Most of this site is about things you can do yourself, because most damp problems genuinely are. This page is about the other set — the situations where continuing on your own costs more than stopping.
It is also about who to call, because the wrong specialist produces the wrong diagnosis. A damp proofing company sells damp proofing.
Call someone about mold when
- The affected area is larger than about 10 square feet — the EPA threshold where DIY cleanup stops being sensible
- It is inside the HVAC system — ducts, the evaporator coil or the drain pan. Growth there distributes spores through the whole house
- It followed sewage or flood water rather than clean water
- Anyone in the house is immunocompromised, has significant asthma or is undergoing treatment that suppresses immunity
- You can smell it clearly but cannot find it after checking the obvious places — it is inside a cavity
- Cleaning it has already failed twice and you do not know why
Call someone about the structure when
- A damp patch has a defined edge and worsens after rain, and you cannot find the external defect from the ground
- A wall shows a level tide mark with salt deposits and lowering the ground level did not help
- The water meter moves with everything turned off
- There is standing water in a crawl space or basement after rain
- Floor boards are cupping, or a floor feels soft underfoot
- You are considering work costing four figures or more on any diagnosis you are not certain of
Who to call for what
| Situation | Who | Watch for |
|---|---|---|
| Mold over 10 sq ft, or in ducts | Mold remediation contractor | Ask about containment and what gets removed vs cleaned |
| Suspected rising or penetrating damp | Independent damp surveyor | Not one employed by a company that sells the treatment |
| Clammy house with plenty of cooling | HVAC contractor | Ask about run time and latent capacity, not tonnage |
| Crawl space or basement water | Waterproofing or drainage contractor | Get external drainage assessed before internal tanking |
| Attic frost, ice dams, roof sheathing | Insulation and air-sealing contractor | Air sealing should come before added insulation |
| Cupped or damaged wood floor | Flooring specialist | Do not let anyone sand it while it is still wet |
| Persistent allergy or respiratory symptoms | Doctor, then an allergy test | Identify the trigger before rebuilding the house |
How to be a good client
Bring the evidence. Dated photographs of the patch over several weeks, your hygrometer readings, whether it changes with rain, and what you have already tried are worth more than any description.
Pay for the diagnosis separately from the work wherever the sums are significant. A free survey from a firm that sells the cure is a sales visit, and the damp proofing industry is the clearest example of that on this site.
Get the cause named, not just the treatment. A quote that says what will be installed without saying what is wrong is not a diagnosis — and if the answer is condensation, the correct treatment may cost nothing at all.
Finally, be willing to hear that it is condensation. It is the most common cause by a wide margin, it is the cheapest to fix, and it is the one an honest surveyor will tell you about for free.
Common questions
How much mold can I clean myself?
Up to roughly 10 square feet, per EPA guidance, on non-porous surfaces or sound drywall. Above that, in HVAC ducts, or after contaminated water, bring in a remediation contractor.
How do I choose a damp surveyor?
Pick one who sells the diagnosis rather than the cure, and pay for it separately. Ask them to name the cause before quoting a treatment — if the answer is condensation, the fix may cost nothing.
Who do I call about condensation?
Usually nobody. Condensation is the most common cause of household damp and the cheapest to fix — ventilation, habits and warming cold surfaces. Call someone when the pattern says penetrating damp, rising damp or a leak instead.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions