South-Facing Glazing and Overheating in a Cold-Climate Multifamily Project

The pattern is a solar-control problem, not a heating problem. A 45% window-to-wall ratio with 60% south glazing and SHGC 0.38 admits substantial solar gain. In a cold climate that gain offsets winter heating, which is why heating demand is only 12 kWh/m²·yr. In summer, the same unshaded aperture drives high cooling demand and long hours above 25°C, especially in living rooms with internal gains from people, lighting, and appliances. The proposed 4-ton chiller treats the symptom: it adds first cost, electrical demand, and source energy, and it may push the project over its Phius source-energy limit. It also does nothing for comfort next to hot glass or for passive survivability. The passive sequence is to reduce gains before adding mechanical capacity: lower the south window-to-wall ratio, specify a lower SHGC on south and west glazing, add exterior shading sized for summer noon and afternoon sun, and use operable windows or night flush where security and acoustics allow. The ERV is a ventilation device, not a cooling device; upsizing it will not solve the overheating and may increase fan energy. Require parametric WUFI Passive runs showing comfort hours, cooling demand, peak load, and source energy before accepting any chiller.

Start with no-cost and low-cost measures: reduce the south window-to-wall ratio from 45% to about 35%, redistribute glazing to north and east where summer gains are lower, and set a south SHGC target of 0.25-0.30 while keeping a reasonable visible transmittance. Next, add exterior shading. A fixed overhang sized to block June-September midday sun can be inexpensive on a wood-frame building, but it must be modeled with the actual window height and orientation; exterior roller shades or louvers are more flexible and better for west-facing units. Keep the triple glazing and the 6 in. mineral wool; do not trade envelope performance for cooling equipment. If budget remains, add ceiling fans and a night-flush strategy in south units, and verify that operable windows have sufficient free area and secure ventilation openings. For each measure, require the modeler to report cooling demand, overheating hours above 25°C, peak cooling load, heating demand, and source energy, not just a single pass/fail. Also require product submittals for SHGC, visible transmittance, and shading device projection, plus a construction sequence showing shading is installed before occupancy. The chiller should remain a contingency only if passive measures cannot bring comfort hours into the target range.

The certification case must rest on installed, verifiable features, not modeling assumptions. Require a window schedule with orientation, U-value, SHGC, visible transmittance, and frame type, matched to the WUFI Passive inputs. Require shading-device shop drawings with dimensions, mounting height, and projection, plus photos after installation. The air barrier and insulation are also part of the passive cooling story, because air leakage and thermal bridging affect comfort at the perimeter. Require a blower-door test at 0.6 ACH50 or better, with the test report showing pre-test conditions and any temporary sealing. For ventilation, require commissioning airflow measurements and a balanced ERV report; do not accept an ERV as a cooling substitute. During construction, require a mock-up of a typical south window with shading and air sealing, and inspect a sample of units before drywall. At completion, require a comfort and operations check: verify that shading is operable where specified, that windows open, and that occupants receive guidance on night flushing and shading use. The Phius QA file should include the model version, all parametric runs, product data, installation photos, and the final blower-door and ventilation reports.

Advise that the chiller does not resolve the underlying passive failure. It may keep peak temperatures down, but it increases source energy and peak electric demand, adds maintenance, and leaves occupants dependent on mechanical cooling. It also weakens the project's resilience case, which matters for affordable housing. If the owner will not change the façade, require a documented trade-off analysis: model the chiller option with realistic part-load performance, fan energy, and refrigerant assumptions, and compare it with at least one passive option on first cost, source energy, comfort hours, and operating cost. Check whether the project still meets the Phius CORE source-energy limit and any program requirements. If the chiller is chosen, require a commissioning plan, a thermostat strategy that does not encourage overcooling, and a written acknowledgment that the south units will still have hot-glass discomfort unless shading or glazing changes are made. Do not let the team call the project 'passive cooling' if the cooling is predominantly mechanical.