Basically, residential buildings have a notable share of 22% annual energy consumption consume in the U.S. From this share of energy in the residential buildings sector, the building envelope devotes 42% of the consumed energy just by the primarily effects of heating and cooling loads. More generally, building envelopes account for 36% of the total building sector energy use due to heat gains and losses. For example, a 2,500 [ft2] single-family house in a cooler climate area of the U.S. DOE Climate Zone 5 with 5,500 heating degree-days, will require only 80 million Btu (23 MWh) annually for space heating, which impose cost about $800 at current typical residential natural gas rates which is responsible for 5.5 tons annual CO2 emission as well. To address these concerns, retrofit suggestions are always recommended. The presented study evaluates a case study of a single-family home of 2,400 square feet in Boulder CO built in 1975 is considered as a base model with no or minimal thermal insulating properties. The retrofit solution is "composite insulation boards containing foam-encapsulated vacuum insulation panels" for walls.
The energy demand is increasing gradually due to population growth, economic development and technological advancement. International Energy Agency (IEA) revealed that most of the world energy was mainly met by fossil fuels accounting for 81% while the rest were supplied by nuclear and renewable resources. From 2000 to 2014, there was an annual increase of 2.3% in the world total primary energy production (Enerdata, 2015). In the United States (US), the building sector is responsible of 40% of the national energy usage and contribute to about 33% of the greenhouse gases emissions (UNEP). Moreover, the gradual increase in the energy production from fossil fuels has detrimental environmental impacts including climate change and global warming (Wang, Wang, & Liu, 2017).
This project aims at investigating the impact of possible retrofitting measures for an existing wood-frame single-family housing located in Boulder Colorado built before 1975, currently comes with no wall insulation. The new retrofit should meet the 2015 International Energy Conservation Code for climate zone 5B accordingly R24. Also, the suggested retrofit resolution must provide some to prevent moisture issues through the wall. Furthermore, cost analysis of the suggestions is presented to show the cost-effectiveness of this technology.
2. METHOD OF APPROACH
While there are several methods proposed to retrofit an existing wall of a single-family house, I personally prefer to take advantage of Vacuum Insulation Panels (VIP) concept because of some properties such as very high thermal and moisture resistance provided by this type of materials. In a vacuum insulation, heat cannot travel through the air by conduction or convection; which limits the heat transfer through the wall and as a result leads to a high thermal insulating performance with absolutely very high R-value. Baetens et al (2010) subdivided the vacuum-based insulating solutions into three categories: Vacuum insulation panels (VIPs), vacuum insulating sandwiches (VISs) or sheet-encapsulated vacuum insulation panels.
Saber et al in 2015 predicts a more reasonable cost for this technology by improving production capacity and efficient manufacturing techniques as well as alternative and less expensive core material for VIPs that will results more cost-effective for building envelopes.
Schiavoni at in 2016 describes VIPs structure including three parts: a core material installed with getters, desiccants and opacifiers and covered by a multilayer envelope. The core is made of a small porous material (diameter about 10 nm) with low thermal conductivity, mostly made by fumed silica. Getters and desiccants are installed to adsorb respectively gases and water, whereas opacifiers hinder the propagation of infrared radiation. The vacuum insulation panel can reach thermal conductivity values down to 0.004 W/mK with a thickness of 5mm and a service life between 60 and 160 years.
There are several researches done on the application of VIP in the building envelope. Meanwhile, I decide to utilize "composite insulation boards containing foam-encapsulated (polyisocyanurate) VIP" on the exterior side, which is based on a recent research done by Biswas et al., 2018, with a high advantage of low-cost vacuum insulation called modified atmosphere insulation (MAI), as it involves significantly fewer processing steps- which makes the product is more cost- effective compare to traditional VIPs. In this method, the composite foam-vacuum insulation boards were created in a semi-automatic operation in a foam. The production process of MAI is much simpler than traditional vacuum insulation manufacturing, and it has the potential for significant cost reduction at the same thermal performance. As far as thermal resistance, the composite board can achieve a of 4.5m2K/W at 5.1 cm thickness; in comparison, current foam and fibrous building insulations can only achieve 2.1 m2 K/W or less at the same thickness.
The composite foam boards consist of 2.5 cm thick MAI panels that are sandwiched by1.3 cm high-density (HD) polyisocyanurate cover boards and 1.3 cm standard polyisocyanurate (PIR) foam; the total composite board thickness is 5.1 cm. The HD board acts as a rigid substrate to which the MAI panels can be adhered, followed by lamination with standard PIR insulation in an in-line manufacturing process. This is similar to existing HD composites. PIR insulation is the most widely used insulation material in low-slope roofs in the U.S., with ≥70% of the market share, comes with a thermal conductivity similar to polyurethane (0.02–0.03 W/m K), which is the lowest among current building insulations.
Finally and as far as the cost, high-performance MAI panel production process cost reduced to only $0.15/R-value/ft2 thanks to the new manufacturing technology, which is considered in a reasonable cost range compare to $0.06-0.10/R-value/ft2 per each inch-thick foam insulation boards. In this case, although the resident takes advantage of very high-insulated vacuum concept in his retrofitted façade, he's paying almost in the same order of magnitude of a conventional retrofit solution. In addition, the installation cost is negligible as these panels could be attached to the exterior without paying any labor cost.
3. CONCLUSIONS AND RECOMMENDATIONS
The presented study aimed at evaluating energy retrofit measures for an existing single-family house in Boulder CO. The retrofit solution is based on vacuum concept, which provides an excellent compressed thermal performance of RSI 4.5 m2K/W with only 5.1 cm thickness, that could easily meet IECC 2015 requirements as well as reducing CO2 emission, and save the space as well. Also, due to the vacuum concept utilized in walls, moisture would not be an issue anymore in the envelopes. Furthermore, those technologies are still located in the reasonable cost range. Finally, indoor air quality is much higher compare to a traditional insulation system, as basically the vacuum concept provides a clean environment compare to a traditional blanket insulation, also because of the location of insulation which would be at the exterior- compare to adding some insulation to the indoor which also will not occupy the interior space.