Condensation in Domes

A great to-do is going on here in the metropolitan area about mold that is occurring in public buildings, and that the mold is affecting people’s health. In fact, it is so bad in several office buildings that the buildings are being demolished. What is that telling us about moisture (humidity) in our homes and our workplaces?

To me, it means that not enough attention is being paid by the engineers and architects who design homes and public buildings, relative to the amount of moisture in the living environment and how it relates to the building itself. The appearance of mold means that there is more moisture in an area than should be, and that mold is thriving in that damp condition.

Wall Cavity Moisture

The dome is particularly vulnerable to wall cavity moisture, and if nothing is done to correct it, that moisture can cause structural as well as health problems. We have contended for a long time that since moisture enters the wall cavity, to make the building secure and healthy, a system of removing that moisture must be available.

Here is what happens:

A basic law of physics states that moisture (humidity) moves from hot to cold, which means that the humidity inside the dome will force its way into the cavity. If it were possible to provide a vapor barrier that would be airtight, it might be possible to stop that movement. But we know that this can’t be done with today’s materials and procedures.

Moisture can also infiltrate the cavity from the outside, either through the roofing (shingles) or around windows. In any case, we have long felt that there must be a system in place to remove that moisture. Some dome companies instruct their customers to take precautions to make certain that the internal humidity does not get too high in the building. We have long recommended the use of an air-to-air heat exchanger, as one method of containing excess humidity. Also dryers and baths should be vented to the outside. However, even with these precautions, some humidity will find its way into the wall cavity.

My Minnesota Dome

The dome that I have lived in for the past fourteen years has 14.5″ walls with 12″ of insulation. My average temperature is 72^o F with 30 percent humidity. And just to verify that the cavity ventilating system is not ruining the efficiency of the dome, my average gas bill is $20.85 per month.

Wintertime here in Minnesota has caused some homes to lose so much humidity that it is necessary to introduce additional moisture to the environment to maintain comfort. My home has almost perfect balance by keeping the temperature at 72^o F and the humidity at 30 percent, even in sub-zero weather, without the addition of extra humidity being added to the environment.

We, and other dome companies who have designed domes in these northern climes, where temperatures range from   –20^o F to +100^o F have long recommended that a system of ventilating the wall cavity, and providing a vapor path for moisture to exit the cavity was an absolute necessity.

Metal Strut Connectors  

 Some criticism has been leveled against metal strut connectors, as being the cause of cavity moisture problems, but that has no merit as to the real cause. The fact remains that when moisture enters the cavity, and it will, and the inside of the exterior panel is cold, moisture will condense on that panel in the form of frost.

If you have ever been in a poorly vented attic in sub-zero weather, you will see frost collected on the roof boards. You will see what I mean. The same thing occurs in a dome cavity, and when the weather warms, if no system is provided to remove the moisture, it runs down and despoils the interior finish of the structure. If left long enough, this can cause structural damage, as well as the possibility of mold.

The Ventilating System

The recommended ventilating system in a hub and strut dome, using fiberglass insulation, is to drill vents in the bottom of the dome shell, at the riser wall. Next is to install a small attic fan in the top vent. This fan is slowed down to about one-fourth of its top rpm. This provides a vapor path that will clear the cavity of any ingested moisture. This free flow system works well in hub and strut systems, but needs some alteration when applied to the panelized domes. We now respectfully suggest that there be some consideration of venting these systems with their individual panels. We are recommending drilling several holes into each of the struts at the panel edge. This will, in effect, give a common air connection to all of the panels. In addition, a small attic roof fan is installed in the top pent panel. It is necessary to place a control on the fan to reduce its speed to about one-fourth of its original capacity. Since some effect of convection currents occur within the wall, a very small amount of air is necessary to carry it out of the cavity.

Fiberglass Insulation

We have several reasons why we recommend fiberglass insulation:

1. It is the lowest cost insulation per BTU per square foot available.
2. It can be installed by the owner, if he or she chooses.
3. If it becomes wet, it will dry out, if given a chance and return to is original R value.

The rigid insulation salesmen are around. But here is the reason I object to that type. If it were possible to guarantee that no moisture would ever penetrate the roofing system, then this type of material might be acceptable. Since no one can guarantee that, then here is what happens. This information comes from the early years of dome construction. Moisture will (at some time) enter the wall cavity. With no way of escaping, it proceeds to rot the panel. This situation will never happen in a ventilated wall design.