What this covers
-
How Much Water a Household Puts Into the Air
-
What the Building Standard Assumes You Will Do
-
The Four Ordinary Reasons It Is Not Working
-
Where the Moisture Ends Up
-
The Cheapest Measurement Available
-
Why the Finding Should Come From Somebody With No Work to Sell
Owners of recently built homes are often surprised to find growth on a wall, because the assumption is that a new building is a dry building. The surprise is reasonable and the assumption is wrong, and the reason is worth understanding rather than treating as a fault.
A tighter house is not a drier house. It is a house where the moisture has fewer places to go.
How Much Water a Household Puts Into the Air
The quantities involved are larger than most people expect, and none of it comes from a leak.
Respiration releases water vapor continuously, more from a sleeping body over eight hours than most people would guess. Showers and baths release it in concentrated bursts. Cooking, particularly anything boiling or simmering uncovered, releases a great deal. Drying laundry indoors releases essentially all of the water that was in the fabric. Houseplants contribute. So does an aquarium.
In an older building most of that leaves without anybody noticing. Gaps around window frames, an unsealed floor, uninsulated cavities and a draughty roof space mean air exchanges with the outside constantly, diluting indoor moisture and carrying it away whether or not the occupants open anything.
Seal the envelope and that process stops. The generation does not change at all.
What the Building Standard Assumes You Will Do
California’s Title 24 standards require whole-building mechanical ventilation in new residential construction, referencing the ASHRAE 62.2 residential ventilation standard. The requirement exists precisely because a building constructed to modern air-tightness levels no longer ventilates itself.
So a new home is designed as a system with two halves: an envelope that keeps conditions in, and equipment that deliberately exchanges a designed quantity of air. The envelope half is permanent and silent. The equipment half has an off switch, a filter and a service interval.
When the second half is not working, the building does exactly what it was built to do, which is retain everything, including the water vapor.
The Four Ordinary Reasons It Is Not Working
It was switched off. Usually because somebody found it audible at night, occasionally because a guest turned it off and nobody noticed. This is the most common single cause and the cheapest to correct.
It was never commissioned. A unit installed and left at a default setting may move a fraction of its design airflow. It appears to be running, it makes the expected noise, and it is not achieving the rate the house was designed around.
The filter is overdue. Airflow falls as a filter loads. In a coastal location filters load with salt-laden fine particulate as well as ordinary dust, and a unit that has not been touched in several years can be moving very little air while sounding unchanged.
Something was altered. A renovation closed off a transfer path, a door was replaced with one that has no undercut, or a return was blocked by built-in furniture. The equipment runs correctly and the air cannot complete the circuit it was designed to complete.
Where the Moisture Ends Up
Water vapor stays in air until it meets a surface cold enough to release it. In a well-sealed home the coldest surfaces are predictable: the inside face of an exterior wall, particularly on the shaded elevation, and particularly behind something that stops air moving across it.
That is why the marks appear behind a dresser, inside a closet on an outside wall, behind a headboard, or in the corner of a room that is never used. Those are not places where water is entering. They are places where the air cannot move and the surface runs cold, so whatever the house is carrying condenses there first.
Near the shore the margin is thinner than it is inland, because outdoor air already carries more moisture more of the time and a ventilation strategy has less dry air to work with.
The Cheapest Measurement Available
Before anybody is called, a hygrometer left in the least ventilated room for a week is the single most useful thing an owner can do. It costs very little and it answers a question a single spot reading cannot.
Humidity moves through the day. A reading taken at two in the afternoon says almost nothing about what the room is doing at six in the morning after eight hours of occupancy with the door shut. Sustained readings above sixty percent explain a great deal, and a daily cycle that peaks overnight points firmly at occupancy and ventilation rather than at anything entering from outside.
That evidence also changes the survey. An assessment that knows the humidity profile can spend its time on surface temperatures, air exchange and the equipment, rather than searching the roof for an intrusion that is not there.
Why the Finding Should Come From Somebody With No Work to Sell
This matters more here than in an intrusion case, because the gap between the correct answers is enormous.
One end of the range is switching a unit on, having it commissioned properly and changing a filter. The other end is specifying and installing new equipment, or opening up walls on the assumption that water is entering from outside.
Most firms that assess a property also sell the remediation that follows, so the business deciding which end of that range applies also prices it. An assessor who performs no remediation has no stake in the answer and can report that the ventilation was switched off and the fix costs nothing. Arranging an independent mold inspection in Manhattan Beach on that basis is the difference between a finding and a quotation, and the practice can be found on Google Maps here.
Three free steps first. Check the ventilation is running. Pull the furniture an inch off the wall that keeps marking. Put a hygrometer in the quietest room and read it in a week.
