Here is our cozy Mineral Wool Sweater before the addition of the rainscreen.
The most time-consuming of these two features is the exterior insulation and all that installation details that entails. So we (I say we, but it is, of course, Carlton and his crew that have done this) covered the entire house with a cozy mineral wool sweater, so to speak. This 2 inch thick sweater of insulation (total R value of 8) provides an uninterrupted layer over the house that is insect, fire, water resistant and helps maintain the desired interior temperature with much lower energy use. It reduces thermal bridging - the transmission of heat through the parts of the building that are more conductive such as the wood studs and beams. Remember the R-value (defined as resistance to heat flow) for wood is about 1 per inch whereas most insulation ranges from 3 to 6.
This is a drawing of a thermal scan showing where the heat loss occurs in a normal stud framed house. You can see that more intense heat loss occurs through the windows but a significant amount happens through heat flow across the wood studs. In the average home built with 2' x 4' or 2' x 6' framing, approximately 25% of the exterior is wood which means LOTs of thermal bridging.
Here is an actual thermal image showing the interior of a house where you can clearly see the wood studs and bracing in the wall. The deep purple is probably due to air leaks through the intersection of the studs in the corner.
|From Erik North's Energy Auditing Blog|
When I re-sided my house about 3- 4 years ago , I covered the entire house with Polyisocyanurate foam sheets (polyiso for short) to reduce the thermal bridging. I chose polyiso because it has the highest R-value for common insulation materials and was relatively easy to install. Also that was early in my studies on energy efficient design. Disadvantages of polyiso are that it is made from oil, somewhat flammable, insects love to nest in it, and it has "global warming potential" from the blowing agents used in the making of the foam. So at that early stage in my studies, that's what I chose.
After realizing that there are many other options out there, we chose Roxul Rockboard 80 for John's house. It's much more of a pain to work with because it itches (some people aren't bothered by it, but I try not to touch it without gloves on) and the sheets come as 2' x 4' rather than 4' x 8' for foam sheets. Truthfully, I did not calculate the cost difference because I was set on Roxul and was not going to consider foam. I feel that this is the proper choice for John's house based on fire, insect, and water resistance.
Later in the construction process, we will do an energy audit so we can see how effective the cozy mineral wool sweater actually is.