Why this is hard
Pre-1920 masonry buildings were not designed for the kind of envelope that contemporary HVAC presumes. They were designed to breathe: lime mortar, soft brick, single-glazed window, an inch and a half of plaster on lath, no vapor barrier. The wall is a hygroscopic mass that absorbs and releases moisture seasonally; the historic preservationist’s phrase is “the wall is the climate system.” That phrase contains a problem: contemporary HVAC presumes an envelope that is not part of the climate system. When the two meet, the wall sweats. The plaster spalls. The mortar weeps. We have walked too many historic retrofits where a well-meaning HVAC firm tightened the envelope to support a Class A or AA setpoint and then watched the lime mortar fail at the inside face of the brick within three winters.
The work, then, is to engineer the seam between an envelope that cannot support a tight regime and a collection that asks for one. The answer is almost never a tighter envelope. The answer is almost always a less ambitious setpoint, a more capable mechanical, and a controlled level of envelope improvement that the masonry can tolerate.
The trade-offs
Most of our historic retrofits land at Class B (±5 °C, ±10% RH, with seasonal RH adjustment of ±10%) or Class C (a controlled-RH-only regime). Class A is occasionally achievable; Class AA is essentially never achievable inside an unmodified pre-1920 envelope. We will tell you, at Gate M2, what the wall can support — and we will refuse to design a Class A regime in a building that cannot hold one. The risk register names the wall as a stakeholder. Every job we’ve walked away from has been walked away from at the wall.
A Class B or C envelope is not a failure. For a vast range of cultural-heritage collections — books, archival paper, ceramics, decorative arts, ethnographic textiles — Class B is plenty. The tighter regime would not measurably improve preservation outcome, and it would cost ten times the energy.
The approach
- Read the wall first. A two-week instrumented walk-through plus, often, an external partner doing a hygrothermal model of the wall (we use WUFI for the model). The model tells us, before we touch anything, where the wall’s dew point is going to land under the proposed regime.
- Decide the class against the wall. This is where we earn our keep. The class is not what the collection asks for; it is the closest the collection can get to what it asks for, given what the wall will tolerate.
- Mechanical sizing for envelope inadequacy. The chiller and the dehumidifier are oversized relative to the building load, because the load is variable in ways the building doesn’t bound. We build for the worst week, then run smart controls that throttle for the typical week.
- Reversible interventions. Every penetration, every flush-mount sensor, every duct run is reversible. A future restoration team should be able to remove the mechanical we install with a hand tool and a drill; nothing is grouted in.
- Slow ramp. Equilibration takes longer in historic fabric than in any other context; the wall has to keep up with the room. The longest ramp we have run in this protocol was 14 months for the cordage warehouse.
Conservation partners
We work, on every historic retrofit, with at least one of the following partners. We will name them on every project; this is a short list and a real one.
- Pittsburgh History & Landmarks Foundation — advisory on PHLF-listed properties.
- NPS Technical Preservation Services — for tax-credit projects under the Federal Historic Preservation Tax Incentives program; a Part 2 application typically constrains the mechanical scope, and we want to know about that constraint at Gate M1, not Gate M3.
- APT Bulletin archives — the Association for Preservation Technology’s journal, where most of the conservation-HVAC literature for North American historic buildings actually lives.
- Getty Conservation Institute — the institutional source for the Michalski 2007 paper and for the Sustainable Climate Strategies for Cultural Heritage initiative.
- Local historic-preservation architects (we work most often with three Pittsburgh firms; we’ll name names on a call).
Cost & schedule
Historic retrofits are the longest projects we run. Twelve months is fast; eighteen to twenty-four months is typical; thirty-six months is not unheard-of, particularly when the project is being run in coordination with a building-wide restoration. The cordage warehouse case 031 ran 26 months from Gate M1 to final commissioning; the spinning-mill case 041 is in its 18th month and is not yet commissioned.
Pricing is highly variable. A small parish-archive retrofit (one room, ~600 ft², Class C) lands at $80,000–$220,000. A full-building Class B retrofit of a 12,000 ft² Carnegie-era branch library lands between $1.4M and $4.2M. The variance is at the wall.
Begin a historic retrofit
If your building is on the National Register, on a state register, or in a local historic district, the first call should be with Sloane and a member of your historic-preservation team simultaneously. We will explain why on the call. Begin a historic project.