DOME MAGAZINE: Winter 1993-94, Vol. 6 | No. 2
This article will deal with a subject that I have discussed in the past on several occasions. The explanation and importance of the double-wall structure is necessary, as the advent of alternative wall systems has placed some information in the public domain that is not only suspect, but incorrect.
There are two dome companies, that I am aware of, which manufacture both double-wall and single-wall dome designs. Both of these are manufacturers of hub-and-strut systems. Both of these dome companies are located here in Minnesota.
One company is Natural Spaces, and the other is Energy Structures, Inc. To those of you who know Minnesota, you are well aware that seasonal swings of temperatures of –20o F to +100o F are not uncommon. These wide temperature swings have provided a good laboratory for the testing and development of the double –wall design. And, I might add, to its success. Together, these dome companies have more than 25 years of cumulative experience in the research, design and manufacture of double-wall domes.
The Double-Wall System
The double-wall system, when used in the framework of domes, is comparatively cost effective. This design actually takes less material than the standard six-inch wall, panelized dome system, for the following reasons.
The mainframe of the system is standard building material, 2 x 6, No. 2 or better, construction-grade lumber. The use of “standard building materials” assures low-cost construction because of the volume of use by the building industry, as opposed to exotic and expensive building materials.
The interior wall of this particular type of construction is standard, 2 x 4 construction-grad lumber. The exterior mainframe carries the building load and is structural. The interior wall is non-load bearing and is located 14.5” to 16” from the outside strut. The two walls are joined by plywood gussets.
This space is designed to accept two –six-inch bats or more of un-faced fiberglass insulation. Once again, this material is standard building product with its subsequent economy. Two six-inch bats are recommended over one twelve-inch bat, because the twelve-inch bat costs almost two times the cost of two six-inch bats. The reason, once again, is that the six-inch bats are standard building material with large production, and the twelve-inch bats are special, with low production and use.
You will note that the space between the struts is 14.5”, and the recommended insulation thickness is 12”, providing a nominal R value of 42 or more over the whole dome surface. Add to this high R value and the reduced surface area of the dome and you have exceedingly low design heat losses. This leaves a 2.5-inch air space to provide a vapor path to make certain design heat losses. This leaves a 2.5” air space to provide a vapor path to make certain that any moisture that does get into the cavity has an avenue of escape.
Wall Ventilation
This raises the question of cavity or wall ventilation, used in residential construction. That subject is almost unheard of in standard wall construction by commercial home builders and architects. For some strange reason, proponents of the single-wall systems have failed to comprehend a basic law of physics, or, they simply do not want to talk about it.
That law simply states that moisture (humidity, if you will) moves from hot to cold. That means that household humidity is constantly migrating through the wall to get to the cooler outside surface, where it condenses on the interior surface of the exterior panel. This is true regardless of the type of insulation that is used.
Care is taken, when the building is being constructed, to slow down this moisture migration process, as much as possible, by the installation of a six-mil poly vapor barrier on the inner side of the insulation. But it is almost impossible to make the total interior surface completely air tight. Which means that it is almost impossible to keep the humidity from entering the cavity. For that reason, the 2.5” space on the exterior of the insulation provides a space for the collection of the moisture, and a vapor path to take the moisture out of the cavity and to make certain that the cavity remains dry.
This is where we part ways with standard single-wall design. It is probable that if some method of keeping the interior of the wall dry is not designed into the system, ultimately, the wall will fail. Some method of keeping that cavity dry must be provided. That ventilation detail works this way. Openings are provided at the base of the dome shell that provide for the entering of dry air from the atmosphere. The struts have holes or notches installed when manufactured, and, of course, the struts are open at the connectors.
The installation of spacers on the interior of the exterior panel keeps the insulation away from the exterior shell and provides a path for the spelling of any possible moisture accumulation. Several methods are used to ensure that the moisture escapes from the cavity. One is the use of a powered roof (attic) ventilating fan that ensures positive cavity ventilation. Other options are ventilating cupolas, or standard wind-powered turbines. Any of these systems will facilitate the removal of moisture from the cavity.
The proponents of rigid insulation, which costs considerably more than fiberglass, fail to consider what happens to moisture that will inevitably enter that wall when there is no way out of the wall cavity.
Humidity Control
Humidity control is a definite problem in residential housing. Cooking. Breathing, bathing, etc., does add humidity to the house, but during winter months, many times, during the heating season, it is necessary to increase or decrease the humidity level by the use of auxiliary equipment to reach a point of comfort. This is also true during the summer months, when it is sometimes necessary to decrease the humidity using dehumidifiers or air conditioning.
Though I do not doubt the sincerity of the proponents of the straw and clay wall, or those who propose no vapor barrier, I have difficulty in understanding the theory that this system would ensure a comfortable and healthy humidity level throughout extremes in temperatures.
Fuller’s Dream
The understanding of the double-wall theory, and how it works, prescribes to the basic philosophy of “more with less”, a theory to which Buckminster Fuller dedicated his life. After reading and rereading The Grunch of Giants, as well as Nine Chains to the Moon, I am more and more convinced that Buckminster Fuller was one of the greatest human beings in all history. Part of his dream was that the dome system would take 50 years to become the housing of choice. Those 50 years have passed, and it still looks like the average residential builders and buyers have their collective heads in the sand. Though the thought of the dome becoming the housing of choice may still seem remote, the world is changing rapidly. Would we have seen the worldwide changes that were unthinkable only a few short years ago? So, collectively to all of the dome manufacturers, hang onto your dream. That day may be closer than you think.