Reading a moisture risk map
How to interpret a moisture risk map: it shows regional conditions, not buildings. Colour tells you the background pressure in an area; the variation within one block is usually larger.
Step by step
Establish what the colour actually encodes
Before reading anything, find out what quantity is shaded. Flood zone, precipitation normal, humidity, hazard index and modelled moisture burden are five different maps that look alike, and a legend that says only "risk" is not a legend. If the source will not tell you the quantity and its vintage, the map is decoration.
Check the resolution against the decision
A map shaded by county, metro or postal code cannot say anything about a building. Regional moisture burden is 20 of the model's 100 points, and it is identical for every property sharing a postal code. Using an area colour to judge one house means acting on a fifth of the picture.
Look for the terrain the map has flattened
Drainage varies in metres. One house at the foot of a shallow depression and another sixty metres up the rise have different drainage classes on the same street, inside one shaded polygon. Where a map matters most, it resolves least.
Read the housing stock behind the shading
Areas built out in one period carry that period's failure modes at scale. A district that is largely 1990s construction is carrying the model's worst era — tightened envelopes without matched ventilation, barrier EIFS, polybutylene — regardless of what the climate layer says. That is not on the map and it often matters more than what is.
Use it to choose where to look, then leave it
The honest use of an area map is triage across a search area or a portfolio. Once you have candidate properties, the map has given you everything it has, and the next question needs an address.
What an area map can and cannot support
It can support comparison between markets — which is what the metro index does explicitly, with the drivers, weights and vintages published so the ranking can be argued with rather than taken on trust.
It can give context to a score you already have. A property scoring Elevated in a low-burden area is being driven by its building or its ground, which narrows what to inspect. The same score in a high-burden area may be mostly climate, which is nobody's fault and not addressable.
It cannot tell you whether a house has a problem, and it cannot rank two houses on a street. Eighty of the hundred points in a property score are about the property.
Why flood maps get misread as moisture maps
Flood mapping is about inundation from outside — riverine, coastal, surface water. It is well made and it answers a specific question.
Most moisture damage in housing has nothing to do with it. Failed supply lines, condensation on cold surfaces, roof and flashing details, missing capillary breaks: none of these appear on a flood map, and a property outside every flood zone can be permanently damp for any of them.
The reverse misreading is just as common. A property in a mapped flood zone that has never taken water is being judged on exposure rather than on what happened, and exposure is a prior, not a finding.
Data vintage
Climate layers should be long-run normals — thirty years is the standard — and should say so. A layer built from one recent year is weather, not climate, and will move next year.
Hazard and claim layers carry the period they cover, which matters because a claim record starting in 1996 and one starting in 1978 describe different amounts of history. Any published figure that does not name its window cannot be compared with one that does.
Questions people ask
- Is a moisture risk map the same as a flood map?
- No. Flood maps cover inundation from outside. Most moisture damage in housing comes from plumbing, condensation and envelope details, none of which are mapped.
- My whole area is shaded high. Does that mean my house is at risk?
- It means the background pressure is high, which is a fifth of a property score. Whether your building is affected depends on its ground, its construction and its history.
- Why not publish a property-level map?
- A map dense enough to be property-level would publish an estimate about every individual building, most of them thin, and it would be read as a finding. Area comparison is published as a ranking with its method attached; property-level output goes to whoever asked about that property.