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Building Performance Begins on the Blueprints

Thermal shorts and good design don't mix
March 28, 2016

Stopping leaks in houses begins with stopping leaks on paper. The Pen Test* is a way to eliminate leaks on paper before construction begins.

Wherever one material or component meets another is a place that can leak. If there is no plan to stop water, heat, and air from flowing through the gaps, the gaps will compound into callbacks and unhealthy houses.

The joint between the foundation and the earth is one such place where water can move through capillary action and heat can flow through conduction. Insulation below a slab will slow most heat flow, but often the edges are not entirely detailed—which paves the way for capillary flow and cold floors.

Similarly, the framing represents an uninsulated bridge to the outdoors. Air can also leak through these joints. If the air leak is in the slab, it can draw soil gasses and radon into the living space.

Where framed walls meet, framed floors afford many more leaky spots. Heat can leak through framing that bridges indoors and out, and air can leak through gaps in the framing.

Some holes are engineered into the wall assembly—called windows—which amount to three-dimensional holes that can be tricky to visualize. Gaps along the perimeter add up to pretty good-sized holes, so it’s smart to go the extra mile inside and out.

Again, wall/roof intersections are like the other spots: thermal bridges and 3-dimensional air passageways that allow air to flow into stud cavities, through holes cut for wires and outlets, and all over the place.

Spray foam insulation in a stud cavity is a great way to seal the cavities, but it won’t solve the thermal bridging problem or the problem of air leaks through framing gaps. That can be done with a layer of rigid insulation on the outside. Of course, thickening the walls means rethinking how they align. In this case, moving the wall solves the thermal bridge and air leak at the bottom of the wall.

To solve the problems at the slab, Make sure it is isolated with insulation, and the walls are sealed to the floor. The huge capillary connection can be solved with paint-on waterproofing over the footing before the wall is poured and over the inside of the wall.

Filling gaps in connections between building materials in wall, floor, and roof assemblies can improve home performance and help you pass the pen test with flying colors.

*Editor's note: The Pen Test has become a fairly popular method for double-checking one's three-dimensional logic. It is also a good punchline, which helps to explain its origins in the early days of Building Science Corporation and Dr. Joe Lstiburek's tough-love approach to training young engineers and architects. We just want to take a minute to nod to our buddy and BS mentor, Dr. Joe.

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