ASHRAE Journal - June 2011 - 83

can be added through the exterior of the existing walls, or if the studs or sheathing of the house is exposed during the retrofit directly into the wall cavities. The easiest retrofit is to add blownin or spray insulation to wall cavities without insulation. To do so, holes are drilled through the exterior of the home, allowing access to these cavities. Blown-in insulation can be cellulose fibers, fiberglass, or even soybean based polyurethane.5,9,10 Advantages of blown-in insulation that is properly installed is that it effectively surrounds all obstructions in the wall cavity such as pipes and wire conduits, successfully insulating the walls. If the home retrofit includes new siding, high-density foam can be installed over the existing sheathing, creating a new vapor and air barrier for the home. Insulation installed over the sheathing not only provides added R-value but aids in air sealing and removing thermal breaks.10 Additionally, if the existing sheathing or studs are exposed during retrofit, it is suggested that an air barrier is used; this consists of wrapping the house in a material permeable to moisture, but not to airflow.5 The common attic and roof retrofit solution is to add more insulation to the attic. Alternative efficient retrofits include the construction of a cool roof, or installation of a PV-PCM roof. One company installs a cool roof in the following manner: • After removing the existing roof the sheathing is covered with a synthetic roof underlayment. • Over the underlayment, 1.5 in. (38.1 mm) or 3 in. (76.2 mm) of rigid foam is installed using 2X4s to frame the insulation. • Additional 2X4s are used as spacers to create a gap between the rigid insulation and the new roof sheathing. The new roof, its sheathing, and roof ventilation and drip guards are installed. The addition of insulation with an air gap not only increases the roof’s R-value but also enables the roof to maintain a constant temperature in cold climates. As a result, water damage from melting
June 2011

Year of Construction Before 1950 1950 – 1960 1960 – 1970 1970 – 1980 1980 to present

Percent of Total Housing Units* 20.40% 11.50% 11.60% 16.70% 39.90%

Wall Insulation R-Value 4 3 or 6.2 6.2 or 11 11 and 5† 11 and 5;† 19 and 5†,‡

* There are roughly 127.7 million homes in the U.S., according to data taken from 2005 – 2009. † “and” indicates use of multiple types of insulation. ‡ Each set of numbers represents the R-value for a common insulation combination used.

table 1: U.S. housing stock and typical insulation R-values per construction year.5,6
Window Type Single-Glazed With Storm Window Double Glazed Double Glazed With Low-e No Window (Insulated Walls Only) Super Window Annual Cost $1,310 $1,228 $1,120 $1,000 $960

table 2: Heating costs (average house in a heating climate) with different window types.8 snow leaking and/or refreezing as ice is minimized or eliminated.11 PV-PCM roofs are installed over the existing roof using metal roof panels, and PV laminates with phase-change material (PCM) heat sinks. The intent is to reduce the home’s heating load by using solar energy to maintain a warm attic space through thermal release from the PCM. The intended and test environment for these roofs are homes in southern climates. The solar energy absorbed by the PV panels that is not converted to electric energy is stored in the PCM. At night, the PCM releases the stored energy to maintain high attic air temperatures, which reduce heat loss through the roof. When compared to traditional shingled roofs, 18% to 30% lower weekly heat loss was measured.12 Walls and roofs are often broken up by windows and skylights. Windows can be retrofitted or completely replaced. Complete replacement is best done as part of a major remodeling project; when windows are completely replaced exterior damage is usually incurred as the window glass and frame are both removed from and replaced in the house.13 A retrofit window is installed into the old frame and damage to the interior and exterior walls and trim of the house is avoided. Window retrofits are specified based on a window’s U-value, or heat transfer coefficient, and solar heat gain coefficient (SHGC). The type of window used is climatedependent. Heating climates require low-U, high-SHGC windows to decrease heating losses while taking advantage of solar heat gain. Cooling climates require low-SHGC windows to assist in keeping the conditioned space cool and well lit.1 Through a simulation of window performance in a heating climate in an average home, the DOE calculated the annual costs for heating based on type of window installed shown in Table 2. DOE and private investors are funding increased research and manufacturing of “smart windows.” Smart windows are electrochromic or thermochromic units that tint manually or automatically based on sunlight intensity and outdoor thermal conditions. They allow for a broader spectrum of control over the visible light transmitted, and heat gained or lost through windows. In addition to furthering the development and production of smart windows, the DOE, in conjunction with Lawrence Berkeley National Laboratory, plans to produce a cost-effective R-5 window.1 DOE is also pursuing the development of R-10 windows, or windows having less heat loss than reasonably well-insulated walls.
ASHRAE Journal 83

ASHRAE Journal - June 2011

Table of Contents for the Digital Edition of ASHRAE Journal - June 2011