The Butterfly Effect

Peeling paint and powdering plaster were the first indications something was amiss at the Blairmore Courthouse, a Provincial Historic Resource in the Crowsnest Pass. A leak in the cedar shingle roof, replaced just the previous year, was immediately suspected. Detailing around the dormers in particular, part of the 1922 building’s distinctive Spanish Colonial Revival design by architect R.P. Blakey, is tricky and vulnerable to water penetration.

Nippon School of Technology, which owns the building and runs a technical school and exchange program for Japanese engineering students there, inspected the roof from the attic and found no active leaks. Puzzled, N.I.T. engaged a conservation architect to inspect the building and identify sources of moisture causing the paint and plaster failure. The findings were at once surprising and (in hindsight) credible.  

During the 2014 re-roofing project, several vents were introduced near the ridge to complement existing vents beneath the eaves. Adequate attic ventilation is good roofing practice, and a combination of eave and ridge ventilation on this large hip roof would facilitate air movement by convection and help warm, moist air escape. Inadequately vented attics can lead to such classic old building problems as ice dams where melting and re-freezing on warm roofs diverts water through the roof and causes all sorts of damage. A more insidious but still costly effect of poor roof ventilation is reduced drying beneath the roof which, in the case of cedar shingles, promotes cupping and splitting. Although the previous roof had lasted nearly thirty years, the intent was to enhance ventilation where it appeared to be lacking and thereby extend the new roof’s life.

Southern Alberta and the Crowsnest Pass in particular are famously windy. Unbeknownst to us, increased attic ventilation upset a delicate moisture balance sustained for decades in the partly vented attic, with unintended consequences. Inspection revealed that enhanced airflow was scouring the loose fill insulation, stripping it from the attic floor. Chilling of the ceilings and walls below showed up clearly in infrared photographs, where cold surfaces appear as darker hues, and was most striking where dormers project from the roof and are especially susceptible to heat loss. Cold surfaces are naturally damp and, once the “dew point” is reached, moisture in the air condenses into liquid water.

To add insult to injury, extra venting drew in drifting snow through the eaves vents and even pulled liquid moisture through defective caulked joints around windows and hairline cracks in the historic stucco. The cracked stucco was a known issue, but it took on new significance with powerful negative air pressure now sucking water through tiny gaps in the exterior walls.

Another source of moisture came from within. In Alberta, interiors are heated for months on end and, compared to outside air, are relatively warm and humid from occupants and such activities as showers and doing laundry. Normally this isn’t a problem; early twentieth-century construction lacks vapour barriers but porous historic materials can absorb moisture and usually dry gradually with no ill effect. The inherent permeability of old buildings, which has generally served them well, is one reason retrofits with insulation and vapour barriers can be tricky. At the Blairmore Courthouse, added roof ventilation and powerful negative attic air pressure boosted the “vapour drive”, or diffusion through the lath and plaster ceilings. With the now poorly insulated materials hovering at or below the dew point, condensation was inevitable. The lime-gypsum traditional plaster absorbed the liquid water, swelled as the gypsum hydrated and caused the paint layer to fail, and then turned to powder as the wall eventually dried – exactly the problems the owner observed.

Interventions into historic buildings are carefully considered but can still upset a subtle equilibrium – a “butterfly effect” of sorts where a change within a complex system – new airflow within a building envelope – can have manifold and far-reaching consequences. At the Blairmore Courthouse, what seemed to be a marginally ventilated roof in fact turned out to be perfectly calibrated to windy site conditions. Although the plaster and paint damage must be repaired, the underlying fix appears to be relatively straightforward; new vents will be restricted or closed entirely, loose fill insulation will be replaced with mineral wool batt or rigid foam insulation with baffles to deflect airflow from eave vents, and defects in the stucco and brick exterior will be repaired.

Written By: Fraser Shaw, Heritage Conservation Adviser