As we continue with our discussion about (affordable zero energy homes) and High Performance Construction Methods, our subject for today’s blog will focus on a very important connection point where double-stud exterior walls intersect with the roof system. Here is where a lot of energy treatments must be installed correctly. Unfortunately, the temptation for many builders is to get the roof on as quickly as possible and it’s all too easy to skip right on by and then attempt to do it later. Sadly, some things just can’t be done as well later.
One spot where a lot of energy can be lost is at the edge of the house where the ceiling insulation intersects with the exterior wall. We already know that the double stud wall will perform at R45, but if the insulation in the attic isn’t installed in such a way as to completely overlap the full thickness of the stud wall below, valuable heat will find its way to the outside at this location. Think of it like climbing into a warm bed on a cold night and finding out that the quilt you are covering yourself with isn’t long enough to cover your feet. I guarantee that you will want a longer quilt. It’s not much different with attic insulation. If the attic insulation blanket doesn’t overlap the exterior wall, much of the heat will escape and create a cold corner at the ceiling where moisture can condense and create a condition for mold to grow.
Another factor to keep in mind is that wind can blow its way into attic insulation that isn’t protected with blocking and air sealing around the entire perimeter. This can cancel out the entire performance of the insulation around the perimeter of the home where it’s needed the most. This blocking is often referred to as “wind blocking” or “snow blocking.” It’s very easy to do before installing plywood on the roof. It’s even easier to apply caulking and foam around all the gaps. Wind blocking must be the same height as the thickness of the insulation but should stop shy of the roof plywood to allow for free air to ventilate over the insulation. Builders who are in a hurry will rarely get this right. It may look ok from the floor, but if it’s not totally air sealed to prevent drafts from infiltrating, then it’s all for show.
When ordering trusses, we are asked by the truss manufacturing company what the “heel height” will be. The height of truss heels are determined by adding 2-3 inches to the thickness of the proposed insulation blanket. For example, if you are planning for R60 blown cellulose insulation in your attic, you will need about 20” of blanket thickness – blown cellulose performs at R 3.13 per inch. So when you are ordering your attic trusses the energy heel of the truss should be 20” (same as insulation) plus an additional 3” to provide an unrestricted air shaft over the insulation for proper ventilation. In the drawing provided with this blog, you will see a proposed energy heel of 22.”
Next time: The science of air sealing – Should a house breathe?