Feasibility of adding ridgid insulation to the outside of an existing single family home
Many different things can be done to improve a building envelope, such as improving the windows, doors, air tightness, and amount of insulation in or on the walls. This outline will describe to potential benefits to Adding additional insulation to the outside of a home. Naturally, any additional form of insulation will improve the R value of a wall. Expanded polystyrene (XPS) however seems the best choice due to its properties. It “works well in wet conditions, and can act as a vapour retarder” (©1996 – 2011 CMHC-SCHL). It also has one of the highest R ratings per inch, roughly 4.5 - 5.0 (information gathered from Canada Mortgage & Housing Corp.) The feasibility of adding XPS to a house is dependent on a number of things, such as how much insulation will be added, whether it will be added to the outside or the inside, and the overall size of the house. Adding the XPS to the outside would most likely be more efficient, since you would simply have to remove the exterior cladding rather than the inside layer of drywall. Also note that after so many layers of insulation, the heat retention capability of the wall does not increase proportionally. More than 2-4 inches of insulation will make the project unfeasible due to cost.
Other advantages to adding XPS to the outside of a house would be to minimize thermal gaps created over time when the batt insulation inside the stud wall starts to slump, creating air gaps at the tops of the walls. It also minimizes thermal bridging caused by the studs themselves. “When only wall cavities are insulated, fully one quarter of the wall surface is left un-insulated” (© The Dow Chemical Company (1995-2011).)
One of the main disadvantages of adding XPS to the outside of the wall is its susceptibility to moisture condensation and subsequent moisture build-up inside the wall. “According to some builders, rigid foam insulation on the exterior of the building carries an increased risk of water damage inside exterior walls, because the foam can trap moisture and prevent walls from drying to the exterior.” (©Sept. 2010 greenbuildingadvisor.com) Another issue would be how to attach the siding back on the house with the extra insulation sandwiched in-between. There are some ways that would work, such as “installing 1x3 or 2x4 horizontal strapping, 24 inches on center, on top of the foam” (©Sept. 2010 greenbuildingadvisor.com), and then attaching the old siding to the house. Nevertheless, if done right it will improve the the reduction of a buildings heat loss through the walls. The cost of the materials and labour for the house in question would roughly be $2430 and $1511 respectively (see cost estimate sheet). This overall cost will then be deducted from heat savings by the heating system annually until finally the project will have paid itself off. In order to calculate the cost savings annually, heat loss calculations on both the improved and old wall system will have to be done. These calculations will be studied in detail during the fall semester in the 3rd year ARET 3500 course. (Cost Estimate Sheet not included)
