The #1 Question Architects Ask Us: "When Should We Bring You In?" — The Answer Is Earlier Than You Think, and Here's the Curve That Shows Why.
By the time construction documents are stamped, most of the decisions that determine a building's energy performance are already locked in. Orientation. Massing. Glazing ratios. Wall assembly. Structural depth. Mechanical strategy. Every one of them shapes the PHPP outcome — and every one of them gets harder, and more expensive, to change as the project moves forward.
Here's what a building science consultant can actually influence at each design phase, and what the cost of late engagement looks like in practice.
We get the same question from architects in nearly every first call: "At what phase should we bring you in?"
The answer — backed by the MacLeamy curve, our own project record, and twenty years of integrated-design research from the AIA and CURT — is feasibility or early schematic design. Not design development. Not construction documents. Not after the energy code review flags something.
This isn't a sales pitch for earlier engagement. It's a description of how the cost of decisions changes over the life of a project. The earlier a building science consultant is in the room, the more design freedom is still available — and the cheaper every performance-relevant decision becomes. After construction documents, we're mostly doing damage control on choices that were made before we were called.
The MacLeamy curve, in one paragraph
In 2004, Patrick MacLeamy (then CEO of HOK) published the now-famous "MacLeamy curve" through the Construction Users Roundtable. Two lines, one chart.
The first line — the ability to influence cost and functional capabilities — starts high and drops sharply as the project moves from pre-design to construction. The second line — the cost of design changes — starts low and rises just as sharply. The two lines cross somewhere in design development. After that crossover, changes rapidly become more expensive relative to the value they add.
The MacLeamy argument was that the industry's traditional fee curve (heavy effort in CDs and CA) is upside down. Fees should be loaded earlier, when decisions are still cheap to make.
For a code-minimum project, the curve is forgiving. For a Passive House or net-zero project, where envelope, mechanical, and ventilation systems are tightly coupled, the curve is unforgiving. A late envelope change drives a mechanical change, which drives an electrical change, which drives a framing change.
What we can still influence at each phase
Feasibility (pre-schematic) — almost everything is still flexible. Site orientation, massing, surface-area-to-volume ratio, basic glazing-to-wall ratio, structural system choice, mechanical strategy at the concept level.
At this phase, we can run a PHPP feasibility model with placeholder values for assembly U-values and ψ-values, test whether the proposed massing can hit the target heating demand, and flag the decisions that need to be locked in early to make Passive House (or PHIUS, or ZERH) achievable at the targeted cost. The PHPP file at this stage is a design tool, not a compliance document.
Schematic design — big moves are still cheap. Window placement, shading geometry, wall assembly direction (interior or exterior insulation; mass-wall or light-frame), preliminary thermal-bridge strategy at the most critical junctions (foundation-to-wall, parapet, balcony or cantilever, rough opening).
At this phase, we can model multiple wall assembly options against the PHPP target, compare cost-per-kWh-saved across options, and converge on the assembly that meets the performance target at the lowest installed cost for the project team's preferred trades and materials. A common approach on our projects is a self-adhered, vapor-open exterior WRB (Pro Clima Solitex Adhero, or the equivalent SIGA Majvest SOB) paired with a Pro Clima Intello smart vapor membrane as the interior airtight layer, with an interior service cavity inboard of the membrane so electrical, low-voltage, and most plumbing rough-ins don't penetrate the air barrier at all. That choice is made here, with the architect, not later.
Design development — equipment and details get locked in. Final glazing specifications (U-value, SHGC, frame type, installation depth), final mechanical sizing (heat pump capacity at the PHPP-modeled load, ERV selection altitude-derated for Colorado, hydronic vs. forced-air vs. mini-split distribution), final thermal-break details for cantilevers (Schöck Isokorb, Armatherm), parapet caps, slab edges.
At this phase, we coordinate the mechanical drawings with the PHPP file so the equipment schedule reflects the modeled load, the duct routing fits in the planned chases, and the ERV intake/exhaust geometry meets ASHRAE 62.2 separation rules. We also produce thermal-bridge calculations in Therm or Flixo for any junction whose ψ-value materially affects the energy balance.
Construction documents — documenting decisions, not making them. Detail-level coordination of the air control layer, vapor control layer, and rain control layer through every junction. PassivSure compliance documentation prepared in jurisdictions where it's currently recognized (including Denver's Green Code R408, among others). Specifications for tapes, sealants, membranes, and gaskets keyed to the actual assemblies.
If a major decision still needs to be made at CDs, the project is already late.
Construction administration — protecting the performance ceiling, not raising it. Pre-rough-in site training for the framing, sheathing, and air-sealing crews. Pre-drywall blower door, with an interim target set inside the project's certification airtightness threshold — which varies by program (PHI Classic, PHIUS+, ZERH, or a local code-compliance target) — so the team has margin before the final test. Mechanical commissioning verification against the PHPP-modeled flow rates and equipment performance. Submittal review on every product with a performance-relevant spec.
By the time we're in CA, the building's performance ceiling is set by the choices made before bid. We're protecting that ceiling from erosion, not raising it.
What gets locked in by the time most teams call
The pattern is consistent. The decisions that most affect a building's performance — orientation, massing, glazing area, structural depth, wall assembly direction, and the basic mechanical strategy — are made early, often before a building science consultant is in the conversation. Once they're locked in, every later optimization is constrained by them.
A few examples of how that plays out:
Overheating risk from a south-facing glazing decision can't be solved with a better window spec. It's solved with overhangs, shading, or less glass — design moves that have to happen before the elevations are signed off.
Thermal-bridge losses from a structural cantilever can't be solved with more insulation. They're solved with a structural thermal break that has to be specified before the steel or concrete schedule is fixed.
Oversized mechanical equipment can't be undone after submittals. The heat pump is sized off whichever load model the engineer ran first.
None of this is unusual. It's the routine cost of late engagement on a project with PH-level performance targets — and it's the part that doesn't show up in the fee comparison until the project is well past the point of fixing it cheaply, often discovered during value engineering, when scope cuts hurt the most.
What "early" actually costs
There's a fair pushback to all of this: bringing a consultant in at feasibility costs more total hours than waiting until CDs. That's true in raw fee terms. It's not true in net project cost.
Our experience — confirmed by the integrated-design literature — is that feasibility-phase engagement costs a few thousand dollars and saves five-figure scope and equipment costs downstream by preventing the late-stage ripples that drive the real costs of high-performance construction. Bringing us in at feasibility on a $2M single-family custom home typically runs $5,000–$12,000 in additional pre-schematic fee. The downstream avoided costs — oversized equipment, late envelope re-coordination, overheating mitigation, custom thermal-break details, redesign of duct routing — are routinely an order of magnitude larger than that.
For a Passive House project where the certification itself is the goal, early engagement isn't an upgrade. It's the cheapest way to actually hit the target.
The takeaway
The MacLeamy curve has been on architectural-practice slides for twenty years. It's true. It's especially true on high-performance work, where the building's performance is the deliverable, not just the code-compliance footnote.
If you're an architect, builder, or owner planning a Passive House or other high-performance project, the practical version of the question isn't "when should we bring in a building science consultant?" It's "how much design freedom do we still have when we do?"
Answer that clearly, and the timing question answers itself.
Architects — when was the last time it was already too late, and what tipped you off? The candid stories below help the rest of the field.
#PassiveHouse #BuildingScience #IntegratedDesign #MacLeamyCurve #HighPerformanceBuilding #ColoradoConstruction #PHPP #PassivhausUSA
Sources
Construction Users Roundtable (CURT) — Collaboration, Integrated Information, and the Project Lifecycle in Building Design, Construction and Operation (WP-1202, 2004): the original publication of what became known as the MacLeamy curve, attributed to Patrick MacLeamy of HOK. https://www.curt.org/
American Institute of Architects — Integrated Project Delivery: A Guide (2007): formal codification of the cost-of-change curve and the case for front-loading effort earlier in the design phases. https://info.aia.org/SiteObjects/files/IPD_Guide_2007.pdf
ASHRAE — Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Residential Buildings: ventilation rate definitions, intake/exhaust separation, and air-distribution requirements. https://www.ashrae.org/technical-resources/standards-and-guidelines
PassivSure — Energy Compliance Approvals & Planning for Utilities: structured submittal layer for PHPP-derived performance data into code-compliance and utility-incentive workflows. https://www.passivsure.com
Passive House Institute — Certified Passive House Requirements (PHI Classic ≤0.6 ACH50); PHIUS — PHIUS CORE / ZERO Certification Targets (airtightness varies by climate zone and building type); U.S. DOE — Zero Energy Ready Home Program Requirements. Project-specific airtightness targets are set against the applicable certification standard. https://passivehouse.com/02_informations/02_passive-house-requirements/02_passive-house-requirements.htm
Passive House Institute — PHPP User Manual: Frequency-of-Overheating Output and the 10%-of-the-Year Threshold. https://passivehouse.com/04_phpp/04_phpp.htm
