The Vizcaya Mansion Grotto

Last May I was lucky enough to be included in a team, along with Courtney Magill and led by Frank Matero, to travel down to the Vizcaya Mansion in Miami, Florida to have a discussion of the conservation of the grotto there, the ceiling of which was the canvas for one of Robert Chanler’s few remaining breathtaking murals.

Conditions assessments had been carried out in the past, one of which was the focus of a recent historic preservation grad from Penn. The focus of our trip in particular, however, was to take a closer look at underlying, root causes and not just the symptoms of the issues that have manifested at the grotto since its completion in 1916. As far as sources of vulnerabilities are concerned, the grotto’s environment is paramount.

“Conservation based on risk mitigation is defined as “preventive,” or a process of conservation achieved through indirect action. If risk can be reduced or controlled, deterioration will be slowed, thus the integrity of the cultural resource can be preserved and maintained to a higher degree than if no action were taken. Conservation based on direct actions is defined as “remedial,” and includes activities such as structural stabilization and material restoration. The merits of indirect or preventive conservation can be longer retention of original material and avoidance of expensive episodic campaigns of restoration. Nevertheless, despite the importance of a preventive conservation approach for the pool ceiling mural, existing damage and serious future structural instability will also necessitate remedial stabilization.”
-Frank Matero

My charge, in a nutshell, was to supplement the ongoing discussion of material and structural vulnerabilities with that of environmental risk. Of the numerous conversations had in preparation for our May trip, during, and in the multiple debriefings that followed, most began with the misconception: “Well, the humidity here is always incredibly high. So, obviously that is root cause here.” In fact, the contrary is equally, if not more detrimental to the behavior of plaster ceiling. Before arriving, Lauren Meyer–a Penn historic preservation alum and current conservator at Vizcaya–provided me with some past data that had been gathered in the grotto space, which immediately presented the smoking gun that gave rise to that argument.

“The vulnerability of the grotto ceiling mural is governed by those risks associated with and resulting from its exposure to an exterior environment. While sheltered, which provides sanctuary from such environmental elements as direct solar gain and ultraviolet exposure and rain, the grotto design provides for an unrestricted equilibrium between the ‘interior’ and exterior regarding ambient temperature and atmospheric moisture—as a grotto should. An insight into the risks facing the grotto ceiling begins with an understanding if its environment at a resolution capable of revealing not only the macro-scale, anticipated vicissitudes of seasonal climate, but more importantly the micro-scale, rapid diurnal fluctuations.”


The jumping-off point for our discussion in May, and in the conversations to follow, thus became the severe fluctuations in not only ambient temperature but also relative humidity that occur during the winter months, from November through May. That is, the focus of concern shifted away from “always wet” to “always wet, and then suddenly, rapidly drying.”

Our fieldwork included some pilot monitoring to measure ambient conditions in the grotto space below, and in the interstitial space above the ceiling and below the reinforced concrete slab that supports a (now) conditioned, interior space above. Three sensors were placed into the space above the ceiling, branching out radially to try and capture the different conditions present. The key to understanding the behavior of the plaster ceiling, which effectively acts as a membrane between two varying environmental conditions, is to understand those two environmental conditions. The grotto geometry may be biaxially symmetrical, however the conditions present for any given location will vary, not the least of which is the fact that the volume of air and proximity to the cold, concrete slab above varies from point to point; additionally, to the north there is proximity to an exterior wall that responds to the hygrothermal conditions of the outside environment (hot, from solar gain wet from RH and rain); and to the south/east/west there are cold, interior walls that enclose dehumidified, and and air-conditioned spaces.

There was also the attempt to correlate reflected surface temperature with moisture content using an infrared camera. Ultimately, the conditions were inadequate for quantitative thermography (lack of temperature differential through the surface, varying thickness of the plaster elements); however, a wealth of unanticipated information was uncovered, namely the presence of thermal bridges between the reinforced concrete beams and the ceiling. Furthermore, as the IR images were accompanied by an along, resistance moisture reading, I was able to map the moisture readings on the ceiling to uncover some interesting phenomena to add to the discussion as well. Most notable, the clear relationship between thickness of plaster and the corresponding moisture value, i.e. thicker elements (toward the perimeter of the grotto where Chanler’s use of appliques–particularly at the pilaster capitals–over the plaster substrate and scratch-coat layers significantly increased the overall thickness of the cross section) were ‘wetter’ and areas toward the center of the ceiling were significantly ‘dryer.’

Composite image illustrating the reflected ceiling plan of the grotto ceiling including readings taken with a resistance moisture meter of the plaster.
Again, this was simply an observation, however when you isolate the deterioration of the ceiling, the pattern becomes clear. To do this, I processed a high resolution, orthorectified image of the ceiling and clustered the colors of the image. Using those colors that closely matched the color of the underlying, white scratch-cloat plaster layer as an upper and lower threshold, I was able to create a mask of ‘deterioration’ by proxy. Understanding that thinner plaster elements, such as those at the apex of the grotto ceiling, have less hygric and thermal inertia to self-regulate against sudden, and rapid environmental fluctuations, as might be present diurnally during the winter months, is key to understand the environmental pathologies in the Vizcaya grotto. Furthermore, as there is significantly less air volume above this portion of the ceiling, air having a tremendous capacity for moisture and thus heat, also adds to this understanding. In fact, at the exact apex of the ceiling, the plaster physically contacts the concrete beams (see earlier IR image).

A clustering of 20 color groups of the original, high-resolution orthorectified ceiling image. Note the distinctly off-white and beige color groupings, which were used as a lower/upper threshold for the mask.

Now layer onto the above observation the understanding that at certain times of the year, when the surface temperatures of the concrete walls, ceiling, and iron armatures drop below the dew point temperature, liquid moisture is added into the mix, sometimes directly onto the ceiling itself. In analyzing the sample data, both during June and July this condition may have occurred, especially when you consider the 3-5°F temperature differential between the ambient temperature and the surface of the concrete above. This phenomenon would certainly only be exacerbated during the winter months, when the temperature differential is more drastic.

From the fieldwork a list of variables of concern was compiled to be monitored, moving forward:

  • Living room (conditioned interior space above grotto) – Relative humidity, temperature (RH/T)
  • Interstitial space (above plaster ceiling) – RH/T
  • Interstitial space (underside of RC slab) – Time of wetness/condensation (Or surface-mount thermocouples to measure if surface temp drops below dewpoint (measured from ambient sensors)
  • Grotto – RH/T
  • Grotto pool (water within grotto) – Temperature
  • Grotto (archways to exterior) – Air Velocity (hot wire anemometer)

What exactly would ‘moving forward’ look like? Well, the first, and most straightforward answer is the formulation of a more informed, evidence-based conservation plan by the Vizcaya conservators. A plan that begins to contextualize treatments within a greater framework of environmental cause and effect, this begins to avoid the ‘heroic’ restoration campaigns, ones that have a finite half-life. The team at Vizcaya already collects environmental data in some of their interior spaces, and have occasionally collected data in the grotto itself; however, the collective data have not been looked at comprehensively to understand the underlying patterns involved. For instance, while visiting I managed to get the facilities engineers to sit down and have a conversation with the conservators to the benefit of learning that a phased HVAC retrofit plan was underway, and would directly affect the conditioning (heating/cooling and dehumidifying) of the spaces around the grotto. Meaning, in 6 months to a year, the environmental context above and adjacent to the ceiling will change, and this needs to be acknowledged.

How to use the data from the monitoring protocol? Monitoring the necessary variables within the grotto at a near real-time basis provides for evidence-based, reactive decision making regarding the environmental control of the space. That is, by analyzing recorded data, one can make a change to one or more of the variables to affect a change in the system. A more efficient protocol is one which involves analyzing past data to proactively elicit a future system response to predicted conditions. This category of control involves the real-time implementation of dynamic setpoints informed by analysis of past system behavior, rather than static setpoints that do not change (Kramer et al, 2016). Such an implementation allows for a smooth response to seasonal fluctuations, and avoids sudden changes to system variables.

A conservative application of this strategy involves the restoration of the existing grotto fountains. Historically, such fountains were relied on to shape the atmospheric quality of the space, but also undeniable to affect the environmental perception of the space by humidifying the air. In coordination with the constant breeze into this space this would have also provided additional evaporative cooling. At once, these elements are as significant to the historic architectural interpretation of the grotto space as the historic environmental interpretation. The latter is often neglected in the conservation and interpretation of projects located in areas subjected to intense environmental loading.

If in fact the grotto fountains are to be restored to operational condition, it should be considered to provide for temperature control of the supply water. By control the temperature of the water, it can be possible to modulate its evaporative potential. During the winter, if the conditions are present (air velocity through the archways, ambient temperature relative humidity) it may be possible to, in a limited capacity, humidify and dehumidify the air in the grotto space. A similar method was used in the Manitoba Hydro Place building in Winnipeg, Canada to passively condition their naturally ventilated atria (Fig.32). It is hypothesized that the periods of low humidity, in fact, are more detrimental than the prolonged periods of humidity. Typically paired with periods of low temperature, this hygrothermal shock is believed to cause not only condensation formation on elements within the interstitial space (and possible on the plaster surface itself) and rust formation of these elements, but also differential drying (causing displacement, delamination of finishes, and cracking). It is also possible to considering the introduction of a new element, such as a water curtain over the grotto archways that can more actively provide this effect, like the Manitoba Hydro Place atria water features. These multi-story Mylar cables allow water to trickle down them, without aerosolizing/misting; and by modifying the temperature of the water it is possible to humidify or dehumidify the incoming air as necessary. In a location that sees similarly heavy environmental loading, albeit on the opposite end of the spectrum, it is possible to passively provide nearly 100% natural ventilation year-round.

One thinks, “Oh, Miami–it’s always humid there.” Which, for a majority of the time is true, however there are periods when the humidity, as can be seen in both the provided and fieldwork data, when the RH drops below 50%. And when you have a fragile ceiling composed of entirely hygroscopic materials that is saturated nearly year-round and then all of a sudden it begins drying, you have problems. If it is possible to simply smooth out those spikes on the chart, there might be a chance to lengthen the lifespan of any new treatments. What started as a half-serious comment over coffee, quickly turned into a “That actually might work.” And it’s completely reversible to boot.

This all being said, I have to take the time to say that our gracious hosts and colleagues at Vizcaya have suffered severe damage, both to the architecture on the grounds and to the grounds themselves in the wake of hurricane Irma and could use your help. Please consider visiting this link to donate.


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