SUSTAINABLE EXTERIOR WALL – WITH BILLBOARDS?

Minnesota is an extreme place to live. The state’s climate is well known throughout the U.S. for frigid winter temperatures – most Minnesotans I’ve met have a cold-weather anecdote where a friend of a friend comes surprisingly close to death after committing some minor error in judgment on a 30 below evening. The state’s summer heat and humidity is less notorious, but temperatures can reach 95 and higher with 70+ degree dewpoints. In any given year, outside temperatures can swing 120 degrees and relative humidity in your home can climb to 95% or drop as low as 10%.

Designing a high-performance insulating wall is tough with that kind of temperature and humidity range. Walls are barriers that attempt to block nature from achieving balance; heat will always drive towards cold, humid air will always migrate towards dryer environments. Architects and builders ignore physics at their peril, if not designed properly, a wall will trap moisture and start to degrade. Soon afterwards, the resulting mold will spoil indoor air quality.

There are other competing interests when designing walls. More structure and insulation, which is desirable for obvious reasons, is at odds with adding more windows. Exterior claddings that are more labor intensive (stucco and brick for example) are expensive – and porous! Less expensive materials (vinyl and metal sidings for example) can be toxic to manufacture or may not hold up well over time. In our practice, we design as close as we can to the ideal – walls where we increase insulation, increase light and views, provide adequate structure, reduce maintenance, and create dynamic space.

Here’s how we did it at nowHaus. For the exterior layer, we chose polycarbonate sheet. It’s manufactured for commercial greenhouses, is impervious to water, and won’t require the maintenance of typical siding (i.e. staining, painting, tuckpointing). This outer “skin” protects the more fragile layers of the wall behind.

The next layer, directly behind the polycarbonate, is the most abundant and least expensive on the entire project – air. Varying in thickness from 1.5 inches to 4 feet, the air space serves several functions. It acts as a weep (a tool to get water out). Any water or condensation that might get behind the polycarbonate has a chance to dry out, as the sheets are installed with micro vents top and bottom of the wall. This prevents mold from forming inside the wall. Secondly, the air provides a thermal blanket for the house on sunny days in the winter. When the sun hits the polycarbonate, it warms the thin air layer. This is critical to energy performance; it effectively reduces the temperature differential between inside and outside, thus reducing heat loss over our 7 month heating season. The air layer also slows heat loss at night.

The third layer of the system utilizes a waste product from the advertising industry – discarded vinyl billboards. These huge vinyl sheets are extremely durable, yet – ironically – are manufactured for short campaigns with a brief shelf life. We acquired full sheets from a local billboard company, cut them into 48″ strips, and used them like giant siding. The strips fulfilled the function of the building’s moisture barrier (think Tyvek), while the gaps between strips allow the layers behind to breathe. Since the polycarbonate is translucent, the billboards play an aesthetic role on the exterior, transforming the façade under different lighting conditions.

Behind the billboards, we installed 4′ X 8′ sheets of ThermaCal polyisocyanurate insulation, using long screws to fasten the insulation to the outer surface of the plywood sheathing. Since these sheets are on the outside face of the 2X4 framing of the wall, there are no gaps – thermal bridges for quicker heat loss – in the insulation.

Behind the insulation, we installed a self-sealing vapor barrier. This location is particularly important, as it ensures this thin membrane remains a minimum of 4” away from the inner and outer finishes, minimizing the risk of inadvertent punctures from construction, mechanical or electrical work, or a nail to hang a painting.

Behind all these layers stands a standard 2X4 wall with exterior sheathing. All the systems that typically reside in the wall – wiring, ducts, etc. – have plenty of room, as we installed the insulation outbound of the structural wall.

nowHaus Wall Section

We completed the wall design with an interior finish of 1/2″ pre-finished birch veneer fiberboard. At about the same installed cost of gypsum board, we feel the wood is a much better solution than drywall. It recalls a Minnesotan Scandinavian tradition, gives warmth to a modern space, and has a depth not achieved with paint finishes. As with all layers of the system, this layer works on multiple levels – it is economical, expressive, and sustainable.

We’ve been asked questions about our choices at nowHaus. Wouldn’t another organic product have been better than polycarbonate? Aren’t you giving tacit approval, by creating a second market, to the manufacture of vinyl billboards? Wouldn’t reclaimed denim insulation be a better eco choice than polyisosanurate? While all valid questions, we feel this design a) addresses a common building problem (moisture inside exterior walls) with a new solution that gives thermal benefits, b) calls attention to a questionable practice (manufacturing vinyl billboards) yet also provides an end use for the material (which is commonly shipped over the U.S. border for disposal), and c) provides an unbroken, stable, and inert thermal enclosure that should save energy and resources over its usable life. As with all our work, we try to weave creativity, innovation, research, and sustainable thinking into new ways of building cost effectively. There’s always a better way to do something, and we’re always looking for it.