The Pressure Pan That Told Two Stories
You are performing an energy assessment on a 24-unit, two-story masonry building with a central hallway and individual unit entry doors. A blower-door test yields 4,100 CFM50 for the whole building, which is roughly 0.9 ACH50 at the tested volume. You run pressure pan readings at 14 receptacles on exterior walls and get values ranging from 0.2 to 9.8 Pa; the two highest are in a first-floor unit on the windward side, and one receptacle reads negative. The building has a sealed combustion boiler in a basement mechanical room with a louvered door to the hallway. The client wants to know whether air sealing is worth the cost and whether the basement should be included in the thermal boundary.
1. Interpret the pressure pan results. What do the high, low, and negative readings mean, and how do they change your air-sealing scope?
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Pressure pan readings are a relative leak indicator, not an absolute flow measurement. With the building depressurized, a reading near zero means the receptacle is on a well-sealed assembly or is not connected to the exterior; a high reading (roughly above 2-3 Pa) means a significant air path connects that box to the outside or to an unconditioned zone. The 9.8 Pa readings on a windward first-floor wall are consistent with wind-driven infiltration through the wall assembly and box penetrations. The negative reading is the important anomaly: it suggests the box is connected to a zone that is at a different pressure than the house, or that the reading was taken with the probe in a location affected by a duct or chase. It should be re-tested and traced before any scope is written.
The practical implication is that air sealing should be prioritized by measured leak, not by count. The high-reading boxes warrant gasketed, air-sealed covers and sealing of the wall cavity behind them. Low-reading boxes need no treatment. The negative reading must be resolved, because sealing a box that is actually a combustion-air or chase path could create a safety problem.
Because the building is only 0.9 ACH50, it is already relatively tight for masonry construction; the marginal savings from generic air sealing will be modest. Target the measured leaks and the basement connection rather than a whole-building sealing campaign.
2. The client asks whether to include the basement mechanical room in the thermal boundary. What is your recommendation and reasoning?
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The basement contains a sealed combustion boiler, so it does not need indoor air for combustion, but it does contain distribution piping and possibly the DHW tank. The louvered door to the hallway means the basement is currently coupled to the conditioned space, so it is effectively inside the boundary whether or not it was designed that way.
Two defensible options exist. Option A: leave the basement inside the boundary, seal the louvered door and any rim-joist and penetrations, and insulate the basement walls. This captures distribution losses and reduces stack-driven infiltration through the basement. Option B: move the boundary to the basement ceiling, insulate and air-seal that plane, and isolate the basement. This is usually cheaper and protects piping from freezing only if the space stays above freezing.
Given a sealed combustion boiler and a masonry building, Option B is often the better value: the ceiling plane is easier to air-seal continuously than rubble or block walls, and it removes a large stack-leak area from the conditioned envelope. However, the decision must account for freeze risk to any water piping in the basement and for whether the boiler and DHW equipment can tolerate a colder space. The recommendation should be documented with the pressure-boundary drawing, because ambiguity here is the most common cause of failed air-sealing scopes.