Which Trade Jobs Change House Pressure and Moisture

Why this matters

Most of the pressure and moisture problems in a house are side effects. Nobody set out to backdraft the water heater or grow mold in the closets; somebody replaced the windows, sealed the attic or hung a bigger hood, did good work, and left. The problem shows up weeks later and gets called in to a different trade, which finds nothing wrong with the thing that is complaining. The only person who knew what changed was the one who changed it. This card maps the common jobs to what each one does to the house, so the trade doing the job can ask the one question that matters before it leaves: what did this do to everything else?

Four things a job can move

Every job on the list below touches one or more of these:

• Leakage. How many holes the house has. Fewer holes means less uncontrolled air exchange, and every fan in the house pulls it harder. See related: The House as One Pressure System.
• Pressure. How negative or positive the house, or one room of it, runs. Fans, doors, duct leaks and the boundary's location all move it.
• Humidity. How much moisture the indoor air holds. Leaks dilute indoor moisture with outdoor air; in winter that dries the house, in a humid summer it wets it. See related: Relative Humidity Versus the Moisture Actually in the Air.
• The combustion appliances that remain. Whether any natural-draft appliance is left, and whether the job made its room more negative or its vent weaker. See related: How a House Pulls Flue Gas Back Down a Vent.

The matrix

  WHAT EACH JOB TENDS TO DO

  job                 LEAK   PRESS   RH     NAT-DRAFT
  ------------------  -----  ------  -----  ---------
  replace windows     down   fans+   up     risk up
  attic air sealing   down   fans+   up     risk up
  bigger range hood   same   neg+    vary   risk up
  move dryer / door   same   room-   same   risk up
  seal ducts          small  mixed   same   mixed
  sealed-comb furn    small  same    same   WH alone
  fireplace insert    down   mixed   up     mixed
  finish basement     down   room-   local  risk up
  encapsulate crawl   down   fans+   down   risk up

  LEAK  house leakage
  PRESS fans+ = same fans pull harder, neg+ = more
        exhaust, room- = one room more negative
  RH    winter indoor humidity tendency
  NAT-DRAFT  effect on natural-draft appliances left
  WH alone = water heater orphaned in the old vent

These are tendencies, not test results, and each row has exceptions noted below.

Job by job, and where each one hands off

Window replacement. New windows usually seal far better than the old ones at the sash and, if the rough opening is sealed, at the frame. Leakage drops, winter humidity rises because less dry outdoor air leaks in, and the existing fans pull the house harder. Condensation on the new glass in the first winter is the common callback; it is the house's humidity showing, not a window defect. Hand off: tell the customer the house is tighter, and if a natural-draft appliance remains, recommend a combustion-safety check and confirm CO alarms are installed.

Attic air sealing. The single biggest leakage reduction most houses get, because it closes the high holes where stack effect pushes air out. Everything above for windows, more so, plus the neutral pressure plane drops, which makes the basement less negative from stack alone even as the fans pull harder. See Stack Effect and the Neutral Pressure Plane. Hand off: weatherization practice is to test combustion appliances before and after air sealing; a shop doing the sealing without that capability routes it to someone who has it.

A bigger range hood. Leakage unchanged, exhaust added, often the biggest single fan in the house. In winter it dries the house while it runs; in humid weather it pulls in humid air. Hand off: Kitchen Hood Makeup Air has the code trigger and the reasoning; the cabinet or appliance installer flags it before hanging the hood, not after.

Moving the dryer or adding a laundry door. Leakage unchanged; one room goes negative while the dryer runs. Dangerous only if that room holds a natural-draft appliance, which it often does. Hand off: see The Clothes Dryer as the Fan Nobody Counts; add a transfer path or route a worst-case test.

Duct sealing. Mixed, because it depends on where the leaks were. Sealing supply leaks outside the boundary removes a depressurizing effect; sealing return leaks outside removes a pressurizing one, which can leave the house slightly more negative than before. Sealing a return leak in the room with the water heater removes a direct pull on that room and lowers the risk. Hand off: the HVAC tech sealing ducts knows which leaks were sealed and should say so on the work order. See Duct Leakage Outside the Envelope.

Sealed-combustion furnace replacing a natural-draft one. House pressure barely changes, though the old furnace's draft diverter was a hole in the room that is now gone. The real effect is on the water heater left behind: it is now alone in a vent or chimney sized for two appliances, and its draft may be too weak. Hand off: this is Combustion Safety After a Partial Conversion, and the HVAC installer owns raising it the day the old furnace comes out.

Fireplace insert or a sealed glass-door unit. Closing an open masonry fireplace removes one of the largest high holes in the house. Leakage drops and the remaining fans pull harder; but an open wood fire was itself a big exhaust device whenever it burned, and a direct-vent gas insert removes that. Net effect depends on how the fireplace was used. Hand off: the chimney or hearth installer flags any natural-draft appliance left on another flue.

Finishing a basement. New walls and a door around the utility area turn it into a closed combustion appliance zone. Framing against foundation walls adds cold surfaces where humid air can condense behind the drywall, and a new basement bath adds exhaust. Hand off: the finishing contractor leaves a transfer path or a louvered door to the utility room, keeps clearances to the appliances, and flags the new bath fan. See The Combustion Appliance Zone and the Door That Closes It.

Crawlspace encapsulation. The crawlspace moves inside the boundary, ground moisture is cut off and the house usually gets drier and tighter. If the encapsulation uses the exhaust method, a continuous crawlspace exhaust fan is a new, always-on line in the exhaust budget. Hand off: the encapsulation crew notes which method it used so nobody later caps it, and flags any natural-draft appliance. See The Attic and Crawlspace, Inside or Outside the House.

Worked example: three good jobs in one year

A 1970s two-story with a natural-draft gas water heater in the basement and a sealed-combustion furnace installed a few years ago. Blower door result before any work: 3,200 CFM50. Existing exhaust: a dryer at an illustrative 150 cfm and a bath fan at 80, 230 cfm when both run.

Using the depressurization estimate from Kitchen Hood Makeup Air, pressure equals 50 times exhaust over CFM50, raised to about 1.54:

The 2,700 and 2,200 figures are illustrative results for those two jobs; in practice each would come from a test-out.

Check the last line. 530 over 2,200 is about 0.241. Raised to 1.54 that is about 0.112, and 50 times 0.112 is about 5.6 Pa.

What the table shows. No single job did it. The windows moved the dryer-and-fan case from 0.9 to 1.1 Pa. The attic sealing moved it to 1.5. Neither would alarm anyone. The hood added to the original, leaky house would have produced about 3.2 Pa: 530 over 3,200 is about 0.166, raised to 1.54 is about 0.063, times 50 is about 3.2. Added to the house after two rounds of tightening, the same hood produces 5.6, about 1.8 times as much. The kitchen remodeler is the one standing in the house when the number crosses into territory a natural-draft water heater cannot handle, and the kitchen remodeler did the least to cause it.

Humidity moved too. The house's leakage fell from 3,200 to 2,200 CFM50, about 31 percent less: 1,000 divided by 3,200. Under the same weather, uncontrolled winter air exchange through the leaks falls in roughly the same proportion, and the moisture the family produces is diluted by that much less air. Closet and window condensation the next winter would not be a surprise. See related: Why Air Sealing Changes What a House Needs for Ventilation.

Who should have asked. All three. The window installer had the smallest effect and the easiest conversation: "your house is tighter now; if you add exhaust, have the water heater checked." The air sealer should have tested, or routed testing, as standard practice. The remodeler had the Kitchen Hood Makeup Air question in front of them regardless of the earlier work. Any one of the three prompting a combustion-safety check would have caught it, and the durable answer, replacing the orphaned natural-draft water heater with a sealed-combustion, power-vented or heat-pump unit, would have taken the safety question off the table for every later job.

The one question, by trade

• Anyone who seals: is there a natural-draft appliance left, and has anyone tested it since the house got tighter?
• Anyone who adds exhaust: what is the house's total exhaust now, and where does the makeup air come from? See Counting Every Exhaust Device in the House.
• Anyone who adds a door or a wall: did that enclose an appliance, a return, or a room with a supply and no return?
• Anyone who moves the boundary: which spaces are now inside, and what in them was designed for outside?

Write the answer on the work order. A one-line note, "house tightened, natural-draft water heater remains, customer advised to have combustion safety checked," is the record that tells the next trade where to look.

References

• See related: The House as One Pressure System; Counting Every Exhaust Device in the House
• See related: Kitchen Hood Makeup Air; Combustion Safety After a Partial Conversion
• See related: How Moisture Moves Through a Building; Moisture Control in Residential Construction
• See related: Radon Mitigation Basics Reference (tightening and depressurization both affect soil gas entry)