Headlines Team
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What this covers
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Most of what gets written about damp in older houses treats age as the variable. The house is from 1938, therefore it is a risk. The reasoning is intuitive and it is close to useless, because it does not distinguish between a 1938 house that has been left alone and a 1938 house that has been extended three times.
The second one is where the findings are, and the reason is structural rather than chronological.
Contents
- What an Addition Actually Creates
- Why the Original Structure Usually Performs Better
- Four Common Additions and How Each One Fails
- The Reason the Wall Reads Dry From Inside
- What a Thermal Camera Shows on Solid Wall Construction
- The Buried Window
- Coastal Air Accelerates the Whole Process
- What Gets Measured, and in What Order
- Where an Inspection Sits Relative to the Work
- Why the Person Measuring Should Not Be the Person Repairing
What an Addition Actually Creates
An addition creates a junction between old framing and new. That sentence is the whole subject.
Two structures built decades apart do not share a coefficient of movement, a code, a fastener standard or a flashing detail. They are tied together at a seam, and that seam was detailed by whoever held the contract in whatever year the work happened. Everything downstream of that follows from the quality of one joint that nobody can see once the drywall goes up.
The consequence is that the moisture path in an extended house is not the moisture path the original builder designed. Water that once shed off a continuous roof plane now meets an interruption. Air that once moved through a ventilated crawlspace now meets a poured slab. The building has two logics inside one envelope.
Why the Original Structure Usually Performs Better
This surprises people, and it should not.
An original structure has already survived every storm season since it was built. Its failures happened long ago and were either fixed or the building would not still be standing. What remains has been selected for by time.
An addition has not been through that filter. A rear extension from 2004 has faced twenty years of weather, and twenty years is enough for sealant to go and not enough for anyone to have noticed. The newer half of a house is frequently the wetter half, which runs directly against the assumption that drives most inspection requests.
Four Common Additions and How Each One Fails
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Addition type |
Where the water gets in |
What it looks like inside |
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Second story over an original roof |
Roof-to-wall junction, old rafter tails |
A ceiling stain well away from the entry point |
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Rear extension on a new slab |
Step between raised foundation and slab |
Persistently high crawlspace readings on the newer side |
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Converted garage |
Slab poured for a car, no vapor barrier beneath |
Damp at the base of the walls, musty at floor level |
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Enclosed porch or deck |
Original exterior wall now an interior wall |
Nothing visible at all until a cavity is opened |
The pattern in that table is worth naming. In three of the four cases the visible symptom appears somewhere other than the source, and in the fourth there is no visible symptom. That is why the diagnostic question in an extended house is never “where is the stain” but “where are the joints”.
The Reason the Wall Reads Dry From Inside
A wall can be wet inside its cavity and dry on the surface a person touches. Interior paint, particularly a modern acrylic, is far less permeable than the plaster or lime beneath it. It behaves as a membrane. Moisture arriving from the cavity side reaches that membrane and stops, and the room stays comfortable while the cavity does not.
This is the mechanism behind a complaint that comes up constantly: the wall was fixed, and a year later it came back. Sealing the interior face of a wall that is being fed from behind stops the water appearing. It does not stop it arriving. It goes somewhere else, usually to the floor junction, and reappears as a different problem that gets treated as unrelated.
What a Thermal Camera Shows on Solid Wall Construction
Thermal imaging reveals temperature differences behind finished surfaces. On a modern cavity wall that is useful. On an older solid-wall house that has been extended, it is close to indispensable, for a reason specific to this construction.
The original structure and the addition have different thermal masses. They hold and release heat at different rates. Point a camera at the finished interior and the outline of the original building appears through the plaster, because the two halves are at measurably different temperatures. The joint that was invisible is now the most visible thing in the frame.
That does not, on its own, prove moisture. It locates the geometry. The moisture meter then reads material moisture through the finish at the points the camera identified, and the two readings together are what a finding is made of.
The Buried Window
Enclosing a porch, a side return or a deck converts an exterior wall into an interior one. The wall does not know that.
Its windows are still flashed as exterior openings, sloped to shed water outward into what is now a wall cavity. Its cladding is still detailed to drain. Its weep holes, if it has them, now vent into an enclosed space with no airflow.
This is a case that almost nothing written about home additions covers, and it is one of the more reliable findings on any house where a porch was closed in. It also explains findings that otherwise look inexplicable, because the source is a component nobody thinks of as a component any more. From inside the finished room, there is no window there.
Coastal Air Accelerates the Whole Process
Marine air corrodes flashing faster than inland air. Salt is hygroscopic, so it holds moisture against a metal surface rather than letting it dry, and galvanized flashing, fasteners and fixings in a coastal band degrade on a shorter timeline than the same components a few miles inland.
Add the marine layer, which keeps surfaces damp through the morning and shortens the drying window every day, and a junction detail that would have lasted thirty years somewhere dry gets considerably less near the water. Nothing about that is dramatic. It simply moves the maintenance interval, and most owners are working from the interval they read about rather than the one their location imposes.
What Gets Measured, and in What Order
Sequence matters more than equipment.
- Moisture meter readings across every wall plane at multiple heights. The pattern up a wall says whether water is arriving from below, from behind or from above, and that alone eliminates most of the possibilities.
- Thermal imaging to map the extent of any anomaly and to locate the construction joints.
- Physical inspection of every crawlspace, sub-floor and roof space that can be reached, because a great deal of what matters is visible if somebody gets in there.
- Sampling, only where the readings justify it. Air sampling compares indoor spore counts against an outdoor control taken the same day, and surface sampling identifies what is actually present. Without the outdoor control the indoor number means nothing.
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Step |
What it answers |
What it cannot answer |
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Moisture meter |
Where material is wet, and at what heights |
What is growing, or how long it has been there |
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Thermal imaging |
Where the joints are, and how far a wet area extends |
Moisture directly; it reads temperature |
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Physical inspection |
What is actually visible in the cavity |
Anything behind a closed finish |
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Sampling |
What is present, against an outdoor control |
Which specific junction let the water in |
Steps one to three answer most questions. Step four exists to document, to quantify, and to satisfy a third party who was not present.
Where an Inspection Sits Relative to the Work
Ahead of it, and not only for moisture reasons.
Under SCAQMD Rule 1403, an asbestos survey is required before demolition or renovation activity in Los Angeles County. A house of the age this article is describing may also carry lead in its paint layers, since lead-based paint was banned for residential use in 1978 and repainting covers a surface without removing what is beneath it.
Both of those questions belong before a wall is opened rather than after, which puts them at the same point in the schedule as the moisture survey. Firms that carry out all three tend to run them on one visit, which is a scheduling convenience rather than a technical one, but it is the reason the three are so often quoted together.
Why the Person Measuring Should Not Be the Person Repairing
An independent inspector performs no remediation. That distinction decides what the report is able to say.
A company that inspects and then abates has written the scope of its own next invoice. It is not that such a report is necessarily wrong. It is that it cannot easily conclude that nothing needs doing, and on extended older houses that conclusion is reached more often than owners expect, because a great many of the findings turn out to be a drainage detail or a planter against a wall rather than anything inside the building.
Where the finding is real, a report from a firm with nothing downstream can be handed to three contractors who then bid against the same document, which is the only arrangement under which those bids are comparable. Independent mold inspection near the Hermosa Beach pier is carried out on that basis by a South Bay firm whose El Segundo office covers the coastal cities, and which does not perform removal work of any kind.
The short version. The age of a house is a weak predictor. The number of times it has been joined to something else is a much better one.
Anyone commissioning an inspection on an extended property is better served asking what the inspector intends to do about the junctions than asking what they think of the year it was built.
