We're in Southern Maine and have a 30ft, 5/8 geodesic dome with a hydroseal roofing paint exterior, 2x4/plywood construction with one inch of sprayfoam and another inch or two of blown cellulose(some sort of recycled paper product)... with no interior walls and plexiglass for windows. Its our fix-up project, but learning about condensation, domes, cathedral ceilings, etc., I am stuck deciding what to leave underneath the sheetrock. Up until now, the walls and struts seem great and undamaged with no moisture build up since there never seems to have been any interior walls put up... no place for moisture to collect. But after 30 years you can imagine the dust.

I've read up on dome builders that insist on spray foam with heat/air exchangers(HE) installed, and then some that use a cathedral ceiling style insulation job, with a soffit around the base of the dome and a ridge vent around the cupola at the top, leaving a one inch air gap on the cold side of the walls. We already have what we have and it would make more sense just building a new dome rather than try to remove all the insulation to do either of these methods... we'd be scraping the frame of this place for months.

We will deal with interior air ventilation more as we add a cupola to the top and open up the back with some larger windows/doors. We might also get a HE as we've been told we'd be better safe than sorry with sprayfoam.

The Problem?

It's the air circulation in the walls that concerns me. With no attic to allow moisture from the walls to exit like a traditional home, and no soffit/ridge construction like a cathedral ceiling, I fear I will soak the walls wet if I seal them up with sheetrock. There's also the odd surface of the old blown on cellulose that needs to be finished on top with more insulation to avoid huge air pockets.

The Solution?

I'm not worried about the exterior, as the roofing paints and products create a seemingly impenatrable vapor barrier. On the interior, i was going to marry 2x2s up to the 4x4s, and then sheetrock, which would allow space for a layer of 3 1/2" fiberglass insulation and a vapor barrier to be layed up on top of the uneven cellulose and fill any gaping air pockets. It would then be 1' sprayfoam, 2"cellulose, 3~" fiberglass.

wall sketch

The 2x2s would be notched on their underside allowing a small amount of air movement through the fiberglass and cellulose in the walls similar to a traditional home. The top ring of the dome would then have a ridge vent to mimick the open spaces of an attic where a traditional home's moisture would end up and be removed with vents. There would be no soffit vents below since this setup would not be on the cold side of the insulation. I liken it to being half traditional using half a cathedral setup, and half sprayfoam with an HE. Nuts...

roof sketch

And that's all I know so far. I hope I'm not insane. Would attempting to mimick a traditional home like this, along with a HE installed, be a problem? Or rather, what problems could anyone forsee happening? Am I missing something catastrophic? As I've mentioned, researching has led me to a lot of horror stories dealing with moisture, rot, blackmold and air quality. So I'm a bit nervous getting things started and sealing this place up. Any help or ideas or nasty comments would be appreciated.

The primary reason you have not experienced any moisture issues up to now is most likely the Dome has not been lived in. Once People move in and the space gets conditioned Moisture begins to collect and things begin to happen.

If you spray foam the entire ceiling and leave no air gaps you should not worry about moisture issues within the ceiling cavity.

The reason we see ventilation in modern attics is to allow for heat and moisture to excape from the attic space. It also helps insulation to work better.
If an attic was incorrectly vented, we end up seeing more mold issues then one that is done correctly.

In your case the spray foam insulation will act as a vapor barrier preventing the moisture from entering into the cavity where it could condense.

However you must control the humidity within the living space to prevent the condensation from forming on the upper ceilings. A good quality ceiling fan that runs during the colder periods would suffice. As this would bring the warm moist air down to the lower living area.

Heat exchangers do nothing other then lower humidity within a home and provide fresh air into the environment. But you can control the humidity within the home without these expensive units. Simply install and use quality bath vents and kitchen vent appliances.

Your also talking about the walls and air circulation within them. First off if the insulation was done correctly within the wall cavity you should experience little to no air movement in this cavity. The general construction of the wall is such that top plates should prevent any air movement from getting into the dome area. In addition any insulation installed should be fitted so its snug within the wall. Thus preventing any air movement. Add a plastic vapor barrier and no moisture should be getting into the wall as well.

As far as the exterior acting as a vapor barrier. Its not. And its nothing to worry about either.

To understand this answer you must first understand vapor movement within a wall cavity and how it relates to inside and outside temps.

In the northeast during the winter the air becomes dry. You observe this as frost on the ground and on the car in the early AM. The colder air condenses and freezes on these surfaces.
On the inside of the house you have moisture as well. This moisture is observed on the outside of a glass of ice water. It gets wet. Again the cold surfaces are causing condensation to form. Just like the outside.
Moisture also travels from areas that are damp to areas that are dry. Thus the reason for the vapor barrier on the inside of the home.

If moisture was allowed to travel through the wall (which it does if no vapor barrier is installed) it would eventually get to a point within the wall or ceiling cavity where it would get cool enough to change to condensate.

It is apparent that the greater the amount of moisture within our living environment the greater the chance of condensate to form on walls and exterior surfaces. This is the reason why they suggest the heat exchanger.

If you seal the walls properly and use foam insulation within the ceiling cavity you should not experience any moisture within the walls or ceiling.

As far as the reason of the ventilation in the attic areas. Ideally any heat that is removed below the roof covering the easier it is for the insulation to prevent it from moving through and getting into the living side of the ceiling. However in your case you would be better off filling the cavity completely thus preventing any moisture or air movement and then deal with the extra heat in the summer by proper ventilation on the ceilings. Rather then installing a air gap within the ceiling and then loosing the extra insulation thickness with translates to higher R values which you need in the northern climate.