-half of the occupied space is air-conditioned.

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HVAC and appliance efficiencies are preemptively regulated at the Federal level. See

• The occupied space is not heated.

• Solar, wind, or other renewable energy source supplies are not less than 80 percent of the service water heating energy.

• Glazing in the conditioned space has a SHGC of less than or equal to 0.40, or has an overhang with a projection factor equal to or greater than 0.30.

• Permanently installed lighting is in accordance with the requirements of the 2015 IECC.

• The exterior roof surface complies with one of the options specified in the commercial provisions or the roof/ceiling has insulation with an R-value of R-15 or greater. If present, attics above the insulation are vented and attics below the insulation are unvented.

• Roof surfaces have a minimum slope of 1/4-inch per foot of run and the finished roof does not have water accumulation areas.

• Operable fenestration provides ventilation area equal to not less than 14 percent of the floor area in each room. Alternatively, equivalent ventilation is provided by a ventilation fan.

• Bedrooms with exterior walls facing two different directions have operable fenestration or exterior walls facing two directions.

• Interior doors to bedrooms are capable of being secured in an open position.

• A ceiling fan or ceiling fan rough-in is provided for the bedrooms and the largest space that is not used as a bedroom.

This code change applies to a portion of homes considered to be part of climate zone 1 in the 2012 IECC determination analysis. Based on new construction starts data compiled from the 2010 Census data in the 2012 determination analysis, approximately 50 percent of the single-family construction starts attributed to climate zone 1 were in Hawaii (Mendon et al. 2013). To estimate the energy impact of this code change in the present analysis, a new climate zone called “climate zone 1-tropical” is added to the existing list of 15 climate zones. This is done solely for the ease of post-processing and aggregation of results in a streamlined fashion. The IECC did not change the climate zone map to reflect a new climate zone for the tropical areas and hence this does not impact work done by Briggs et al. (2003) referenced in Section 3.2. The representative climate location selected for the energy simulation of the tropical climate zone is Honolulu, Hawaii, consistent with cost-effectiveness analysis conducted for the State of Hawaii (BECP 2012b, Mendon et al. 2013).

While not mentioned specifically, the requirements to demonstrate compliance will likely apply to single-family homes only. Thus, this determination analysis assumes that new single-family homes built in climate zone 1-tropical, which represents approximately 50 percent of all new single-family homes built in climate-zone 1, would be eligible for this code change. No data are available to indicate how many eligible homes will use the tropical climate zone alternative. However, a building codes expert in Hawaii provided useful information for formulating a set of assumptions to quantify the energy impact of this code change1:

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Based on discussions with Mr. Howard Wiig, Department of Business, Economic Development and Tourism, State of Hawaii.

• Although the Hawaii state residential building code requires that all new single-family dwellings built in the State of Hawaii have a solar water heater, about 25 percent of new homes submit a “Request for Variance from Mandatory Solar Water Heater”; these are all accepted.

• No direct statistical data exist to arrive at an exact number of new homes that meet all the

requirements listed in CE66-13 Part II. However, it is considered reasonable to assume that perhaps 35 percent of all new homes built in the State of Hawaii meet the prerequisites and hence can opt for the proposed tropical climate zone alternative.

Because Hawaii dominates the new residential construction shares in the tropical areas and data required to segregate these between the different islands are not available, the current determination analysis assumes that 35 percent of all single-family homes built in the tropical climate zone will opt for the proposed alternative. Modeling this code change involves a change to the baseline single-family prototype building to match its characteristics with the homes that already meet the prerequisites. Thus, the 2012 IECC single-family prototype building models are modified to be semi-conditioned homes with solar water heaters such that:

• Only the second story of the single-family home is considered to be conditioned while the first story is kept unconditioned.

• The conditioned space is not heated.

• The energy required by the domestic water heater is assumed to be provided by a solar water heater based on the Hawaii state residential code.

A second set of models, i.e., the 2015 IECC models, is then created to match the requirements listed in this code change proposal:

• Glazing in the conditioned space is assumed to have a SHGC of 0.40. • The ceiling insulation level is assumed to be R-15.

The difference in energy between the 2012 IECC models and the modified prototype building models designed to imply compliance with the tropical climate zone alternative is aggregated with the other modeled code changes as described in chapter 3.0.