DOME MAGAZINE: Spring 1996, Vol. 8 | No. 3
The environmental differences between the geodesic dome and rectilinear structures are not very well documented. Problems of cavity ventilation, stratification, and particularly condensation, are not always well defined. Sometimes problems of simple creature comforts are not always disclosed. Why is my second floor bedroom so much warmer than the first floor? Of all the items that need special attention, the most important is the control of condensation. Controlling this problem in cathedral ceilings used in square houses is similar, and it is of particular importance in dome structures.
Also, the extent of condensation problems is more severe in climates that have extremes in temperatures. We received our experience, sometimes in the school of hard knocks, since we have had the fortune, or misfortune, as it were, of gaining this information in an area where we had both extremes in temperatures of –30o F to +100o F with which to deal. We have been able to find some of these answers.
Condensation Problems
One of the first calls I received from an irate customer, back in 1978, was to answer the question of why his comparatively new dome was leaking – and it had not rained for several days. This was early in December and we had had some extremely cold weather. The dome was a 2 x 6 wall structure, with fiberglass insulation. The insulation was covered with 6 mil ply, and the interior panels were finished with tongue and groove cedar. Several stain spots appeared on the pentagon panels on the dome top where some dripping had occurred. We had, previous to that time, recommended a return air duct to the second floor area. At that time, the reason was more practical. We wanted the temperature to remain relative between the first and second floor.
Natural Laws of Physics
Because of natural laws of physics, convection currents carry the warmed air to the dome top. If the dome were a single story structure, extra heat in the dome top would not represent any particular discomfort. However, most dome designs contain second floors, and the extra heat caused by these convection currents made that area uncomfortable. Especially since many plan designs have bedrooms in that location, all of our plans include a return air duct to the upper portions of the dome.
In this case, the customer did his own thing, and installed his heating system without a return air duct to the upper part of the dome. When I asked him where it was, he told me that “the heating contractor told him that he did not need it.” He did comment that his second floor bedrooms were warmer than the first floor.
We suggested that he run a duct to the upper portions of the dome, and using a small squirrel case fan, to pull the heated air from the dome top, back to the lower portions of the dome. He did. This caused the temperature to equalize, and in addition, it stopped the condensation problem. While it was pretty much theory at that time, and not much experience, we came to this conclusion. Since convection currents carry the warm air to the dome top, they also carry moisture (humidity).
Without any encouragement to move on, the air simply stays stacked up there until it cools off, and then it returns to the lower levels. In the meantime, that moisture laden air comes into contact with the cooler plastic and condenses. And when enough moisture condenses, it drips through the paneling. Since that time, we have recommended forced air systems for all domes, with a single return air duct to the dome top, indicated in all of our plans. Occasionally, owner/builders want other types of heating systems besides forced air, but in most cases, a return air duct is installed to keep the first and second floor temperatures relatively the same. This also forestalls any possible condensation problems.
Since that time in 1978, we have had no problems with condensation caused by moisture collection on the interior of the plastic vapor barrier, where the heating systems are properly installed. However, I have been talking to some dome owners of other manufactured domes on the Internet who are having a similar problem. It is possible, that those domes built in moderate climates where it never freezes do not experience this problem, but I am convinced that if the outside temperature drops to below freezing, that interior condensation may occur. If the shingling is adequate, and the extra warm air is drawn off the ceiling, before it has an opportunity to condense, there are no more condensation problems. It has been suggested that the Casablanca® fan might do the job, but experience proves otherwise. That type of fan blows the air in a downward motion, but the air currents surge back to the dome top. These fans, though aesthetic, sometimes cause uncomfortable air currents.
Positive Ventilation
A problem developed early on in dome construction also included condensation in the interior of the dome wall (shell). We started by drilling holes in the framing struts. We then provided openings in the exterior panels at the base of the shell. What we did, was to provide a vapor track, so that any moisture could be expelled to the atmosphere.
We first installed ordinary flat roof vents, but found that they were not too reliable, because, quite often, they became conduits for moisture infiltration, instead of moisture out. The next step was to substitute wind turbine vents. They too, leaked in heavy rains, if there was no wind to turn them. They do have a propensity to fail. They also did nothing to relieve the cavity problem, when the wind did not blow. In standard wall design, the wall cavity is isolated from the interior environment by a 6 mil poly covering the insulation.
More Physics
Another law of physics states that heat moves from hot to cold, by way of conduction. Though the vapor barrier is supposed to stop this infiltration, it is almost impossible to seal off the cavity at 100 percent. An ideal situation would be for the vapor barrier to be 100 percent tight. This, however, is almost impossible to achieve with ordinary construction methods.
This means that some of the normal humidity of the interior of the structure is carried into the cavity. In extremely cold weather, some of that moisture will collect on the inside of the exterior panel. When the weather warms, that condensation will melt, and once again you have a moisture problem.
To get rid of this moisture, the answer came in the form of a positive power ventilator attic fan installed in the dome top. These fans are furnished with all dome kits. They can be operated automatically or manually, preferably after a strong freeze, when the temperature rises above 32o Fahrenheit.
Though the system sounds complicated, it is relatively simple. Most dome companies who operate in the colder climates have variations of this system and have solved the problems of dome condensation.