What building science means for a custom home

A finished house is judged by what you can see: the millwork, the stone, the way light comes through the windows. How long all of that keeps looking the way it did on move-in day is decided by things you cannot see. How the house sheds water. Where its air leaks are. Whether its walls and roof can dry out after a wet spring and a long winter. That is the territory of building science, and over the next several months we will take it up here one piece at a time.

What is building science?

The phrase gets used loosely, often as a label for whatever product is being sold. Used properly, it means the physics of how heat, air and moisture move through a building, combined with practical knowledge of materials and construction, and tested against what houses actually do over years of use. John Straube, one of the field’s leading researchers, describes it as the place where those three meet. A habit that has never been checked is not building science. Neither is a calculation that looks good on paper and fails in the field.

For us, it comes down to a simpler question we ask on every project.

What happens to this house when it rains sideways, when it is ten below for a week, and when the family inside is cooking, showering and living in it?

A house works as one system

The roof, walls, foundation, windows and mechanical equipment are not separate decisions. Each one changes the others. Make the walls tighter and the house needs a planned source of fresh air. Move the insulation up to the roof and the attic becomes part of the house. Add a powerful range hood and the air it removes has to be replaced from somewhere. The Department of Energy calls this the whole-house approach, and it works best at the start of design, before anything is ordered or framed.

What the walls and roof have to do

The enclosure, everything that separates inside from outside, has more jobs than most people realize. It holds the house up against wind, snow and soil. It carries the finishes. And it has to manage four things: liquid water, air, heat and water vapor. Building scientists and the trade press, from Building Science Corporation to Fine Homebuilding and Energy Vanguard, rank them in about that order, and for a practical reason. Each depends on the ones before it. Insulation does little in a wall that takes on rain, and a carefully placed vapor retarder accomplishes nothing if air is leaking around it.

Water first, then air

When we open up an older house, the damage we find almost always traces back to water: soft sheathing under a window, a stained ceiling below a valley, paint that will not stay on one wall. Most of it is rain and snowmelt that found a path in. That is why the roof, flashing, the drainage layer behind the siding or stone, and the grading around the house get attention first.

Air is next, and it is the one most often underestimated. In winter, warm indoor air that leaks into a wall or attic brings moisture with it, far more than ever soaks through solid materials, and leaves it on the first cold surface it reaches. The same leaks make rooms drafty and waste energy. Controlling air drives much of what we do in the walls and roof of a new house, and it is the subject of an entry in November.

Why it matters here

Metro Detroit has long heating seasons and humid summers. Moisture is pushed outward through the walls in winter and inward in summer, so assemblies have to be able to dry in at least one direction. Snow and freeze-thaw cycles concentrate stress where roofs meet walls, and that junction is where many problems start; it is also where the water, air and insulation layers of the roof and walls have to connect without a gap.

Much of our work is on older homes being renovated or expanded. Adding insulation and airtightness to an old house changes how its walls dry. Done thoughtfully, it makes the house more comfortable and durable. Done without thinking it through, it can create problems the house never had.

None of this replaces craftsmanship. It protects it. In the weeks ahead we will take these ideas one at a time, alongside entries on design and craft: airtightness and ventilation in November, then attics, ice dams and winter humidity in December.

Further reading: U.S. Department of Energy, Whole-House Systems; Fine Homebuilding, Crash Course in Control Layers; Energy Vanguard, Breaking Down a House and The Basics of Control Layers; Building Science Corporation, BSI-152: What is Building Science?.

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The Journal shares general information about home design and construction for educational purposes. It is not professional advice for any specific property and does not replace guidance from a licensed architect, engineer or your local building department. Codes, products and site conditions vary, and recommendations change over time. Views expressed are those of the author. Reading this article does not create a client relationship with Mosher Dolan; any work we perform is governed solely by a written contract. Full Journal disclaimer.